The hydrothermal transformation of calcium aluminate hydrates were investigated by in situ synchrotron X-ray powder diffraction in the temperature range 25 to 170 °C. This technique allowed the study of the detailed reaction mechanism and identification of intermediate phases. The material CaAl2O4·10H2O converted to Ca3Al2(OH)12 and amorphous aluminum hydroxide. Ca2Al2O5·8H2O transformed via the intermediate phase Ca4Al2O7·13H2O to Ca3Al2(OH)12 and gibbsite, Al(OH)3. The phase Ca4Al2O7·19H2O reacted via the same intermediate phase to Ca3Al2(OH)12 and mainly amorphous aluminum hydroxide. The powder pattern of the intermediate phase is reported.
A new method for the investigation of the structure of silicates - solid-state high-resolution 29Si NMR spectroscopy - was used for the investigation of 14 Å, 11 Å and 9 Å tobermorites prepared under different conditions. It is possible to determine selectively the individual SiO4 structural units in tobermorites (end groups, chain middle groups, branching sites, etc.) and to draw conclusions about the type and structure of the silicate anions in the lattice. Synthetic 14 Å tobermorite consists predominantly of single chains, the 11 Å and 9 Å tobermorites contain, depending on the method of preparation, double chains and/or single chains.ZusammenfassungDie neuentwickelte hochauflösende 29Si Festkörper-NMR-Spektroskopie wurde zur Strukturuntersuchung unterschiedlich hergestellter 14 Å, 11 Å und 9 Å Tobermorite angewandt. Es ist möglich, die einzelnen SiO4-Baueinheiten der Tobermorite wie Endgruppen von Ketten, Mittelgruppen und Verzweigungsgruppen, zu bestimmen und daraus Schlußfolgerungen über die Struktur der Silikatanionen im Kristallgitter zu ziehen. 14 Å Tobermorit ist vorwiegend aus Einfachketten aufgebaut, während die 11 Å und 9 Å Tobermorite in Abhängigkeit von ihren Herstellungsbedingungen Einfachketten oder Doppelketten enthalten.
The crystal structure of Kuzel's salt has been successfully determined by synchrotron powder diffraction. It crystallizes in the rhombohedral R3̅ symmetry with a = 5.7508 (2) Å, c = 50.418 (3) Å, V = 1444.04 (11) Å3. Joint Rietveld refinement was realized using three X-ray powder patterns recorded with a unique wavelength and three different sample-to-detector distances. Kuzel's salt is the chloro-sulfoaluminate AFm phase and belongs to the layered double hydroxide (LDH) large family. Its structure is composed of positively charged main layer [Ca2Al(OH)6]+ and negatively charged interlayer [Cl0.50·(SO4)0.25·2.5H2O]−. Chloride and sulfate anions are ordered into two independent crystallographic sites and fill successive interlayer leading to the formation of a second-stage compound. The two kinds of interlayer have the compositions [Cl·2H2O]− and [(SO4)0.5·3H2O]−. The crystal structure explains why chloride and sulfate anions are not substituted and why the formation of extended solid solution in the chloro-sulfate AFm system does not occur.
In the current study, MRI was applied to investigate lithium and sodium ion diffusion in cement paste and mortars containing inert sand and borosilicate glass. Paste and mortars were treated by complying with ASTM C 1260. Lithium and sodium distribution profiles were collected at different ages after different treatments. Results revealed that sodium ions had a greater diffusion rate than lithium ions, suggesting that Na reaches the aggregate particle surface before Li. Results also showed that Na and Li ions had a competitive diffusion process in mortars; soaking in a solution with higher [Li] favored Li diffusion but hindered Na diffusion. In mortars containing glass, a substantial amount of Li was consumed by the formation of ASR products. When [Li] in soaking solution was reduced to 0.37 N, a distinctive Na distribution profile was observed, indicating the free-state Na ions were continuously transformed to solid reaction products by ASR. Hence, in the modified ASTM C 1260 test, [Li] in the storage solution should be controlled at 0.74 N, in order to completely prevent the consumption of Na ions and thus stop ASR.
Among the numerous tests prescribed for assessing alkali–silica reactivity, ASTM C 1260 has become the preferred test method because of rapidity of the test procedure. However, a general concern about this method is the severity of the test conditions. The authors have evaluated the functionality of the method after modifying some of the important test parameters such as water–cement ratio, normality of test solution, length of test period, and curing. Combinations of high- and low-alkali cement with and without Classes C and F fly ash were included in the test program. The results are presented in this paper.
