Hydration mechanisms of supersulfated cement

Considerable attention has been given to special cements, capable of reducing CO₂ emissions, energy and limestone consumption. Supersulfated cements are made of blast furnace slag (GBFS), calcium sulfate (CS), and small quantities of activator, but achieving their optimal proportions is complex. In this paper, the effects of the both CS and alkali activator (KOH) contents were studied. The main results showed that the compressive strength, heat of hydration, and consumption of anhydrite phase were strongly influenced by the alkaline content, while low calcium sulfate or alkaline content increased the formation of CSH. The instability of ettringite was verified: with low CS, the probable hypothesis was its conversion into monosulfate due to the scarcity of sulfate; with high CS, it was associated with intense, rapid consumption of anhydrite with high KOH content, followed by the precipitation of ettringite on the surface of slag grains and its conversion into monosulfate.     

Thermokinetic analysis of the hydration process of calcium phosphate cement

A microcalorimeter (Setaram c-80) was used to study the thermokinetics of the hydration process of calcium phosphate cement (CPC), a biocompatible biomaterial used in bone repair. The hydration enthalpy was determined to be 35.8 J g^–1 at 37.0°C when up to 80 mg CPC was dissolved in 2 mL of citric buffer. In the present study, parameters related to time constants of the calorimeter were obtained by fitting the recorded thermal curves with the function θ=Ae^–?t(1– e^–?2t). The real thermogenetic curves were then retrieved with Tian function and the transformation rate of the hydration process of CPC was found to follow the equation α=1–[1–(0.0075t)³]³. The microstructures of the hydrated CPC were examined by scanning electron microscopy. The nano-scale flake microstructures are due to crystallization of calcium phosphate and they could contribute to the good biocompatibility and high bioactivity.     

Resistance of supersulfated cement to strong sulfate solutions

One of the principal uses of supersulfated cement has been for structures exposed to sea water and sulfate bearing ground waters. The resistance to such environments has been related to the absence of calcium hydroxide and the combination of much of the free alumina into ettringite during hydration. This paper reports the resistance of SSC to sulfate solutions in which ettringite has been decomposed. Prism samples were subjected to initial water storage at 25°C for both 28 days and 6 months. Samples were also cured for 6 months at 95°C and at both 11% and 100% R.H. The control samples of 28 days were compared with the 6 months samples of a more mature undecomposed SSC paste. After curing the prisms were measured and all the samples were immersed in three sulfate solutions (0.7M Na₂SO₄ , 0.7M MgSO₄ and saturated CaSO₄), and water at the same time. Measurements of linear expansion over 6 months were carried out. Core and surface material following immersion was examined by DTG and XRD. The study indicated that SSC is resistant to sodium and calcium sulfate solutions. Strong magnesium sulfate solutions decomposed the samples under all conditions. A possible mechanism for this attack is suggested. This revised version was published online in July 2006 with corrections to the Cover Date.     

Study on optimization of hydration process of blended cement

To optimize the hydration process of blended cement, cement clinker and supplementary cementitious materials (SCMs) were ground and classified into several fractions. Early hydration process of each cementitious materials fraction was investigated by isothermal calorimeter. The results show fine cement clinker fractions show very high hydration rate, which leads to high water requirement, while fine SCMs fractions present relatively high hydration (or pozzolanic reaction) rate. Cement clinker fractions in the range of 8–24 μm show proper hydration rate in early ages and continue to hydrate rapidly afterward. Coarse cement clinker fractions largely play “filling effect” and make little contribution to the properties of blended cement regardless of their hydration activity (or pozzolanic activity). The hydration process of blended cement can be optimized by arranging high activity SCMs, cement clinker, and low activity SCMs in fine, middle, and coarse fractions, respectively, which not only results in reduced water requirement, high packing density, and homogeneous, dense microstructure, but also in high early and late mechanical properties.     

Calorimetric determination of the effect of additives on cement hydration process

Possibilities of a multicell isoperibolic-semiadiabatic calorimeter application for the measurement of hydration heat and maximum temperature reached in mixtures of various compositions during their setting and early stages of hardening are presented. Measurements were aimed to determine the impact of selected components?? content on the course of ordinary Portland cement (OPC) hydration. The following components were selected for the determination of the hydration behaviour in mixtures: very finely ground granulated blast furnace slag (GBFS), silica fume (microsilica, SF), finely ground quartz sand (FGQ), and calcined bauxite (CB). A commercial polycarboxylate type superplasticizer was also added to the selected mixtures. All maximum temperatures measured for selected mineral components were lower than that reached for cement. The maximum temperature increased with the decreasing amount of components in the mixture for all components except for silica fume. For all components, except for CB, the values of total released heat were higher than those for pure Portland cement samples.     

