Three-dimensional computer modeling of slag cement hydration

A newly developed version of a three-dimensional computer model for simulating the hydration and microstructure development of slag cement pastes is presented in this study. It is based on a 3-D computer model for Portland cement hydration (CEMHYD3D) which was originally developed at NIST, taken over in the authors’ group and further developed. Features like the digitized 3-D microstructure, the cellular automata (CA) algorithm for simulating the random walking, phase transformation for simulating the chemical reactions, are retained. But, the 3-D microstructure was reconstructed allowing for slag particles as binder in the system. Algorithms and rules are developed to account for the interaction between Portland cement hydration and slag reaction in the paste, of which the mechanisms were revealed in the studies by Chen and Brouwers [(2007) J Mater Sci 42(2):428; (2007) J Mater Sci 42(2):444] Methods for considering the various factors on the reactivity of slag in hydrating slag cement pastes are proposed, mainly for the oxide composition of slag and the alkalinity in the pore solution composition. A comparison between the model predictions and the experimental results in literature shows that the presented computer model can successfully predict the hydration process and the microstructure development of hydrating slag cement paste.     

Influence of cement type on resistance to attack from two carboxylic acids

In a number of circumstances, concrete may be required to possess resistance to organic acids. These are frequently carboxylic acids. This paper examines the effect of two such acids – acetic and butyric – on hardened cement paste specimens made from three cement types – Portland cement (PC), a combination of PC and fly ash (PC/FA), and a calcium sulfoaluminate cement (CSA). Specimens were exposed to solutions of the acids and deterioration characterized in terms of mass loss and pH measurements, micro-CT scanning, and chemical and mineralogical analysis. Additionally geochemical modelling was used to further examine the mechanisms involved during acid attack. The CSA cement was most resistant to attack, with the PC paste displaying the least resistance. This resistance has been partly attributed to the higher acid neutralization capacity of CSA cement. However, this paper demonstrates that the enhanced performance is most probably the result of a denser microstructure.     

Admixtures in cement: a study of dosage rates on early hydration

The authors have shown from earlier work that the electrical response of cement paste to an alternating electrical field can be used as a technique for monitoring hydration and phenomena such as setting and hardening. This paper demonstrates an area in which the technique could prove effective, by showing the influence of a range of commercial admixtures on electrical response. It is clearly indicated that admixtures have a definite effect on the response characteristics.     

Experimental investigation and modeling of hydration and performance evolution of fly ash cement

A systematic study on several mix designs has been carried out to correlate the microstructural properties, i.e. degree of hydration, C–S–H composition, capillary porosity to the mechanical properties, such as compressive strength and elastic modulus. For this purpose, thermodynamic modeling was used to calculate the type and amount of hydrates formed during ongoing hydration of ordinary Portland and blended fly ash cements. Obtained results of the phase development over the hydration time agree well with the measured amount of the phases. Additionally, by plotting the measured compressive strength versus the modeled porosity, a quasi-unique relationship for both, OPC and OPC blended systems, is obtained. This new finding gives further insights allowing us to develop a micromechanical model linking the mineralogical composition of anhydrous systems to corresponding mechanical properties. On the other hand, it provides a possibility to improve the composition of cementitious systems to improve their properties and performance.     

A positron annihilation study on the hydration of cement pastes

Positron annihilation lifetime spectroscopy experiments were carried out in various ordinary Portland cement pastes, in an attempt to monitor the porosity of the pastes. It is found that positronium intensity is well correlated to the time evolution of the total porosity and it is influenced by the water-to-cement ratio. This parameter is also sensitive to the delayed hydration process induced by adding methanol to the water–cement mixture.     

Alongterm study of MgO hydration in cement pastes

The effect of various additives on the longterm hydration of Magnesium Oxide, (MgO) in cement pastes cured in water at 18±2°C up to 12 years was studied by X-ray diffraction (XRD), seanning electron microscopy (SEM) and EPMA. It was found that cement with high MgO content, is stabilized even after a longterm period of hydration by active pozzolans.     

Effect of nanosilica and montmorillonite nanoclay particles on cement hydration and microstructure

The need to produce sustainable cements has driven research towards nanotechnology. The main cement hydration product, calcium silicate hydrate, is nanosized; hence, the addition of nanoparticles to blended Portland cement formulations can remarkably modify mechanical strength, porosity and durability. The present paper discusses the material aspects of two different nanoparticles, nanosilica and montmorillonite nanoclay, the complications that arise from their addition to cement pastes and ways to mitigate these limitations. It is deduced that nanosilica solids in blended cement pastes should be limited to 0.5%, whereas nanoclay solids to almost 1?mass-% binder. Competitive reactivity of nanoparticles with other constituents is expected, and the possible pozzolanic activity is critically addressed. Notwithstanding progress made, there are significant potentials related to inorganic nanoclays.     

