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Effect of sodium carbonate and sodium phosphate on hydration of cement paste

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Abstract

Both sodium carbonate (NC) and sodium phosphate (NP) could immediately precipitate in the presence of calcium ions (Ca²⁺), and these precipitates would deposit on the surface of cement as one layer in cement paste. However, these precipitates have different effects on the hydration of cement. In this study, effect of NC and NP on cement hydration was discussed, and the different effect between these two was investigated. Setting time, hydration heat, conductivity of cement suspension, and compressive strength of cement paste were examined, and the hydrates were analyzed with XRD and TG-DTG. The precipitates synthesized in saturated calcium hydroxide solution were characterized with XRD, TEM, and SEM. Then one model was proposed to illustrate the mechanism of the effect of NC and NP on cement hydration. The results indicated that NC accelerated cement hydration, but NP retarded it. And the results of TEM and SEM showed that calcium carbonate (the precipitate formed by NC and Ca²⁺)) could well crystalize with directional growth, there is a large amount of interspace in the calcium carbonate-based layer, which would benefit the transports of water molecules and dissolved ions. However, the calcium phosphate precipitate (the precipitate formed by NP and Ca²⁺)) seemed to have a large amount of amorphous structure with a much more compact layer structure than calcium carbonate. The difference in composition and structure of the layer was responsible for the effect of NC and NP on cement hydration. The mechanism behind accelerating effect of NC was due to the interspace in the calcium carbonate based layer which facilitated the ion dissolution of system, while that for retarding effect of NP was because the calcium phosphate-based layer formed on the surface of cement grains seemed compact enough to restrict the transports of water molecules and dissolved ions. Such consequences were expected to give deeper insight into the effect of the surficial structure of cement grains on early hydration of cement paste.

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... The approximately identical mechanical properties of PO samples at 7 and 28 days (Fig. 4) also ascertain this hypothesis. Previous works assumed that calcium phosphate (amorphous and/or crystalline precipitate) might produce and retard cement hydration [84]. However, due to the variability in mineralogy and reactivity in cementitious phases of such phosphate-binder systems, no clear and recognized conclusion was ever drawn that reveals the interaction in complex processes. ...
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... Nevertheless, some rhombic cube-like particles are generated ( Fig. 9(b)), which can fill part of the pores and refine sample microstructure [51]. It is known that the morphology of CaCO 3 (calcite) particle is rhombic cube-like [52][53][54], moreover, the calcium, carbon, and oxygen elements were observed in the EDS results. Therefore, the marked particles in Fig. 9(b) are CaCO 3 (calcite) particles. ...
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This paper aims to study the influence of rheological parameters of cement paste on the dispersion of carbon nanofibers (CNFs) and the subsequent electrical resistivity. Three groups of cement paste with water-cement ratio (w/c) of 0.4, 0.35, and 0.3 and three groups with w/c of 0.4 incorporating 10% silica fume (SF), 20% granulated blast-furnace slag (SL) and 0.4% carboxymethylcellulose sodium (CMC) were prepared. The CNFs were incorporated at 2.25% of binder volume. Different dosages of superplasticizer were added for each group, to obtain variable rheological performances of fresh paste. The average value and variability coefficient (Cv) of electrical resistivity for 16 specimens were determined for each mixture. Scanning electron microscopy (SEM) observation and piezoresistivity were tested on selected samples. Results showed that slump flow increased and the viscosity decreased with more addition of superplasticizer and higher w/c. For each group of cement paste, the slump flow of around 200 mm is most favorable for enhancing the dispersion of CNFs. The addition of CMC and SF improved the dispersion of CNFs, but SL had a disadvantageous influence. The lower Cv value and better dispersion of CNFs due to the rheological performance of cement paste led to the improved piezoresistivity.
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The effects of three superplasticizers, β-naphthalenelfonic acid-based superplasticizer (BNS), aminosulfonic acid-based superplasticizer (AS) and polycarboxylate acid-based superplasticizer (PC), and two retarders, citric acid (CA) and sodium gluconate (SG), on the fluidity, flow loss and compressive strength of sulphoaluminate cement were studied. The competition adsorption effects between the retarder CA and the superplasticizers of BNS and AS were intensively and lead to low initial fluidity. Besides, there is not any competition effect between SG and superplasticizer of BNS and AS, and the combinations of the two retarders and superplasticizer of PC had better influence on the fluidity and flow loss. Furthermore, superfluous retarders would lead to decrease of compressive strength due to the inactivity of cement particle.