Calorimetry in the studies of cement hydration

Calorimetry was applied to an investigation of the early hydration of Portland cement (PC)–calcium aluminate cement (CAC) pastes. The heat evolution measurements were related to the strength tests on small cylindrical samples and standard mortar bars. Different heat-evolution profiles were observed, depending on the calcium aluminate cement/Portland cement ratio. The significant modification of Portland cement heat evolution profile within a few hours after mixing with water was observed generally in pastes containing up to 25% CAC. On the other hand the CAC hydration acceleration effect was also obtained with the 10% and 20% addition of Portland cement. As one could expect the compressive and flexural strength development was more or less changed—reduced in the presence of larger amount of the second component in the mixture, presumably because of the internal cracks generated by expansive calcium sulfoaluminate formation.     

Investigation of the cellulose ethers effect on the Portland cement hydration by thermal analysis

Cellulose ethers (CE) are introduced in almost all cement-based dry mortars in order to retain water in mortar mass avoiding losing it too quickly by substrate absorption or water evaporation. In this way the workability of the fresh material, the adherence to the substrate and internal-strength characteristics of mortar, render or tile adhesive are improved. One of the side effects of cellulose ethers is the Portland cement hydration delaying. The influence of six commercial cellulose ethers, hydroxyethylmethyl cellulose (HEMC) type, on the hydration of Portland cement CEM I 42.5 R, was followed by thermal analysis (TG and DTA curves). Three of these cellulose ethers are unmodified, and have different viscosities, while three of them have the same viscosity but differ in the degree of modification (unmodified, one with medium modification and one with high modification). The interest of dry mortars producers for the effects of these cellulose ethers, is generated by the wide offer available on the market and by the absence of systematic data on the effect of different viscosities and degrees of modification on dry mortars properties. In order to quantify the effect of the CE on the cement hydration, the surface area of the endothermic effect corresponding to the dehydration of portlandite (Ca(OH)₂), formed after 1, 3, and 7 days of hydration, was defined. It was noted that the proportion of Ca(OH)₂ in samples containing CE after 1 day was 30–40 % lower than in reference sample. After 3 and 7 days of hydration the proportion of Ca(OH)₂ in samples containing CE approaches that of reference sample (10–20 % less). For the same period of hydration, the different viscosity, and different degree of modification of cellulose ethers cause variations in narrow limits of the proportion of Ca(OH)₂, and the degree of cement hydration, respectively.     

Effect of ZnO on the hydration of Portland cement

ZnO added to the system Portland cement — water changes the kinetics of the hydration process substantially. Amorphous zinc hydroxide is formed and inhibits the reaction of tricalcium silicate with water, resulting in an induction period prolongation. This effect depends on the amount of ZnO added to the hydrated paste. The transformation of zinc hydroxide into calcium hydrozincate provokes the further hydration.

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Zusammenfassung Durch den Zusatz von ZnO zum System Portlandzement-Wasser wird die Kinetik des Hydratationsprozesses grundlegend verändert. Amorphes Zinkhydroxid wird gebildet, was einen Inhibitor für die Tricalciumsilikatreaktion mit Wasser darstellt, wodurch die Induktionsperiode verlängert wird. Dieser Effekt hängt von der Menge ZnO ab, die dem Zementbrei zugesetzt wurde. Die Umwandlung von Zinkhydroxid zu Calciumhydrozinkat führt eine weitere Hydratation herbei.

     

Influence of chromate reducers on cement hydration

Present research compared the effect of chromate reducers such as ferrous sulphate heptahydrate (FeSO₄·7H₂O) and stannous sulphate dihydrate (SnCl₂·2H₂O) on the hydration of cement paste, using water?cement ratio of 0.5 and sealed in plastic bags without curing for 28 days. Uncured hydration properties of cement paste are investigated in detail by thermogravimetric analysis (TGA) and verified with the use of scanning electron microscopy (SEM) and X-ray powder diffractometry (XRD). This research concluded that the cement paste with 0.1% additives showed better hydration in the uncured condition than the control.

     

Use of emanation thermal analysis to characterize microstructure development during Portland cement hydration

Portland cement hydration has been investigated by emanation thermal analysis (ETA), based on the application of radon atoms as radioactive indicators. This method enabled us to characterize continuously changes in the microstructure of the cement paste at selected temperatures. The numerical simulation of time dependences of the emanating rate during cement hydration was carried out. An agreement between the mathematical model and experimental results of the ETA was obtained.     

