Effect of hydration temperature on the solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pastes

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement and white Portland cement pastes were measured during the first 28 d of curing at temperatures ranging from 5–50 °C. Saturation indices with respect to solid phases known to form in cement paste were calculated from a thermodynamic analysis of the elemental concentrations. Calculated saturation levels in the two types of paste were similar. The solubility behavior of Portlandite and gypsum at all curing temperatures was in agreement with previously reported behavior near room temperature. Saturation levels of both ettringite and monosulfate decreased with increasing curing temperature. The saturation level of ettringite was greater than that of monosulfate at lower curing temperatures, but at higher temperatures there was effectively no difference. The solubility behavior of C-S-H gel was investigated by applying an appropriate ion activity product (IAP) to the data. The IAP_CSH decreased gradually with hydration time, and at a given hydration time the IAP_CSH was lower at higher curing temperatures.     

Thermodynamic modelling of the hydration of Portland cement

A thermodynamic model is developed and applied to calculate the composition of the pore solution and the hydrate assemblage during the hydration of an OPC. The calculated hydration rates of the individual clinker phases are used as time dependent input. The modelled data compare well with the measured composition of pore solutions gained from OPC as well as with TGA and semi-quantitative XRD data. The thermodynamic calculations indicate that in the presence of small amounts of calcite typically included in OPC cements, C-S-H, portlandite, ettringite and calcium monocarbonates are the main hydration products. The thermodynamic model presented in this paper helps to understand the interactions between the different components and the environment and to predict the influence of changes in cement composition on the hydrate assemblage.     

Alkali binding in hydrated Portland cement paste

The alkali-binding capacity of C-S-H in hydrated Portland cement pastes is addressed in this study. The amount of bound alkalis in C-S-H is computed based on the alkali partition theories firstly proposed by Taylor (1987) and later further developed by Brouwers and Van Eijk (2003). Experimental data reported in literatures concerning thirteen different recipes are analyzed and used as references. A three-dimensional computer-based cement hydration model (CEMHYD3D) is used to simulate the hydration of Portland cement pastes. These model predictions are used as inputs for deriving the alkali-binding capacity of the hydration product C-S-H in hydrated Portland cement pastes. It is found that the relation of Na⁺ between the moles bound in C-S-H and its concentration in the pore solution is linear, while the binding of K⁺ in C-S-H complies with the Freundlich isotherm. New models are proposed for determining the alkali-binding capacities of C-S-H in hydrated Portland cement paste. An updated method for predicting the alkali concentrations in the pore solution of hydrated Portland cement pastes is developed. It is also used to investigate the effects of various factors (such as the water to cement ratio, clinker composition and alkali types) on the alkali concentrations.     

A preliminary investigation of the in vitro bioactivity of white Portland cement

Freshly-mixed and partially-cured ordinary Portland cement (OPC) pastes have been shown to exhibit good biological compatibility with a range of cells and tissue-types; particularly those associated with bone formation. Formulations based on OPC have been used as dental restoratives and are now being investigated for their potential use in orthopaedic repair. Despite the current clinical interest in OPCs, very little is known about their chemistry in the physiological environment. In this respect, research to investigate aspects of the interactions between a white Portland cement (WPC) paste and simulated body fluid (SBF) has been carried out in vitro. Exposure to SBF has been found to promote the precipitation of a layer of ‘bone-like’ hydroxyapatite on the surface of WPC paste which underpins its ability to integrate with living tissue. The dissolution of portlandite and formation of calcite were also observed on contact with SBF.     

The hydration products of Portland cement in the presence of tin(II) chloride

The hydration products of Portland cement pastes cured using water containing tin(II) chloride have been compared with those using distilled water. In the latter case, the expected products—portlandite, ettringite and calcite—were observed. The X-ray diffraction patterns of the cement pastes cured in the presence of tin(II) chloride showed several additional peaks that have been attributed to the formation of calcium hydroxo-stannate, CaSn(OH)₆, and Friedel's salt (tetracalcium aluminate dichloride-10-hydrate), Ca₃Al₂O₆·CaCl₂·10H₂O. The amount of portlandite formed was reduced in the presence of tin(II) chloride. Calcium hydroxo-stannate contains tin in the +IV oxidation state and equations are presented to account for the oxidation of Sn(II) to Sn(IV) preceding the formation of CaSn(OH)₆ and Friedel's salt.     