Article
This research provides a fundamental understanding of the early stage hydration of Portland cement paste modified by 2 and 4 wt.% of sodium carbonate. An excess of CO32- ions retards the Ca(OH)2 development and enables an intensive rise and growth of CaCO3 crystals in hydrated cement. This process lasts very intensively from 6 to 24 hours. A high conversion of unreacted clinker minerals to hydration products in the cement - Na2CO3 pastes takes place rapidly between 1 and 24 hours. Later the conversion of clinker minerals to the hydrate phase is reduced and higher contents of calcite and vaterite relative to that of Ca(OH)2 in comparison with those found in the Portland cement paste are observed. As a consequence of this, differences in strength, dynamic modulus of elasticity and porosity between hardened Portland cement paste and those modified by Na2CO3 are found. The decrease in two-year compressive strength and elasticity modulus of cement - Na2CO3 pastes relative to Portland cement pastes is caused by the loss in binding capability due to preferential CaCO3 formation at the early stage of cement hydration and consequent growth of CaCO3 crystals due to gradual carbonation, particularly in air with 60 % of relative humidity. In this respect the higher volume of non-affected products of the hydration process by the action of CO2, the better compressive strength and elasticity modulus the cement paste becomes.
Article
The workability of a superplasticised cement paste can be improved by a higher fluidity and a lower flow loss. The effects of two retarders, sodium gluconate (SG) and citric acid (CA), on the workability of pastes containing 0.7 wt% β-nap hthalenesulfonic acid-based superplasticiser (BNS) were determined. At the same BNS dosage, improved workability only occurred with the addition of 0.03-0.09 wt% SG, and not with 0.12-0.15 wt% SG or with 0.03- 0.15 wt% CA. The reasons for this phenomenon were investigated by way of the adsorbed amount of BNS on C 3S, C 2S, C 4AF-gypsum, and C 3A-gypsum surfaces at different SG or CA dosages using ultraviolet spectrophotometry. SG and CA were found to reduce the adsorbed amounts of BNS on C 3S, C 2S, and C 4AF-gypsum surfaces at 5 and 60 min because of the competitive adsorption between the retarders and BNS. CA also had a more remarkable reducing effect than SG. On the other hand, the adsorbed amount of BNS on C 3A-gypsum was enhanced because the retarders accelerated the formation rate of ettringite. The adsorbed BNS amount was also higher in the presence of CA than that of SG, indicating that CA enhances the consumption of BNS in the structure of ettringite. Overall, 0.03-0.09 wt% SG distributes BNS better on different minerals and improves paste workability.
Article
An overview of current PCE compositions and synthesis methods is provided, followed by novel applications for PCEs including C-S-H-PCE nano-composites and a description of still unresolved challenges for PCE technology. In addition, the functionality of chemical admixtures in specific applications for low-carbon cements and concrete systems is discussed. The action mechanisms of retarders and the recycling system of sludge water by using retarder are introduced. Furthermore, the influence of fluoride ion and the effectiveness of PCE polymers in blended cements and the effect of non-adsorbed polymer are presented. And the impact of special interface modifying materials, of a refined pore structure and of chemical admixtures, particularly shrinkage-reducing agents, is described. The article concludes that more accurate quantitative micro-analytical methods and modeling tools will be needed to obtain a holistic understanding of factors affecting the microstructure of concrete, with the final goal of achieving a more durable concrete.
