混凝土化学组成对其抗折性能试验研究

混凝土化学组成的研究,对提升水泥混凝土强度有着十分重要的作用。主要研究了胶凝材料体系的硅酸盐水泥的化学组分与其相对掺量、还有配合比对混凝土强度的影响。综述了不同胶凝材料的化学组成与其反应机理,要特别说明的是C_3S和4CaO·Al_2O_3·Fe_2O_3的含量对强度的影响最大。水泥熟料发生的水化反应得到的水化产物,也对其产生了一定影响。通过优化胶凝材料不同的化学组分掺量,可以有效提高水泥混凝土强度。     

Hydration in high-performance cementitious systems containing vitreous calcium aluminosilicate or silica fume

The influence of a relatively new high-performance cement replacement material—vitreous calcium aluminosilicate (VCAS)—on the hydration behavior in cementitious systems, and its comparison to silica fume (SF) are presented in this paper. VCAS is shown to have no cementitious qualities, but exhibits significant pozzolanicity, which has been quantified using strength activity index and electrical conductivity change. VCAS modified pastes are found to consume more water during hydration than the corresponding SF modified pastes. Based on a normalized calcium hydroxide content defined in this paper, it is seen that the pozzolanic reaction of VCAS does not happen until 7 days while that of SF occurs as early as the first day. The degrees of hydration of the modified pastes are predicted using a model that employs the change in non-evaporable water resulting from the use of these replacement materials. VCAS modified pastes show lower later age porosities as compared to the plain and SF modified pastes. However, at equal degrees of hydration, SF modified pastes show the lowest porosity.     

Shear-thickening behavior of high-performance cement grouts — Influencing mix-design parameters

Rheology of concrete is of great importance to its flow performance, placement and consolidation. A full understanding of fresh concrete flow behavior can be achieved through a good understanding of paste rheology. Cement pastes exhibit a complex rheological behavior affected by several physical and chemical factors, including water-to-cement ratio (w/c), high-range water-reducer (HRWR) type and dosage, and cement characteristics. An experimental investigation was carried out to evaluate the effect of w/c, HRWR–cement combinations, and supplementary cementitious materials (SCM) on the pseudoplastic behavior of high-performance cement grouts. Grout mixtures proportioned with w/c of 0.30, 0.33, 0.36, and 0.40, various cement–HRWR combinations, and cement substitutions by 8% silica fume were investigated. The incorporation of HRWR can lower the yield stress of mixtures, thus enhancing deformability, while silica fume improves mechanical and durability performances.High-performance structural grouts are shown to exhibit shear-thickening behavior at low w/c and shear-thinning behavior at relatively higher w/c. Mixtures made with polycarboxylate HRWR acting by steric effect exhibited greater shear-thickening behavior compared to those made with polynaphthalene sulfonate-based HRWR acting by electrostatic effect. The paper discusses the effect of mixture parameters on non–linear rheological behavior of various grout mixtures prepared with different w/c, HRWR–cement combinations, and silica fume.     

A physico-chemical basis for novel cementitious binders

The drive towards sustainability in construction is shaping our attitudes towards alternatives to Portland cement. Although the cement and concrete industry is essentially sustainable with respect to raw materials supply, and concrete manufacture actually gives relatively low CO₂ emissions per unit volume compared to most competitive construction materials, the current focus on climate change has led to concerns about cement industry-generated CO₂. Thus, there is interest in developing alternative cements with lower associated CO₂ emissions. This paper seeks to provide a context for innovative development through a review of what is meant by a hydraulic cementitious binder, identification of key physico-chemical properties of successful binders and how novel systems generally rely on similar factors. Concepts such as reactivity, availability of reactive species and physico-chemical drivers for the formation of cementitious systems are discussed as a basis for introducing and reviewing recent developments in the search for ever more environmentally sustainable cements.     

