Electrical conductivity and rheological properties of ordinary Portland cement-silica fume and calcium hydroxide-silica fume pastes

Two groups of solids mixtures were prepared: (i) the first group includes four mixes having different ordinary Portland cement/silica fume (OPC/SF) weight ratios and (ii) the second group consists of four blends having different Ca(OH)₂/SF molar ratios. Electrical conductivity measurements were carried out on the pastes of the first group mixes using two initial water-to-solid (W/S) ratios of 0.55 and 0.70 by weight; while the W/S ratios used for the second group mixes were 1.00 and 1.20 by weight. The measurements were done at 25 and 45 °C for each paste during setting and hardening processes after gauging with deionized water. Rheological properties were studied at room temperature for all mixes using various W/S ratios. The results obtained indicate clearly the effect of SF and W/S ratio on the rheological properties and electrical conductivity of all pastes under investigation. The relation between the electrical conductivity and rheological properties for different mixes were discussed based on the chemical nature and physical state of the hydration products formed at early ages of hydration.     

Electrical conductivity and phase composition of calcium aluminate cement containing air-cooled and water-cooled slag at 20, 40 and 60 °C

Calcium aluminate cement (CAC) pastes containing Egyptian air-cooled slag (AS) or water-cooled slag (WS) were prepared using different amounts of slag, namely, 5, 10, 15, 20 and 25 mass%. The pastes were prepared with deionized water using the required water of standard consistency to produce normal workability. The variations of electrical conductivity with the hydration time were measured at 20, 40 and 60 °C. The results demonstrate that electrical conductivity is a useful technique to study the change in the phase composition at different temperatures during the setting and hardening of calcium aluminate cement as well as reflecting the role of AS and WS, preventing the conversion occurring during the CAC hydration.     

不同温度下矿渣对铝酸盐水泥早期水化行为的影响

通过凝结时间、抗压强度、电阻率、浆体内部温度测试和水化产物分析,研究了20 ℃、35 ℃和50 ℃下矿渣(GGBFS)对铝酸盐水泥(CAC)早期水化行为的影响。结果表明,掺入矿渣会逐渐减小CAC 72 h的化学收缩,降低化学收缩速率峰值。20 ℃时,电阻率变化曲线出现了明显的晶相转变期,化学收缩曲线存在明显的诱导期; 35 ℃时,凝结时间延长,掺入矿渣抑制了电阻率的发展;50 ℃时,电阻率在接近24 h时显著降低,凝结时间显著缩短,掺入矿渣缓解了24 h电阻率的减小。矿渣-铝酸盐水泥体系的水化产物和抗压强度受养护温度的影响较大。20 ℃时,掺入40%(质量分数)矿渣减少了CAH₁₀的生成量,降低了硬化浆体的强度;35 ℃和50 ℃时,1 d水化产物主要为C₂AH₈和少量C₃AH₆,掺入矿渣延缓了强度的倒缩。在28 d龄期时,不同养护温度下掺入矿渣均能促进C₂ASH₈的生成。     

Thermal properties of hydrating calcium aluminate cement pastes

As the hydration of calcium aluminate cements (CAC) is highly temperature dependent, yielding morphologically and structurally different hydration products that continuously alter material properties, a good knowledge of thermal properties at early stages of hydration is essential. Thermal diffusivity and thermal conductivity during CAC hydration was investigated by a transient method with a numerical approach and a transient hot wire method, respectively. For hydration at 15 °C (formation of mainly CAH₁₀), thermal diffusivity shows a linear decrease as a function of hydration degree, while for hydration at 30 °C there is a linear increase of thermal diffusivity. Converted materials exhibited the highest values of thermal diffusivities. The results on sealed converted material indicated that thermal conductivity increased with an increase in temperature (20-80 °C), while thermal diffusivities marginally decreased with temperature. The Hashin-Shtrikman boundary conditions and a simple law of mixtures were successfully applied for estimating thermal conductivity and heat capacity, respectively, of fresh cement pastes.     