Effect of crystal seeding on the hydration of calcium phosphate cement

In this paper, the effect of crystal seeding on the hydration of calcium phosphate cement (CPC) has been carefully investigated. The setting time of the CPC slurry not containing any crystal seeds was 150 min, while the setting time for the specimen containing 5 wt% low crystallinity hydroxyapatite used as a crystal seed was 7 min. This improvement in the setting time was due to HAP serving as a substrate for heterogeneous nucleation which accelerated nucleation. In addition, the compressive strength of the specimen containing the crystal seeding was deduced and we report values different from those previously reported in the literature. The calorimetric curve indicated that crystal seeding could reduce the induction period. A.c. impedance spectroscopy revealed that at the beginning of hydration, the rate of reaction increased and also that the mean diameter and porosity decreased as the seed content increased. At the end of the hydration reaction the situation was changed with the mean diameter and porosity in the sample without any seeds being a minimum, which indicated that the compressive strength was a maximum. This result could be explained by the dissolution and reprecipitation of small hydration products produced by the high rate of reaction produced by the introduction of the crystal seeds.     

Effect of barium hydrosilicates on the early hydration rate of Portland cement

We have studied the effect of a barium hydrosilicate-based modifier on the phase composition of Portland cement hydration products. The results demonstrate that the addition of a modifier containing barium hydrosilicates, silicic acid, and calcium carbonate makes it possible to reduce the nucleation rate of crystals of new phases during the induction period of the hydration process, increase alite (3СaO · SiO₂) hydration, and reduce the rate of aluminate phase (3CaO · Al₂O₃) hydration. The use of such a modifier increases the degree of cement hydration and the amount of calcium hydrosilicates and reduces the amount of forming portlandite and ettringite, thereby improving the mechanical properties and durability of the set cement.     

An experimental investigation of the thermal conductivities of esters of carboxylic acids

This paper gives the results of an experimental investigation of the thermal conductivities of a homologous series of esters of saturated monobasic acids. Equations for calculating the thermal conductivity of liquids in relation to temperature and composition are presented.     

Sorption of phenolics and carboxylic acids on polybenzimidazole

The sorption of phenolics and carboxylic acids on the new ion-exchange resin polybenzimidazole (PBI) has been measured using aqueous solutions of the sorbates and compared with the sorption on another weak-base resin, poly(4-vinyl pyridine) (PVP). The phenolics used include phenol, p-cresol, p-chlorophenol and m-aminophenol, while the carboxylic acids chosen are formic, acetic, propionic and butyric acids. For aminophenol and phenol, the equilibrium sorption capacities of PBI and PVP are comparable, while for more acidic p-cresol and p-chlorophenol, PVP has significantly higher equilibrium sorption than PBI. On the other hand, for carboxylic acids at low concentrations, PBI exhibits considerably higher sorption capacity than PVP, the difference being more pronounced for acids with smaller acidity. At low concentrations, formic acid has several-fold higher sorption than phenol on PBI, which thus offers a simple method for separation of formic acid from phenol in dilute solutions. PBI has fast sorption kinetics for both phenols and carboxylic acids and a fast rate of stripping and regeneration with dilute NaOH, being much superior to PVP in both these respects.     

Effect of PCs superplasticizers on the rheological properties and hydration process of slag-blended cement pastes

The effect of polycarboxylate (PC) superplasticizers with different structure on the rheological properties and hydration process of slag-blended cement pastes with a slag content between 0 and 75% has been studied. Fluidizing properties of PCs admixtures are significantly higher in slag-blended cement with respect to non-blended Portland cement. Also, it has been observed that the rise of the fluidity induced by the PCs on the cement pastes increases with the slag content. This effect is mainly attributed to a decrease in the amount of C₃A available to adsorb and consume admixture to form an organo-mineral phase. Consequently, the PC admixtures are absorbed onto the silicate phases of the clinker and onto the slag particles, inducing a repulsion and the concomitant reduction in yield stress despite a reduction in the zeta potential. The rheological results allow us to conclude that the highest increase of the fluidity is caused by the admixtures with highest molecular weight due to the higher steric repulsion induced. As a consequence of the adsorption of the PCs, a delay of the hydration process of the pastes has been observed.     

Influence of finely ground limestone on cement hydration

Some work has been carried out on the effect of calcium carbonate on cement paste, but there is no general agreement on the relative effects of different amounts of calcium carbonate on cement paste properties. The objective of the present work is to assess the effect of various amounts of calcium carbonate on the hydration of tricalcium silicate in order to explain the physico-chemical changes occurring during Portland cement hydration. It is shown that calcium carbonate has an accelerating effect on C₃S and cement hydration and leads to the precipitation of some calcium carbosilicate hydrate.     