Leach characteristics of 137Cs and 60Co radionuclides from both ordinary Portland cement and cement mixed with two different ratios of silica fume and ilmenite have been studied using International Atomic Energy's (IAEA) standard leach method. A mathematical model has been simulated to predict the release rate of each nuclide from cubic geometry waste matrix and the predicted values are discussed in relation to experimentally observed leach rates to confirm the validity of the proposed mechanism in the model. The effect of temperature on the radionuclides leaching rates was also studied and the effective diffusion coefficients were obtained at different temperatures. The net fractional release of the two radionuclides from different waste forms showed a decreasing pattern as 137Cs>60Co, indicating the largest diffusion coefficient for cesium in all waste matrices.
MCC-1 static leaching experiments were carried out for a cementitious waste form in distilled water for up to 64 days at 5°C and 20°C in order to examine the leaching behavior of carbon-14. The complicated leaching behavior of carbon-14, meaning that the leached carbon-14 activity did not increase with (time)0.5, was attributable to the precipitation of calcite and the formation of colloidal particles in leachates, which were mainly dependent on the pH value and calcium concentration of leachate. The normalized elemental mass loss of carbon-14 was about 7.5 × 10−4 g/cm2 at 20°C for 64 days, which was lower than those of cement constitute elements such as calcium, sodium and aluminum. Especially, the leach rate of aqueous carbon-14 was lower than that of carbon-14 in the suspended leachate by a factor of about 10.
In their inspiring paper, Sanahuja et al. (CCR (37) 1427–1439) add a new dimension and degree of freedom for implementing the Materials Science Paradigm between microstructure and properties of cement-based materials, which is the particle aspect ratio of C–S–H elementary building block. The question addressed in this discussion is how far this departure from the perfect disordered morphology of C–S–H is truly relevant for cement-based materials.
To test the ability of the Virtual Cement and Concrete Testing Laboratory (VCCTL) software to predict cement hydration properties, characterization of mineralogy and phase distribution is necessary. Compositional and textural characteristics of Cement and Concrete Reference Laboratory (CCRL) cements 151 and 152 were determined via scanning electron microscopy (SEM) analysis followed by computer modeling of hydration properties. The general procedure to evaluate a cement is as follows: (1) two-dimensional SEM backscattered electron and X-ray microanalysis images of the cement are obtained, along with a measured particle size distribution (PSD); (2) based on analysis of these images and the measured PSD, three-dimensional microstructures of various water-to-cement ratios are created and hydrated using VCCTL, and (3) the model predictions for degree of hydration under saturated conditions, heat of hydration (ASTM C186), setting time (ASTM C191), and strength development of mortar cubes (ASTM C109) are compared to experimental measurements either performed at NIST or at the participating CCRL proficiency sample evaluation laboratories. For both cements, generally good agreement is observed between the model predictions and the experimental data.
The long-term corrosion process of reinforced concrete beams is studied in this paper. The reinforced concrete elements were stored in a chloride environment for 17years under service loading in order to be representative of real structural conditions. At different stages, cracking maps were drawn, total chloride contents were measured and mechanical tests were performed. Results show that the bending cracks and their width do not influence significantly the service life of the structure. The chloride threshold at the reinforcement depth, used by standards as a single parameter to predict the end of the initiation period, is a necessary but not a sufficient parameter to define service life. The steel–concrete interface condition is also a determinant parameter. The bleeding of concrete is an important cause of interface de-bonding which leads to an early corrosion propagation of the reinforcements. The structural performance under service load (i.e.: stiffness in flexure) is mostly affected by the corrosion of the tension reinforcement (steel cross-section and the steel–concrete bond reduction). Limit-state service life design based on structural performance reduction in terms of serviceability shows that the propagation period of the corrosion process is an important part of the reinforced concrete service life.
Several studies independently have shown that concrete strength development is determined not only by the water-to-cement ratio, but that it also is influenced by the content of other concrete ingredients. High-performance concrete is a highly complex material, which makes modeling its behavior a very difficult task. This paper is aimed at demonstrating the possibilities of adapting artificial neural networks (ANN) to predict the compressive strength of high-performance concrete. A set of trial batches of HPC was produced in the laboratory and demonstrated satisfactory experimental results. This study led to the following conclusions: 1) A strength model based on ANN is more accurate than a model based on regression analysis; and 2) It is convenient and easy to use ANN models for numerical experiments to review the effects of the proportions of each variable on the concrete mix.
Under deep oil-well conditions of elevated temperature and pressure, crystalline calcium silicate hydrates are formed during Portland cement hydration. The use of silica rich mineral additives leads to the formation of crystalline hydrates with better mechanical properties than those formed without the additive. The effects of silica flour, silica fume (amorphous silica), and a natural zeolite mixture on the hydration of Class H cement slurries at 180 °C under externally applied pressures of 7 and 52 MPa are examined in real time using in-situ synchrotron X-ray diffraction. For some compositions examined, but not all, pressure was found to have a large effect on the kinetics of crystalline hydrate formation. The use of silica fume delayed both C3S hydration and the formation of crystalline silicate hydrates compared to what was seen with other silica sources.