Fire resistance of fired clay bricks-fly ash composite cement pastes

This work aims to study the effect of substitution of fly ash for homra on the hydration properties of composite cement pastes. The composite cements are composed of constant proportion of OPC (80%) with variable amounts of fly ash and homra. The addition of fly ash accelerates the initial and final sitting time, whereas the free lime and combined water contents decrease with fly ash content. The fly ash acts as nucleation sites which may accelerate the rate of formation of hydration products which fill some of the pores of the cement pastes. The fire resistance of composite cement pastes was evaluated after firing at 250, 450, 600, 800 °C with rate of firing 5 °C/min with soaking time for 2 h. The physico-mechanical properties such as bulk density and compressive strength were determined at each firing temperature. Moreover, the phase composition, free lime and microstructure for some selected samples were investigated. It can be concluded that the pozzolanic cement with 20 wt% fly ash can be used as fire resisting cement.     

Early hydration and setting of oil well cement

A broad experimental study has been performed to characterize the early hydration and setting of cement pastes prepared with Class H oil well cement at water-to-cement ratios (w/c) from 0.25 to 0.40, cured at temperatures from 10 to 60 °C, and mixed with chemical additives. Chemical shrinkage during hydration was measured by a newly developed system, degree of hydration was determined by thermogravimetric analysis, and setting time was tested by Vicat and ultrasonic velocity measurements. A Boundary Nucleation and Growth model provides a good fit to the chemical shrinkage data.Temperature increase and accelerator additions expedite the rate of cement hydration by causing more rapid nucleation of hydration products, leading to earlier setting; conversely, retarder and viscosity modifying agents delay cement nucleation, causing later setting times. Lower w/c paste needs less hydration product to form a percolating solid network (i.e., to reach the initial setting point). However, for the systems evaluated, at a given w/c, the degree of hydration at setting is a constant, regardless of the effects of ambient temperature or the presence of additives.     

Effect of calcium formate as an accelerator on the physicochemical and mechanical properties of pozzolanic cement pastes

The aim of the present work is to study the effect of calcium formate (CF) as an accelerator on the properties of pozzolanic cement pastes. Three types of cements were used in this investigation. These cements were OPC and pozzolanic cements containing 80 mass% OPC and 20 mass% silica fume (SF) or 20 mass% ground clay bricks (GCB). The dosages of CF were 0.00, 0.25, 0.50, and 0.75 mass% of cement. The compressive strength, total porosity, and hydration kinetics such as free lime and combined water contents were investigated. The results obtained in this study showed that the addition of CF shortens the initial and final setting times and increases the compressive strength and combined water content as well as gel/space ratio at all ages of hydration. On the other hand, it decreases the total porosity. CF activates the liberation of Ca(OH)₂ of OPC pastes. The free lime content of pozzolanic cement in the presence of CF increases up to 7 days, then decreases at the later ages of hydration.     

Properties and hydration of blended cements with calcareous diatomite

In this paper the effect of diatomite addition on blended cement properties and hydration was studied. Calcareous diatomaceous rocks of Zakynthos Island, Ionian Sea, containing mainly CaCO₃ and amorphous silica of biogenic origin with the form of opal-A were used. Cement mortars and pastes, with 0%, 10%, 20% and 35% replacement of cement with the specific diatomite, were examined. Strength development, water demand and setting time were determined in all samples. In addition, XRD, SEM and weight loss at 350 °C were applied in order to study the hydration products and the hydration rate in the cement-diatomite pastes. Blended cements, having up to 10% diatomite content, develop the same compressive strength, as the corresponding Portland cement, while the presence of diatomite leads to an increase of the paste water demand. Diatomite is characterized as natural pozzolana, as it satisfies the requirements of EN 197 1 concerning the active silica content. The pozzolanic nature of the diatomite results to the formation of higher amounts of hydrated products, specifically at the age of 28 days.     

Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO₃, CC?). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO₄ solution under different conditions (room temperature, 60 °C, and drying-immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC? improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying-immersion cycles was determined. The drying-immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.     

