Thermal behavior of cement matrix with high-volume mineral admixtures at early hydration age

Thermal cracks that usually occur in mass concrete are closely related to the thermal behavior of cement matrix, such as heat liberation, temperature rise and thermal shrinkage. Cement pastes added with large-volume mineral admixtures that are usually used for thermal controlling were cast into well-sealed plastic cylinder and covered by heat insulation materials to simulate the pseudo-adiabatic condition of mass concrete. The deformation and temperature rise of cement specimens under the heat insulation condition have been examined at early hydration age. Results show that with addition of fly ash, coal gangue and blast furnace slag the heat liberation and peak temperature of cement paste decrease, while its total shrinkage increases.There is no shrinkage but expansion of the pastes during the temperature rise process, which may be ascribed to the complete compensation of the shrinkage by thermal dilation of the pastes. The thermal dilation coefficient (TDC) of cement paste changes drastically with the hydration duration, and it is also related to the addition of mineral admixtures.     

Effects of three Turkish fly ashes on the heat of hydration of PC-FA pastes

Effects of the type and amount of fly ash substitution on the heat of hydration of portland cement-fly ash pastes were investigated. Three Turkish fly ashes were used. One of them was a high-calcium and the other two were low-calcium fly ashes. The specimens contained 0, 10, 20, and 40% fly ash by weight of portland cement. The tests were carried out as described in ASTM C 186 however one separate set of specimens were first subjected to an early external temperature of 67±2°C for six hours followed by the standard temperature until time of test. The results revealed that the low-calcium fly ashes, regardless of their type, reduce the heat evolution when used for partial cement replacement. The high-calcium fly ash, on the other hand, does not produce significant changes in the heat of hydration.     

Electrochemical studies on the corrosion performance of steel embedded in activated fly ash blended concrete

The use of fly ash to replace a portion of cement has resulted significant savings in the cost of cement production. Fly ash blended cement concretes require a longer curing time and their early strength is low when compared to ordinary Portland cement (OPC) concrete. By adopting various activation techniques such as physical, thermal and chemical methods, hydration of fly ash blended cement concrete was accelerated and thereby improved the corrosion-resistance of concrete. Concrete specimens prepared with 10-40% of activated fly ash replacement were evaluated for their open circuit potential measurements, weight loss measurements, impedance measurements, linear polarization measurements, water absorption test, rapid chloride ion penetration test and scanning electron microscopy (SEM) test and the results were compared with those for OPC concrete without fly ash. All the studies confirmed that up to a critical level of 20-30% replacement; activated fly ash cement improved the corrosion-resistance properties of concrete. It was also confirmed that the chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

Performance characteristics of high-volume Class F fly ash concrete

More than 88 million tonnes of fly ash is generated in India each year. Most of the fly ash is of Class F type. The percentage utilization is around 10 to 15%. To increase its percentage utilization, an extensive investigation was carried out to use it in concrete. This article presents the results of an experimental investigation dealing with concrete incorporating high volumes of Class F fly ash. Portland cement was replaced with three percentages (40%, 45%, and 50%) of Class F fly ash. Tests were performed for fresh concrete properties: slump, air content, unit weight, and temperature. Compressive, splitting tensile, and flexural strengths, modulus of elasticity, and abrasion resistance were determined up to 365 days of testing.Test results indicated that the use of high volumes of Class F fly ash as a partial replacement of cement in concrete decreased its 28-day compressive, splitting tensile, and flexural strengths, modulus of elasticity, and abrasion resistance of the concrete. However, all these strength properties and abrasion resistance showed continuous and significant improvement at the ages of 91 and 365 days, which was most probably due to the pozzolanic reaction of fly ash. Based on the test results, it was concluded that Class F fly ash can be suitably used up to 50% level of cement replacement in concrete for use in precast elements and reinforced cement concrete construction.     

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.     

