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Polyacrylamide induced flocculation of a cement suspension
Abstract
The use of cellulose, instead of asbestos, in the fibre cement composites manufacture, using the Hatschek process, reduces cement retention and makes necessary to use a flocculant which is crucial for the plant productivity. The use of different types and doses of polyacrylamides (PAM) as well as the addition process, have been studied to obtain an in-depth knowledge of floc properties. A real-time methodology has been used to study size, shape, strength and reversibility of formed flocs, based on a focused beam reflectance measurement (FBRM) system. The results have been corroborated by particle vision and measurement (PVM) analysis. This paper shows that anionic PAM (A-PAM) are the most suitable to induce cement flocculation and to obtain optimal properties of the formed flocs. This is because the flocculation process is enhanced by the interaction of the Ca2+ ions, produced by the cement hydration, with the carboxylic groups of the polymer chains. Cations not only increase the stiffness of the chains, improving floc strength, they also enhance the importance of the patching aggregation mechanism when the initial bridges are broken. Higher molecular weight polymers improve initial aggregation but the effect of anionic charge is more important for a stable flocculation with time under variable shear conditions. Results show that the optimal dosage is between 100 and 200 ppm.
... 2 flocculation, retardation, and bridging effects generated by PAMs in cement pastes [9][10][11][15][16][17][18][19][20][21][22][23][24]. ...
... The characteristics of the molecular structure of PAMs, which have only been outlined in the literature, have significant impacts on the cement properties. Negro et al. [17] found that APAM is the most suitable among the three main ionic types of PAMs for inducing the cement flocculation process. They [14] further concluded that the higher molecular weight of APAM significantly reduces the bending strength of the cement composite owing to its density reduction, whereas the higher anionic charge increases its bending strength. ...
... At the age of 3 d, the flexural strength of the APAM-modified cement increases by 40% compared with that of the blank group. APAM can form microgels that can be adsorbed on the surfaces of different cement particles by complexing Ca 2+ [10,17], which is conducive to improving the attraction between cement particles, thus improving the cement flexural strength. This test result can also be supported by the improvement in the microstructure at the aggregate-cement interfacial transition zone in a previous study [26]. ...
Polyacrylamides (PAMs) have been widely used as chemical admixtures to modify the workability of cement-based materials. However, polymers possess variable molecular characteristics that have different effects on the macro-and micro-properties of cement, which need to be further explored. In this study, three different ionic PAMs, namely anionic PAM (APAM), cationic PAM (CPAM), and nonionic PAM (NPAM), were applied to investigate their effects on the hydration and mechanical properties of Portland cement. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) tests were carried out to reveal the evolution of cement phase compositions. The results indicate that APAM significantly delays the cement hydration, increases the cement drying shrinkage, and reduces the cement strength development. However, CPAM and NPAM promote total heat release from the cement. CPAM and NPAM have minor effects on the cement drying shrinkage and strength development compared with APAM. Phase composition analysis reveals that APAM inhibits the dissolution of C 3 S and promotes the formation of Ca(OH) 2 after 1 d. In contrast, CPAM and NPAM mainly enhance the dissolution of C 3 S before 4 h.
... Water-soluble polymers mainly include polyvinyl alcohol (PVA), cellulose ether (CE) and polyacrylamide (PAM), which possess abundant polar and hydrophilic groups such as carboxyl, hydroxyl, amide and amine groups. Such special molecular structures make water-soluble polymers can dissolve in molecular form in water without surfactants [1][2][3][4][5] and exhibit unique chemical or physical properties and functions such as flocculation, viscosity enhancement, drag reduction and adhesion [6][7][8][9][10]. Thus, water-soluble polymers have extensive applications in the fields of sewage and drinking water treatment, paper and textile industries, oil exploitation, building materials and so on. ...
... As a typical water-soluble polymer, PAM is widely used as an organic thickener in the field of building materials. PAM exerts great effects on the early properties of fresh cement-based materials due to the interaction with cement particles as well as the ions in the interstitial solution of fresh cement slurry [6,[22][23][24][25]. Cement slurry modified with PAM displays a better cohesive quality and stability in cast states, thus reducing the separation risk of the cement-based materials components during transport and consolidation [26]. ...
... In addition, PAM can mainly be divided into three kinds of ionic types: anionic polyacrylamide (APAM), cationic polyacrylamide (CPAM), and nonionic polyacrylamide (NPAM). Negro [6] has concluded that APAM had the stronger ability to induce the cement flocculation process than CPAM and NPAM due to the interactions between calcium ions and the anionic carboxylic groups. However, there is still a lack of comprehensive studies on cement flocculation, hydration and structural build-up of different ionic PAMmodified cement slurries. ...
Polyacrylamide (PAM) has been widely applied as a viscosity modifier to optimize the workability of cement-based materials, but PAMs with different ionic types may show various effects, which is often ignored in practical engineering. In this paper, the early properties including the rheology, flowability, and setting of cement pastes modified by three ionic PAMs (anionic APAM, cationic CPAM, and nonionic NPAM) were contrastively studied. The absorption and hydration of cement pastes with different ionic PAMs were further investigated. Moreover, the contributions of physical flocculation and chemical hydration on the structural build-up of cement pastes were quantified by combining the static yield stress and heat release at the same time scale. The results indicated that three ionic PAMs showed different effects on the structural build-up, fluidity and setting of fresh cement pastes, which were dominated by the different physical flocculation and hydration retardation effects of three ionic PAMs on the cement pastes. The order of flocculation and hydration retardation effects of these three ionic PAMs was APAM ≫ NPAM > CPAM, which was concentration-dependence and evolved over times. The reason is mainly due to the distinct molecular structures and physicochemical properties of three ionic PAMs and their different adsorption behaviors with cements particles.
... The combination of nanoparticles with polymeric admixtures is a promising alternative to improve rheology control over time of CBM [10,11]. The synergetic effects produced by their combination can be related to size, shape and morphology of NC [12][13][14][15][16][17], and also to polymer chain, morphology and molecular weight of VMAs [10,[18][19][20][21][22][23][24]. ...
... Polymeric admixtures, as viscosity modifying admixtures (VMA) and high range water reducing admixtures (HRWRA), are also used to adjust rheological properties of cement pastes [18][19][20][21][22][23]25,[33][34][35]. VMAs are water-soluble organic polymers that increase plastic viscosity, enhancing rheology of liquid phase of CBM. ...
... The combination of VMAs and HRWRA in adequate proportions can produce mixtures with high segregation resistance and homogeneity [10,18,20,24,25]. Several types of VMAs are used to adjust cement paste rheology, as natural polymers (welan gum), semi-synthetic polymers (methylcellulose-based) and synthetic polymers (polyacrylamide, polyacrylate) [18,19,[21][22][23]35]. Four modes of action of VMAs have been described: a) water retention, b) entanglement (polymer-polymer interaction); c) bridging flocculation (polymer-particle interaction); d) depletion flocculation (difference in polymer concentration may increase the osmotic pressure and hence its yield stress) [10,18,20]. ...
Nanoclays, nanosilica and viscosity modifying admixtures can be used to tune rheological properties of cement pastes, in order to fulfill the requirements for non-conventional cast-in-place techniques. An experimental study on cement paste rheology was carried out to evaluate their combined effects on fluid cement pastes’ properties. A reference paste with cement blended with limestone filler and a low water to binder ratio was designed. A high range water reducing admixture (HRWRA), three types of viscosity modifying admixtures (VMA), and five nanocomponents (nanosilica and four different nanoclays) were added. Flowability, initial rheological properties and structural build-up of cement pastes were assessed with mini-cone slump test and dynamic shear rheometer test (DSR). The synergies between nanoclays, nanosilica and VMAs were evaluated. It was observed that an adequate combination of admixtures and nanoclays reduced the amount of HRWRA required to achieve target flowability, regarding the same components uncombined. The type and amount of VMAs and nanocomponents also produced changes on yield stress, viscosity and structural build-up. Their effect on structural build-up of fresh paste was related to reversible and non-reversible mechanisms, comparing the reduction of spread diameter of fresh samples stirred and left at rest over time measured with the mini-cone.
... Some recent studies have estimated the zero-point of charge using the streaming potential, which is easy to measure, and can be used as an in-situ method for online monitoring [204]. ...
... Unlike FBRM, PVM is better suited to lower suspended solids concentrations [204]. ...
In recent years, researchers have investigated a variety of approaches to remove natural
organic matter (NOM) from source water since it has been problematic for water treatment
techniques. NOM is a complex organic matrix which contaminates freshwater bodies and
is the cause of numerous issues in water treatment plants such as membrane fouling, filter
clogging and algal growth. Most importantly, the NOM pool contains disinfection byproduct (DBP) precursors. Even though NOM has no direct adverse health effects, some
DBPs are toxic, and thus, NOM removal is a key part of water treatment.
The coagulation process is one of the generic water treatment methods used to remove
NOM. The primary mechanism of the coagulation process is surface charge neutralisation
which is done by coagulants. Finding the optimal coagulate dose is challenging without
measuring the surface charge. Therefore, this study elaborates a method that monitor the
surface charge, that is streaming potential (SP) measurements. The coagulation process can
be reliably optimised with SP measurements because SP data can be measured in real time.
Therefore, other factors influencing the coagulation process, such as pH, coagulant aids,
temperature and turbidity, can be optimised efficiently with SP measurements. The pH
value plays a critical role in the coagulation process. Suspended NOM changes its surface
according to the variation of the pH value of the solution. The further hydrolysis of
inorganic coagulants depends on the pH value. This study also investigates the effect of
coagulant aid on aggregation. Bentonite and kaolinite clays are applied in the coagulation
process. The results indicate that kaolinite is an effective aid for fast flocculation owing to
its self-aggregation ability, and also it is a non-swelling clay with less surface charge than
bentonite (swelling clay). Further, polyelectrolytes are charged (negatively/positively) and
therefore, they change the total surface charge when added after the primary coagulants.
Thus, the sequential addition of coagulants is important. This study, therefore, suggests that
metal coagulants should be added after adding polyelectrolytes.