Influence of limestone on the hydration of Portland cements

The influence of the presence of limestone on the hydration of Portland cement was investigated. Blending of Portland cement with limestone was found to influence the hydrate assemblage of the hydrated cement. Thermodynamic calculations as well as experimental observations indicated that in the presence of limestone, monocarbonate instead of monosulfate was stable. Thermodynamic modelling showed that the stabilisation of monocarbonate in the presence of limestone indirectly stabilised ettringite leading to a corresponding increase of the total volume of the hydrate phase and a decrease of porosity. The measured difference in porosity between the “limestone-free” cement, which contained less than 0.3% CO₂, and a cement containing 4% limestone, however, was much smaller than calculated.

Coupling of thermodynamic modelling with a set of kinetic equations which described the dissolution of the clinker, predicted quantitatively the amount of hydrates. The quantities of ettringite, portlandite and amorphous phase as determined by TGA and XRD agreed well with the calculated amounts of these phases after different periods of time. The findings in this paper show that changes in the bulk composition of hydrating cements can be followed by coupled thermodynamic models. Comparison between experimental and modelled data helps to understand in more detail the dominating processes during cement hydration.     

Binding of chloride and alkalis in Portland cement systems

A thermodynamic model for describing the binding of chloride and alkalis in hydrated Portland cement pastes has been developed. The model is based on the phase rule, which for cement pastes in aggressive marine environment predicts multivariant conditions, even at constant temperature and pressure. The effect of the chloride and alkalis has been quantified by experiments on cement pastes prepared from white Portland cements containing 4% and 12% C₃A, and a grey Portland cement containing 7% C₃A. One weight percent calcite was added to all cements. The pastes prepared at w/s ratio of 0.70 were stored in solutions of different Cl (CaCl₂) and Na (NaOH) concentrations. When equilibrium was reached, the mineralogy of the pastes was investigated by EDS analysis on the SEM. A well-defined distribution of chloride was found between the pore solution, the C-S-H phase, and an AFm solid solution phase consisting of Friedel's salt and monocarbonate. Partition coefficients varied as a function of iron and alkali contents. The lower content of alkalis in WPC results in higher chloride contents in the C-S-H phase. High alkali contents result in higher chloride concentrations in the pore solution.     

激发条件对脱硫石膏-矿渣体系水硬强度的影响

利用正交实验研究了硅酸盐水泥和其他两种矿物组分复合激发对脱硫石膏-矿渣体系强度的影响,用SEM、XRD分析了水化样品的微观结构.研究结果表明:硅酸盐水泥等多组分复合激发下,脱硫石膏-矿渣体系在水中标准条件养护,3 d抗压强度达17 MPa以上,28 d抗压强度达58 MPa以上.复合激发剂3种组分的优化组合为6:6:5,复合激发剂的用量为脱硫石膏-矿渣体系质量的17%左右.脱硫石膏-矿渣体系在复合激发条件下的水化产物主要是钙矾石和C-S-H.大量钙矾石、石膏晶体相互交叉连生,未水化石膏、矿渣颗粒所填充其间,在C-S-H凝胶的胶结下,形成了较为致密的晶胶搭配构成的微观结构.     

Microstructural development of early age hydration shells around cement grains

An important microstructural aspect of the early hydration of Portland cement (PC) is the formation of a shell of hydration products around cement grains. There is, at present, limited information on the mechanism of formation of the shell and of the chemistry of the phases that constitute the shells. Through the use of STEM imaging of early age hydrated cement pastes as early as 2 h, the present work shows that the shells correspond to the first C-S-H type product formed which has a distinct morphology compared to C-S-H formed later when the main reaction occurs (nucleation and growth stage at setting time). The shells form only around the silicate part of the grain and are not empty but filled with a fragile fibrous C-S-H which appears to have a lower (packing) density than the rest of the hydration products. The cement grains underneath the shells are seen to react unevenly and the hydration seems to follow a reaction front, leaving striations up to 1 µm deep on the grains. Over the long term, the original fragile product seems to densify and gives rise to the usual inner C-S-H. High resolution EDS chemical analysis and mappings were used to get insight into the chemistry associated with the formation of these early age products. The C/S ratio of all C-S-H (inner and outer shell) is the same (within the limits of the analysis accuracy) and evolves insignificantly over the first 24 h of hydration. High concentrations of sulfate are associated with the C-S-H formed during the early development of the microstructure, but these decrease later, the sulfate being mainly incorporated into ettringite.     