Enhanced durability performance of fly ash concrete for concrete-faced rockfill dam application

The main purpose of this research was to enhance the durability in both the design and construction of dams. Especially, in case of rockfill dams, the durability of face slab concrete in a concrete-faced rockfill dam (CFRD) is achieved by optimizing the fly ash replacement for cement. The effect on durability corresponding to the increasing replacement of fly ash was evaluated, and the optimum value of fly ash replacement was recommended. The results show that 15% of fly ash replacement was found to be an optimum level and demonstrated excellent performance in durability.     

Use of ground coarse fly ash as a replacement of condensed silica fume in producing high-strength concrete

This paper presents a method of improving coarse fly ash in order to replace condensed silica fume in making high-strength concrete. The coarse fly ash, having the average median diameter about 90-100 μm, yields a very low pozzolanic reaction and should not be used in concrete. In order to improve its quality, the coarse fly ash was ground until the average particle size was reduced to 3.8 μm. Then, it was used to replace Portland cement type I by weights of 0%, 15%, 25%, 35%, and 50% to produce high-strength concrete. It was found that concrete containing the ground coarse fly ash (FAG) replacement between 15% and 50% can produce high-strength concrete and 25% cement replacement gave the highest compressive strength. In addition, the concrete containing FAG of 15-35% as cement replacement exhibited equal or higher compressive strengths after 60 days than those of condensed silica fume concretes. The results, therefore, suggest that the FAG with high fineness is suitable to use to replace condensed silica fume in producing high-strength concrete.     

Properties of concrete incorporating high volumes of class F fly ash and san fibers

The results of an experimental investigation to study the effects of replacement of cement (by mass) with three percentages of fly ash and the effects of addition of natural san fibers on the slump, Vebe time, compressive strength, splitting tensile strength, flexural strength and impact strength of fly ash concrete are presented. San fibers belong to the category of “natural bast fibers.” It is also known as “sunn hemp.” Its scientific (botanical) name is Crotalaria juncea. It is mostly grown in the Indian subcontinent, Brazil, eastern and southern Africa and some parts of the United States (Hawaii and Florida). A control mixture of proportions 1:1.4:2.19 with W/Cm of 0.47 and superplasticizer/cementitious ratio of 0.015 was designed. Cement was replaced with three percentages (35%, 45% and 55%) of class F fly ash. Three percentages of san fibers (0.25%, 0.50% and 0.75%) having 25-mm length were used.The test results indicated that the replacement of cement with fly ash increased the workability (slump and Vebe time), decreased compressive strength, splitting tensile strength and flexural strength and had no significant effect on the impact strength of plain (control) concrete. Addition of san fibers reduced the workability, did not significantly affect the compressive strength, increased the splitting tensile strength and flexural strength and tremendously enhanced the impact strength of fly ash concrete as the percentage of fibers increased.     

An improved basis for characterizing the suitability of fly ash as a cement replacement agent

Fly ash is a critical material for partial replacement of ordinary portland cement (OPC) in the binder fraction of a concrete mixture. However, significant compositional variability currently limits fly ash use. For example, the performance of OPC‐fly ash blends cannot be estimated a priori using current characterization standards (eg, ASTM C618). In this study, fly ashes spanning a wide compositional range are characterized in terms of glassy and crystalline phases using a combination of X‐ray fluorescence (XRF), X‐ray diffraction (XRD), and scanning electron microscopy with X‐ray energy‐dispersive spectroscopy (SEM‐EDS) techniques. The compositional data are distilled to a unitless parameter, the network ratio (N_r), which represents the network behavior of atoms that form alkali/alkaline earth‐aluminosilicate glasses that make up fly ashes. N_r is correlated with known composition‐dependent features, including the glass transition temperature and amorphous XRD peak (“hump”) position. Analysis of heat release data and compressive strengths are used to evaluate the impact of fly ash compositions on reaction kinetics and on the engineering properties of cement‐fly ash blends. It is shown that fly ashes hosting glasses with a high network ratio (ie, having a less stable glass structure) are more reactive than others.     

