石膏掺量对高阿利特水泥防腐抗渗性能的影响研究

研究了石膏掺量对高阿利特水泥抗海水侵蚀和抗渗性能的影响,并与普通水泥进行了比较。利用XRD、SEM—EDS等测试方法对水泥水化产物的物相组成和形貌进行分析、观察;用压汞法对水泥硬化浆体的孔结构进行了分析。结果表明,石膏掺量对高阿利特水泥硬化浆体的致密性有较大影响,进而影响水泥砂浆的抗海水侵蚀性能,石膏的适宜掺量为5％,在此掺量下高阿利特水泥的抗蚀系数达1．01,而普通水泥的抗蚀系数仅为0．87,高阿利特水泥的有害孔较少,总孔隙率较低,抗渗性能得到较大改善。     

Solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pore solutions as a function of hydration time

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement (OPC) and white Portland cement (WPC) pastes were measured during the first 28 days of hydration at room temperature. Saturation indices (SI) with respect to various solid phases known to occur in cement pastes were calculated from a thermodynamic analysis of the elemental concentrations, resulting in good agreement between the two pastes. In agreement with other published work, gypsum was saturated during the first several hours of hydration and then undersaturated thereafter, while portlandite was modestly supersaturated after the first few hours. High levels of supersaturation with respect to ettringite and calcium monosulfoaluminate were calculated, particularly prior to the consumption of gypsum at around 10 h. Results are consistent with published thermodynamic studies that show calcium monosulfoaluminate is metastable with respect to ettringite under normal hydration conditions. Three different ion activity product (IAP) equations for C-S-H were applied to the data. From 10 h onward, each of the IAP values declined gradually over time and the values for the OPC and WPC pastes were in close agreement. The same IAP equations were applied to experimental data from the pure CaO-SiO₂-H₂O system, resulting in good agreement between the cement paste pore solutions and the equilibrium between portlandite and the upper, or metastable, C-S-H solubility curve.     

Hydration of calcium sulfoaluminate cements — Experimental findings and thermodynamic modelling

Calcium sulfoaluminate cements (CSA) are a promising low-CO₂ alternative to ordinary Portland cements and are as well of interest concerning their use as binder for waste encapsulation. In this study, the hydration of two CSA cements has been investigated experimentally and by thermodynamic modelling between 1 h and 28 days at w/c ratios of 0.72 and 0.80, respectively.The main hydration product of CSA is ettringite, which precipitates together with amorphous Al(OH)₃ until the calcium sulfate is consumed after around 1-2 days of hydration. Afterwards, monosulfate is formed. In the presence of belite, strätlingite occurs as an additional hydration product. The pore solution analysis reveals that strätlingite can bind a part of the potassium ions, which are released by the clinker minerals. The microstructure of both cements is quite dense even after 16 h of hydration, with not much pore space available at a sample age of 28 days.The pore solution of both cements is dominated during the first hours of hydration by potassium, sodium, calcium, aluminium and sulfate; the pH is around 10-11. When the calcium sulfate is depleted, the sulfate concentration drops by a factor of 10. This increases pH to around 12.5-12.8.Based on the experimental data, a thermodynamic hydration model for CSA cements based on cement composition, hydration kinetics of clinker phases and calculations of thermodynamic equilibria by geochemical speciation has been established. The modelled phase development with ongoing hydration agrees well with the experimental findings.     

Experimental studies and thermodynamic modeling of the carbonation of Portland cement,metakaolin and limestone mortars

The carbonation of Portland cement, metakaolin and limestone mortars has been investigated after hydration for 91 days and exposure to 1% (v/v) CO₂ at 20 °C/57% RH for 280 days. The carbonation depths have been measured by phenolphthalein whereas mercury intrusion porosimetry (MIP), TGA and thermodynamic modeling have been used to study pore structure, CO₂ binding capacity and phase assemblages. The Portland cement has the highest resistance to carbonation due to its highest CO₂ binding capacity. The limestone blend has higher CO₂ binding capacity than the metakaolin blends, whereas the better carbonation resistance of the metakaolin blends is related to their finer pore structure and lower total porosity, since the finer pores favor capillary condensation. MIP shows a coarsening of the pore threshold upon carbonation for all mortars. Overall, the CO₂ binding capacity, porosity and capillary condensation are found to be the decisive parameters governing the carbonation rate.     

