The influence of alkali content on the electrical resistivity and transport properties of cementitious materials

This paper examines the role of alkali content on the transport properties of cementitious materials. While cement with high alkali content may be a concern due to the potential for alkali silica reaction (ASR), the alkali content also influences the electrical properties of pore solution and the bulk cementitious system. Higher concentrations of alkalis decrease the resistivity of the pore solution. However, the alkalis may also impact the microstructure formed during hydration. This paper examines the role of alkali content on both the pore solution and on the microstructure that forms. Water vapor desorption measurements indicate that systems with a higher alkali content have a lower overall porosity. This dense microstructure is not caused simply by a higher degree of hydration (DOH) (i.e., an increase in reacted products), as suggested by loss on ignition (LOI), but rather possibly from the improved morphology and distribution of calcium hydroxide, as observed with scanning electron microscope (SEM).These improvements in the microstructure alter the electrical resistivity and the results of steady and non-steady state migration tests. This indicates that with higher alkali content, cement may have a beneficial influence on reducing chloride ion transport. Furthermore, alkali leaching during saturated curing is observed, quantified, and discussed as it relates to the interpretation of electrical measurements and microstructure formed.     

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 alkalis on porosity percolation in hydrating cement pastes

Loss-on-ignition (LOI) measurements and low temperature calorimetry (LTC) are used to study the properties of hydrating cement pastes with various quantities of alkalis. In addition to the well-known acceleration of early age hydration and “retardation” of later age hydration, the alkalis are observed to have a significant effect on the percolation of the porosity in the hydrating systems, as assessed using the LTC technique. At equivalent degrees of hydration, the capillary pores in cement pastes with sufficient added alkalis may depercolate while those in lower alkali cement pastes remain percolated. A simple dissolution/precipitation three-dimensional microstructural model is applied to examine the potential effects of hydration product morphology (random, needles, and plates or laths) on pore space percolation. The model suggests that the observed experimental results could be consistent with the higher alkali levels modifying the morphology of the C–S–H gel to produce more lath-like hydration products, as has been observed by others previously using electron microscopy. Potential implications for the transport properties and durability of these materials are discussed.     

The capacity of ternary blends containing slag and high-calcium fly ash to mitigate alkali silica reaction

The efficiency of ternary blends containing high-calcium fly ash and slag in mitigating alkali-silica reaction (ASR) was evaluated. The concrete prism expansions showed that the ternary blends did not offer significant advantage over binary blends of portland cement and either of the individual material at the same total SCM content. The ability of a particular blend to mitigate ASR was related to its capacity to retain alkalis in its hydration products, as evaluated by an alkali leaching test. For the slag and fly ash used in this study, the capacity to retain alkalis increased with the ability of the blend to consume Ca(OH)₂ during its pozzolanic reaction. For the blends investigated here, the alkali leaching test was more realistic than the accelerated mortar bar test in predicting the 2-year expansion of concrete prisms. The adopted alkali leaching test is proposed to be used as a tool to compare the efficacy of different cementing blends to mitigate ASR.     

Hydration of slag-blended cements

In this paper, the hydration of slag in blended cements is investigated through the measurement of hydration reaction indicators such as portlandite content, non-evaporable and free water, and hydration heat. Three substitution rates of cement by slag were used (30%, 50% and 70%). The tests were performed at two constant temperatures (20 °C and 40 °C) in order to assess the activation energy of the different components. A multiphasic hydratation model is proposed to take account of the difference of kinetics of each main phase (clinker and slag) and the hydration kinetic law proposed considers interactions between the two phases. It includes the activation of the dissolution of slag by alkalis released by the clinker phases in the pore solution, the portlandite consumption by slag and the effect of temperature and moisture content on the reaction kinetics. The model is able to simulate the evolution of hydration products and adjust the hydration product stoechimetry to the rates of slag and the current temperature automatically and instantaneously. Its reliability is shown through its ability to fit the whole experimental plan results with a single parameter set. Among these parameters are the hydration heat of slag and its water consumption. The model and its parameters should be useful to simulate other types of slag-blended cement.     