The Elon Farnsworth Battery, a concrete structure completed in 1898, is in an advanced state of disrepair. To investigate the potential for rehabilitation, cores were extracted from the battery. Petrographic examination revealed abundant deposits of alkali silica reaction products in cracks associated with the quartz rich metasedimentary coarse aggregate. The products of the alkali silica reaction are variable in composition and morphology, including both amorphous and crystalline phases. The crystalline alkali silica reaction products are characterized by quantitative X-ray energy dispersive spectrometry (EDX) and X-ray diffraction (XRD). The broad extent of the reactivity is likely due to elevated alkali levels in the cements used.
The uncertain environmental properties of cements, when used in construction materials, during these materials' service life and any “secondary life” (construction debris), have been raised as matters of concern due to the increasing use of alternative fuels and raw materials in the manufacture of cement clinker. A comparison of the leaching behavior of a range of traditional cement types, assessed in standard mortar prisms, with two non-traditional special cements, made using alternative fuels/raw materials, has shown that the leachability from these special cements does not exceed the leachability from the traditional cements. For relevant constituents, such as Cr, even a lower leachability is observed in spite of a higher total composition. This illustrates that an evaluation of cement based on total concentration of either the anhydrous cement or the cement-based product, in which it is used, is not a valid means of judging environmental impact. The emphasis in environmental evaluation has been on the properties during service life of cement-based products. This stage of life of a cement-based product has proved to be of limited concern. The emphasis should be focused on the “second life” of cement-based products. If construction debris is reused as aggregate in concrete, again leachability is of limited concern as the chemical environment dictated by the cement matrix ensures a low leachability. When construction debris are reused as hydraulically unbound aggregate, e.g., in road stabilization, environmental issues prove to be relevant as oxyanionic species (e.g., chromate, sulfate, molybdate, vanadate) may exceed critical limits according to Dutch regulations. With the possible exception of Cd, metals such as Pb and Zn are unlikely to become critical environmentally, even in the “second life” of cement-based products. When environmental criteria for cement-based products are developed they should be based on leachability of standard mortar and not on concentration to guarantee environmental compatibility during service life, “second life” and “end of life” scenarios.
We describe a quantitative mineralogical study of the hydrothermal reactions of an oil well cement with added silica and alumina, hydrated at temperatures from 200 to 350 °C. We compare the products with pure end member systems and find phase stability can be altered radically, even by small amounts of additive. The upper temperature limits of α-C2SH (< 250 °C), and 1.1 nm tobermorite C5S6H5 (< 300 °C) are increased. C8S5, reported in a cement-based system for the first time, is stable to 300 °C and is believed to prevent foshagite C4S3H formation below 350 °C. Hydrogarnet C3AS3−yH2y is the only aluminum bearing phase at < 300 °C but it coexists with C4A3H3 and bicchulite C8A4Si4H4 at higher temperatures. The presence of alumina increases the stability of 1.1 nm tobermorite greatly and also to a lesser degree of gyrolite.
Hydroceramic compositions in the CaO–Al2O3–SiO2–H2O (CASH) system have potential as geothermal well sealants as well as autoclaved construction materials. We report new data on phase compositions and reaction rates in hydrothermal syntheses at 200 °C and 250 °C using a commercial API Class G oilwell cement alone, and at 200 °C with additions of silica flour and of corundum (alumina). Curing times were in the range 1–240 h. We use both ex-situ laboratory X-ray diffraction and in-situ synchrotron energy-dispersive X-ray diffraction to track rates of reaction. When cement only is hydrated, jaffeite, α-C2SH and portlandite are formed. When silica flour is added a precursory gel forms prior to the crystalline calcium silicate hydrate phases xonotlite and gyrolite. Both XRD and EDD data suggest that the addition of silica flour retards the hydration of the cement at early times (< 24 h). In alumina-containing systems the rate of consumption of clinker phases is the same as in cement only systems. Jaffeite and α-C2SH occur as intermediates but the major end product is a siliceous katoite-type hydrogarnet. Quantitative phase analysis using Rietveld refinement of ex-situ diffraction data gives results which are mostly consistent with stoichiometric constraints in all three systems examined here.
Phase relations in the title system were studied using crystalline and amorphous precursors. These were treated in sealed steel alloy autoclaves for periods ranging up to 12 months. Many of the synthetic precursors crystallised to give high purity, single-phase preparations.Although the CaO–SiO2–H2O system is marked by metastable phase formation, it is demonstrated that a number of reactions important to establish the low-temperature phase relationships can be shown to occur reversibly and therefore define the phase equilibrium. New stability data are presented for hillebrandite, afwillite, xonotlite, tobermorite and jennite. Synthetic jennite is shown to have a Ca/Si atomic ratio ∼1.45, rather less than the reported 1.5 ratio. A phase diagram revised in light of new knowledge is presented.