Article
Four novel polycarboxylate superplasticizers (PCEs) with different hydrophobic groups incorporating acrylic acid (AA) and polyether (PEO) macromonomers (C4-M-1, C5-M-2, C6-M-3, and C11-M-4) as repeating unit were synthesized by the free radical polymerization. Characterization of the molecular structures of the PCEs by Gel permeation chromatography (GPC), Nuclear Magnetic Resonance (1H NMR), Fourier-transform infrared (FT-IR), and Thermogravimetric analyses (TGA) were performed. After meticulous studying on the impact of the different hydrophobic groups of polycarboxylate superplasticizers on dispersion, adsorption, and flow retaining behaviors of cement mortar, it was found that the longer length of hydrophobic chains of PCEs should be more effective in dispersion maintaining abilities of cement mortars in spite of their weaker initial water-reducing abilities. The difference of the performance in adsorption and zeta potential was attributed to configuration variation of PCEs when the polymers were adsorbed on the cement particles. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers
Article
The presence of Ca(OH)2 may adversely influence resistance to sulfates, while its absence reduces the pH, leading to corrosion of the steel in reinforced concrete. Test results of the present study showed that adding NaHCO3 allowed carbonation to occur uniformly within the cement paste, resulting in the consumption of Ca(OH)2. Despite the consumption of Ca(OH)2 , the pH was found to increase by production of NaOH. Strength test results showed that an addition of NaHCO3 of less than 5% enhanced the strength, but that exceeding 5% had an adverse effect. The results here provide new insight into potential means to improve the durability of concrete.
Article
Sodium gluconate is commonly used to delay the setting of cement and concrete. This study investigated the effects of sodium gluconate on the physical properties and structure of Portland cement. Sodium gluconate improved the compressive strength at 3 and 28 days, delayed cement setting and increased the fluidity of the Portland cement mortar. Less than 0.03% sodium gluconate promoted the formation of ettringite (AFt) at early age. A dosage of 1.0% sodium gluconate significantly inhibited the reaction between C3A and CaSO4·2H2O. Sodium gluconate delayed the hydration reaction of C3S, which increased the duration of the induction period. Sodium gluconate had only a slight effect on the hydration reaction of the ferrite phase. The pore distribution and porosity of the cement paste were not improved due to the decrease in hydration.
Article
Sucrose and zinc oxide (ZnO) are effective cement hydration retarders. The goal of this study was to provide a new look into the ZnO cement hydration mechanism and to investigate impacts of various supplementary cementitious materials (SCMs) on retardation behavior of ZnO and sucrose. Changes in the pore solution composition and reaction kinetics were measured for cementitious systems with ZnO or sucrose that contained rice straw ash (RSA), wheat straw ash, silica fume, metakaolin, and fly ash. Among the SCMs used, RSA dramatically suppressed ZnO and sucrose retardation. Experimental results indicated that the mechanism by which ZnO retards hydration reaction could be nucleation and/or growth poisoning of C-S-H. Reduced retardation of paste samples containing RSA was attributed to the ability of RSA to provide nucleation sites for C-S-H precipitation. This study provides a better understanding of the interaction between SCMs and cement hydration retarders essential in predicting retarder–dose effects.
Article
The adsorption amount, ζ-potential of cement particles and fluidity of cement paste were tested to research the competitive adsorption between naphthalene superplasticizer (FDN) and STPP. The experimental results showed that the presence of STPP could significantly improve the fluidity of cement paste and reduce the fluidity loss with FDN. There existed a competitive adsorption between STPP and FDN. STPP and calcium ions formed complexes; they preferentially adsorbed onto surface of cement particles and preempt adsorption points of FDN; and it reduced adsorption amount of FDN. In the absence of STPP, saturation adsorption amount of FDN was 5.93 mg/g; but when the dosage of STPP was 0.1%, it reduced to 4.3 mg/g (about 72.5%). The adsorption amount of FDN was reduced by STPP, but ζ-potential of cement particles enhanced and fluidity of cement paste increased because of strong negative charge effect of the complexes. Adsorption of the complexes would delay Ca2+ into liquid and inhibit formation of active adsorption points. Then, content of FDN in liquid increased with the addition of STPP and ζ-potential of cement particles became stable. In this way, fluidity loss of cement paste reduced.
Article
The effects of two retarders, sodium gluconate (SG) and citric acid (CN), on the fluidity and flow loss of pastes containing aminosulfonic acid-based superplasticizers (AS) were investigated. The appropriate SG dosages resulted in a better workability of cement pastes than when only AS was used. However, at the same dosages, CN did not improve workability. The mechanism behind this phenomenon was investigated by analyzing the amount of AS adsorbed on the cement particles and on each mineral, ettringite formation in C3A–gypsum pastes, and the degree of tricalcium silicate (C3S) hydration using ultraviolet spectrophotometry and thermogravimetric analysis in the presence of AS and different SG or CN dosages. The effects of SG on the competitive adsorption and ettringite formation enhancement were weaker than those of CN. In addition, SG retarded C3S hydration. Thus, the workability of the pastes plasticized with AS was improved.