Upscaling quasi-brittle strength of cement paste and mortar: A multi-scale engineering mechanics model

It is well known from experiments that the uniaxial compressive strength of cementitious materials depends linearly on the degree of hydration, once a critical hydration degree has been surpassed. It is less known about the microstructural material characteristics which drive this dependence, nor about the nature of the hydration degree–strength relationship before the aforementioned critical hydration degree is reached. In order to elucidate the latter issues, we here present a micromechanical explanation for the hydration degree–strength relationships of cement pastes and mortars covering a large range of compositions: Therefore, we envision, at a scale of fifteen to twenty microns, a hydrate foam (comprising spherical water and air phases, as well as needle-shaped hydrate phases oriented isotropically in all space directions), which, at a higher scale of several hundred microns, acts as a contiguous matrix in which cement grains are embedded as spherical clinker inclusions. Mortar is represented as a contiguous cement paste matrix with spherical sand grain inclusions. Failure of the most unfavorably stressed hydrate phase is associated with overall (quasi-brittle) failure of cement paste or mortar. After careful experimental validation, our modeling approach strongly suggests that it is the mixture- and hydration degree-dependent load transfer of overall, material sample-related, uniaxial compressive stress states down to deviatoric stress peaks within the hydrate phases triggering local failure, which determines the first nonlinear, and then linear dependence of quasi-brittle strength of cementitious materials on the degree of hydration.     

International Summit on Cement Hydration Kinetics and Modeling Special Edition Québec City, July 27, 28 and 29, 2009

The performance of portland cement concrete relies upon a series of complex events that begin with raw minerals and end many years after the concrete is placed. Between these points, the life of this dynamic material is dominated by chemical reactions called hydration. While much is known about hydration, unfortunately, there is no unifying theory that describes the kinetics (rates) of these complex transformations from anhydrous cement to hydrous cement paste. Other industries including metalugy, petrochemicals, pharmaceuticals and semiconductors have asserted process control by developing a fundamental, mechanistic understanding of the kinetics of the chemical reactions and phase transformations that define their products. Might the concrete industry be moving along a similar trajectory?     

Hydration of cementitious materials, present and future

This paper is a keynote presentation from the 13th International Congress on the Chemistry of Cement. It discusses the underlying principles of hydration and recent evidence for the mechanisms governing this process in both Portland cements and other cementitious materials. Given the overriding imperative to improve the sustainability of cementitious materials, routes to reducing CO₂ emissions are discussed and the impact of supplementary materials on hydration considered.     

Modeling the hydration of concrete incorporating fly ash or slag

Granulated slag from metal industries and fly ash from the combustion of coal are industrial by-products that have been widely used as mineral admixtures in normal and high strength concrete. Due to the reaction between calcium hydroxide and fly ash or slag, the hydration of concrete containing fly ash or slag is much more complex compared with that of Portland cement. In this paper, the production of calcium hydroxide in cement hydration and its consumption in the reaction of mineral admixtures is considered in order to develop a numerical model that simulates the hydration of concrete containing fly ash or slag. The heat evolution rates of fly ash- or slag-blended concrete is determined by the contribution of both cement hydration and the reaction of the mineral admixtures. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and mineral admixture substitution ratios.     

Early age strength enhancement of blended cement systems by CaCl2 and diethanol-isopropanolamine

The enhancement of the 1 day strength of cementitious systems by a combination of calcium chloride (CaCl₂) and diethanol-isopropanolamine (DEIPA) was studied, particularly in blended cement systems. A combination of quantitative X-ray diffraction with Rietveld refinement (QXRD), scanning electron microscopy (SEM)/backscattered electron image analysis, thermogravimetric analysis (TGA), and isothermal calorimetry were used to investigate the mechanism of strength enhancement by the additives. The additives were found to increase the early age mortar strength by enhancing the cement hydration, with the DEIPA enhancing primarily the aluminate hydration. DEIPA also affected the morphology of portlandite which was formed as thin plates. In parallel, the calcium-to-silica ratio of the C-S-H was found to increase with the use of DEIPA, possibly because of the inclusion of microcrystalline portlandite. After 48 h DEIPA was found to directly enhance the rate of reaction of granulated blast-furnace slag and fly ash.     

Interactions between shrinkage reducing admixtures (SRA) and cement paste's pore solution

Shrinkage reducing admixtures (SRA) have been developed to combat shrinkage cracking in concrete elements. While SRA has been shown to have significant benefits in reducing the magnitude of drying and autogenous shrinkage, it has been reported that SRA may cause a negative side effect as it reduces the rate of cement hydration and strength development in concrete. To examine the influence of SRA on cement hydration, this study explores the interactions between SRA and cement paste's pore solution. It is described that SRA is mainly composed of amphiphilic (i.e., surfactant) molecules that when added to an aqueous solution, accumulate at the solution-air interface and can significantly reduce the interfacial tension. However, these surfactants can also self-aggregate in the bulk solution (i.e., micellation) and this may limit the surface tension reduction capacity of SRA. In synthetic pore solutions, SRA is observed to form an oil-water-surfactant emulsion that may or may not be stable. Specifically, at concentrations above a critical threshold, the mixture of SRA and pore fluid is unstable and can separate into two distinct phases (an SRA-rich phase and an SRA-dilute phase). Further, chemical analysis of extracted pore solutions shows that addition of SRA to the mixing water depresses the dissolution of alkalis in the pore fluid. This results in a pore fluid with lower alkalinity which causes a reduction in the rate of cement hydration. This may explain why concrete containing SRA shows a delayed setting and a slower strength development.     