粉煤灰粒径分布对硅酸盐水泥水化性能的影响

为研究粉煤灰粒径对硅酸盐凝胶材料水化性能的影响,经球磨仪研磨得到3种不同粒径的粉煤灰,探讨其对硅酸盐水泥水化放热速率、水化放热总量、水化反应程度和粉煤灰自身水化反应程度的影响。结果表明:随粉煤灰粒径的减小,粉煤灰的水化活性明显增大,水化反应程度增大,养护龄期为7 d时,水化程度增加20.7%;粉煤灰粒径分布对硅酸盐水泥水化放热总量的影响较小,主要影响其水化放热速率、水化反应程度,养护龄期为28 d时,胶凝材料水化程度增加3%。     

A conduction calorimetric study of early hydration of ordinary Portland cement/high alumina cement pastes

Conduction calorimetry was applied to an investigation of the early hydration of ordinary Portland cement (OPC)/high alumina cement (HAC) pastes. Three different rate of heat-evolution profiles were observed, depending on the HAC/OPC ratio. Relevant processes affecting heat development include ettringite formation, HAC and OPC hydration. Results from SEM examination and X-ray diffraction studies are also presented. An acceleration of OPC hydration was observed in pastes containing less than 12.5% HAC. A similar acceleration effect on HAC hydration was also obtained with the addition of OPC. A large amount of ettringite was formed and OPC hydration delayed in the pastes containing 15%–30% HAC. The latter could be one of the factors attributed to poor strength development in these HAC/OPC systems. Early hydration mechanisms of OPC/HAC systems are also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Flash-calcined dredging sediment blended cements: effect on cement hydration and properties

Dredging of docks and waterways generates a large and continuous supply of sediments currently destined for disposal. Transforming this currently wasted materials into new resources still requires meeting technical challenges. One of the options is to process the sediments into a supplementary cementitious material by flash-calcination. This paper describes the effect of cement replacement by flash-calcined dredged sediments on cement hydration and key properties. The hydration kinetics, products and microstructure are studied to explain changes in cement properties such as compressive strength development and workability. The flash-calcined dredging sediments show clear pozzolanic activity which surpasses that of typical coal combustion siliceous fly ash (V, EN 197-1). This is manifested in (1) the rate of compressive strength development, (2) reduced portlandite and (3) increased ettringite and bound water contents. The results show that calcination can transform wasted dredging sediments into a new supplementary cementitious resource for producing large volumes of low-CO₂ blended cements.     

铅离子对复合磷酸盐磷酸镁水泥水化硬化特性的影响

研究了铅离子对复合磷酸盐磷酸镁水泥水化硬化特性的影响及其在复合磷酸盐磷酸镁水泥中的稳定性。实验结果表明,复合磷酸盐磷酸镁水泥抗压强度随着铅离子掺量的增加而降低,其中硝酸铅掺量达到10%时,复合磷酸盐磷酸镁水泥的各个龄期的抗压强度发生明显下降。铅离子对复合磷酸盐磷酸镁水泥凝结时间没有明显影响。在复合磷酸盐磷酸镁水泥水化过程中,铅离子对水泥体系的pH值影响不大,但能够造成水泥水化放热峰出现的时间延迟,水化放热的总量减少并影响主要水化产物的结晶程度。在复合磷酸盐磷酸镁水泥水化反应后期,当硝酸铅掺量达到10%以上时,在水化产物中出现了较为明显的Pb2P2O7的衍射峰。复合磷酸盐磷酸镁水泥固化铅离子的浸出毒性试验结果(43μg/L)远低于国家标准要求(5mg/L)。     

硬化水泥净浆基于纳米压痕的相态识别与水化程度计算

采用纳米原位压痕手段测量硬化水泥净浆中单一相态的代表性微观力学性能,并采用纳米点阵压痕研究各相态的含量。研究对象囊括水灰比为0.3、0.4、0.5的纯水泥净浆和水灰比为0.3情况下含50%、70%矿渣掺量复合体系,共5种配比,以表征它们的相态分布和微观力学性质的异同点。掺矿渣的试件中含有明显多的复合相,因此提出三相模型测算复合相中未水化物的体积分数。此外,提出基于纳米压痕技术计算纯水泥和掺矿渣水泥试件水化程度的方法,结果吻合于热重分析的结果,其中纯水泥净浆中复合相较少,计算得到的水化程度优于对掺矿渣水泥试件的计算。