水泥窑灰-粉煤灰体系的水化与强度发展(英文)

研究了配合比与养护温度对水泥窑灰(cement kiln dust,CKD)—粉煤灰(fly ash,FA)净浆的水化与强度发展的影响。净浆采用5种不同的水泥窑灰与粉煤灰比例配制,部分试件添加硅酸盐水泥作添加剂。试件在24、38℃及50℃的条件下进行养护。采用热重分析与X射线衍射测试试件的水化产物。结果表明在50℃的养护条件下,75%CKD+25%FA与45%CKD+45%FA+10%OPC试件的28 d与56 d强度分别达到了100%OPC水泥净浆强度的70%与80%以上。CKD-FA体系中的主要结晶水化产物是钙矾石。无论CKD与FA比例多大,CKD-FA浆体中钙矾石的含量显著高于水泥净浆。CKD-FA体系中钙矾石在90d的龄期仍可保持稳定。     

Impact of zeta potential of early cement hydration phases on superplasticizer adsorption

The zeta potential of early hydration products of cement was found to be a key factor for superplasticizer adsorption. A highly positive zeta potential results in a strong superplasticizer adsorption whereas a negative zeta potential does not allow adsorption. Synthetic ettringite precipitated from solution shows a highly positive zeta potential, hence it adsorbs great amounts of negatively charged superplasticizer. Monosulfate (AF_m) has a less positive zeta potential. Therefore, it adsorbs smaller amounts of superplasticizers. For syngenite, portlandite and gypsum, the zeta potential is around zero or negative. These phases do not adsorb superplasticizers. Consequently, a hydrating cement grain is best represented by a mosaic structure, with superplasticizer molecules mainly adsorbed on ettringite and some on monosulfate and C-S-H nucleated at surface.     

The C-S-H gel of Portland cement mortars: Part I. The interpretation of energy-dispersive X-ray microanalyses from scanning electron microscopy, with some observations on C-S-H, AFm and AFt phase compositions

Scanning electron microscopy (SEM) microanalyses of the calcium-silicate-hydrate (C-S-H) gel in Portland cement pastes rarely represent single phases. Essential experimental requirements are summarised and new procedures for interpreting the data are described. These include, notably, plots of Si/Ca against other atom ratios, 3D plots to allow three such ratios to be correlated and solution of linear simultaneous equations to test and quantify hypotheses regarding the phases contributing to individual microanalyses. Application of these methods to the C-S-H gel of a 1-day-old mortar identified a phase with Al/Ca=0.67 and S/Ca=0.33, which we consider to be a highly substituted ettringite of probable composition C₆A₂S?₂H₃₄ or {Ca₆[Al(OH)₆]₂·24H₂O}(SO₄)₂[Al(OH)₄]₂. If this is true for Portland cements in general, it might explain observed discrepancies between observed and calculated aluminate concentrations in the pore solution. The C-S-H gel of a similar mortar aged 600 days contained unsubstituted ettringite and an AFm phase with S/Ca=0.125.     

Effect of temperature on sulphate adsorption/desorption by tricalcium silicate hydrates

Sulphate adsorption from internal sources was studied in hydrating systems containing C-S-H gel and gypsum with respect to delayed ettringite formation. The influence of C₃A addition on sulphate desorption was also investigated. Research indicates that C-S-H gel will adsorb sulphate faster at high temperature resulting in quick depletion of the gypsum phase in C-S-H - gypsum mixtures. Sulphate adsorbed at high temperature is desorbed more slowly than that adsorbed at normal temperature. Slower release of sulphate from an internal sulphate source may be a critical condition for delayed ettringite formation in high temperature cured Portland cement paste.     

Changes in C3S hydration in the presence of cellulose ethers

The influence of cellulose ethers (CE) on C₃S hydration processes was examined in order to improve our knowledge of the retarding effect of cellulose ethers on the cement hydration kinetics. In this frame, the impacts of various cellulose ethers on C₃S dissolution, C-S-H nucleation-growth process and portlandite precipitation were investigated. A weak influence of cellulose ethers on the dissolution kinetics of pure C₃S phase was observed. In contrast, a significant decrease of the initial amount of C-S-H nuclei and a strong modification of the growth rate of C-S-H were noticed. A slowing down of the portlandite precipitation was also demonstrated in the case of both cement and C₃S hydration. CE adsorption behavior clearly highlighted a chemical structure dependence as well as a cement phase dependence. Finally, we supported the conclusion that CE adsorption is doubtless responsible for the various retarding effect observed as a function of CE types.     

Change in pore structure and composition of hardened cement paste during the process of dissolution

An understanding about the dissolution phenomena of cement hydrates is important to assess changes in the long-term performance of radioactive waste disposal facilities. To investigate the alteration associated with dissolution, dissolution tests of ordinary Portland cement (OPC) hydrates were performed.Through observation of the samples after leaching, it was confirmed that ettringite precipitation increased as the dissolution of the portlandite and the C-S-H gel progressed. EPMA performed on cross-sections of the solid phase showed a clear difference between the altered and unaltered parts. The boundary between the two parts was termed the portlandite (CH) dissolution front. As the leaching period became longer, the CH dissolution front shifted toward the inner part of the sample. A linear relationship was derived by plotting the distance moved by the CH dissolution front against the square root of the leaching time. This indicated Ca ion movement by diffusion.     