So far, a few factors have been identified as being potentially significant in the coagulation
process. The number of suspended particles in source water is another significant factor
that affects the coagulation process. Suspended particles create turbidity in water. Thus,
turbidity is an excellent indicator of suspended particles. However, there is scant literature
on the effect of temperature in the coagulation process. In Australia, the water temperature
changes from 1 °C to 40 °C approximately with seasonal changes. Hence, this study
investigates the coagulation process during temperature variations. The results indicate that
the coagulant dose is proportional to the temperature because the solubility of metalxvii
coagulants (Fe3+) and the surface charge of NOM depends on the temperature. Further, the
results show that floc settling is considerably slow at low temperatures (<10) and high
temperatures (>30). Turbidity removal efficiency was calculated to be 83% at 2 °C and
reduced to 78 % at 40 °C. Thus, the sludge recirculation method is employed with
coagulation in the temperature range of 2 °C- 40 °C. It can be concluded that the sludge
recirculation method improves coagulation-flocculation efficiency, and NOM removal
efficiency improved by more than 20 %. This study also investigated hydraulic conditions
on floc settling, and G is 20.5 s-1 to 28.7 s-1 (40-60 rpm) range is better for faster settling of
flocs formed by NOM coagulation by the dual coagulation system.
The climate is critical in summer in Australia. Due to extreme high-temperature levels,
bushfires occur in many parts of Australia. Bushfires increase the temperature of water
sources, while bushfire residuals leach inorganic and organic materials in water bodies.
This is one of the primary reasons for water pollution. Hence, this study evaluates the
impact of bushfire residuals, including NOM on water pollution and the treatment method
as coagulation. The results show that of the bushfire residuals tested, fly ash had the most
significant impact on water quality: pH increased from 7 to 9, alkalinity increased to 750
mg/L as HCO13−, turbidity increased to 24.5 NTU. Whereas soil that is affected by bushfire
shows fewer pollutants in water than the ash. i.e. heated soil (300 °C) leached organic
matter and showed UV254 absorbance of 0.069, and it was reduced to 0.036 when the soil
that was heated at 600 °C leached in water. Which means, that NOM is absent after
complete oxidation when the soil is burnt at 600 °C.
The literature indicates that the coagulation process has been modified with many
engineered components, including centrifuge coagulation reactors and laminar clarifiers.
However, few researchers have investigated venturi oxidation coupled with the coagulation
process. Therefore, this study attempts to investigate the effects of venturi oxidation on the
coagulation process. The venturi setup injects fine air bubbles into the coagulation reactors.
The study investigates whether the dissolved oxygen level increases rapidly and reaches
saturation level. Additionally, the coagulation system coupled with the venturi setup
effectively removes both suspended and dissolved NOM. Also, Venturi setup provides
good flocculation conditions and thus improves particle collision frequency. As a result,
turbidity and UV254 removal increased to 82% and 87%, respectively, with the venturi
application.
... Based on this property, PAM could be used in the field of shotcrete, single-coat plastering mortar or tile adhesives [6]. According to [7][8][9], the flocculation properties of fiber reinforced cement-based composites prepared by polyacrylamide could be appropriately tuned to ensure the casting process smoothly. Due to its ability of reducing bleeding, segregation, and significantly increasing the internal cohesion of the traditional concrete, PAM was always employed as the antiwashout admixture in underwater pouring, filling, or repair engineering [10][11][12]. ...
... This is because PAM belongs to the absorbable polymer, which could stick on the surface of cement particles through chelation or hydration bonds as the anchorage points [33]. Its long-chain structure of polymers could simultaneously adsorb onto several cement grains and bridge them together [2], through the interaction between cement particles and polymer intermolecular as the connection nodes, resulting in the intertwining structures in the polymer system [7,40]. With the progress of shear protocol, the network floc structure in fcps is gradually destroyed. ...
In this article, the rheological and thixotropic properties of fresh cement pastes (fcps) with nonionic polyacrylamide (PAM) are investigated through the coaxial rotary method. The B-HB model is resorted to fit the shear stress versus shear rate diagram, from which the slurry plastic viscosity is particularly discussed. The results show that with the development of PAM, the plastic viscosity increases first, then decreases, and then increases again, exhibiting the “up-down-up” trend, which is consistent with the fcps fluidity. The adsorption, lubrication, and entanglement mechanisms of PAM are successfully used to interpret this phenomenon. Combined with the suspension density, the relationship among plastic viscosity, flowability, and density is successfully established follower via a multivariate linear regression method. The Durbin-Watson coefficient, variance inflation factor, μ significance, ρ significance, and R 2 are 2.122, 1.024, 0.014, 0.004, and 0.776, respectively, demonstrating the feasibility of fitting formula. Besides that, the PAM containing slurry which exhibits the explicit thixotropy is also found according to the appeared hysteresis curve during one shearing cycle. With the increase of PAM dosage, the thixotropic indexes including Th1 and Th2 decreased, until the PAM exceeded 0.5%-0.6% dosage, both of them yielded negative values. The phenomenon that the fcps final shear stress exceeds its initial value occurs.
... So far, only a few researchers studied 3D printed cementitious materials modified by adding another major water-soluble polymer, polyacrylamide (PAM) [19,[23][24][25]. However, some progress has been made in the physicochemical action of PAM in cement-based materials. ...
... Between cement particles, the van der Waals force and electrostatic repulsion were changed by the adsorption effect of APAM micro-gels, which can explain that APAM has ability to improve the workability of the 3D printed cementitious materials. In addition, Negro et al. [25] found that APAM was responsible for the interaction between polymer and cement coagula in water saturated in Ca(OH)2. ...
Weak interlayer interface is a major drawback of three-dimensional (3D) printed cementitious materials technology. In this paper, the effect of anionic polyacrylamide (APAM) with different dosages on the workability, interlayer interface properties (i.e., interlayer durability, shear bond strength) and interface microstructure of 3D printed cement mortar were studied. The relationship between the workability regulated by APAM and the interlayer interface properties was analyzed. The results showed that the interlayer interface properties of printed artifacts had a good correlation with the workability of fresh cement pastes. With the increase of APAM content, the plastic viscosity and structural build-up rate of cement paste improved. Meanwhile, the interface microstructure defects increased, and the porosity became higher, hence resulting in the decline of the interlayer durability and shear bond strength. In addition, the introduction of APAM can improve the cohesiveness and layer stacking performance of 3D printed cement mortar.
... PAM are usually used as thickeners or flocculants in various industries of building materials industry. In the production process of fiber cement composites, the addition of PAM can make them flocculate and suitable for pouring [31][32][33][34], which lead to the rapid development of fiber reinforced concrete. According to [35][36][37], PAM can increase the density of industrial products, which increase the mechanical strength. ...
... PAM is also used as an anti-washing additive in concrete applications because PAM molecules can reduce bleeding and significantly increase the cohesion of fresh concrete when poured underwater [36,38]. With the extensive use of flocculants in fiber reinforced concrete, many scholars began to focus on the effect of PAM on strength of fiber reinforced concrete [32][33][34][35][36][37]. Negro found that the greater the molecular weight of PAM, the worse the flexural properties of the fiber reinforced concrete [32,33]. ...
Deep understanding of the influence and mechanism of A-PAM on the early properties of the cement-based materials is of both scientific and practical importance. In this paper, the effects of anionic polyacrylamide (A-PAM) with different mass ratios on the early properties of cement, including hydration, structural build-up, setting time and early strength development were studied. The interaction between A-PAM and cement particles was also investigated. The relationship between the adsorption, retardation of A-PAM and the early properties of cement paste was then discussed. The results showed that the retardation and bridging effect generated from A-PAM jointly affect the early performances of cement. A-PAM can be adsorbed onto the surface of cement particles until a saturation point is reached. When the A-PAM dosage is below the saturation point, A-PAM mainly acts as flocculant, bridging cement particles and increasing the connection between cement particles. This accelerates the structural build-up and setting of cement paste. When the A-PAM dosage is above the saturation point, the dominant role of A-PAM is retarder, which highly retards the cement hydration and setting, thus slows down the structural build-up and reduces the early strength.
... Some recent studies have estimated the zero-point of charge using the streaming potential, which is easy to measure, and can be used as an in-situ method for online monitoring [131]. However, there is some controversy about optimisation using streaming potential because the isoelectric point may not indicate the point at which flocculation will occur for some coagulation mechanisms. ...
... Particle mobility, size, and shape can easily be tracked via image analysis, and the method has been applied to tailings and mineral systems in a mining context to optimise flocculation processes. Unlike FBRM, PVM is better suited to lower suspended solids concentrations [131]. Clearly in-situ measurement, such as the streaming potential, FBRM, and PVM provide the opportunity for plant operators to quickly dose and titrate a water system for optimum NOM removal. ...
Natural organic matter (NOM) represents a range of soluble and insoluble material which can have considerable impact on drinking water quality. In addition to creating problems with taste, odour, clarification, and colour, removal of NOM is problematic because it can initiate the formation of disinfection by-products, which can adversely affect human health. Numerous technologies and methods have been employed to remove NOM in water treatment, with the most common processes involving the use of coagulants and similar technologies. This paper provides an overview of the most widely studied coagulants, coagulant aids, dual coagulants, and alternative coagulants. The paper also investigates the effects of operating parameters such as temperature, coagulant dose, pH, use of inorganic salts, inorganic polymeric coagulants, and organic polyelectrolytes in terms of charge neutralisation, polymer adsorption, and polymer bridging. Finally, emerging technologies and the use of novel coagulants are investigated.
... Further, this result can be attributed to the high degree of flocculation that occurs when the cement powder is mixed with water [7]. This phenomenon has been attributed to the various physical actions and chemical reactions that occur during the mixing process, namely, the attraction between the positive and negative charges, van der Waals forces that exist between the particles, formation of hydrogen bond, and Brownian motion, which leads to collision flocculation [8][9][10]. From a macroscale perspective, the degree of flocculation of cement slurries can be quantified in terms of their rheological properties [11,12]. ...
... On the other hand, the cement particles are strongly hydrophilic and have high surface potential energy because of their small size. In order to reduce the surface potential energy, flocculation occurs spontaneously between the particles [9]. The increase in water-to-cement ratio leads to decrease in the density of cement particles per unit volume and increase in the particle spacing, which reduces the electrostatic effect and surface tension between the particles. ...
... adsorbed on unhydrated cement particles and inhibited the hydration reaction (Sukmak et al., 2020). Additionally, the grafted PAM on NR may interact with Ca 2+ released from the cement hydration resulting in a decrease in Ca 2+ for further production of the portlandite phase (Negro et al., 2006). Fig. 2c presents DSC thermograms of SC samples and the corresponding enthalpy (ΔH) are summarized in Table 1. ...