Rheology of latex-modified grouts

The rheology of grouts containing latex was investigated. The two latex additives used were carboxylated styrene-butadiene and acrylic. The influences of superplasticizer, fly ash, and blast furnace slag on the rheology of latex-modified grouts were addressed. Shear stress-shear rate curves were determined for a variety of mix proportions. The time-dependent behaviour of selected grouts was also studied. It was determined that the yield stress and apparent viscosity are influenced by latex content and that the grouts are shear thinning at low water/cement ratios. Latex imparts stability and thixotropy in grouts. Partial replacement of cement with either fly ash or slag diminishes the effect of latex on rheology.     

Influence of steam curing on the compressive strength of concrete containing supplementary cementing materials

Heat treatment is widely used to accelerate the strength-gaining rate of concrete. In general, the ultimate strengths of the heated-treated concrete are lower than those of the standard cured specimens. When ultrafine fly ash (UFA) is included in concrete, the pozzolanic reaction is accelerated through the heat treatment. Sometimes, various chemical activators were used to activate the reactivity of fly ash. In the current study, UFA and slag were used as a replacement for cement, steam curing and chemical activators were used to accelerate hydration of cement and fly ash, and then compared with moist curing. This paper presents the influence of steam curing on the compressive strength of concrete containing UFA with or without slag. The experimental results indicated that the concrete containing UFA has low early strength after 13-h steam curing and that the difference between the 28-day compressive strength of concrete through 13-h steam curing and that of moist-cured concrete is large, but the concrete with UFA and CaSO₄ or Ca(OH)₂ has a high early strength, thus, the reactivity of fly ash must be accelerated. Concrete containing UFA and ground slag was prepared, whose compressive strengths were improved.     

Effect of fly ash and nanosilica on compressive strength of concrete at early age

The replacement of cement by mineral admixtures in concrete has been of increasing interest in the construction industry. Nevertheless, several of the potential replacements, such as fly ash class F, lower the compressive strength of concrete at early age. This project investigates the use of nanosilica to compensate for such loss of compressive strength. A statistical experimental design involving mixtures of Portland cement, fly ash and nanosilica, in addition to water/binder ratio as an external factor, is proposed to study their combined effect on the compressive strength of concrete. This design allows estimating a cubic regression model that properly accounts for the effects of the mixture components within a constrained experimental region. The range of each factor was selected according to levels normally used in the industry. Finally, an optimisation strategy permits to recommend the use of nanosilica when high percentages of cement replacement by fly ash are present.     

The fluidity of fly ash-cement paste with superplasticizer

The influence of various fly ashes on the fluidity of fly ash-cement paste with superplasticizer was investigated in connection with the amounts of superplasticizer adsorbed on fly ashes, the change of ζ-potential and the physical properties of fly ashes. Fly ash indicates a high negative ζ-potential, and by adding superplasticizer, an even higher negative ζ-potential is indicated. However, the fluidity of fly ash paste is not improved by addition of superplasticizer as much as in the case of cement. The fluidity of fly ash-cement paste mixed with superplasticizer is influenced by the kinds of fly ashes. The bulk specific gravity of fly ash may be an index for judging the influence of fly ash on the fluidity of fly ash-cement paste.     

蒸养条件下两性聚羧酸减水剂对胶砂及混凝土强度的影响

合成了阴离子型和两性型聚羧酸减水剂,研究了两类聚羧酸减水剂对水泥水化热、蒸养胶砂和蒸养混凝土强度的影响.结果表明:在蒸养条件下,与阴离子型聚羧酸减水剂相比,掺两性型聚羧酸减水剂的水泥水化温峰更高;在相同水灰比时,掺两性聚羧酸减水剂的蒸养胶砂和蒸养混凝土的强度也更高.XRD分析可知,掺入两性聚羧酸减水剂在蒸养条件下生成更多的AFm和氢氧化钙,促进了C3S和C2S的水化.     