The early hydration of Ordinary Portland Cement (OPC): An approach comparing measured heat flow with calculated heat flow from QXRD

Heat flow was calculated from XRD data and compared with measured heat flow from calorimetric experiments. It was shown that the heat released during the hydration of a commercial Ordinary Portland Cement can be assigned mainly to three mechanisms, the silicate reaction (sum of dissolution of alite and precipitation of C-S-H-phase and portlandite), the dissolution of C₃A, and the precipitation of ettringite. The contributions made by anhydrite dissolution and gypsum dissolution to the heat released during hydration turned out to be quite small. It is possible to explain, on the basis of the data produced, the origin of the heat flow curve of the cement used.     

Study on the hydration and microstructure of Portland cement containing diethanol-isopropanolamine

Diethanol-isopropanolamine (DEIPA) is a tertiary alkanolamine used in the formulation of cement grinding-aid additives and concrete early-strength agents. In this research, isothermal calorimetry was used to study the hydration kinetics of Portland cement with DEIPA. A combination of X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC)–thermogravimetric (TG) analysis and micro-Raman spectroscopy was used to investigate the phase development in the process of hydration. Mercury intrusion porosimetry was used to study the pore size distribution and porosity. The results indicate that DEIPA promotes the formation of ettringite (AFt) and enhances the second hydration rate of the aluminate and ferrite phases, the transformation of AFt into monosulfoaluminate (AFm) and the formation of microcrystalline portlandite (CH) at early stages. At later stages, DEIPA accelerates the hydration of alite and reduces the pore size and porosity.     

Effects of Al2O3 on the hydration activity of municipal solid waste incinerator fly ash slag

This study investigates the effects of slag composition on the hydration activity of slag-blended cement (SBC) pastes. Synthetic slag samples were prepared by melting Al₂O₃-modified, municipal solid-waste incinerator (MSWI) fly ash. In addition to the original slag (containing 25.0% CaO and 17% Al₂O₃), two other synthetic slag types, A1 and A2 slag, were prepared, having a 15% or 5% Al₂O₃ content, respectively. These synthetic slags were blended with ordinary Portland cement (OPC) at weight ratios ranging from 10% to 40%. The results indicate that the incorporation of 10% A1 slag tended to enhance the degree of hydration in SBC pastes during the early ages (3-28 days), but at later ages, significant difference in the degree of hydration between the OPC and SBC pastes with 10% A1 slag was not observed. The tendency of the 10% A2 slag case was similar, but with a limited enhancement during the early ages (3-28 days). However, samples that incorporated the Al₂O₃-modified slag (AMS) showed decreased degrees of hydration. The degree of hydration of the 40% blend ratio sample decreased significantly.     

The influence of water removal techniques on the composition and microstructure of hardened cement pastes

The removal of water from hardened cement paste for analysis or to arrest ongoing hydration has been reported to affect the composition of hydrated phases and microstructure. The effect that arresting the hydration of hardened cement paste by replacing the pore water with acetone before drying, and by removing the water by freeze, vacuum and oven drying has on the hardened cement paste has been investigated. Two pastes were studied, a cemented iron hydroxide floc where a high proportion of ordinary Portland cement (OPC) had been replaced by pulverised fuel ash, and a pure hydrated OPC. The results showed that none of the water removal techniques caused any major deterioration in the composition and microstructure of the hardened cement pastes studied, but the pores appeared better preserved after arresting hydration using acetone quenching. Freeze drying appeared to cause more cracking of the microstructure than the other water removal techniques.     

掺煅烧石膏水泥早期水化过程的研究

利用DTA，XRD，IR测定水泥水化浆体的化学结合水和Ca(OH)2的生成量，研究了煅烧石膏，二水石膏对硅酸盐水泥早期水化过程的影响。结果表明：在水化龄期相同时，掺煅烧石膏水泥浆体中水化产物同掺二水石膏相比，Ca(OH)2生成量大；在1d前无钙钒石（AFt)生成，结合水量在1d前，前者高于后者，而1d后则相反。指出了煅烧石膏加快水泥水化产物形成的机理在于：由于它的溶解度较低，在水泥水化初期（1d前），存在于水泥中的铝酸盐相不能形成AFt，从而减缓了AFt对水泥水化的延缓作用，加速了整个熟料矿物相的水化。