Investigation of the microstructural evolution of calcium sulfoaluminate cements by thermoporometry

Thermoporometry was applied to the investigation of the microstructural evolution of cementing systems. A pure calcium sulfoaluminate cement – CSA – and one mixed CSA–Portland system, together with a reference Portland cement were considered. Specimen preparation was carefully optimized in order to minimize any structural damage and the repeatability of results was checked through the utilization of inorganic standard. Nitrogen adsorption/desorption was used for comparison.A wide set of information could be acquired regarding the microstructure of the investigated materials: a) both the CSA and the mixed CSA–Portland system mainly revealed ink-bottle pores; b) a much more rapid development of hydrated structures was observed for all CSA cements than for the Portland cement; c) melting and freezing curves allow to gain information about the pore size distribution and the presence of pore entries of preferential size, about tortuosity and connectivity of the cement microstructure, and about the existence of isolated pores.     

Hydration characteristics of waste sludge ash that is reused in eco-cement clinkers

The study reports on the hydration characteristics of eco-cement clinkers produced with waste sludge ash as raw components. The tested mixtures were composed of different types of waste sludge ash, including sewage sludge ash, water purification sludge ash, limestone, and ferrate, prepared using the optimum proportioning method. The mixtures were burned at 1400 °C for 6 h. The clinkers thus obtained were quantified and the hydration characteristics of the eco-cement pastes prepared from the waste sludge ashes. The setting time, compressive strength, hydrates and porosity distribution were examined at various ages. The 28-day compressive strength of the early high strength developing of eco-cement C paste outperformed that of ordinary Portland cement paste by 3 MPa. It is supposed that the large quantity of limestone used provided CaO, which in turn enhanced the formation of C₃S, leading to the greater compressive strength development in the eco-cement C paste. From the porosity distribution, shown by the Mercury Intrusion Porosimetry results, it was found that, with increasing curing ages, the gel pores (<0.01 μm) increased and the total porosity and capillary pores (>0.01 μm) decreased—a result that shows that hydrates had filled the pores. This resulting densification and enhanced later strength were caused by the shifting of the pore size distribution to a smaller diameter range.     

水泥-硅灰/粉煤灰体系强度、收缩性能与微观结构研究

为研究硅灰及粉煤灰对不同养护龄期的水泥浆体强度及收缩性能的影响,以水胶比为0.29的水泥浆体为基体,设计制备了五种硅灰及粉煤灰掺量的复合水泥浆体,借助量热仪和压汞仪测试表征了不同复合水泥浆体的水化放热特性以及孔结构组成,分析了水化放热量、孔隙率等参数随硅灰和粉煤灰掺量增加的变化规律,建立了复合浆体抗压强度与孔结构以及水化特性与收缩应变之间的量化关系。结果表明,掺入粉煤灰会大幅降低水泥净浆早期抗压强度,但对减小自收缩应变和干缩应变极为有利。掺入硅灰能明显提高净浆3 d抗压强度,但当硅灰掺量超过10%(质量分数)后,净浆3 d自收缩应变及28 d干缩应变增加极为明显。掺入硅灰会使水泥水化诱导期开始和结束的时间提前,还会增加水化反应级数和各阶段的反应速率常数值,导致水泥-硅灰复合浆体的水化放热总量和放热速率相较于水泥-粉煤灰体系大幅增加。粉煤灰和硅灰的掺入均能有效细化水泥浆体内部孔结构,提高凝胶孔比例,大幅降低大孔比例。复合浆体的72 h水化放热总量和3 d自收缩应变呈现正相关关系,而孔隙率和抗压强度呈现明显的负相关关系。     

无机盐对水泥石水化程度和孔结构的影响(英文)

采用压汞法研究了水灰比为0.3和0.5的掺加无机盐外加剂[CaCl2,Na2SO4,NaNO2和Ca(NO3)2]水泥石在3 d和28d时的孔结构,并测试化学结合水含量.结果表明:CaCl2,Na2SO4 NaNO2能促进水泥水化:CaCl2促进水泥水化作用最为明显,并可降低水泥石大孔和毛细孔孔隙率;Na2SO4增大了大孔孔隙率;NaNO2能显著减小28d时毛细孔连通孔径和毛细孔孔隙率;Ca(NO3)2在前3d对水泥水化没有明显的作用,在3d时水泥石中大孔和毛细孔孔隙率以及毛细孔连通孔径增大.     