Hardened portland cement pastes of low porosity III. Degree of hydration. Expansion of paste. Total porosity

The degree of hydration, the expansion during hydration, and the total porosity of low-porosity portland cement pastes were investigated at hydration times ranging from 1 hour to 180 days. The effects of the type of cement (Type I and II), the grinding aid, the surface of the cement, the water-cement ratio (0.2 and 0.3), and the temperature of hydration (5°, 25° and 50°C) were determined.     

Effect of delaying addition of some concrete admixtures on the rheological properties of cement pastes

The effect of delayed addition of two concrete admixtures, namely melamine formaldehyde sulfonate (MFS) and naphthalene formaldehyde sulfonate (NFS), on the rheological and adsorption properties of ordinary Portland cement (OPC), sulfate-resisting cement (SRC) and silica fume-ordinary Portland cement (SF-OPC) pastes was investigated. The admixture addition was delayed by 1, 3, 6, 10 and 13 min after the addition of mixing water. The shear stress, as well as the apparent viscosity of these cement pastes, was determined at different shear rates. Total organic carbon (TOC), Ca²⁺ concentration and conductivity of the filtrate and the combined water content of the precipitated cement pastes were determined. The results show that delaying the admixtures addition increases the cement pastes workability than that of simultaneous addition.     

Autogenous deformations of cement pastes: Part I. Temperature effects at early age and micro-macro correlations

A micro-macro experimental study has been performed, from the end of mixing up to 2 years, on a set of plain cement pastes prepared with the same type I ordinary Portland cement (OPC) and various water-to-cement ratios (W/C), and cured at various constant temperatures. In this part I of the paper, volumetric autogenous shrinkage has been analysed in relation to various parameters characterizing the hydration process: chemical shrinkage, degree of hydration of the cement, Ca(OH)₂ content and Vicat setting times, within the early-age period (≤24 h). The effects of the curing temperature (ranging from 10 up to 50 °C) have in particular been investigated. Its effects recorded on both the rate and the magnitude of volumetric autogenous shrinkage vs. time have pointed out the irrelevance of the usual maturity concept to describe such phenomenon within the whole early-age period. An improved maturity concept has hence been proposed. It is based on separating the early-age period in different phases and on using chemical shrinkage data for the calculation of the apparent activation energy applied to the prediction of autogenous deformations occurring after the setting period. Furthermore, micro-macro relationships have been pointed out, illustrating in particular the determining role of Ca(OH)₂.     

固井水泥浆早期水化过程电导率研究

研究纯水泥浆在不同水灰比条件下的电导率与温度变化之间存在的规律,同时探究了电导率与浆体水化过程的联系.结果表明:当水灰比相同时,浆体的电导率峰值随着实验温度(4～30 ℃)的增大而逐渐减小;通过水泥浆滤液测试,得到了与水泥浆电导率相同的变化趋势,即先增大后减小;实验采用水化热分析和扫描电子显微镜(SEM)观察,进一步解释了电导率的变化与水泥浆体水化过程及浆体结构之间所存在的关系.     

Electrical conductivity of concrete containing silica fume

The influence of silica fume on concrete properties represents an important technical research. In general, silica fume tends to improve both mechanical characteristics and durability of concrete. Thus the electrical properties of concrete containing silica fume can be studied to clarify its physical performance during hydration. The electrical conductivity of neat cement, mortar and concrete pastes was measured during setting and hardening. The ordinary Portland cement was partially replaced by different amounts of silica fume by weight. The changes in the electrical conductivity were reported during setting and hardening after gauging with water. The results of this study showed that the electrical conductivity can be used as an indication for the setting characteristics as well as the structural changes of the hardened pastes made with and without silica fume.     

Electrical conductivity of cement pastes

The electrical conductivity of cement- and clinker mineral pastes was measured during setting and hardening. Conductograms give interesting and structurally important data on the beginning period of cement hydration.     