Prediction of chloride ingress and binding in cement paste

This paper summarizes recent work on an analytical model for predicting the ingress rate of chlorides in cement-based materials. An integral part of this is a thermodynamic model for predicting the phase equilibria in hydrated Portland cement. The model’s ability to predict chloride binding in Portland cement pastes at any content of chloride, alkalis, sulfates and carbonate was verified experimentally and found to be equally valid when applied to other data in the literature. The thermodynamic model for predicting the phase equilibria in hydrated Portland cement was introduced into an existing Finite Difference Model for the ingress of chlorides into concrete which takes into account its multi-component nature. The “composite theory” was then used to predict the diffusivity of each ion based on the phase assemblage present in the hydrated Portland cement paste. Agreement was found between profiles for the Cl/Ca ratio predicted by the model and those determined experimentally on 0.45 water/powder ratio Portland cement pastes exposed to 650 mM NaCl for 70 days. This confirms the assumption of essentially instantaneous binding where quasi-equilibrium is established locally. This does not imply steady state diffusion however. It simply implies that incremental increases in the concentration of diffusing ions in the pore solution will rapidly re-equilibrate with the hydrates present locally, where, the greater the ratio of bound to free ions, the greater the buffering effect which slows down the rate of ingress. In the case of chlorides, this buffering effect is greatest at high contents of AFm (AFm is aluminium ferrite compounds with a single (mono) formula unit CaX.) phase and low alkali metal contents.     

Micro-structural development of gap-graded blended cement pastes containing a high amount of supplementary cementitious materials

To clarify the strength improvement mechanism of gap-graded blended cements with a high amount of supplementary cementitious materials, phase composition of hardened gap-graded blended cement pastes was quantified, and compared with those of Portland cement paste and reference blended cement (prepared by co-grinding) paste. The results show that the gap-graded blended cement pastes containing only 25% cement clinker by mass have comparable amount of gel products and porosity with Portland cement paste at all tested ages. For gap-graded blended cement pastes, about 40% of the total gel products can be attributed to the hydration of fine blast furnace slag, and the main un-hydrated component is coarse fly ash, corresponding to un-hydrated cement clinker in Portland cement paste. Further, pore size refinement is much more pronounced in gap-graded blended cement pastes, attributing to high initial packing density of cement paste (grain size refinement) and significant hydration of BFS.     

An alternative to Portland Cement for waste encapsulation--the calcium sulfoaluminate cement system

Currently, Portland Cement (PC) is used extensively in the solidification/stabilisation of a wide variety of wastes. In the nuclear industry, low and intermediate level radioactive wastes are encapsulated or immobilised within composite PC cement systems based on high replacement with blast furnace slag or fly ash. However, the high alkalinity of these PC-based systems will corrode reactive metals found in some wastes releasing hydrogen and forming expansive corrosion products. Alternative cement systems could provide a different hydration chemistry, which would allow wastes containing these metals to be encapsulated with lower reactivity. Calcium sulfoaluminate (CS A) cement is one such cement. It combines economy of cost and low emission of CO(2) with rapid strength gain and compatibility with other construction materials. Hydration provides an internal pore solution where the pH is considerably lower than that of PC. The main hydration product, ettringite, can incorporate a number of ions into its crystal structure, making it an ideal candidate for waste immobilisation. This paper details some results from a commercial CS A system that examines aspects of mixing, hydration of different formulations and aluminium corrosion behaviour. The fluidity of mixes can be adjusted by changing the formulations. All designed mixes were set within 24 h with little bleeding and the pH values were in the range of 10-11.5. In addition, a significant reduction in Al corrosion was observed compared to a composite OPC system. Although these results provide encouragement for the idea that CS A cement can provide a possible alternative to PC in the immobilisation of difficult and reactive wastes, further investigation is needed.     

Structural evolution of hydrated cement compacts

This paper provides a simplified and repeatable method for research on morphology and structure evolution of hydrated cement. Dry cement powders were compacted in a steel mould to absorb water for hydration. Cement compacts and hydrated compacts were characterized by capillary absorption, MIP, SEM–EDS or synchrotron small-angel X-ray scattering (SAXS). Results show water is fully saturated in the homogenous compacts. Followed by repeatable preparation of cement compacts, pore structure of hydrated cement compacts are also repeatedly. Hydration products are preferentially produced near cement particles due to difficulty of ions diffusion at later hydration of cement compact. Therefore, lager pore far from cement particles is less filled and smaller pore near cement particles is more filled. Simplified processing and structural evolution of hydrating cement will benefit to design and control of cement materials.     

碱式硫酸镁水泥耐酸腐蚀性能研究

为了研究碱式硫酸镁水泥耐酸腐蚀性能,将不同配比的水泥试样分别在柠檬酸溶液及水中浸泡不同龄期,再进行质量变化测定及抗折强度和抗压强度试验。采用XRD与SEM技术分析不同配比水泥试样浸泡于两种溶液后的物相组成和显微形貌。结果表明,掺入的矿渣和粉煤灰对碱式硫酸镁水泥具有良好的密实填充作用,降低了水泥的孔隙率,有效阻止了侵蚀介质的进入,其耐酸腐蚀性能与未掺矿渣和粉煤灰的碱式硫酸镁水泥相比有明显提升,其中,掺矿渣的碱式硫酸镁水泥耐酸腐蚀性能更优。     