Article
Cationic surfactants, didodecyldimethylammonium bromide (DDAB), 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br) and DDAB/[C12mim]Br mixture were used to induce the formation of calcium carbonate (CaCO3) crystals at ambient temperature. The obtained CaCO3 particles were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The morphologies of CaCO3 crystals changed from laminated cube to sphericity and string shape with the increase of DDAB and [C12mim]Br concentration, respectively. Flower-shaped CaCO3 crystals were synthesized in the mixed DDAB/[C12mim]Br system. More importantly, it was found that the complete conversion from calcite to vaterite was achieved at room temperature only through changing DDAB concentration. The regulations of DDAB and [C12mim]Br to CaCO3 crystals have been compared with that of dodecyltrimethylammonium bromide (DTAB) in our previous work and possible mechanisms have been proposed. It is shown that the cationic surfactants can control the crystallization of CaCO3 and the number of hydrophobic alkyl chains of cationic surfactants might be more effective in modulating the crystallization of vaterite than the head groups.
Article
A new superplasticizer of poly-carboxymethyl-β-cyclodextrin (PCM-β-CD) was prepared by ring-opening polymerization of epoxy chloropropane and CM-β-CD using sodium hydroxide as initiator. The structure of PCM-β-CD was characterized by FTIR and NMR as well as GPC. The performances of the CM-β-CD were tested by measuring paste luidity, setting time, adsorption and zeta-potential. The result showed the performances of PCM-β-CD closed to the control polycarboxylate superplasticizer (PCs) used in this study, especially the fluidity loss is smaller and the setting time is longer than the PCs. The reason might be the synergism of the steric hindrance with the hollow truncated cone and the electrostatic repulsion from –COO− groups. The mechanism of dispersion and retardation by PCM-β-CD was elucidated based on the results of adsorption properties and zeta‐potential of the cement particles. The study offers a new approach to synthesize superplasticizer based on β-CD and an alternative to concrete engineering.
Article
Most concrete produced today includes either chemical additions to the cement, chemical admixtures in the concrete, or both. These chemicals alter a number of properties of cementitious systems, including hydration behavior, and it has been long understood by practitioners that these systems can differ widely in response to such chemicals.In this paper the impact on hydration of several classes of chemicals is reviewed with an emphasis on the current understanding of interactions with cement chemistry. These include setting retarders, accelerators, and water reducing dispersants. The ability of the chemicals to alter the aluminate–sulfate balance of cementitious systems is discussed with a focus on the impact on silicate hydration. As a key example of this complex interaction, unusual behavior sometimes observed in systems containing high calcium fly ash is highlighted.
Article
The use of cement by nuclear industry for confining low radioactive waste knows a constant increase. The interest bore to this material is due to its mechanical and chemical properties. However, these properties may be modified by the nature and the amount of wastes introduced with the mixing solution especially with large amount of phosphate. Precipitation of hydroxylapatite in the cement paste at the very beginning after mixing was emphasized from a particular concentration of orthophosphate. It was also found that this phenomenon leads to an early rigidification of the cement paste. From a kinetics point of view, the evolution of hydration was both delayed and slowed down according to a pessimum of phosphate concentration. These results indicate a competition between a adsorption process of phosphate on the cement phases which delay the hydration of the cement pastes and a process of precipitation of hydroxylapatite which does not.
Article
Tricalcium silicate dissolution in the presence of orthophosphate ions was monitored by measuring the concentrations of calcium and silicate ions in dilute suspensions using a special dissolution cell coupled to an optical emission spectrometer. Results show that increasing adsorption of orthophosphate ions slows down the dissolution of Ca3SiO5 and that a calcium-phosphate precipitate may form at certain orthophosphate concentrations. These observations are correlated with results of calorimetric experiments carried out during the hydration of silica-rich cement pastes in the presence of the same salts.
Article
The kinetics of the conversion of amorphous calcium phosphate (ACP) to hydroxyapatite (HA) at pH 8 at 26.0°, 37.5 and 48.0 C, in the presence of Mg has been studied in two sets of experiments in which (a) Mg-free ACP was added to solutions containing different amounts of Mg, or, (b) ACP precipitated in the presence of Mg was left in contact with its mother liquor. If the Mg/Ca ratio in the system exceeded 0.2 no conversion was observed. In the range of Mg/Ca ratios found in bones and teeth (mole ratios from 0.004 to 0.04), the induction period of the transformation (time before first HA crystals observed) increased with increasing Mg concentration, but the rate of the first order transformation was independent of Mg content. It is shown that the Mg effects the transformation by reducing the ACP solubility.