Natural rubber latex grafted with polyacrylamide (NRL-g-PAM) was employed as the cement admixture for cement pastes. During the preparation of cement pastes, adding NRL-g-PAM improved the homogeneity of freshly prepared cement pastes compared to unmodified NRL. The presence of NRL retarded the hydration of cement, and the retardation was more pronounced with increasing NRL content in the cement pastes. However, the chemical structures of the hydration cement pastes remained unaffected by the presence of natural rubber (NR). Scanning electron microscope (SEM) images of NRL-g-PAM-modified cement pastes revealed the formation of polymeric films in the cement matrix. Beneficially, the polymeric film of NR in the cement pastes enhanced the toughness due to the effective energy absorption of an elastomeric NR. Compared with unmodified cement paste, adding NR-g-PAM improved the flexural strength, which arises from enhanced rubber-cement matrix bonding to compensate for a reduction of cement bonding and cementitious products.
... Both admixtured mortars MDry0.5 and MSol0.5 have a dry granular-shaped structure, which reduces the cohesive strength of the entire material and, by the way, the strengths. This finding is probably due to a flocculation mechanism of FM on cement grains previously described by Dickinson and Eriksson [42] and confirmed by Negro et al. [43]. This macroscopic structure is representative of the low bulk density of these mortars. ...
Biobased mortars are gaining interest because of the opportunity to upgrade agricultural byproducts and to reduce the environmental impact of cementitious materials. However, to overcome the drawbacks linked to the use of flax byproducts, such as mucilage and its hygroscopy, it’s necessary to study the impact of the water-to-cement ratio and the addition of mucilage on mortars. The W/C ratios range from 0.5 to 0.8 and the mucilage is added anhydrous with cement or pre-dissolved in the water. High W/C ratios affect both fresh and hardened properties of mortars. A high ratio accelerates the setting process and generates more heat released on unmodified mortars while decreasing the induction period. Conversely, when mucilage is added, the setting time is increased due to a poisoning effect affecting the hydrate growth by a chemisorption mechanism on Ca2+. This delay is higher as the W/C ratio increases and the mucilage is in-solution. Anhydrous mucilage accelerates the setting until the highest water content is reached. The undissolved polysaccharides of the mucilage cannot impact the hydration process at a W/C ratio below 0.8. Increasing the water content is beneficial for the mortar strength but an excessive W/C ratio is harmful for the unmodified mortar. Mucilage polysaccharides generate a lack of strength compared to the standard. This reduction in strengths is lowered in flexural tests due to an elastic behavior conferred by the mucilage. The changes in properties are not correlated to any mineralogical modification as shown by FTIR and TGA analyses.
... For instance, the type of ionic group present in PAM has also been found to influence the workability of cementitious materials, where anionic PAM demonstrates superior thickening performance compared to non-ionic and cationic PAMs. The research conducted by Negro et al. [24] and our previous work [25] supported that anionic PAM possesses the best anion adsorption, whereas non-ionic PAM may be partially hydrolyzed and exhibits some degree of adsorption, and cationic PAM has a negligible adsorption ability. ...
... Nevertheless, the maximum values corresponding to different types of PAM dosage were different, and the impact of APAM was the greatest. Negro et al. [92] investigated that APAM had a greater ability to induce cement flocculation, compared with CPAM and NPAM. This is due to the strong interactions between Ca 2+ and COO − groups in the former system. ...
... Normally, polymers with higher molecular weights or more side chains could attach more particles and form denser entangled network structures, thereby resulting in better water resistance [66,67]. Compared with cationic and nonionic types, anionic AWAs may connect more easily with the metal cations in cement particles, thus resulting in higher water resistance and lower fluidity [68]. By contrast, AWCs modified with nonionic AWAs appear to have better fluidity, despite its relatively poor water erosion resistance [69]. ...
Anti-washout concrete (AWC) is a special cement-based material that can be directly employed in underwater environments without dispersion. It was developed around 50 years ago, and more than 150 journal articles and technical reports have been published. This paper provides a holistic review about the basic fresh-state and hardened-state properties of the AWC, such as water (washout) resistance, consistency, bleeding and segregation resistance, mechanical properties, and durability, as well as relevant testing methods. The discrepancies between AWC’s characteristics and those of conventional concrete are clearly presented. The mixture compositions, supplementary cementitious materials (SCMs), and other conditions that affect AWC’s performance are also elaborated. Finally, the paper discusses the specific performance requirements of AWC and corresponding construction strategies for its different applications, including normal construction, marine engineering, bulk filling, and repair practices. Future research needs to stimulate the development of AWC are also discussed.
... Conventional visualization by scanning electron microscopy (SEM) analyses are not applicable for a fresh paste in a hydrous or liquid state [21]. Some innovative in-situ testing methods have been employed (such as ESEM, FBRM/PVM, etc.) to investigate the microstructural features of fresh cementitious mixtures [22][23][24][25][26]. However, such understanding of early-stage AAM mixtures is very limited up to date. ...
This study aims to interpret the early-stage rheology of alkali-activated slag (AAS) paste from microstructure perspectives. The microstructures visualized by cryogenic scanning electron microscopy (cryo-SEM) revealed the essential distinction between hydroxide and silicate-activated slag pastes. The hydroxide-based mixture showed typical suspension features, where slag particles were dispersed in the hydroxide activators. In the hydroxide media, even at very early ages (5 min), the solid grains were attached to each other through rigid connections of reaction products, which resulted in high yield stress. As for the silicate-based mixtures, an emulsion phase has been observed between slag particles, which consists of discontinuous water droplets and continuous silicate gels. Fine emulsions with smaller water droplets were observed as the silicate modulus of activators increased, which dispersed the slag particles but on the other hand improved the viscosity of the paste. With increasing water to binder ratio, both yield stress and viscosity of AAS pastes significantly reduced.
... The cement bleeding includes four physical and chemical processes, which can be distinguished into sedimentation, flocculation, consolidation, and hydration in principle [14,15]. Among them, particle sedimentation and self-weight consolidation are mainly considered when establishing the quantitative model to describe the bleeding phenomenon. ...
The bleeding of cement slurry influences its key rheological parameters, such as the apparent viscosity. In this paper, we proposed an analytical model for describing the development of apparent viscosity during the bleeding process. In this model, the self-weight consolidation theory was employed to interpret cement slurry bleeding, and the water-cement ratio as an intermediate variable to establish the relationship between the bleeding time and the apparent viscosity. The model requires five parameters, all of which can be determined conveniently through laboratory tests. A series of experiments were employed to validate the proposed model. The results show that this model can predict the apparent viscosity variation during the entire bleeding time, and two increase stages of apparent viscosity during the bleeding process were captured. The limitation of this model and further research are discussed briefly.
... The second is the structure caused by slurry sedimentation during the grouting (see Fig. 12 (b)). Under the complicated physical and chemical reaction (flocculation, hydration) of the cement slurry [43,[48][49][50], the cement particles clump together to form groups that larger than original particle size. When the cement slurry passes through the constriction structure, due to the mismatch between the size of cement particle groups and the flow channel, cement particle groups with a larger size block before the constriction structure and the cement particle groups with a smaller size can pass through. ...
Natural fractures are normally accompanied with a certain roughness, which could significantly affect the grout penetration through them and grouting effect. In this paper, an experimental system for studying grout flow through rock fractures with three-dimension rough surfaces was presented. This apparatus can simulate the grouting process in rough rock fracture, and the real-time grout penetration through rough fracture can be directly observed. To evaluate the effect of the surface roughness on grout flow within rough fractures and obtain the distribution characteristics of the cement slurry concentration in rough fracture, two different grouting tests were conducted. Results show that the slurry penetration front advanced unevenly under the radial and quasi-one-dimensional diffusion form. Water was bleeding from the slurry penetration front when the slurry penetrated at a low speed. The concentration of cement slurry decreased along the direction of grout penetration, showing an obvious flow thinning trend. The distribution of cement slurry concentration along the penetration zone was related to the penetration length and penetration width. This test system is capable of providing potential guidance for evaluating the grouting performance in construction engineering.
... Others have used aggregate imaging to help elucidate structural information from FBRM data. Negro et al. (2006) Previous studies have sought to describe the structural characteristics of aggregates as formed in different industrial solid-liquid separation applications (Barany et al., 2009;Liu et al., 2017;Moruzzi et al., 2017), but there is still limited knowledge on how such structures influence dewatering in the minerals industry, especially as related to flocculation conditions within thickener feedwells. This work is part of a broader study into the impact of the applied mixing duration on aggregate structures formed by polymer bridging flocculation, and an important aspect of that is to establish how changes in structure can be reliably determined. ...
Mineral processing produces large volumes of dilute tailings that are flocculated and thickened, returning water to the process. The drive towards more sustainable processing has seen seawater considered to replace fresh water usage and an increased focus on how flocculation conditions can influence the fractal-like aggregates formed. The fragility of such structures and the likelihood that they change with time makes their study challenging.
A synthetic tailings slurry of quartz and kaolin in seawater was flocculated at different mixing times and flocculant dosages, with aggregate size monitored over time by focused beam reflectance measurement (FBRM) and hindered settling rates determined after set reaction times. A model relating hindered settling rate to size as estimated by FBRM was used to derive an aggregate fractal dimension (Df) at each time. With flocculation at typical tailings solids concentrations rapid, aggregate breakage will normally dominate after very short reaction times. Under mild mixing conditions, settling rates are higher than expected from sizes measured in real time due to aggregate growth continuing after mixing ceases, leading to inflated Df estimates. An apparent decline in Df under more intense mixing was consistent with an increased presence of high aspect ratio (low effective porosity) aggregate fragments, supported by in situ aggregate imaging at different reaction times. In contrast, the observed trends for a reduction in Df with mixing intensity and an increase with dosage are both thought to represent the expected aggregate structure changes for the synthetic tailings.
While estimating aggregate structural properties by relating FBRM sizes to settling rates can offer useful insights, it is important to recognise the aggregation state observed by FBRM under mixing applied to induce flocculation may differ from that which exists during hindered settling. Values of Df derived in this manner must therefore be viewed with caution.
