Susceptibility of Portland cement and blended cement concretes to plastic shrinkage cracking

The market share of different types of blended cements is increasing year by year. Generally, blended cements are ground to higher fineness and exhibit a slower development of mechanical properties compared to Ordinary Portland Cement (OPC), which might affect the concrete performance in terms of shrinkage cracking at early ages.In this paper, the performance of concretes made with different cement types is compared according to the ASTM C1579-13 standard for plastic shrinkage cracking. The cracking behavior was further correlated to the deformations of both unrestrained and restrained specimens measured by a 3D image correlation system. The main factors influencing the cracking behavior were discussed based on poromechanics. It is concluded that the bulk modulus evolution has a dominant effect on controlling the plastic shrinkage cracking. Concretes made of more reactive cements, in particular with higher clinker content, are less susceptible to plastic shrinkage cracking. For cements with the same clinker content, increasing the cement fineness reduces the risk of plastic shrinkage cracking.     

Simulation of the hydration kinetics and elastic moduli of cement mortars by microstructural modelling

The ability of the VCCTL microstructural model to predict the hydration kinetics and elastic moduli of cement materials was tested by coupling a series of computer simulations and laboratory experiments, using different cements. The novel aspects of this study included the fact that the simulated hydration kinetics were benchmarked using real-time measurements of the early-age phase composition during hydration by in situ X-ray diffraction. Elastic moduli are measured both by strain gauges (static approach) and by P-wave propagation (dynamic approach). Compressive strengths were measured by loading mortar prisms until rupture. Virtual samples were generated by VCCTL, using particle size distribution and phase composition as input. The hydration kinetics and elastic moduli were simulated and the numerical results were compared with the experimental observations. The compressive strength of the virtual mortars were obtained from the elastic moduli, using a power-law relation. Experimentally measured and simulated time-dependence of the major cement clinker phases and hydration product phases typically agreed to within 5%. Also, refinement of the input values of the intrinsic elastic moduli of the various phases enabled predictions of effective moduli, at different ages and different water-to-cement mass ratios, that are within the 10% uncertainty in the measured values. These results suggest that the VCCTL model can be successfully used as a predictive tool, which can reproduce the early age hydration kinetics, elastic moduli and mechanical strength of cement-based materials, using different mix designs.     

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.     

Numerical simulation of heat and mass transport during hydration of Portland cement mortar in semi-adiabatic and steam curing conditions

A model that describes hydration and heat-mass transport in Portland cement mortar during steam curing was developed. The hydration reactions are described by a maturity function that uses the equivalent age concept, coupled to a heat and mass balance. The thermal conductivity and specific heat of mortar with water-to-cement mass ratio of 0.30 was measured during hydration, using the Transient Plane Source method. The parameters for the maturity equation and the activation energy were obtained by isothermal calorimetry at 23 °C and 38 °C. Steam curing and semi-adiabatic experiments were carried out to obtain the temperature evolution and moisture profiles were assessed by magnetic resonance imaging. Three specimen geometries were simulated and the results were compared with experimental data. Comparisons of temperature had maximum residuals of 2.5 °C and 5 °C for semi-adiabatic and steam curing conditions, respectively. The model correctly predicts the evaporable water distribution obtained by magnetic resonance imaging.     

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。     

基于CT扫描技术的水泥净浆微观结构及水化程度

进行了0.3和0.5两种水灰比的水泥净浆从1~40天龄期的CT扫描试验,成功重构了水泥净浆尺度上的三维微观模型,观测了未水化水泥颗粒、水化产物和孔隙在这期间的形态变化。基于CT扫描的结果量化未水化水泥颗粒体积含量,进行了水化程度的计算。计算结果与常用的TGA方法进行比较,发现可比性好, CT扫描为一项较可靠的量化水泥水化程度的方法,但需要高精度的CT扫描设备。     

The influence of mechanical activation by vibro-milling on the early-age hydration and strength development of cement

This paper presents results from an experimental investigation that evaluated the mechanical activation of portland cement using vibro-milling. In this investigation, the duration of the vibro-milling was systematically varied and its influence was evaluated using mortar samples. In addition, the amount of activated cement used in the mortar samples was varied and evaluated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate differences in hydration products and the structure of activated cement and mortars. The activated cements were tested to determine the influence of activation on the rate of hydration and compressive strength development. The test results suggested that the use of mechanical activation can improve early-age structure formations and compressive strength. A 32% and 25% increase in 1-day strength were observed for the systems with Type I and Class H cements, respectively. This increase in 28-day strength was 16% and 58% for Type I and Class H cement, respectively. It was observed that longer milling times did not necessarily improve performance, and 15 min appeared to be sufficient vibro-milling time to provide valuable benefits.     