Article
In a self-levelling mortar based on the ternary binder system ordinary Portland cement (OPC), calcium aluminate cement (CAC) and anhydrite, a polycarboxylate-based superplasticizer (PC) showed no plasticizing effect in combination with citrate retarder while good flowability was observed with tartrate. The mechanism behind the incompatibility between PC and citrate was investigated by means of adsorption and zeta potential measurements. Also, anionic charge densities of the admixtures were compared. Adsorption measurements revealed that, in presence of citrate, PC adsorption drops dramatically to less than 10% of dosage added, implying a complete loss of fluidity in the paste. In presence of tartrate, however, PC adsorption remains high enough to still provide good flowability. In contrast, adsorption of casein biopolymer is not much affected by addition and type of retarder. Thus it provides high fluidity with both retarders. Comparison of specific anionic charge density of Ca2+ retarder complexes and PC reveals a direct correlation between their adsorption behaviour and anionic charge density. Admixtures with higher anionic charge density show higher affinity to the binder surface and thus adsorb preferredly. When several admixtures are present, molecules with lower anionic charge density will adsorb only if, after adsorption of the admixture with higher anionic charge, a cationic surface charge and enough adsorption area still exists. The incompatibility problem between PC and citrate in the self-levelling mortar formulation was solved by increasing the anionic charge density of the PC molecule. Similar to casein, adsorption of this modified PC is not much influenced by retarder molecules.
Article
The specific anionic charge density of polycarboxylate superplasticizers can be determined experimentally by titration with a cationic polyelectrolyte. In this study, the anionic charge densities of several polycarboxylates based on methacrylate ester chemistry were measured in aqueous solution at pH 7 and 12.6, resp., and in cement pore solution. The anionic charge of the polycarboxylates increases with increasing pH value as a result of deprotonation of the carboxylate groups in the polymer backbone. Addition of Ca2+ ions generally causes a decrease of the anionic charge density. The reduction in anionic charge varies and depends on the architecture of the polycarboxylate. The effect results from the binding of calcium ions by the carboxylate groups, both through complexation and counter-ion condensation. Consequently, the effective anionic charge density of polycarboxylates in cement pore solution can differ significantly from the charge density which is calculated based on the chemical composition. Generally the –COO− functionality may coordinate Ca2+ as a monodentate or bidentate ligand. The type of coordination depends on the steric accessibility of the carboxyl group. In PC molecules possessing high side chain density, the –COO− group is shielded by the side chains and coordinates as bidentate ligand, producing a neutral Ca2+–PC complex. Accordingly, this type of PC shows almost no anionic charge anymore in cement pore solution. In PCs possessing high amount of –COO−, Ca2+ is coordinated monodentate, resulting in an anionic complex. Consequently, this type of PC shows significant anionic character in pore solution. Its adsorption behaviour is determined by a gain in enthalpy which derives from the electrostatic attraction between the PC and the surface of cement. This way, by utilizing the relatively simple method of charge titration, it is possible to assess the electrostatic attraction which, besides entropy gains, is the driving force behind the adsorption of polycarboxylates on the cement surface and thus determines their effectiveness as dispersing agent. The findings are generally applicable to other anionic admixtures used in cement.
Article
The early hydration of alite, in particular the reason for the onset of the induction period, has been a subject of controversy for many decades. Several theories have been proposed, principally the formation of a protective phase inhibiting dissolution or delayed nucleation and growth, but none have successfully taken into account all the experimental data available. This paper proposes a new mechanism, based on a geochemical approach to crystal dissolution that fully explains the origin of the induction period. It implies that during cement hydration, dissolution is initially dominated by the formation of etch pits on surfaces and later becomes limited to step retreat from such pits. This change in mechanism alone can account for the rapid decrease in reaction after first contact with water, without the need to invoke the formation of a protective phase. Furthermore it can explain all the experimental findings in the literature. While this geochemical view of dissolution explains many features of the induction period it does not account for its end. This remains a question for further research, but the most probable explanation appears to be the onset of rapid growth of C–S–H.