... However, some of these methods partially destroy the ice crystal structure during sample preparation, and none of them have been able to directly measure the ice crystal size in the exit stream of the product during the freezing process. Recently, online techniques such as the focused beam reflectance method (FBRM) have been developed for in situ monitoring of CSD in the crystallisation processes of chemical and pharmaceutical products (Barrett and Glennon, 2002;Negro et al., 2006). In the case of ice crystallization, Haddad et al. (2010) have successfully used the FBRM technique to follow the evolution of ice crystal size during batch freezing of sucrose/water solutions. ...
La congélation partielle du mix dans un échangeur de chaleur à surface raclée (ECSR)est l'étape la plus critique dans la fabrication d'un sorbet, car c'est la seule étape où de nouveaux cristaux de glace se forment; par la suite ces cristaux ne font que grossir. L'objectif principal est de produire un grand nombre de cristaux les plus petits possibles afin d'obtenir une texture onctueuse. Pendant le procédé de congélation, le produit est soumis à des interactions couplées d'écoulement du fluide, de transfert de chaleur, de changement de phase et de cisaillement. Ces interactions sont déterminées par les conditions opératoires du procédé de congélation et affectent l'évolution de la distribution de taille des cristaux de glace, ainsi que la texture finale du produit. Ce travail présente la caractérisation expérimentale et la modélisation du procédé de congélation d'un sorbet. La congélation du sorbet à été effectuée dans un ECSR à l'échelle pilote. L'objectif principal de ce travail est l'étude de l'influence des conditions opératoires du procédé de congélation sur les caractéristiques finales du produit: distribution de taille de cristaux de glace, température du produit, fraction volumique de glace et viscosité apparente. Le comportement de l'écoulement du produit dans l'ECSR a été caractérisé par une étude expérimentale et une modélisation de la distribution du temps de séjour (DTS). Une approche de modélisation de la cristallisation de la glace couplant le modèle de DTS avec des équations de transfert de chaleur et de bilan de population des différentes classes de taille de cristaux a été développée. À partir d'une première estimation des paramètres, le modèle de cristallisation prédit de façon satisfaisante les tendances expérimentales et donne un bon aperçu de l'évolution de la distribution de taille des cristaux de glace au cours du procédé de congélation dans l'ECSR.
... According to other studies, this might also be related to the collapsed network under the steric repulsions among HPMC molecules in solution [49] or the transient network with less stability in XG solution [50]. Thirdly, partially hydrolyzed VMA molecules with anionic charges may interact with calcium ions (a typical example as the complexation of carboxyl group with Ca 2+ for APAM) and be adsorbed simultaneously on several cement grains to enhance the flocculation state due to bridging effect [51,52]. HPMC molecules can also be adsorbed on cement particles, which possibly competes with the adsorption of SP and degrade the dispersion effect of SP [53][54][55]. ...
The influence of viscosity-modifying agent (VMA) on air-void system of self-compacting concrete has attracted considerable attention in recent years. But for conventional concrete with a sharp loss of air content due to vibration operation, the role of VMA on air-void system remains unclear. This paper aims to validate and clarify the positive effects of VMA on air bubbles stability and air voids homogeneity in vibrated concrete. Three typical VMAs with variable dosages were incorporated to prepare twenty air-entrained fresh mixtures. Rheological properties of mortar matrix portion of fresh concrete, surface tension of aqueous solution and time-dependent air content were evaluated for every mix. The air-void system in hardened concrete was inspected at three casting depths of prism specimen. Results showed that the surface tension of VMA solution played a key role in the stabilization effect on air bubbles, which strongly varied with VMA molecules. A higher plastic viscosity induced by the higher VMA dosage was favorable for maintaining the homogeneous distribution of entrained small air bubbles and reducing the formation of large voids in vibrated concrete. Therefore, a proper incorporation of VMA can improve the freeze-thaw resistance of vibrated concrete with lower spacing factor and smaller chord length.
... Indeed, polymers, and in particular neutral (PAM) or partially hydrolyzed (HPAM) polyacrylamides are used in a wide range of industrial applications, such as water treatment by flocculation of particles [1], soil stabilization [2,3], agriculture [4], cement flow control [5][6][7], and last but not least, in the field of enhancement oil recovery (EOR) where acrylamide based polymers are massively used mainly to increase the viscosity of the injected fluid [8]. The study of competitive adsorption is of particular interest in this last case where PAM or HPAM of very high average molar mass with broad molar mass distributions are involved. ...
The present work aims at studying the outcome and kinetics of competitive adsorption between low and high molar mass chains of neutral polyacrylamide (PAM) or partially hydrolyzed polyacrylamide (HPAM) in aqueous solution at the surface of siliceous material. Monodisperse neutral and charged polymers, PAM and HPAM, were synthesized by RAFT/MADIX polymerization and their adsorption, alone or in competitive process, was studied. The analysis of adsorption measurements were carried out quantitatively by size exclusion chromatography which allows for distinguishing the relative adsorption of the chains depending on their molar masses. The effect of the average molar mass, dispersity as well as the kinetics of the displacement of pre-adsorbed polymer chains onto the surface by longer chains was investigated. We evidenced that both PAM and HPAM exhibit the same trend. Close to surface saturation, the high molar mass chains desorb polymer chains of lower molar mass. Moreover, by mixing different monodisperse polymers we were able to model the behavior of the polydisperse systems usually encountered in practical applications. By following adsorption over time, our results show that while low molar mass polymers diffuse rapidly and adsorb first, they are displaced by high molar mass ones once they reach the surface. This result remains unchanged even if the low molar mass chains are added several hours before the higher ones, showing that there is no kinetic limitation to the displacement of low molar mass PAM and HPAM by higher molar mass ones. The understanding and prediction of this complex competitive adsorption behavior over time is of utter importance for the control of the mechanism of action of these polymers notably in industrial processes. Can be downloaded on
https://authors.elsevier.com/c/1ZPhf3IywUBZJx
... The most common flocculants used in C-FCC production by the Hatscheck process are anionic polyacrylamides (APAMs). This is because the flocculation process is enhanced by the interaction of the Ca 2+ ions, produced by the cement hydration, with the carboxylic groups of the polymer chains [118] (Figure 5a). However, the use of APAMs can reduce the flexural strength of the composite. ...
Nanocelluloses (NCs) are bio-based nano-structurated products that open up new solutions for natural material sciences. Although a high number of papers have described their production, properties, and potential applications in multiple industrial sectors, no review to date has focused on their possible use in cementitious composites, which is the aim of this review. It describes how they could be applied in the manufacturing process as a raw material or an additive. NCs improve mechanical properties (internal bonding strength, modulus of elasticity (MOE), and modulus of rupture (MOR)), alter the rheology of the cement paste, and affect the physical properties of cements/cementitious composites. Additionally, the interactions between NCs and the other components of the fiber cement matrix are analyzed. The final result depends on many factors, such as the NC type, the dosage addition mode, the dispersion, the matrix type, and the curing process. However, all of these factors have not been studied in full so far. This review has also identified a number of unexplored areas of great potential for future research in relation to NC applications for fiber-reinforced cement composites, which will include their use as a surface treatment agent, an anionic flocculant, or an additive for wastewater treatment. Although NCs remain expensive, the market perspective is very promising.
Background
Removal of suspended kaolin at low concentration (0.25 wt.%) is tedious hence not investigated at random. An architecturally decorated semi-synthetic flocculant derived from Xanthan gum or XG, has been synthesized and specifically used to unsettle the stable kaolin suspension with high efficiency.
Methods
Hyperbranched Polyacrylamide or HBPAM was grafted onto XG following Strathclyde method whereby the extent of hyperbranching of PAM was controlled by the molar ratios of brancher (methylene-bis-acrylamide or MBA) to chain transfer agent (dodecane thiol or DDT). The chemical structure was established through various analytical characterization.
Significant findings
It was evident that the copolymer with maximum extent of branching (70.5%, designated as H3) exhibited fastest settling time as well as lowest turbidity simultaneously, especially at pH 7 (58 secs, 70 NTU) and at the isoelectric point (pH 4, 76 secs, 30 NTU) as compared to other hyperbranched and linear grafts of XG.
For some new mines, the utilization rate of tailings is not satisfactory when using unclassified tailings as backfill aggregate for cemented backfill. At the same time, with the progress of mineral processing technology, the tailings discharged by the concentrator gradually become finer. Therefore, cemented filling with fine-grained tailings as aggregate will become the development direction of filling technology in the future. In this paper, the feasibility of fine particle tailings backfill is studied by taking the particle tailings of-200 mesh as aggregate in Shaling gold mine. The calculation shows that the utilization rate of tailings is increased from 45.1% to 90.3% by using-200 mesh tailings as filling aggregate. The response surface central composite design method (RSM-CCD) was used to study the strength of backfill with alkali-activated cementitious material as binder by taking the mass concentration of backfill slurry and sand-binder ratio as input factors. The results show that the 28-day strength of the backfill with graded fine-grained tailings as filling aggregate can reach 5.41 MPa when the sand-binder ratio is 4, which can fully meet the needs of the mine for the strength of the backfill. The thickening test of-200 mesh fine particle tailings was carried out by static limit concentration test and dynamic thickening test. In the case of adding 35 g/t BASF 6920 non-ionic flocculant, the concentration of 64.74% tail mortar can reach 67.71% after 2 h of static thickening, and the concentration can reach 69.62% after 2 h of static thickening. The feeding speed of thickener should be controlled between 0.4 and 0.59 t/(m² h). In this case, the underflow concentration of thickener is relatively high, which is 64.92–65.78%, and the solid content of overflow water is less than 164 ppm. The conventional full tailings thickening process was improved by using the design of high-efficiency deep cone thickener and vertical sand silo. The feasibility of fine-grained tailings as filling aggregate was demonstrated by combining the filling ratio test of fine-grained tailings, the data of thickening test and the improved thickening process. The research results can provide reference for other mines to use fine-grained tailings as filling aggregate to design filling system.
Purpose
The purpose of this study is to investigate the authors' previously prepared and fully characterized poly (methacrylamide)-chitosan nanoparticles (CNPs) graft copolymer having 50.2% graft yield with respect to flocculation efficiency for ferric laurate aqueous dispersions. This was done to compare the ability of the latter cheap, biodegradable and ecofriendly hybrid natural-synthetic polymeric substrate as a flocculant in comparison with higher cost, nonbiodegradable and harmful polyacrylamide as a well-known synthetic flocculant counterpart.