Hydration of high-volume fly ash cement pastes

The hydration processes of high-volume fly ash cement paste were investigated by examining the non-evaporable water content, the CH content, the pH of pore solution and the fraction of reacted fly ash, curing at either 20°C or elevated temperatures after an initial curing at 20°C. The replacement percentage levels of fly ash were 40%, 50% and 60% by weight, respectively. The results revealed that the non-evaporable water content in high-volume fly ash cement pastes does not develop as plain cement pastes does, so it may be improper to apply the non-evaporable water content to evaluate the hydration process in high-volume fly ash cement matrix. The reduction in CH content increases with the progressing of hydration process and varies linearly with the logarithm of curing age. The addition of 3.0% of Na₂SO₄ could accelerate the pozzolanic reaction of fly ash at early ages. At 20°C, the pH of pore solution of high-volume fly ash cement paste was reduced to a great extent at early ages and it continued to decline at later ages due to the inclusion of large amount of fly ashes. At elevated temperatures, however, this trend was not found. The fraction of reacted fly ash directly reflects the pozzolanic reactivity of fly ash both at normal and elevated temperatures. There is some inherent correlation between the reduction in CH content, the pH of pore solution and the fraction of reacted fly ash. For specified matrix, the consumption of CH and the pH of pore solutions change linearly with the increase of the fraction of reacted fly ash.     

低场核磁共振技术在水泥基材料研究中的应用及展望

阐述了低场核磁共振技术在水泥基材料研究中的应用现状,认为现有的研究主要集中于水泥水化进程和水在硬化浆体中的扩散特征,也包括对硬化水泥浆体孔结构和比表面积的测试。分析了低场核磁共振技术在实际应用中面临的挑战,展望了该技术在新拌水泥浆体结构性能研究中的应用前景。     

Hexavalent chromium in tricalcium silicate: Part I Quantitative X-ray diffraction analysis of crystalline hydration products

The hydration products of CrVI-doped tricalcium silicate (C3S) have been investigated. C3S is the main constituent of Portland cement responsible for the strength and stability of hardened Portland cement paste. Chromium trioxide (CrO3), added as a dopant to C3S, simulates hexavalent chromium waste that may be stabilized in ordinary Portland cement. X-ray diffraction was used to monitor the development of the hydration reaction products from the early stages to the late reaction stages. Leaching studies were carried out to evaluate the stability of the CrVI-containing phases in the hydrated C3S matrix.In monolithic waste forms containing hexavalent chromium, CaCrO4·2H2O was found to form within a few minutes of the hydration reaction. With increasing concentration of Ca2+ in the pore solution, Ca2CrO5·3H2O became the stable species. The chromium in this phase was found to be very mobile in a standard acetic acid leaching test.     

A simple method for predicting the chloride diffusivity of cement paste

A simple method for predicting the chloride diffusivity of cement paste is presented. In this method, cement paste is modeled as a two-phase composite material, consisting of solid matrix and pore space. By incorporating the classical percolation theory into the effective medium approach, an explicit solution is formulated for the chloride diffusivity of cement paste. Two parameters involved in the solution are determined by fitting the solution to experimental data. After the validity of the simple method is verified with experimental results obtained from the literature, the effects of the water/cement ratio and the degree of hydration on the chloride diffusivity of cement paste are evaluated in a quantitative manner. The paper concludes that the proposed simple method can predict the chloride diffusivity of cement paste with reasonable accuracy.     

HLC—RS高效减水剂对水泥石微观结构的影响

本文通过运用MIP、XRD、SEM、TG—DTA等无机物测试分析方法,探讨高效减水剂HLC—RS对水泥水化过程和水泥石结构的影响。检测结果分析表明,掺有HLC—RS能够显著有效降低混凝土中的孔隙率和孔径,改善孔的结构分布,使得水泥石中凝胶产物增多,提高了}昆凝土的强度和耐久性能。     

碳酸化钢渣和矿渣作硅酸盐水泥混合材水化性能研究

通过对钢渣碳酸化前后的硅酸盐相提取及水化放热性能和将碳酸化钢渣和矿渣作为混合材的硅酸盐水泥的胶砂强度和水化产物种类的测定,以及对它们微观形貌的观察,研究了碳酸化钢渣对胶凝体系水化性能的影响.结果表明,碳酸化使钢渣中硅酸盐相的含量由47.06％下降至14.38％;碳酸化促进了钢渣的早期水化,抑制其后期水化;在配比相同的条件下,碳酸化钢渣-矿渣-硅酸盐熟料体系试样的3、28d抗压强度较未碳酸化钢渣-矿渣-硅酸盐熟料体系试样的高;碳酸化生成的CaCO3促进了熟料的水化;碳酸化钢渣促进了胶凝体系中AFt的生成,且生成水合碳铝酸钙.     