Effect of curing time on the physicomechanical characteristics of the hardened cement pastes containing limestone

The effect of curing time on the physico-mechanical properties of the hardened Portland cement pastes containing limestone was studied. Five cement-limestone blends were prepared using 0%, 5%, 10%, 15%, and 20% of limestone as a partial substituent of Portland cement. The cement pastes were prepared using the standard water of consistency of 0.255, 0.255, 0.258, 0.261, and 0.263, respectively. The fresh pastes, thus produced, were moulded into 2×2×2-cm cubes. The pastes were first cured within the moulds at 100% relative humidity for 24 h, then the specimens were demoulded and cured under tap water for 3, 7, 14, and 28 days. At each hydration age, the hardened pastes were tested for bulk density, compressive strength, differential scanning calorimetery (DSC), and X-ray diffraction analysis (XRD). The results obtained were related as much as possible to the mechanical properties of the hardened cement pastes. The inclusion of limestone results in a notable improvement of the mechanical properties of the cement pastes containing limestone.     

Percolation of capillary pores in hardening cement pastes

This paper presents a study of the pore structure and permeability of hardening cement pastes. Cement pastes with water/cement (w/c) ratio 0.4, 0.5 and 0.6 from 1 up to 28 days of age were tested with mercury intrusion porosimetry. In parallel, the water permeability was measured. With the help of a numerical simulation model, correlations between pore structure and permeability were found. Water permeability is related more to the pore size distribution and to the effective porosity than to the total porosity of the pastes. No depercolation of the capillary pores was evident neither in the experiments nor in the numerical simulations during the first 28 days after casting.     

Tieftemperatur-DTA-untersuchungen des zementsteingefüges bei unterschiedlichem hydratationsgrad

Using quantitative DTA in a temperature region between + 20°C and ? 60°C it was possible to study the hydration of hardened cement paste (Ordinary Portland Cement and Slag Cement) in the age between 1 and 28 days. It has been observed that the volume of the bigger gel- and capillary pores (R_H > 10 nm) is decreasing and the volume of the medium gel pores (3 nm < R_H < 10 nm) is increasing during hydration. Therefore quantitative low temperature DTA could be an improved measuring technique to evaluate the degree of hydration and to elucidate the kind of hydration products.     

碳化过程中大水灰比水泥浆体孔结构的变化(英文)

使用特殊的增黏剂与聚羧酸减水剂,制备了掺加石灰石粉、高炉矿渣、硅灰等混合材的普通波特兰水泥浆体和和低热硅酸盐水泥浆体(水粉比为1.0)。这些水泥浆体在20℃的水中养护4年后基本完全水化。这些硬化水泥浆体在5%(质量分数)CO2、相对湿度66%和温度20℃条件下进行碳化,对比研究碳化前后水泥浆体孔结构的变化。结果显示:碳化浆体内孔直径大于10nm的孔体积明显减少;碳化浆体的孔径分布向大孔径范围偏移;掺加混合材的硬化水泥浆体结构明显趋于松散;与不掺加任何混合材的水泥浆体相比,掺加混合材的水泥浆体的孔径更大。     

Hydration of Portland cement with additions of calcium sulfoaluminates

The effect of mineral additions based on calcium aluminates on the hydration mechanism of ordinary Portland cement (OPC) was investigated using isothermal calorimetry, thermal analysis, X-ray diffraction, scanning electron microscopy, solid state nuclear magnetic resonance and pore solution analysis. Results show that the addition of a calcium sulfoaluminate cement (CSA) to the OPC does not affect the hydration mechanism of alite but controls the aluminate dissolution. In the second blend investigated, a rapid setting cement, the amorphous calcium aluminate reacts very fast to ettringite. The release of aluminum ions strongly retards the hydration of alite but the C–S–H has a similar composition as in OPC with no additional Al to Si substitution. As in CSA–OPC, the aluminate hydration is controlled by the availability of sulfates. The coupling of thermodynamic modeling with the kinetic equations predicts the amount of hydrates and pore solution compositions as a function of time and validates the model in these systems.     