Chemical shrinkage of portland cement pastes

Chemical shrinkage of normal Portland cement pastes (0.4 ≤ w/c < 0.8) has been measured at 20°C and of pastes with w/c = 0.5 furthermore at 35, and 50°C by means of measuring the volume change of samples of cement paste during the hydration. A small increase in the chemical shrinkage at “infinite time” was found at increasing water-cement ratio. The influence of the temperature was found to be twofold: Increasing temperature caused an increasing rate of the development of chemical shrinkage and a decrease of the chemical shrinkage at “infinite time”.Earlier studies of chemical shrinkage of Portland cement paste are also reported.     

Rheological properties of cement pastes admixed with some alkanolamines

The influence of addition of some alkanolamines, namely, monoethanolamine (MEA), triethanolamine (TEA) and polytriethanolamine (PTEA) on the rheological properties of the fresh pastes made of sulfate resisting cement (SRC) and ordinary Portland cement (OPC) was investigated. Different cement pastes made with and without these alkanolamines were prepared at a water/cement (w/c) ratio of 0.40. The shear rate-shear stress relationship was established by a co-axial cylindrical Rheotest. It was found that the rheological properties of the various cement pastes were affected by (i) the type and amount of alkanolamine, (ii) the type of cement and (iii) the applied shear rate. The alkanolamines were found to possess a significant retarding effect on the hydration of SRC in the order: TEA>PTEA>MEA.     

Influence of mixing sequence and superplasticiser dosage on the rheological response of cement pastes at different temperatures

An experimental methodology to study the rheological response of superplasticised cement pastes subjected to temperatures ranging from 5 to 45 °C is presented. The content of a polycarboxylate-based superplasticiser (PC) and moment of its addition to the mix (direct or delayed) are investigated. A loop of shear rate ramps is applied to each sample in order to obtain information about apparent viscosities and yield stresses, as well as a measure of their thixotropic behaviour. Results from the experimental campaigns indicate PC saturation dosages depend only slightly on temperature. The evolution of yield stress and thixotropy with temperature shows inverted trends under some conditions.     

磷渣对水泥浆体水化性能和孔结构的影响

通过对水泥浆体凝结性能、水化放热、力学性能和孔结构的测定,以及扫描电镜分析和差热-热重分析,研究了不同掺量磷渣对水泥浆体水化性能和微观结构的影响.结果表明:随着磷渣掺量的增加,浆体的凝结时间延长,水化热减少,早期抗压强度下降.但掺磷渣水泥浆体的后期抗压强度已接近或超过了纯水泥浆体的,磷渣掺量的增加对水泥浆体的后期抗压强度影响不显著.浆体中的Ca(OH)2量随龄期的延长而增加并随磷渣掺量的增加而降低.磷渣的活性效应和填充效应的发挥有效地改善了浆体水化后期的微观结构和孔结构,从而使浆体的力学性能有所提高.     

沸石粉对硫铝酸盐水泥水化行为的影响机理研究

研究了沸石粉对硫铝酸盐水泥浆体流动度、凝结时间和抗压强度的影响规律,并通过自收缩、电阻率和XRD测试分析了沸石粉在硫铝酸盐水泥水化行为中的作用机理。结果表明,掺入沸石粉后水泥浆体的流动度明显降低,凝结时间显著延长,且延长时间随掺量的增大呈先增大后减小的趋势。当沸石粉掺量为5%~15%(质量分数)时,硬化水泥浆体的1 d、3 d、7 d抗压强度均有显著提高;沸石粉掺量为10%时,水泥浆体3 d、7 d、28 d的强度增长幅度最大,和空白组相比,分别增长了21.6%、13.9%和5.4%。掺入沸石粉后水泥浆体的24 h电阻率显著增大,硬化浆体的7 d自收缩减小,且在相同龄期时,硬化浆体的自收缩随沸石粉掺量的增大而减小。XRD分析显示沸石粉的掺入能有效促进硫铝酸盐水泥的水化,有利于1 d、3 d和28 d龄期内钙矾石的形成。