Sustainable development and climate change initiatives

In the present paper we argue that the cement and concrete industry is contributing positively to the Climate Change Initiative by:

? Continuously reducing the CO₂ emission from cement production by increased use of bio-fuels and alternative raw materials as well as introducing modified low-energy clinker types and cements with reduced clinker content.

? Developing concrete compositions with the lowest possible environmental impact by selecting the cement type, the type and dosage of supplementary cementitious materials and the concrete quality to best suit the use in question.

? Exploiting the potential of concrete recycling to increase the rate of CO₂ uptake.

? Exploiting the thermal mass of concrete to create energy-optimized solutions for heating and cooling residential and office buildings.

     

Phase development in normal and ultra high performance cementitious systems by quantitative X-ray analysis and thermoanalytical methods

Quantitative X-ray diffraction (QXRD) and thermogravimetry (TG) methods are used to determine the phase development up to 28 days of hydration in normal and ultra high performance cementitious systems (UHPC) that do not contain aggregate. The phase development in ultra high performance cementitious formulation is quantitatively and kinetically different from that in normal concrete formulation. This is related to the different components employed and their associated reactions. For both formulations the most remarkable changes of the phase contents are recorded between the first and second hydration day and up to the seventh day. After the seventh day less phase content changes are measured. Because of the non sufficient water amount for hydration, considerable amount of cement remains non hydrated in the UHPC formulation. The portlandite content, which is present in the UHPC specimen, gives evidence for non complete pozzolanic reactions even after 28 days of hydration, whereas the absence of calcite in the UHPC specimen indicates an insignificant carbonation in this specimen.     

Hydration and microstructure of ultra high performance concrete incorporating rice husk ash

Rice husk ash (RHA) and silica fume (SF) have a similar chemical composition and a very high specific surface area, but RHA is not an ultra-fine material like SF. The high specific surface area of RHA originates from its internal porosity. For this reason RHA can be expected to behave differently from SF in terms of the hydration and the resulting microstructure of concrete. This still remains unclear in Ultra High Performance Concrete (UHPC). The objective of this research was to study the effect of RHA on the hydration and microstructure development of UHPC. The results are compared to those obtained with a control sample and a sample made with SF.The results show that the addition of RHA can increase the degree of cement hydration in UHPC at later ages. RHA can also refine the pore structure of UHPC and reduce the Ca(OH)₂ content, but less significantly than SF. The thickness of the interface transition zone (ITZ) between sand particles and cement matrix of all samples is very small at the age of 28 days. The compressive strength of the sample made with RHA after 7 days was higher than that of the control sample and the sample made with SF. It is suggested that the porous structure of RHA and the uptake of water in this porous structure results in a kind of is attributable to the internal water curing of the RHA modified mixture.     

Early-age behavior of materials with a cement matrix

Materials with a cement matrix classically present early-age volume variations (shrinkage and/or swelling). This intrinsic early-age behavior strongly influences the length of time the buildings and structures will last because of the micro-cracking and cracking that results from it. One explanation for the macroscopic shrinkage is the presence of pore pressure in the porous medium. In this study, fine modeling of the coupling mechanism behind these internal strains is proposed. The chemical reaction associated with hydration is considered as the main force behind the hydric and mechanical evolutions in an endogenous configuration. Thus, the influence of chemical contraction, porosity, pore-size distribution and pore pressure are central to the study in the light of the numerical and experimental results obtained. A self-leveling layer of mortar of sulfo-aluminous concrete base was used.     