Differential scanning calorimetry study of ordinary Portland cement paste containing metakaolin and theoretical approach of metakaolin activity

This paper aims to investigate the hydration and pozzolanic reactions in cement pastes with different levels of metakaolin replacement, using differential scanning calorimetry (DSC) and theoretical analysis based on reaction stoichiometry. It was found that the DSC technique could follow the hydration process quantitatively by measuring the peak temperature and enthalpy corresponding to decomposition of hydration products, as functions of age. The pozzolanic process can also be followed from the measurements of the changes in the amount and the nature of amorphous material in the paste and the change of the amount of calcium hydroxide. In addition, it was confirmed that a theoretical approach using reaction stoichiometry could give a good estimation of the concentration of calcium hydroxide in a metakaolin concrete.     

Strength, durability and shrinkage characteristics of cement stabilised soil blocks

The paper outlines results of a comprehensive investigation undertaken to assess the influence of soil characteristics and cement content on the physical properties of stabilised soil blocks. The dry density, compressive and flexural strength, durability and drying shrinkage of over 1500 block tests are outlined in the paper. Experimental results are compared with current specifications and used to develop empirical guidelines for cement content requirements for a range of soil plasticity characteristics. An empirical relationship between compressive and flexural strength is proposed as a simple means of field assessment.     

A comparison study of the mechanical properties and drying shrinkage of oil palm shell and expanded clay lightweight aggregate concretes

For making artificial lightweight aggregate, selected raw materials are fed into a rotary kiln at high temperature. Providing such a high temperature is costly and generally, the process of making artificial lightweight aggregate is not environmentally friendly. The use of natural lightweight aggregate for making lightweight concrete can lead to low-cost construction. The use of a solid waste lightweight aggregate namely oil palm shell (OPS) as coarse aggregate, is not only environmentally friendly but leads to a low-cost material. This study is a comparison between some engineering properties of OPS lightweight concrete and an artificial lightweight (expanded clay) concrete with low water to cement ratio, along with having good workability and without any segregation. The test results show that OPS concrete has better mechanical properties and a higher efficiency factor than expanded clay lightweight concrete. The ceiling strength of expanded clay lightweight concrete occurs at an early age; while it happens in OPS concrete at a later age. The crack pattern of the tested specimens shows that OPS is much stronger than expanded clay. On the other hand, the compressive strength of OPS lightweight concrete is more sensitive to lack of curing. Although OPS lightweight concrete shows twice the amount of drying shrinkage than expanded clay lightweight concrete in the short term, this difference reduces significantly at later ages.     

A study on the structural and mechanical properties of nanocrystalline CuS thin films grown by chemical bath deposition technique

We report a chemical route for the deposition of nanocrystalline thin films of CuS, using aqueous solutions of Cu(CH₃COO)₂, SC(NH₂)₂ and N(CH₂CH₂OH)₃ [triethanolamine, i.e. TEA] in proper concentrations and ratios. The films were structurally characterized using X-ray diffraction technique (XRD), field emission scanning electron microscopy (FESEM) and optical analysis [both photo luminescence (PL) and ultraviolet-visible (UV-vis)]. Optical studies showed a large blue shift in the band gap energy of the films due to quantum confinement effect exerted by the nanocrystals. From both XRD and FESEM analyses, formation of CuS nanocrystals with sizes within 10-15 nm was evident. A study on the mechanical properties was carried out using nanoindentation and nanoscratch techniques, which showed good mechanical stability and high adherence of the films with the bottom substrate. Such study on the mechanical properties of the CuS thin films is being reported here for the first time. Current-voltage (I-V) measurements were also carried out for the films, which showed p-type conductivity.