Investigation regarding the effect of viscosity modifying admixtures upon the Portland cement hydration using thermal analysis

The effect of 13 viscosity modifying admixtures (VMA) on the Portland cement hydration was studied in this paper. In this purpose, thermal analyses (DTA and TG) were performed after 1, 7 and 28 days of hydration on cement pastes containing 0.01–0.5 % from the following VMA: diutan gum, welan gum, polygalactomannane ether, natural cellulose fibres, modified polysaccharide, polyacrylamide, high-molecular mass synthetic copolymer, hydroxypropyl starch and a chemically modified starch. It was noticed that the proportion of Ca(OH)₂ from the samples containing polygalactomannane ether and modified polysaccharide was smaller than in the reference sample, which proved their effect of cement hydration delay. For the other VMA, this effect was not detected, on the contrary, the amount of Ca(OH)₂ was higher than in the reference sample.     

Influence of superplasticizers on the course of Portland cement hydration

A multicell isoperibolic — semiadiabatic calorimeter was used for the measurement of temperature and the determination of the hydration heat evolution at earlier period of cement pastes setting and hardening. The measurements were aimed at the determination of the effect of superplasticizers (SPs) on the course of the Portland cement hydration. Commercial polycarboxylate SP was added to the mixtures and the heat effect was measured. With the increasing content of SP, the hydration temperature increased up to a certain value and then decreased. In case of a sufficient amount of water in the mixture to achieve complete hydration of cement, samples with the highest values of the maximum hydration temperature reached the highest values of the released total heat. If there is not a sufficient amount of water to achieve complete hydration, the samples with the highest values of the maximum hydration temperature reach the lowest values of the released total heat.     

Influence of ultradispersed silicas on Portland cement hydration

Solid-state ²⁹Si NMR spectroscopy was applied to examine Portland cement hydration and cement stone composition as influenced by the nature of commercial ultradispersed silicas (microsilica, precipitated silica, and colloidal silica).     

The use of thermal analysis to investigate the effects of cellulose ethers on the Portland cement hydration

Thermal analysis (DTA) was used for monitoring the proportions of Ca(OH)₂ formed at the hydration of simple Portland cement (CEM I 42.5 R) samples, and cement samples with 0.5% addition of unmodified hydroxypropyl methyl cellulose (HPMC), respectively, with the addition of starch ether and polyacrylamide modified HPMC. The proportions of Ca(OH)₂ formed after 1, 3, 7, and 28?days of hydration were assessed by the peak areas of the endothermic effect at the temperature range of 493?C503?°C, caused by the Ca(OH)₂ decomposition. The results obtained based on thermal analysis reflect very well the correlation between the Ca(OH)₂ proportions in the samples after different hydration periods and the retarding effect of the hydration processes caused by the cellulose ether's addition. This retarding effect is also evidenced by the setting times of the studied samples and the evolution of their mechanical strengths.     

Insights into the hydration of Portland cement under hydrothermal curing

Journal of Thermal Analysis and Calorimetry - The combined effect of temperature and vapor pressure on hydration reactions of three different types of Portland cements was studied using a...     

Specific features of portland cement hydration in the presence of sodium hydrosilicates

Specific features of hydration of a portland cement with addition of alkaline silicates and the ability of these additives to affect the concentration of hydroxide ions and Ca²⁺ ions in the liquid phase of the cement paste are considered.     

Study on the hydration product of ettringite in cement paste with ethanol-diisopropanolamine

Journal of Thermal Analysis and Calorimetry - Ettringite (AFt) is a very important hydration product of hardened cement pastes in the early stage. The effect of ethanol-diisopropanolamine (EDIPA)...     

Application of differential thermogravimetry to the hydration of expansive cement pastes

Differential thermogravimetric analysis was used to determine the hydration kinetics of expansive cement and its products at various ages of hydration. Analytical grade reagents, kaolin and Portland cement were used to prepare an expansive cement on the basis of calcium sulphoaluminate. Two mix compositions having the stoichiometric composition of trisulphate and monosulphate were synthesized from pure reagents. Three clinkers were also prepared from kaolin, gypsum and calcium carbonate with different compositions.The hydration of expansive cement prepared from the stoichiometric composition of trisulphate and Portland cement gives ettringite as the stable phase after seven days of hydration. The presence of more CaO than the stoichiometric composition of trisulphate favours the conversion of some ettringite to the monosulphate hydrate. The hydration of expansive cement prepared from the stoichiometric composition of monosulphate and Portland cement shows the presence of ettringite and the monosulphate phase. Ettringite is formed initially, and then transformed to the monosulphate form.