Design/methodology/approach
The graft copolymerization process was carried out at 450°Cfor 120 min using (1.0 g) CNPs, methacrylamide (1.5 g), 100 mmol/l potassium chromate and 80 mmol/l mandelic acid. Fourier transform infrared spectroscopy, thermogravimetric analysis, elemental analysis and specific viscosity were used to characterize and analyze the resultant copolymer. The flocculation efficiency was conferred in terms of transmittance % and weight removal %. The main factors influencing the flocculation process, such as flocculent dose, flocculation medium pH, stirring speed, flocculation temperature and grafting extent, were comprehensively discussed.
Findings
The flocculation efficiency of the prepared copolymers revealed the following findings: increased by increasing the flocculant dose, pH, temperature and stirring speed to a maximum values denoted at 30 ppm, 6.0, 30°C and 50 r/min, respectively, then decreased thereafter; increased by increasing the extent of grafting within the range studied; showed a comparable flocculation efficiency in comparison with polyacrylamide as a synthetic polymeric flocculent; and, finally, a preliminary bridging mechanism representing the attraction between the anionic suspended particles ferric laurate and cationic poly (MAam)-CNPs graft copolymer has been projected.
Originality/value
The advancement addressed here is undertaken with using the authors’ poly (MAam)-CNPs graft copolymers having different extent of grafting (a point which is not cited in the literature especially for the authors’ prepared copolymer) as a hybrid natural-synthetic polymeric substrate as a flocculant for ferric laurate aqueous dispersions in comparison with the high cost and nondegradable polyacrylamide synthetic flocculant.
Penetration grouting is a common grouting method in which the grouting material is injected into the pore spaces of the ground. Bleeding of cement slurry reduces the effectiveness of penetration grouting operations. In this paper, a series of experiments were carried out with water-cement ratio, curing time, and sand size as experimental variables. The influence of slurry bleeding on the performance of penetration grouted sand after grouting diffusion process was investigated. Scanning Electron Microscope (SEM) was used to further examine the microstructure of grouted body after slurry bleeding. The performance attenuation rate was proposed to characterize the spatial attenuation degree of grouted sand performance under the action of slurry. Results show that the performance of penetration grouted sand is negatively correlated with water-cement ratio and sand particle size. The water-cement ratio has a significant influence on performance in space distribution of penetration grouted sand but curing time only affects the global performance of grouted sand.
In this article, underwater cement paste (UWP) was fabricated through using the high molecular nonionic polyacrylamide (PAM) as anti-washout admixtures (AWAs). The traditional coaxial rotary method was used to systematically investigate UWPs rheology parameters. A new B-HB model was successfully used for simulating the obtained shear stress versus shear rate diagram. The results showed that with the increasing of PAM content, the yield stress increased, while the plastic viscosity exhibited the discipline of increase first, then decrease, and then increase again. The obvious shear-thinning behavior was also found in UWPs, and this behavior receded with the growth of PAM dosage. Until the PAM content was beyond 0.7%, the pseudoplasticity index appeared to be around 1.00, showing the typical Bingham fluid characteristics. After that the scanning electron microscopy (SEM) tests were conducted to try to explain these phenomena. The polyacrylamide could adsorb on the surface of cement particles by hydrogen bonding or chelation, and the absorbed components could intertwine together in the form of polymer-cement particles or polymer–polymer entanglement, resulting in the honeycomb-like structure. The mechanisms were shown to be consistent with the measured rheological parameters, demonstrating the feasibility of the B-HB model in UWP systems.
Both polyacrylamide (PAM) and sodium polyacrylate (PANA) are polypropylene based polymer, however it is still uncertain whether there are some differences between their roles on the property of fresh cement paste. Thus this work aimed to compare the property of fresh cement pastes with them, such as shear viscosity, fluidity, bleeding and water evaporation loss ratio. Meanwhile, the ultraviolet visible spectra, infrared spectroscopy and thermogravimetry were carried out to explore the mechanisms that are responsible for these differences. The experimental results indicate that as compared with PANA, PAM increases the shear viscosity and reduces the fluidity of cement paste more significantly. However, PANA more significantly reduces bleeding and water evaporation loss ratio. These differences are in conjunction with their different behaviors in the fresh cement paste, such as promotion in the cement hydration at early age and reaction with cement hydration product.
The rheological and thixotropic properties, as well as the establishment of dynamic and static structures effectively reflect the workability of the cement paste. In order to investigate the effects of organic and inorganic composite thickening agents on fresh cement paste, the rheological curves, structural recovery rate, storage modulus G′ and loss modulus G″ of cement pastes mixed with limestone powder (LP), re-dispersible polymer powder (RPP) and hydroxypropyl methyl cellulose ether (HPMC) were tested by Anton Paar MCR 102 rotary rheometer. The results showed that LP and RPP reduced the yield stress and enhanced workability, and LP also improved the establishment of static structures. The incorporation of HPMC facilitated the recovery rate of static structure, and obviously improved the deformation resistance of the cement paste by reducing the difference between the G′ and the G″. However, a single thickening agent was hard to balance the rheological parameters of the pastes. While the combination of LP, RPP and HPMC could improve the rate of structure establishment and recovery, and increase the deformation resistance as well as the stability of the paste. The dense filling effect of LP, the film-forming and bridging effect of RPP, and the winding and water retention effects of HPMC together made up the synergistic effect to further improve the stability of the pastes.
The rheological properties have an important impact on the migration and diffusion of cement slurry in grouting reinforcement applications. The microscopic interaction between particles affects the macroscopic rheological behaviors of cement slurry. However, the detailed influence mechanisms still remain unclear. In this work, five different fineness cements were selected to investigate the rheological properties. The focused beam reflectance measurement (FBRM) system was used to detect the in-situ flocculation characteristics under different water-to-cement ratios, cement fineness, and shear rates. In addition, the initial and time-dependent rheological properties under various conditions were measured by a rotary viscometer. The results reveal that these three factors influence the number and particle size distribution of flocculent particles, and the degree of flocculation increases linearly with time. Cement slurry with a water-to-cement ratio ranging from 0.6:1 to 1.6:1 follows the Bingham model. The yield strength and plastic viscosity decrease with increasing water-to-cement ratio, and increase with increasing cement fineness. The apparent viscosity is reversible with the change of shear rate. The influence of water-to-cement ratio, cement fineness, and shear rate on the flocculation effect is consistent with the influence on rheological parameters. Three factors and time affect the rheological properties by influencing the formation and destruction of the flocculation structure, and then changing the content of free water and intergranular force in cement slurry. Our study has revealed the evolution of rheological properties from a microscopic point of view, which can provide theoretical guidance for the design and application of cement slurry.
The necessity for an increased dosage of polycarboxylate-ether-based (PCE) superplasticizer in conventional and self-compacting concrete (SCC) is inevitable in hot environments. However, the increased PCE dosage leads mostly to initial instability and segregation of the SCC mixture, which mandates high precautions. Under such conditions, the use of a viscosity-modifying agent (VMA) is highly recommended. Because of that, the possible addition modes of PCE and VMA were investigated in SCC cement pastes with two water-to-cement (W/C) ratios of 0.3 and 0.4 using rheological measurements and image analysis. The mode of action of PCE and VMA with cement is shown to rely on the sequence of their mutual additions. Namely, either PCE or VMA first, followed by the other, or add both together at the same time. The effects of mode of addition on mini-slump flow and the rheological properties of cement pastes, represented by yield stress and plastic viscosity, were evaluated. It is proven that the critical PCE dosage has the maximum power to expel water from both cement paste and VMA-cement paste complex when added to their fresh mixtures. The influence of the critical dosage could be identified using image analysis as a supporting technique, which marks the segregation layer in red as an indicator for segregation regardless of the nature of casting molds. For each W/C ratio, there is an optimum PCE/VMA ratio at which maximum stability with enhanced fresh properties can be obtained. Two opposing mechanisms were also recognized as being water repellent and water attractive. Finally, it is concluded that the net mode of action represents the average effect of both mechanisms as visualized. To that end, mathematical models could be derived and verified.
This chapter describes the chemistry and mechanisms of action of additives used to formulate well-cementing slurries. There is a short section on Portland cement chemistry and nomenclature. The main sections address the types of chemistry used to control the main cement slurry properties, such as viscosity, fluid loss, gas migration, and thickening time. There is also a brief discussion of the additives used to obtain specific set cement properties such as CO2 resistance, self-healing properties, controlled mechanical properties, and postset expansion.
Developing high performance and non-toxic adhesives to replace the petroleum-based adhesives are highly desirable for wood industry. The use of magnesium oxychloride cement (MOC) inorganic adhesive is beneficial for the protection of human health and conservation of fossil resources. However, the low initial viscosity and poor water resistance have limited their application in plywood fabrication. In this study, polyacrylamide (PAM) and sodium polyacrylate (PAAS) were added to the adhesive as organic agents to construct wood-based composites by the organic–inorganic hybrid strategy. The MOC would be solidified by a cold-pressing method, forming a formaldehyde-free wood adhesive with high bonding performance. The addition of PAM efficiently improved the stability and processability of MOC, while PAAS improved the water resistance of MOC by inducing Mg²⁺ chelation and transforming the phase 5 structure. This adhesive exhibited a 53.5% increase in wet shear strength in contrast with the unmodified MOC adhesive. In addition, the MOC adhesive bonded wood-based composites showed high flame-retardant properties.
The particle size and apparent viscosity of cement slurry are the key factors that affect the grouting effect. However, the flocculation mechanism of cement slurry and its influence on the viscosity of macroscopic slurry are unclear. In this study, focused beam reflectance measurement system is used to investigate the in-situ particle characteristics of fresh cement slurry; the viscosity of the slurry is also measured. The results reveal that the water–cement ratio, cement fineness, and content of the water-reducing agent significantly affect the distribution of the chord size and number of flocculent particles. The aforementioned three factors have different influencing mechanisms and affect the viscosity by affecting the flocculation effect. The variation in the viscosity of the slurry is found to be proportional to the flocculation degree under different conditions. Finally, considering the flocculation effect, an effective volume fraction viscosity model based on the chord size and number of flocculent particles is established and verified.