This paper uses the Nernst–Einstein equation to calculate the diffusion coefficient of chloride ions of high-performance concrete (HPC), analyzing and discussing the property of resistance to chloride ion of HPC with fly ash or blast furnace slag. The experimental results show that the diffusion coefficient of chloride ion increases with the rise of the water–binder ratio and decreases with the rise of quantity of fly ash or blast furnace slag. That is to say, the diffusion coefficient of chloride ions is not only related to the water–binder ratio but also to the quantity and the type of additive. Both the fly ash concrete and blast furnace slag concrete have good resistance to chloride ions. Their diffusion coefficients of chloride ions are lower than 10−9 cm2/s.
Disagreement exists in the literature as to whether or not fly ash accelerates or retards the early hydration of fly-ash blended cements. As an outgrowth of this controversy the effects of two fly ashes, a Class C and a Class F, their leachates and leached fly ash residues upon the first 24 hours of cement hydration were studied. It was found that both fly ashes (“as received and to some degree, leached”) retarded a Type I cement hydration; however the fly-ash leachates did not. The retardation phenomenon is apparently related to the presence and condition of the fly ash surfaces.
Pull-out tests were carried out on deformed steel fibers embedded in cement-based matrices. Four matrices including plain cement paste, cement paste with 10% silica fume, cement paste with 10% high reactivity metakaolin (HRM), and cement paste with a combination of 5% silica fume and 5% HRM in the same mix were investigated. Based on the resulting bond-slip curves, HRM was found to be more effective in improving the pull-out performance than silica fume. However, when silica fume and HRM were combined in the same mix, an excessive improvement in the bond occurred which led to undesirable fiber fractures, and consequently, a brittle behavior.
Ettringite is responsible for both the initial set of Portland cement and for premature concrete deterioration. A new method of ettringite crystal growth by combining calcium hydroxide and aluminum sulfate solutions was devised to reliably produce crystals that could be seen with a light microscope (45×–320×). The nucleation, growth, morphology, and stability of ettringite in the presence of over 300 chemicals and admixtures, many of which are present in the concrete environment, was then investigated. The plasticizers sorbitol, citrate, and tartrate were found to inhibit ettringite nucleation and growth, as did certain lignosulfonate air-entraining admixtures. The Type B set retarder borax inhibited ettringite formation at <44 ppm. The consequences and implications of this are discussed.
The solid solution between Al- and Fe-ettringite Ca6[Al1 − xFex(OH)6]2(SO4)3·26H2O was investigated. Ettringite phases were synthesized at different Al/(Al + Fe)-ratios (= XAl,total), so that XAl increased from 0.0 to 1.0 in 0.1 unit steps. After 8 months of equilibration, the solid phases were analyzed by X-ray diffraction (XRD) and thermogravimetric analysis (TGA), while the aqueous solutions were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS). XRD analyses of the solid phases indicated the existence of a miscibility gap between XAl,total = 0.3–0.6. Some of the XRD reflections showed two overlapping peaks at these molar ratios. The composition of the aqueous solutions, however, would have been in agreement with both, the existence of a miscibility gap or a continuous solid solution between Al- and Fe-ettringite, based on thermodynamic modeling, simulating the experimental conditions.
The effects of hydrating a white Portland cement (wPc) in 0.30 and 0.50 M solutions of sodium aluminate (NaAlO2) at 5 and 20 °C are investigated by 27Al and 29Si magic-angle spinning (MAS) NMR spectroscopy. It is demonstrated that NaAlO2 accelerates the hydration of alite and belite and results in calcium-silicate-hydrate (C-S-H) phases with longer average chain lengths of SiO4/AlO4 tetrahedra. The C-S-H phases are investigated in detail and it is shown that the Al/Si ratio for the chains of tetrahedra is quite constant during the time studied for the hydration (6 h to 2 years) but increases for higher concentration of the NaAlO2 solution. The average chain lengths of “pure” silicate and SiO4/AlO4 tetrahedra demonstrate that Al acts as a linker for the silicate chains, thereby producing aluminosilicate chains with longer average chain lengths. Finally, it is shown that NaAlO2 reduces the quantity of ettringite and results in larger quantities of monosulfate and a calcium aluminate hydrate phase.
29Si MAS-NMR measurements of cement have been used to follow the hydration process in cement pastes. Samples prepared using a w/c ratio of 0.45 and type I cement have been cured at temperatures over the range 20 to 55°C with curing times of 3 to 28 days. Compressive strength values for samples subjected to the same time/temperature curing regime were also obtained. The compressive strength is found to show a linear dependence on hydration as characterized in terms of the NMR Q0, Q1 and Q2 silicate polymerization states. Solid state 29Si NMR measurements appear promising as a means of monitoring cement/concrete strength.