Influence of Portland cement composition on early age reactions with metakaolin

The reactivity of two metakaolins, which vary principally in their surface area, and Portland cements of varying composition were examined via isothermal calorimetry for pastes at water-to-cementitious materials ratio of 0.50 containing 8% cement replacement by weight of metakaolin. Both metakaolins examined appear to have a catalysing effect on cement hydration. Calorimetry showed accelerated hydration, a slight increase in cumulative heat evolved during early hydration, and - for some cements examined - apparently an increased intensity of the heat evolved, particularly during the period typically associated with hydration of calcium aluminates. The higher surface area metakaolin had a greater effect. It is proposed that the presence of metakaolin may enhance dissolution of cementitious phases and/or provide additional, well-dispersed sites for nucleation of hydration products, in addition to increasing the early age concentration of solubilized aluminium (due to metakaolin dissolution). The increased intensity of some of the calorimetry data also suggests that some additional exothermic reactions are occurring, which may be related to an increased reactivity of calcium aluminate phases in the cement as well as the reaction of the metakaolin. This effect is apparently increased as the cement equivalent alkali content increases.     

Capillary porosity depercolation in cement-based materials: Measurement techniques and factors which influence their interpretation

The connectivity of the capillary porosity in cement-based materials impacts fluid-and-ion transport and thus material durability, the interpretation of experimental measurements such as chemical shrinkage, and the timing and duration of curing operations. While several methods have been used to assess the connectivity of the capillary pores, the interpretation of some experimental procedures can be complicated by the addition of certain chemical admixtures. This paper assesses capillary porosity depercolation in cement pastes using measurements of chemical shrinkage, low temperature calorimetry (LTC), and electrical impedance spectroscopy. The experimental results are analyzed to identify the time of capillary porosity depercolation. In addition, the factors that influence the interpretation of each technique are discussed. Experimental evidence suggests that capillary porosity depercolation, as defined by Powers, occurs after hydration has reduced the capillary porosity to around 20% in cement paste systems. The influence of capillary porosity depercolation on the transport properties is demonstrated in terms of a reduction in the electrical conductivity of the cementitious material. Special attention is paid to understand and interpret the influence of shrinkage-reducing admixtures (SRAs) on the freezing behavior of cementitious systems, particularly in regard to the inapplicability of using LTC to detect porosity depercolation in cement pastes containing such organic admixtures.     

Dynamic model of temperature rise caused by cementitious materials hydration

This paper presents a dynamic model of temperature rise caused by cementitious materials hydration based on the basic nature of chemical change. There are many models for computation of temperature rise caused by cementitious materials hydration, but the most takes this temperature rise as a function of concrete age only, which does not consider the impact of initial temperature and temperature rise during hydration and thus cannot reflect the real course of temperature rise. This model not only accords with the general law of chemical change, but also coincides well the experimental data and stimulates well the actual course of temperature rise of concrete under any initial temperature.     

有机助磨剂对水泥性能的影响及其有机水泥化学分析(英文)

现代水泥、混凝土中大量使用化学外加剂,特别是有机化合物和高分子聚合物化学外加剂,例如：水泥助磨剂、混凝土超塑化剂、引气剂、增稠剂等,大量有机物的加入改变了水泥水化过程、水化动力学、微观结构的发展,传统的水泥混凝土化学不再能很好地解释其微观结构与宏观性能的关系。为此,提出一个新兴的水泥混凝土化学的补充分支—有机水泥化学,在未来的水泥混凝土研究中该给予更多的重视。以有机化学外加剂—助磨剂为例,说明其对水泥水化动力学、水化产物形态以及水泥浆体的超塑化剂需求量、流变特性、强度发展等宏观性能的影响。水泥中加入微量的助磨剂,不仅改变了水泥颗粒分布,还改变了水化动力学,促进起始离子的溶解和铝酸钙（C3A）和铁铝酸钙（C4AF）的早期水化,明显地提高早期强度和28 d强度。助磨剂吸附在水泥表面改变了水泥的表面性质,其中助磨剂和Ca2＋、Fe2＋螯合起关键作用。     

多元胶凝粉体复合效应的研究

在水泥中掺人具有不同颗粒分布和活性的细掺合料可以获得多元胶凝粉体材料。采用标准稠度用水量法和环境扫描电镜研究了多元粉体体系的紧密堆积效应。提出用标准稠度需水量比作为确定紧密堆积效应的实验方法。用水化热和抗压强度实验研究了多元胶凝粉体材料各组分的水化进程匹配和复合胶凝效应,测定了多元胶凝粉体大体积混凝土的强度和绝热温升过程。实验结果表明：改变胶凝组分的品种、含量和细度可以调控多元胶凝粉体的绝热温升和强度发展过程。以满足配置高性能混凝土的特殊技术需求。