The bleeding characteristic of Portland cement influences its density and rheological behaviour, but the nature and extent of this influence are unclear. This study investigated the development of cement slurry density and rheological characteristics during the entire bleeding period. Portland cement PO42.5 with water-cement ratio (W/C) ranging from 0.8 to 2.0 was used in the experiments. Twelve percentage time periods were divided based on the entire bleeding period, and rheological measurements were carried out at each percentage time period. The results showed that at the end of bleeding, the slurry density with W/C ranging from 2.0 to 0.8 was close to those of the slurry with W/C ranging from 0.8 to 0.6. The slurry with higher W/C behaved a faster density growth speed and degree during the bleeding. The variation of the slurry apparent viscosity as bleeding time showed two stages: a slow growth phase followed by a phase with fast growth. It can be attributed to the weak interaction within cement slurry particles gradually changed into a stronger interaction. The cement slurry behaved like a shear-thinning material after bleeding. The Herschel-Bulkley model could describe rheological characteristics during slurry bleeding better than other empirical models.
Incorporating fly ash in a concrete mix significantly improves its consistency, resulting in lower viscosity. It is believed that the spherical shape of the fly ash particles is the main cause of the low viscosity. However, the physicochemical causality of the fly ash effect has not been deeply studied and there have been only limited experiments on the particles’ interactions. The use of laser backscattering measurement in this paper allowed the investigation of the agglomeration and dispersion of fly ash particles in an in situ paste. It was found that agglomerates composed of mixed particles develop in a fly ash–cement paste. Their dimensions are comparable to those in neat cement paste. More importantly, incorporating nano-scale spherical dispersed fly ash particles in the mix provides a ball-bearing effect between the micro-scale agglomerates. The viscosity of the mix consequently decreases.
This paper reports the effect of mix composition of grout on the fluidity, rheological behaviour and hydro-mechanical behaviour (permeability and compressibility). Factorial design was used in this investigation to assess the combined effects of the four mix composition parameters on fluidity, rheological properties, permeability and compressibility: water/binder ratio (W/B), percentage of limestone filler as replacement of cement (LF), dosage of viscosity modifying admixture (VMA), and dosage of superplasticiser (SP). To evaluate both the rheology of the cement grout and its hydro-mechanical behaviour, mini-slump test, Lombardi plate test, and forced bleeding test, coaxial rotating cylinder viscometer were used. A two-level fractional factorial model was used to model the effect of key variables on the fluidity, the cohesion, permeability and compressibility. The predicted models are valid for mixtures made with W/B ranged from 0.35 to 0.42, LF from 12% to 45% as replacement of cement, VMA (percentage of binder) from 0.02 to 0.7%, and SP from 0.3 to 1.2%. The effects of W/B, LF, VMA and SP were analysed using polynomial regression which can identify the primary factors and their interactions on the measured properties. Prediction models were developed for mini-slump, plate cohesion meter, permeability and compressibility as function of W/B, LF, VMA, and SP. The factorial statistical approach used highlighted the effect of W/B, the dosages of SP and VMA on the various rheological properties, permeability and compressibility.
Hydrate phases form on the cement particles during the hydration of Portland cement.They have a larger volume than the unhydrated constituents. On this basis, some simulation programs describe the hydration by means of shell models.This particle growth can be proved directly during the measuring period up to the initial set by means of the granulometric investigations demonstrated. Different hydration behaviours due to the material composition, the granulometric distribution of the cements and the temperature are reflected in the measurement results.
A methodology to study flocculation processes and floc properties using a non-imaging scanning laser microscope is presented in this paper. This methodology allows us to study floc stability and resistance to shear forces, re-flocculation tendency and reversibility of the flocs. Furthermore, optimal dosage of any polymer and the associated flocculation mechanism can be determined. In order to illustrate the technique, some examples applied to flocculation in papermaking are described. Although in this paper all the examples have been applied to papermaking, the developed methodology can be used in any process in which flocculation phenomena is involved.
On présente dans cet article une méthode pour étudier les procédés de floculation à l'aide d'un microscope laser à balayage sans imagerie. Cette méthode permet d'étudier la stabilité des flocs et leur résistance aux contraintes de cisaillement, la tendance à la refloculation et la réversibilité des flocs. De plus, on peut déterminer le dosage optimal de tout polymère et le mécanisme de floculation associé. Pour illustrer cette technique, quelques exemples appliqués à la floculation dans la fabrication du papier sont décrits. Cependant, même si dans cet article tous les exemples s'appliquent à la fabrication du papier, la méthodologie peut s'appliquer à tout procédé comportant des phénomènes de floculation.
This paper describes recent experimental results concerning investigation of the parameters which determine, on the one hand,
the kinetics of hydration of tricalcium silicate and the thermodynamic, morphological and structural characteristics of C-S-H
and, on the other hand, the evolution of the particle interactions at the origin of setting. It is shown that, in both cases,
lime concentration in solution is the most important parameter. As a consequence, the chemical evolution of the system, which
controls the lime concentration in solution, determines the nature of particle interactions and the physical evolution of
the suspension or paste. In return, the contacts, between particles, resulting from the coagulation of the suspension, seem
to have a role in the kinetics of the chemical reaction.
Cet article décrit les résultats expérimentaux récents concernant la recherche des paramètres déterminant, d'une part, la
cinétique d'hydration du silicate tricalcique et les caractéristiques thermodynamiques, morphologiques et structurales du
C-S-H produit, et d'autre part, l'évolution des interactions interparticuliéres à l'origine de la prise. On montre que, dans
tous les cas, c'est la concentration en ions calcium en solution (ou en équivalent hydroxyde de calcium) qui est le paramètre
déterminant. De ce fait, l'évolution chimique du système, qui fixe la concentration en calcium en solution, détermine la nature
des interactions particulières et l'évolution physique de la suspension ou de la pâte. En retour, le nombre de contacts entre
les particules résultant de la coagulation de la suspension, apparaissent avoir un rôle sur la cinétique de la réaction chimique.
Cet article est une adaption de la conférence présentée à l'occasion de la remise de la Médaille Robert L'Hermite, lors de
la 47ème Réunion du Conseil Général de la RILEM à Haifa, le13 Octobre1933.
An investigation into the interaction between alkaline cations and calcium silicate hydrates (CSH) was conducted by electrokinetic measurements, which provided information on the nature of the interface between the solid and its equilibrium solution. Calcium constitutes for the CSH surface a potential-determining cation. A model of the CSH surface could be proposed, accounting for the experimental evolution of the CSH zeta potential. The necessity of studying the zeta-potential evolution of the system as a function of the calcium activity, instead of its concentration, was underlined. The results obtained suggest a specific interaction between cesium and the CSH surface, whereas sodium and lithium may behave as indifferent cations toward the interface.
The Lasentec focused beam reflectance measurement (FBRM) probe provides in situ particle characterisation over a wide range of suspension concentrations. This is a significant advantage over conventional instruments that require sampling and dilution. However, FBRM gives a chord distribution, rather than a conventional diameter distribution. Both theoretical and empirical methods for converting from chord to diameter data are available, but the empirical method was found to be more successful.
Local dissipation rates were calculated in a corrected way from the data of turbulence energy spectrum measurements to obtain dissipation rate distributions.
For both baffled and unbaffled agitation, dissipation rates showed a wide variation in the vessel space up to several tenfold, with a space-mean value nearly equal to the power input per unit mass. Under geometrically similar conditions of the vessels, these rates tended to change proportionally to the power per mass so that the distribution pattern should be kept unchanged.
The effects of dissipation rate distributions on liquid-liquid and solid-liquid agitation were examined quantitatively. The influence of impeller dimensions on droplet size and solid-liquid mass transfer shown in the literature was explicable based on the change of distribution.
A method for the qualitative and quantitative optimization of the use of polymers in paper mills has been developed. Traditionally optimization of the dosage of polymers in the paper and board industry is achieved by using techniques based on the electric charge of the particles. However, the flocculation phenomenon is based on an increase in the size of the particle aggregates, thus the new method is based on size measurements of the particle aggregates. By applying this technique it is possible to determine the effect of the different chemicals under papermaking conditions; get a better understanding of the interactions occurring in the pulp system; predict the influence of changes in the raw materials or papermaking conditions on chemical efficiency; and control and optimize the chemical flocculation processes.
This paper presents several uses of focused beam reflectance measurement (FBRM) as tool to measure flocculation. It includes information on the interaction of wet-end aids with the different pulp fractions, such as optimal polymer dosage and the resulting floc properties (floc size, strength, etc.). The study of the floc evolution over time under different conditions also allows us to identify flocculation mechanisms. The FBRM methodology provided a real-time, in-process monitor to establish the evolution of the resulting flocculation state and polymer efficiency under the influence of shear forces. The evaluation included a study of deflocculation and reflocculation processes occurring in the sample. We also looked at the problem of contaminants accumulation, as can occur with increased use of recovered paper and the closure of water systems. In addition, FBRM has been used to identify dissolved and colloidal material agglomeration and to study its influence on polymer demand for a given furnish.
The energy dissipation rate distribution in a mixing vessel with and without baffles was determined and the effect thereon of the dimensions of the vessel were studied. The effects of the energy dissipation rate distribution on liquid-liquid dispersion and solid-liquid mass transfer are also investigated.
A fundamental theory in colloid science is the DLVO theory (named after the four contributors: Derjaguin, Landau, Verwey, and Overbeek). This theory and relevant basic concepts are reviewed and applied to cement systems to determine the state of flocculation for cement pastes. Both theoretical and experimental evidence suggest that normal, neat cement suspensions are either flocculated or coagulated. Because of the high ionic strength in the aqueous phase, the degree of flocculation or coagulation is not sensitive to the variation of the zeta potential, for zeta potentials between −20 mV and 20 mV.
A complete population balance equation governing the floc size distribution interpreting both breakage and aggregation mechanisms was obtained in this work. The present work deals with floc formation and breakage in the presence of polymer coagulants. In order to obtain all the parameters involved in the proposed governing equation, a nonlinear least-squares curve fitting procedure was applied. The aggregation collision efficiency was found to be 0.33 for our clay-polymer flocculation system under optimum dosage. The floc splitting frequency was found to be proportional to the product of turbulent shear rate and parent floc volume. The rate of decrease of parent floc volume due to erosion was verified to be proportional to the product of turbulent shear rate and the mean particle volume of erosion fines. The “maximum stable floc size” in a given turbulence field was found to exist in our system.