The low natural abundance and the long spin lattice relaxation time of 29Si lead to long measurement times and/or low signal-to-noise ratios using 29Si magic angle spinning NMR spectroscopy. By contrast, samples containing paramagnetic iron ions have much shorter relaxation times, making measurements up to seven times more efficient, but at the same time making quantitative analysis unreliable. To solve the problem, the spin-lattice relaxation times of ordinary Portland cement (opc) and silica fume with and without iron content has been determined with inversion recovery experiments. The effect of varying the spectrum repetition time on the quantitative analysis is demonstrated for mixtures of opc with silica fume. For opc and silica fume with iron impurities repetition times as short as 5 s has permitted accurate quantitative analysis of the silicates present in these materials.
The degree of hydration of cement and concrete can be followed using 29Si MAS-NMR. Preliminary results of the effects of hydration on the distribution of silicate polymers in concretes of known mix designs, as well as the constituent cement, are presented. The spectra of the sand and aggregate used are also given. The technique appears promising as a means of monitoring concrete silicate anion structure and its possible correlation with mechanical properties.
A new method for the determination of the structure of silicate anions, formed during hydration of tricalciumsilicate Ca3SiO5 (C3S), is presented. High resolution solid state 29Si NMR spectra of C3S and its hydration products, formed during selected reaction times from 6 hours to 130 days, provide the kinetics of formation of the end groups and of the middle groups in silicate chains.ZusammenfassungEs wird eine neue Methode zur Bestimmung der Struktur der Silicatanionen, die im Verlaufe der Hydratation von Tricalciumsilicat Ca3SiO5 (C3S) gebildet werden, vorgestellt. Hochaufgelöste 29Si Festkörper-NMR Spektren von C3S und seinen Hydratationsprodukten, gemessen nach ausgewählten Reaktionszeiten von 6 Stunden bis 130 Tage, ermöglichen es, die Reaktionskinetik der Bildung der End- und Mittelgruppen in Silicatanionen zu verfolgen.
This paper is concerned with the evolution of the microstructure of cementitious materials subjected to high temperatures and subsequent resaturation in the particular context of long-term storage of radioactive wastes, where diffusive and convective properties are of primary importance. Experimental results obtained by mercury intrusion porosimetry (MIP) are presented concerning the evolution of the pore network of ordinary portland cement (OPC) paste heated at temperatures varying between 80 and 300 °C. The consequences of heating on the macroscopic properties of cement paste are evaluated by measures of the residual gas permeabilities, elastic moduli and Poisson's ratio, obtained by nondestructive methods. Resaturation by direct water absorption and water vapour sorption are used to estimate the reversibility of dehydration. The results provide some evidence of the self-healing capacity of resaturated cement paste after heating at temperatures up to 300 °C.
35Cl nuclear magnetic resonance (NMR) T1 and T2 relaxation study of suspensions of the cement hydrate phases portlandite, C4AC̄H11 (an AFm phase), and jennite provides significant insight into the mechanisms of chloride sorption in Portland cement systems. For these three phases, all observed chloride is in rapid exchange (υex>1.6 kHz) between surface and bulk solution sites and is predominantly in a hydrated, solution-like chemical environment. A two-site exchange model for the T1 relaxation rate data yields sorption densities in excellent quantitative agreement with sorption isotherm measurements for portlandite and C4AC̄H11. For jennite, the NMR results underestimate the sorption isotherm results by about 40%, but the sorption densities are low, and the results are probably in agreement within experimental error. The observed sorption densities are much greater than can be accommodated by bonding directly to atoms on the solid surfaces and the amount predicted to be in the Stern and diffuse layers by a Gouy-Chapman calculation assuming sorption of only isolated chloride ions. The significance of understanding the structural and dynamical properties of solution-state chloride and the roles of ion pairs and clusters in the solution near the solid surface are discussed.
The texture and exposed phases of a fracture surface are direct evidence of the mechanical behavior of a cement-based material. Deflection, microcracking and bridging are toughening mechanisms involved in fracture of brittle matrices that affect surface roughness. This study measures crack deflection and branching using 3D surface measurement techniques with confocal laser microscopy of mechanically fractured mortar prisms and 3D stereo pair microscopy of mechanically fractured plain concrete prisms. Image analysis techniques were used to identify phase composition and out-of-surface crack branching from profiles of cracks intruded with a low melting-point alloy. The resulting data was the basis for a micromechanical model to relate surface and phase data and the measured fracture energy to the increase in energy with respect to fracture of the matrix independent from the composite behavior.