In situ imaging of ground granulated blast furnace slag hydration

Ground granulated blast furnace slag (GGBFS) reacts with water in the presence of calcium sulfates and alkalis and is frequently used as a partial replacement for portland cement in concrete. The hydration products are known to be slightly different compositionally and morphologically than those of pure portland cement hydration. In this study, a new technique, soft X-ray transmission microscopy, was used to image the hydration of slag in a variety of solutions to investigate the effects of alkali sulfate and hydroxide activators on the morphology of the resulting hydration products. This microscopy method is unique in that it enables high resolution in situ observation and documentation of the formation of hydration products over time in wet samples at atmospheric pressure.

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Effects of cement size distribution on capillary pore structure of the simulated cement paste

The evolution of tricalcium silicate (C₃S) microstructure during hydration is tri-dimensionally simulated based on an “Integrated Particle Kinetics Model”. The hydration degree, the contact surfaces between the hydrated particles, the hydraulic radius and the capillary pore size distribution of the simulated cement paste at various degrees of hydration are calculated. Three examples of the C₃S microstructure development with different size distributions are presented. The effects of the cement size distribution on pore structure of cement paste are demonstrated and the results are discussed. In these examples, the cement size distribution varies between 3–40, 5–40 and 10–40 μm, respectively.     

Measuring adhesion between steel and early-hydrated Portland cement using particle probe scanning force microscopy

Particle probe scanning force microscopy was used to measure adhesion between steel and early-hydrated cement in the study. Particle probes, created by attaching steel microspheres to microcantilevers, were successfully used to collect adhesive forces between steel and early-hydrated Portland cement in air and in saturated lime water. Mixed Gaussian models were applied to predict phase distributions in the cement paste, i.e., low density calcium silicate hydrate, high density calcium silicate hydrate, calcium hydroxide, other hydrated products and the unreacted components. Consistent correlations were achieved for volume fractions between areas with different adhesion measurements and predictions from the hydration model. Results showed that low density calcium silicate hydrate, high density calcium silicate hydrate and other hydrated products exhibit intermediate adhesion to steel microspheres. Calcium hydroxide exhibits the smallest adhesion, while the unreacted components exhibits the largest adhesion among all groups.     

Decrease of pore solution alkalinity in concrete tested for alkali-silica reaction

Knowledge of alkali concentration in concrete pore solution is key for long-term evaluation of alkali-silica reaction (ASR) expansion. This study aimed at providing experimental data on the alkalinity evolution in concrete tested for ASR in order to determine critical alkalinity values that stop concrete expansion. The pore solution was systematically expressed (i.e. extracted at high pressures) from concrete specimens tested at 38°C over water (Canadian standard). Alkali concentration showed a decreasing trend with time, which can be mainly explained by alkali leaching. After 52 weeks, the decrease in alkalinity was up to 25% of the original Na₂O_eq content in the concrete, contributing to underestimate long-term expansion of reactive concrete. Comparisons were also made between pore solution expressed out of cement paste specimens and concrete specimens; differences (from 25% to 43% higher for cement paste) suggest that results obtained from cement paste are not suitable for concrete as they tend to overestimate the concrete pore solution alkalinity. The alkali concentration threshold varied from one mixture to another (205–335 mmol/l).

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Résumé La connaissance de la concentration en alkalis dans la solution interstitielle du béton est un facteur clé pour évaluer le potentiel d’expansion à long terme des structures touchées par l’alcali-réaction. Cette étude présente des données expérimentales sur l’évolution de l’alcalinité de bétons testés pour leur réactivité afin de déterminer des valeurs critiques stoppant la réaction. La solution interstitielle a été extraite sous pression à partir d’éprouvettes maintenues au-dessus de l’eau à 38°C (norme canadienne). Les analyses montrent une décroissance de l’alcalinité avec le temps, ce qui peut être expliqué en grande partie par le lessivage des alcalis; après 52 semaines, la perte en alcalis atteint jusqu’à 25% de la teneur initiale en Na₂O _eq . Ceci contribue à sous-estimer l’expansion à long terme des bétons réactifs. Des comparaisons entre la solution extraite de pates de ciment et de béton ont également été réalisées. Des différences importantes ont été notées (25% à 40% plus alcalins pour les pates), ce qui indique que les données obtenues sur pate sont difficilement transférables aux bétons car elles tendent à surestimer l’alcalinité de ces derniers. La concentration seuil varie d’un mélange à l’autre (205 mmol/l à 335 mmol/l).