A compartmentalized model for batch flocculation in turbine stirred tanks is presented. Theoretical rates for orthokinetic flocculation at various flow conditions were calculated from published energy dissipation rates. The effective mean shear rate for flocculation is not the same as the mean value obtained from power input per unit mass but rather, it is equal to the volume average value obtained from the first moment of the shear rate distribution. Results from a study of various multi-compartment models indicate that for flocculation processes with small collision efficiencies, a single-compartment stirred tank model incorporating the effective shear rate is adequate for predicting flocculation rates. A method is proposed for estimating the effective shear rate in a Couette apparatus under conditions where secondary flows exist.Experimental flocculation rates in Couette flow and in stirred tanks were measured and were adequately described by a single straight line when plotted against the effective mean shear rate.
A linear pipe reactor (LPR) system was used to study the flocculation of mineral suspensions with commercial flocculants under a variety of operating conditions. Incorporation of a Lasentec focused beam reflectance measurement (FBRM) probe into the system provided real-time, in situ characterisation of the aggregate dimensions during the flocculation process. At constant flow rate, variation of the distance between the flocculant addition point and detection by the FBRM probe enabled control of the reaction time for flocculation.Once flocculant is mixed with the slurry, the size of the aggregates at a given point downstream in the LPR is determined by the relative rates of the competing processes of aggregate formation and rupture. These processes depend upon, amongst others, upon the mixing conditions, flocculant dosage and solids concentration of the slurry. The mixing intensity present within the LPR is determined by the pipe diameter and the slurry flow rate.Both a calcite slurry and a slimes fraction from a sand-processing operation were studied to assess their flocculation performance under different operating conditions (flow rate, flocculant dosage). Following a short induction period, aggregate size increased rapidly to a peak, with the rate of change indicative of the net aggregation kinetics. Rupture processes then dominated with aggregate size decreasing at a slower rate.The flow rate and hence the shear conditions had a marked effect on the aggregate structure. The effective (net) aggregation rate increased as shear increased, but the ultimate aggregate size decreased reflecting the fragility of large aggregates. For both systems, increasing the flocculant dosage increased the maximum aggregate size, but the effective aggregation rate only increased for the sand slimes, while reaching a constant rate for the calcite system. It is proposed that this reflects the degree to which the particle surface is effectively covered by the adsorbed flocculant.
A freshly made Portland cement paste is generally assumed to be in a flocculated state. It has been shown that an estimate of the strength of flocculation may be obtained from its sedimentation characteristics. In order to do that, a general equation describing the sedimentation characteristics of a dispersed system has been obtained; which is then modified for a flocculated system. Comparison of these two equations to the published results showed that a freshly made Portland cement paste has a low flocculatation strength of about 0.004 bar.
A successful industrial crystallization typically requires the development of a robust process in the laboratory. Knowledge of the solubility curve and the stability of the solution in the vicinity of the equilibrium point, as indicated by the metastable zone width, is essential to the successful development, optimization, and scale-up of a crystallization process. Use of the Lasentec Focused Beam Refiectance (FBRM) system, with aqueous potash alum solutions, to determine both the metastable zone width and solubility curve is demonstrated. The measured metastable zone width data are also used to estimate the nucleation kinetics of the system. Lasentec Particle Vision and Measurement (PVM) images are also employed to validate the results. The data collected using the FBRM system compare very well with published literature values for potash alum solubility and metastable zone width in aqueous solutions.
The coagulation of cement particles during early age hydration has been previously identified as the first step of the setting and hardening of cement pastes. By hydrating Ca3SiO5and a silicate-rich clinker under controlled conditions, a correlation between the coagulation of the suspensions and the electrokinetic properties of particles is established. The zeta potential, and hence the surface charge, of particles in suspension depends on the calcium content of the medium. At low concentrations of Ca2+, the zeta potential of Ca3SiO5particles, calcium silicate hydrate (C–S–H), and clinker is negative (
A population balance model (PBM) that incorporates two scales of turbulent motion in the breakup frequency function has been presented. The breakup frequency function is designed such that particles smaller than the impeller-region Kolmogoroff microscale will erode according to a critical velocity related to the local energy dissipation rate. Particles larger than the impeller-region Kolmogoroff microscale will fracture according to a critical velocity related to the impeller tip speed. The two-scale model was found to better predict the experimental steady-state particle size distribution in 5, 28, and 560l tank sizes and with a Rushton turbine and A310 fluid foil impellers. The two-scale PBM was also used to investigate the most appropriate scale-up law for drinking water flocculation processes. In addition, the impact of higher tank average energy dissipation rate, primary particle concentration, and coagulant concentration on the volume mean particle size with increasing tank size and different impeller types was also presented.
The kinetics of particle aggregation, including coagulation and hydrophobic flocculation, in turbulent flow has been studied. It is discovered by both theoretical analysis and experimental results that the macroscopic kinetic model of particle aggregation is closely correlated with the microscopic particle–particle interaction. The energy barrier Umax between the approaching particles determines whether a particle collision can result in effective aggregation or not. While the adhesive strength J of aggregated particles determines whether the formed aggregates are disrupted by the surface erosion in the viscous subrange of turbulence or by the fragmentation in the inertial subrange of turbulence. The maximum aggregate size dmax depends on the ratio of the aggregate strength and the disruption stress of turbulent flow. Due to the apparently distinguished natures of particle–particle interaction in coagulation and hydrophobic flocculation, the aggregation kinetics of the two are different. Experimental results has proven that the coagulation process is generally accompanied with the surface erosion of aggregates in the viscous subrange of turbulence, whilst the hydrophobic flocculation process is mainly affected by the fragmentation mechanism of aggregate disruption in the inertial subrange of turbulence due to the much stronger strength of the flocs. Based on the theoretic analysis and the experimental results a general kinetic model of particle aggregation has been proposed.
An investigation of the feasibility of in situ monitoring of polymorphic transitions is presented. Focused beam reflectance measurement (FBRM) and particle vision and measurement (PVM) are used to study changes in crystal population and morphology during the transition of the δ-polymorph of d-mannitol to the thermodynamically stable β-form. The transformation is confirmed via off-line X-ray powder diffraction (XRPD). Nucleation and growth of β-form d-mannitol was detected and measured by FBRM/PVM when δ-form was added to a saturated solution of β-form d-mannitol. Raman spectroscopy is also introduced as an invaluable in situ identification technique for polymorphic crystallizations. The effect of particle size on the intensity of the Raman spectrum is investigated using slurries of d-mannitol and sucrose in toluene. Raman spectroscopy successfully distinguished a mixture of sucrose and d-mannitol in toluene, and a linear correlation between Raman peak intensity and solids content was observed for both. However, this linear relationship was not observed when the particle size distribution of sucrose was changed. It is clear that, especially in the case of many polymorphic conversions, information on the particle size distribution, along with additional structurally sensitive information such as Raman spectra, is essential in gaining a true understanding of the behaviour of the system.
The synthesis of tris-O-dodecyl-gallic acid (3,4,5-tri-dodecyloxybenzoic acid) was selected as a model system in our program to define design tools and a methodology for fine-chemical scale-up. The just-suspended conditions of potassium carbonate in MIBK have been studied to find a scale-up rule for this system from 1 dm3 lab-scale to an industrial scale of 6000 dm3. By adjusting the relation of Zwietering to the values obtained by visual observations of the lowest stirrer speed at which the solid particles are just-suspended on a 1- and a 10-dm3 scale with different particle sizes and two stirrer types, a scale-up rule was developed which was verified under just-suspended conditions on a 50-dm3 scale. In addition, a prediction was made using this rule as a minimum criterion for a 6000-dm3 reactor where no problems regarding off-bottom criteria are expected. The focused beam rotating measurement (FBRM) and photo video measurement (PVM) Lasentec techniques have been used successfully to determine just-suspended stirrer rates and to gain more insight into the dynamic (de)agglomeration behaviour of potassium carbonate during the tri-alkylation of methyl gallate. These techniques proved to be powerful tools in accelerating process development of heterogeneous reaction systems.
Latex particles with diameters of 60, 380, and 810 nm, respectively, were sheared in a controlled shear environment of circular couette flow after being destabilized by the addition of MgCl2. The evolution of aggregate size and structure, as well as a measure of aggregate mass, was monitored with use of a particle size analyzer (Coulter LS230) operating on the principle of small-angle light scattering. The aggregates of different primary particle sizes displayed distinct behavior in attaining steady state under similar shear conditions, notably at low to moderate shear rates (G ≤ 100 s-1). Restructuring of aggregate structure was favored over fragmentation for aggregates composed of 60- and 380-nm particles, whereas fragmentation and reaggregation were the main mechanism in governing the final floc size and structure for aggregates made up from 810-nm particles. Also presented in this study is a dimensional analysis that yields a correlation between a floc factor (consisting of floc size and structure) and an aggregation factor, which encompasses the fluid properties, applied shear, number concentration and size of primary particles, as well as the estimated bonding force between particles. This relationship provides a better appreciation of other significant aggregation parameters, apart from the shear level and aggregate size, which are often ignored in the more conventional manners of presenting data from flocculation processes.
A fractal is a mathematical description of highly irregular geometric shapes. It is demonstrated that particle aggregates generated in water and wastewater treatment processes possess fractal features. The fractal dimension for the structure of the aggregates, determined in this work, ranged from 1.4 to 2.8. The concept of the fractal dimension is found applicable in the characterization of the aggregates' geometry, in substrate transfer to biological aggregates, and in the process of aggregation.
The Lasentec focused beam reflectance measurement (FBRM) probe provides in situ particle characterisation over a wide range of suspension concentrations. This is a significant advantage over conventional instruments that require sampling and dilution. However, FBRM gives a chord distribution, rather than a conventional diameter distribution. Both theoretical and empirical methods for converting from chord to diameter data are available, but the empirical method was found to be more successful. The FBRM response has been compared to conventional particle sizing techniques (laser diffraction and electrical sensing zone) for a range of sieved aluminium or calcite suspensions. The mode average of the square-weighted chord length was found to be comparable to other sizing techniques over the range from 50 to 400 μm. The square-weighted FBRM results were essentially unaffected by changes to the instrument focal position, the suspension fluid flow velocity, or the solid fraction in the range 0.1-20% w/v.