Cement pastes aged from 1 to 60 days were studied using synchrotron microtomography on the MS-X04SA beam line at the Swiss Light Source. This allowed three dimensional images to be obtained with a resolution approaching that of backscattered electron images in the SEM. From these images, several features can be extracted and studied, both quantitatively and morphologically. In this study, attention was focused on the reacting anhydrous cement grains and porosity. Three dimensional imaging of capillary porosity allowed the connectivity and tortuosity of the pore network to be studied. It is shown that the degree of connectivity of the pore network is very sensitive to both the spatial resolution of the images and the evolution of contrast resolution during ageing of the cement.
Porous media can be considered as interfacial systems where an internal surface partitions and fills the space in a complex way. Meaningful structural features appear on a length-scale where physical chemistry plays a central role either to impose a specific organisation on the material or to strongly modify the dynamics and the thermodynamics of the embedded fluids. A key issue is to understand how the geometrical and interfacial confinement affects numerous phenomena such as molecular diffusion, excitation relaxation, reaction kinetics, phase transitions, adsorption and capillary condensation. We will first review some experimental techniques able to image the 3D structure of disordered porous media. In the second part, we will analyse the geometrical and particularly some topological properties of a disordered porous material. We will discuss the interest and the limits of several strategies for obtaining 3D representations of various pore networks starting from an incomplete set of morphological characterisations. Finally, connection between geometry and diffusive transport will be presented, with emphasis on the application of pulsed gradient spin echo NMR technique as a tool for a multiscale analysis of transport in a confining geometry.
The performance of metakaolin and portland cement in ettringite formation determined by ASTM C 452-68 was analyzed. ASTM C 452-68 testing was conducted on 40 cements, 10 portland cements-6 OPC and 4 SRPC and 30 blended cements containing 20%, 30% and 40% metakaolin. ASTM C 452-68 specimens were manufactured with all cements, and the daily length growth rate was calculated by dividing the measured increase in length, by the number of days lapsing since the preceding measurement. The results showed that in ASTM C 452-68, the pozzolanic reactions involving the reactive alumina present in pozzolans took place during the first 28 days of age, and ettringite from both reactive alumina and C3 were the reaction products in all cases.
The reactivity of Ca2AlMnO5 towards water was studied, in the absence as well as in the presence of gypsum, based on a comparative study regarding the hydration process undergone by Ca2AlFeO5 in identical conditions. In the absence of gypsum, the hydration reaction of Ca2AlMnO5 (C4AMn) is more strongly exothermal than that produced in Ca2AlFeO5 (C4AF) hydration, generating hydrates of the hexagonal type which contain Mn in their structure. These hydrates exhibit a high degree of stability, and their conversion into cubic hydrates is very slow, as it does not set in but after the seven first days of hydration. The presence of gypsum does not have any retarding effect on C4AMn hydration, which occurs speedily and with remarkable heat release. The hydration product formed is an ettringite-like phase with Mn incorporated in its structure. The formation rate of this ettringite phase is much faster than in C4AF hydration.
Outer product C–S–H had a mixture of fibrillar and foil-like morphology in a 28-day-old water-activated paste, and foil- or lath-like morphology in an alkali-activated paste. It was not possible to determine the chemical composition of C–S–H using SEM-EDX because of fine-scale intermixing with other phases; TEM-EDX was necessary. The C–S–H formed in the alkali-activated paste had a lower mean Ca/(Al + Si) ratio than that formed with water. The mean length of the aluminosilicate anions in the C–S–H was similar in both systems and increased with age; those in the Op C–S–H were likely to be shorter than those present in the Ip C–S–H with water activation, but longer (and more protonated) with alkali. The potassium in the alkali-activated paste was present either within the C–S–H structure charge balancing the substitution of Al3+ for Si4+, or adsorbed on the C–S–H charge balancing sulfate ions.
It is well known that the pozzolanic reaction between metakaolin (MK) and calcium hydroxide produces CSH, C2ASH8 (stratlingite), C4AH13 and C3ASH6 (hydrogarnet). However, the presence or absence of these hydrated phases depends on different parameters, such as curing temperature, matrix used, etc. This paper shows the results of a study in order to know the effect of high curing temperature (60 °C) on the kinetics of the pozzolanic reaction in different matrices. MK/lime (calcium hydroxide) and MK-blended cement matrices were studied in samples stored and cured at 60 °C and up to 123 days of hydration. The nature, sequence and crystallinity of the hydrated phases were analysed using differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques.Results showed that the sequence and formation of the hydrated phases was different in both matrices cured at 60 °C. In an MK/lime matrix, C2ASH8, C4AH13 and C3ASH6 were the main hydrated phases; while in an MK-blended cement, stratlingite was the sole hydrated phase issued from pozzolanic reaction. The DTA and XRD data also reveal an important fact: there is no evidence of the presence of hydrogarnet in blended cements.