Gradients of microstructure and diffusion properties in cement paste caused by drying

This paper demonstrates that the microstructure of hydrated cement paste varies with distance from an exposed, drying face. Supporting measurements of cement hydration, adsorption capacity, porosity and diffusion rate are reported for an ordinary Portland cement and an ordinary Portland/pulverised fuel ash blended cement at ages of 28 and 163 days. Possible causes of the microstructural gradients are discussed in relation to the effects of

• 1.a) moisture condition upon cement hydration and pore structure changes and
• 2.b) carbon dioxide upon cement hydrates and porosity.

The practical implications of microstructural gradients are discussed with particular reference to the region immediately below the exposed surface.     

Resistance a la compression simple des pates pures de ciment durcies,temps de durcissement superieur a quatre ans

The objective of this study was to investigate the influence of cement content on the final strength of water cured C.P.A. pastes. To ensure achievement of strengthening, tests were performed on four years old specimens. This is the following of a previous study dealing with mechanical properties of 7, 14, 28 and 56 days old pastes. It then appeared that hardened cement pastes must be characterized by their effective water cement content at the end of bleeding and that, in relation with their compressive strength, they must be separated into two groups. As expected, very old pastes behave as follows. All pastes characterized by a cement content being over a threshold value present, after a long enough water curing time, the same strength which is the absolute maximum reachable, and various degrees of hydration ; they have in common a zero capillary porosity. Strength of other pastes drops as cement content decreases below this threshold value ; all these pastes have in common the same degree of hydration, theoreticaly : 100 %.     

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.     

Effect of some superplasticizers on the mechanical and physicochemical properties of blended cement pastes

Blended cement pastes made of Portland cement and fine sand (known in Egypt as El-Karnak cement) were made using a water–cement ratio of 0.25 by weight. Three pastes containing admixture (water-soluble condensates) were also prepared using a water–cement ratio of 0.25 and condensate (superplasticizer) content of 0.25% by the weight of cement; the superplasticizers used are Na-phenol sulfonate formaldehyde, Na-polystyrene sulfonate, and Na-ß-naphthol sulfonate formaldehyde condensates. All pastes were cured for various time intervals within the range of 0.02–90 days. Compressive strength tests, hydration kinetics, X-ray diffraction analysis, thermal analysis, and surface properties were studied and related as much as possible to the pore structure of the hardened pastes. © 1995 John Wiley & Sons, Inc.     

Pore size distribution of portland cement slurries at very early stages of hydration (influence of curing temperature and pressure)

Pore size distribution of Portland cement pastes has been studied using helium picnometry and mercury porosimetry. Cement samples were hydrated under varying conditions of temperature and pressure and were investigated at very early stages of hydration : thickening, setting, and early hardening. The evolution of pore size distribution with time has been related to physical and chemical properties (compressive strength, shrinkage, and combined water). The interpretation has been based on the repartition between free pores of tubular shape and trapped pores of rounded shape, and a model is proposed for describing cement pore size distribution.     

Influence of superplasticizers on the hydration of cement and the pore structure of hardened cement

This paper describes the influence of various types of superplasticizers such as naphthalene type (β-NS), refined lignin sulfonate type (LS) and polycarboxylate types (P34, S34) on the hydration of cement and the pore structure of hardened cement. Other superplasticizers except β-NS delayed the initial hydration of cement. In any case, it hardly influences the hydration reaction at late stage of cement. The retardation by the addition of superplasticizers is not observed after 28 days of curing. Large pores of 0.1 μm or more for hardened cement with LS or β-NS are larger than those of hardened samples with P34 or S34 cured for 28, 56 and 91 days. This is related to the coagulated structures of fresh cement pastes with various types of superplasticizers. It was presumed that the size of the cluster of aggregated particles became small when S34 or P34 that has a high dispersing ability was added compared to LS or β-NS that has a lower dispersing ability.     

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.