The flocculation of an industrial furnish of TCF Kraft bleached pulp and ground calcium carbonate, induced by polyethylene oxide, and by the dual system formed by polyethylene oxide (PEO) and formaldehyde-phenol resin (PFR), was investigated and compared in order to study the flocculation kinetics, the flocculation mechanism and the properties of the formed flocs. A technique based on the measurement of the particle chord size distribution, called focused beam reflectance measurement, was used to carry out the research. The ability of the polyacrylate contained in the commercial ground calcium carbonate as a PEO cofactor was observed. Results confirmed the formation and the coacervation of the complex PFR-PEO, which loses its flocculant properties over time, confirming the complex bridging flocculation model. The shearing forces effect on the flocculation kinetics and on the properties of the formed flocs was also studied. © 2005 American Institute of Chemical Engineers AIChE J, 51: 1022–1031, 2005
The relationship of flocculation by water‐soluble polymers to the classical phenomena of colloid chemistry is pointed out and the contributions of electrostatic and non‐electrostatic forces are discussed.
Modern flocculants have a high degree of polymerization (e.g. c. 50 000) and an extremely extended configuration when dissolved in a favourable solvent; this factor explains qualitatively some of the peculiarities of flocculation by polymers and indicates the need for rational control of their use if optimum results are to be achieved. It is concluded, however, that a quantitative theory of specific flocculant systems is not possible at present.
Future developments in flocculation technology will almost certainly include introduction of cheaper or more selective flocculants, improved control through the use of auxiliary reagents and more sophisticated consideration of the surface chemistry of materials that have to be flocculated.
DLVO theory has been applied to cement suspensions containing admixtures. Two different batches of the same cement, different only in storage history, are compared. It is found that, although their general sedimentation behavior is similar, differences exist in the zeta potential and basic chemistry. Both the superplasticizer and water-reducing admixture result in all cases in a stable dispersion, contrary to the theoretical prediction that only a coagulated suspension should exist. This finding suggests that steric hindrance plays a larger role compared to electric repulsion in the deflocculation of cement pastes than previously believed. Zeta potential and sedimentation data for CaCl2 and sugar are also presented.
A population balance model is used to model simultaneous coagulation and fragmentation in turbulent shear. The fractal-like aggregate structure is quantified using a mass fractal dimension, Df = 2.05, derived from light-scattering measurements and is incorporated into the model using appropriate kinetic expressions. In addition, fluid viscous effects on aggregate collisions are taken into account using existing theories on viscous retardation. Flocculation experiments of a polystyrene particle/Al(OH)3/water system in a stirred tank are compared to the model results. Good agreement is found for both average-size evolution and steady-state size distribution using only one fitting parameter and assuming binary breakage. The average polystyrene-Al(OH)3 aggregate size initially increases before reaching a constant steady-state value during coagulation–fragmentation in a stirred tank. Increasing the applied shear rate increases the coagulation and fragmentation rates, decreasing the steady-state average aggregate size, and the time lag before steady state. The model developed in this work is applied to laminar shear data from the literature, showing excellent agreement.
Conventional coagulation kinetic models are usually based on Smoluchowski’s work, which employs the coalesced sphere assumption. Much evidence, however, has recently been provided that particle aggregates from natural waters and engineered systems have fractal structures. Consequently, the traditional models should be modified to include the fractal nature of aggregates. This paper describes a modeling approach that simulates changes in particle size distribution (PSD) due to coagulation by incorporating recently proposed fractal mathematics and introducing a new conceptual framework called the coalesced fractal sphere (CFS) assumption. The developed modeling method, which includes the traditional Euclidean case as a subset, was applied to a 2-m settling column system with estuarine sediment particles, and a one-dimensional numerical model was developed. Model simulations were conducted varying the fractal dimension (DF) and the collision efficiency factor (α). For the conventional Euclidean case, the model indicated that coagulation played an important role in the vertical transport of the estuarine sediment particles. The simulations with the fractal cases indicated that both DF and α significantly affected the evolution of PSD, and that with lower values of DF and α, the model predicted a trend of PSD similar to that of the Euclidean case. This finding may be interpreted as dependence of α on the assumed collision models (or DF), that seems to leave a new challenge to our understanding of α. The developed model may be used in various particle aggregation systems.
The effect of shear on floc properties was observed through population balance to comprehend the mechanisms of flocculation, in particular the role of restructuring. Little fundamental attention has been given before to the shear influence responsible for creating compact aggregates, while the floc characteristics might differ in other conditions. It is crucial to understand how aggregates evolve to steady state, if their properties are to be ‘tailored’ to suit subsequent solid–liquid separation. From a previous experimental study (Langmuir 18(6) (2002) 1974), restructuring was observed to occur extensively in the flocculation of latex particles in couette-flow, and was proposed to be responsible for the decrease in floc size on their transition to equilibrium. On the other hand, flocs of larger primary particles were more susceptible to breakage, with densification occurring as a result of fragmentation and re-aggregation. Denser flocs were found when structural deformation dominated, particularly in the initial stage of the process, while comparatively tenuous ones were observed when formation and breakage kinetics were the governing mechanisms. The distinct manners in which aggregates of different primary particle sizes evolved with time, were replicated with a population balance that incorporated the floc structural variation; verifying that restructuring indeed played a crucial role under certain flocculation conditions.
Cement reacts rapidly with a surrounding electrolyte and thus shows a time-dependent zeta potential. This has been measured under various conditions. The effects of the pH value, some simple salts and some surfactants on the transient zeta potential of cement have been studied in more detail. A model of the electrical double layer on cement is proposed, where the time-dependent variations are divided into variations due to the accumulation of ions inside the double layer and variations due to the increasing ionic strength in the bulk elektrolyte. The model is used to evaluate zeta potential measurements on hydrating cement and to discuss the effects observed.
The effect of superplasticizers and air-entraining agents on the Zeta Potential (ZP) of a Portland cement was studied by microelectrophoresis. The purpose of the experiments was to investigate the adsorption mechanism of electronegative polymers and surface-active agents, and the interaction between these when added in combination. As a preliminary experiment the microelectrophoresis technique was investigated in order to obtain the most precise and reproducible method of measuring Zeta Potentials. The results show that the superplasticizer type with the longer polymer chain and thereby the largest amount of electronegative charges per chain gives the highest negative Zeta Potential. This is interpreted as indicating that this type of superplasticizer has a higher dispersing capability. Data from dosage of air-entraining agents show that a part of the air-entraining effect is due to the fact that the molecules are adsorbed on the cement surface, thereby making it hydrophobic. The results of combined use of superplasticizers and air-entraining agents show that only a slightly smaller ZP is obtained than when a superplasticizer is dosed alone, which however still indicates an interaction. Finally the ZP has been determined of cement alone.
Adsorption characteristics of various superplasticizers on portland cement component minerals were investigated. Adsorption isotherms of various types of superplasticizers and ζ-potentials of cement component minerals at the maximum adsorption of the superplasticizers were measured. The value of the adsorption isotherm was calculated from the amount of the superplasticizer adsorbed on a cement component mineral in an equilibrated solution. The maximum amounts of adsorption and the adsorption isotherms varied with types of component mineral and superplasticizer. For all types of superplasticizers, a larger amount of superplasticizer was adsorbed on C3A and C4AF than C3S and C2S. However, the equilibrated concentration of each superplasticizer at the maximum adsorption was not influenced by types of superplasticizer. Without superplasticizer, C3S and C2S had negative ζ-potential. On the contrary, C3A and C4AF had positive ζ-potential. Therefore, accelerated coagulation of cement particles might occur due to their electrostatic potentials that are opposite each other. However, all component minerals of cement had negative ζ-potential when they were mixed with any superplasticizer. Fluidity of fresh cement paste is improved due to electrostatic repulsion acting between particles.
Rapid coagulation by turbulence in stirred tanks was studied for particles and aggregates smaller than the Kolmogorov microscale. The coagulation kinetics are determined by the floc structure and by the hydrodynamic and colloidal interactions between the colliding particles. The collision efficiency for doublet formation in the heterogeneous shear field of a stirred tank follows from particle trajectory analysis of solid particles in simple shear flow, provided the simple shear rate is made to correspond to the residence time weighted turbulent shear rate. Experimentally, the resulting aggregates proved to be fractal-like with their porosity increasing with aggregate size. Porosity within the aggregates results in penetration of the floc surface by the fluid flow, giving rise to enhanced collision efficiencies compared to solid particles. The collision efficiencies between porous flocs may be estimated by a model that pictures a porous floc as consisting of an impermeable core and a completely permeable shell. With the collision efficiencies from this shell-core model the aggregate growth could be described adequately.
Although in the industrial Hatschek process it is necessary to use flocculants to improve retention, dewatering and formation, the use of flocculants may also decrease the strength of the final product. This paper studies the influence of the molecular weight and the anionic charge of anionic polyacrylamides on the flocculation behaviour of fibre cement suspensions and on the bending strength of the final product. Flocculants influence the density of the final product and in-turn the lowering of the density results in strength reduction. Results showed that an increase in the flocculant molecular weight reduces the bending strength of the composites significantly due to its density reduction. However, an increase in flocculant anionic charge increases the bending strength of composites. Therefore, in order to optimise the fibre cement process, it is necessary to use flocculants with high anionic charge and medium molecular weight.
The focused beam reflectance measurement (FBRM), also known as scanning laser microscopy (SLM), was used as a real-time monitor to study the flocculation and reflocculation of clay suspensions under different shear conditions in the presence of single polymer, dual polymer, microparticle and poly(ethylene oxide)/phenolformaldehyde (PEO/PFR) flocculation systems. For initial flocculation, the high molecular weight PEO and cationic polyacrylamide (CPAM) produced larger flocs than others. However, reflocculation of clay suspensions formed by these non- or low-charged polymers was insignificant after the initial flocs were broken under high shear force. In contrast, high charge density polymers, such as poly(diallyldimethylammonium chloride) (PDADMAC), do not form large initial flocs, but they showed significant reflocculation ability under a continuous shear condition. It is concluded that high flocculation can be obtained by effective polymer bridging, but high reflocculation can only be induced by high electrostatic attractive forces between suspended particles.
Floc stability in laminar and turbulent flow
- Mühle
Mühle, K., 1993. Floc stability in laminar and turbulent flow. In: Dobias, B.
(Ed.), Coagulation and Flocculation. Marcel Dekker, New York, p. 355.
Flocculation in stirred tanks
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Koh, P.T.L., Andrews, J.R.G., Uhlherr, P.H.T., 1984. Flocculation in stirred
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