This research presents the experimental results of a study carried out to determine the effect of curing temperature on the reaction kinetics in a metakaolin/lime mixture cured at 60 °C and after 60 months of hydration. The stabilities of hydrated phases formed during the pozzolanic reaction in these working conditions were evaluated. The results obtained in current paper showed that metastable hexagonal phases (C2ASH8 and probably C4AH13) coexist with stable cubic phase (hydrogarnet) in the absence of lime. Also, there is evidence of the possible presence of a calcium aluminum silicate hydroxide hydrate (vertumnite).
When MK reacts with calcium hydroxide, cementitious products are formed. It has been reported that CSH, C2ASH8 and C4AH13 are the most important hydrated phases formed. These phases are stable at 20 °C. However, some of them (C2ASH8 and C4AH13) are metastable phases, converting to hydrogarnet (C3ASH6) for long curing times at elevated temperatures. The partial or total conversion reaction could produce a negative effect on the performance and durability of blended cements, due to a volume decrease associated with the process of transformation.Due to the influence that this conversion could have on the microstructure and durability of a cement paste containing MK, the current paper presents the results of a research programme carried out on blended cements containing 10%, 20% and 25% of MK, cured at 60 °C up to 124 days of hydration.The total, partial porosity and average pore diameter evolution vs. time is determined using mercury intrusion porosimetry (MIP). An estimated degree of hydration of MK-blended cements cured at 60 °C is proposed.The results show that there is no increase in porosity values and average pore diameters with time. Therefore, the hydrated phases produced in MK-blended cements under the test conditions used do not have a negative effect on the microporosity. A suitable correlation between porosity and degree of hydration has been found.
An electromagnetic interference (EMI) shielding effectiveness of 70 dB at 1.5 GHz has been attained in cement paste that contains 0.72 vol.% stainless steel fibers of diameter 8 μm and length 6 mm. The shielding is primarily by reflection. The material exhibits electrical resistivity 16 Ω cm. The presence of sand essentially does not affect the shielding effectiveness. The fibers remain effective in the presence of steel rebars. For comparison, the shielding effectiveness of a solid piece of stainless steel is 78 dB at 1.5 GHz.
Cement-based materials are non-combustible, but the complex chemo-physical mechanisms that drive at elevated temperatures the thermal degradation of the mechanical properties (stiffness, strength) are still an enigma that have deceived many decoding attempts. This paper presents, for the first time, results from a new experimental technique that allows one to rationally assess the evolution of the nano-mechanical behavior of cement paste at elevated temperatures. Specifically, the thermal degradation of the two distinct calcium-silicate hydrate (C-S-H) phases, Low Density (LD) C-S-H and High Density (HD) C-S-H, is assessed based on a statistical analysis of massive nanoindentation tests. From a combination of nanoindentation, thermogravimetry and micromechanical modeling, we identify a new mechanism, the thermally induced change of the packing density of the two C-S-H phases, as the dominant mechanism that drives the thermal degradation of cementitious materials. We argue that this loosening of the packing density results from the shrinkage of C-S-H nanoparticles that occurs at high temperatures, most probably due to the loss of chemically bound water.
The thermal stability of synthetic ettringite was examined in NaOH solutions up to 1 M after 12 h of heat treatment at 80 °C, with or without the coexistence of C3S in the system. Ettringite was found to convert to the U phase, a sodium-substituted AFm phase, over the heat treatment in the absence of C3S. The presence of C3S, leading to C-S-H formation, prevents the U phase formation and results in the conversion of ettringite to monosulfate. Sulfate ions generated from ettringite decomposition mostly remain in the solution, but some is incorporated into C-S-H. During subsequent storage at room temperature, the majority of monosulfate slowly converts back to secondary ettringite under moist conditions, using the supply of sulfate ions from the solution and C-S-H. The observations support the current mechanism of delayed ettringite formation (DEF).
Degree of hydration (DOH) and differential scanning calorimetry (DSC) measurements are used to characterize the effect of early exposure to a 90% relative humidity (RH) environment on cement paste hydration. Early exposure to a 90% RH environment can lead to the consumption of freezable water and altered microstructural development. The minimum duration of 100% RH curing required to eliminate the effects of an unsaturated environment on microstructural development coincides with the appearance of a DSC peak near −30 °C that occurs in the range 1–14 days for the pastes studied. The Jennings colloidal microstructural model is used to argue that the −30 °C peak coincides with the cessation of capillary pore percolation. Alternatively, all samples cured under 100% RH conditions for 7 days prior to 90% RH exposure hydrated at the same rate as those continuously exposed to 100% RH. The application of these results to the formulation of separate curing practices for durability and strength is discussed.
This paper describes an electrical model for cement and concrete applicable over a wide frequency range and at various stages of hydration. The model contains a non-Debye dispersive element to explain the experimental data which are presented.
A.C. impedance spectroscopy has been used to investigate the mechanism of hydration of portland cement paste, from 48 to 380 hours. Interpretation of a.c. measurements obtained over a wide frequency range was complimented by equivalent circuit modelling. The proposed equivalent circuit was chosen so that its RC parameters would physically represent microstructural elements of the cement paste.
The microstructure of very low porosity cement and cement-silica fume paste systems was investigated by A.C. impedance spectroscopy (ACIS). The theoretical relationship between ACIS parameters and the structure of porous materials previously proposed by authors has been confirmed for these systems. Results indicate that a delayed appearance of the high frequency semicircle, present in spectra of normal cement pastes at high water/cement ratios, does not occur in the present cases due to their lower initial porosity. The high frequency arc diameter, R2, increases significantly with hydration time and compacting pressure; this is compatible with results from other measurements, e.g. mercury intrusion porosimetry, helium pycnometry, etc. The densification effect of silica fume in very low porosity systems has also been demonstrated. It was found that there is an optimum addition of silica fume at which R2 has a maximum value. It is apparent that ACIS techniques are applicable to studies of the microstructure of very low porosity cementitious systems.
The influence of silica fume on impedance behaviour of hydrating cement has been carefully investigated. The results demonstrate that impedance measurement is very sensitive to changes in hydration kinetics and microstructure development due to the presence of silica fume. A modified equivalent circuit containing a frequency dependent resistance element is proposed to model hydration of these microporous systems. A.C. impedance spectroscopy is demonstrated to be a useful tool for studying factors that influence pore structure in cement systems containing finely divided silica.
An a.c. impedance technique was applied to the investigation of crack growth mechanisms in portland cement and cement-silica fume pastes, mortars and cement-wollastonite micro-fiber reinforced systems during compressive loading. It was demonstrated that the high frequency arc in the complex plane is very sensitive to microstructural change and micro-cracking in the specimens subjected to compressive load. The experimental results indicate that micro-cracking in different cement-based materials depends on their intrinsic microstructural characteristics.
Conventional reinforcing steel is used in the majority of reinforced concrete structures. In general, steel reinforcement meeting ASTM A615 specifications has been the predominant reinforcement used for these structures. Low-alloy reinforcing steel (ASTM A706) was developed and is being marketed to improve ductility and weldability deficiencies associated with the ASTM A615 reinforcement. Several State Highway Agencies have adopted the use of these low-alloy reinforcing steels. Limited research has been performed on the corrosion characteristics of the steel reinforcement meeting ASTM A706 specifications. This paper presents results from a laboratory study on the critical chloride threshold, macrocell corrosion rates, and mass loss testing for ASTM A706 and ASTM A615 reinforcing steels embedded in concrete and exposed to chloride solution. Results from this study indicate that ASTM A706 reinforcing steel exhibits lower critical chloride threshold levels and higher corrosion rates than ASTM A615 reinforcing steel when embedded in cementitious materials.
For a quantitative understanding of freezing damage on Autoclaved Aerated Concrete(AAC) and fiber reinforced AAC(i.e., RAAC), the influence of water and temperature (R.T. ∼ −20°C) on those materials have been studied by the investigation of AE characteristics, the fracture mechanics J-integral test and SEM observation. Furthermore, using the AE frequency analysis based on the frequency energy density distribution ratio (EDDR), the micro-fracture process for various test conditions has been interpreted.The AE activities and fracture toughness showed a large difference depending on the water content and temperature. All the AE events emitted during the fracture toughness tests could be classified into 6 groups. Also, the AE sources were considered paying particular attention to the micro-crack formation, the friction of inter-matrix and the fiber breaking behaviors at fracture. Noting that the AE is emitted during the drying process, the drying shrinkage damage was discussed.
This paper presents a method for assessing the normalized age factors, which allow accelerated alkali–aggregate reaction (AAR) tests performed at various temperatures (20, 40 and 60 °C) to be related to the conditions encountered in situ in concrete structures. The evaluation of normalized age factors is based on the comparison of many experimental results taken from the literature concerning laboratory tests and in situ measurements. The use of these factors permits us to evaluate, from the results of an accelerated test performed at 60 °C, the protection time against AAR that could be expected for in situ concretes containing mineral admixtures (silica fume and fly ashes). The results show that, in addition to the inhibitory action of mineral admixtures leading to a strong decrease in the final AAR-swelling, the protection against abnormal expansion caused by AAR increases significantly when mineral admixtures are used. Abnormal expansion is expected at 2–4 years for plain concrete compared to 7–50 years for concrete with mineral admixtures.