Effect of TEA on fly ash solubility and early age strength of mortar

The investigations focused on the dissolution behaviour of fly ash in alkaline solution and the effect of triethanolamine (TEA) addition. TEA is known as a grinding aid in cement production and is an Al and Fe chelating agent. To determine the effect of TEA on the dissolution behaviour of fly ash constituents, fly ash was mixed with a KOH solution at pH 13 and different dosages of TEA. Samples were taken after different times and analysed by ICP-OES. The effect of TEA on the heat evolution rates of fly ash cement pastes was investigated using isothermal calorimetry. Strength tests were also conducted to investigate the effect of TEA on plain Portland cement and fly ash/cement mortars. TEA was found to increase the dissolution rate of Al, Ca and Fe from fly ash. A slight, but reproducible, effect on heat evolution rates and an increase in early age strength was observed for fly ash cements.     

高效减水剂与水泥及掺混合材水泥的相容性研究

通过对掺高效减水剂水泥净浆流动性及流动性损失的测定试验,研究了高效减水剂与水泥及掺混合材水泥的相容性。试验结果表明,同一种高效减水剂与不同水泥的相容性不同;矿渣和粉煤灰都能改善水泥与高效减水剂的相容性;复掺粉煤灰和矿渣比矿渣单掺时改善效果好。     

Effect of elevated temperatures on concrete incorporating ferronickel slag as fine aggregate

This study evaluates the effect of elevated temperature exposure on concrete incorporating ferronickel slag (FNS) as a replacement of natural sand. Concrete cylinders were exposed up to 800°C, and the changes in compressive strength, mass, ultrasonic pulse velocity (UPV), and microstructure were investigated. The concretes containing up to 100% FNS aggregate showed no spalling and similar cracking to that of the concrete using 100% natural sand. For exposures up to 600°C, the residual strengths of concretes containing 50% FNS were 7% to 10% smaller than the concrete with 100% sand. Use of 30% fly ash as cement replacement improved residual strength by pozzolanic reaction for exposures up to 600°C. An equation has been found from the correlation between residual strength and UPV. Therefore, UPV can be used as a nondestructive test to estimate the extent of postfire damage and residual strength of concrete incorporating FNS aggregate and fly ash.     

The fluidity of fly ash-cement paste containing naphthalene sulfonate superplasticizer

The zeta potential measurement indicated that the surface potential of fly ash was different from ordinary Portland cement (OPC) in both sign and value. Hence, the Derjaguin-Landau-Verway-Overbeek (DLVO) theory for dispersion-flocculation of heterogeneous particles with different surface potentials was applied to explain the influence of fly ash on the rheology of cement paste containing naphthalene sulfonate superplasticizer. For the fly ash-cement paste without superplasticizer, the sign of zeta potential of fly ash was different from OPC. Thus, the extent of the potential energy barrier between particles was small or even showed negative value, and the change in the rheology of the fly ash-cement paste was mainly dependent on the bulk solid volume of fly ash, which was related to available free water for fluidizing paste. For the fly ash-cement paste with naphthalene sulfonate superplasticizer, fly ash and cement had the same sign and dispersed well due to higher potential barrier. The extent of potential energy barrier depended on the absolute value of surface potential, which was represented by a function of the amount of adsorbed superplasticizer. The bulk solid volume of fly ash also affected the change in flow ability, but the effect of potential energy barrier between particles was superior to that of the bulk solid volume of fly ash.     

粉煤灰再生混凝土中氯离子渗透性能试验研究

通过氯离子自然扩散试验,测定粉煤灰再生混凝土试件中自由氯离子浓度,研究了粉煤灰掺量、干湿循环等因素对粉煤灰再生混凝土中氯离子渗透性能的影响。结果表明:粉煤灰能提高再生混凝土抗氯离子渗透能力,粉煤灰最佳掺量介于10%~30%之间;干湿循环加快了粉煤灰再生混凝土中氯离子的渗透速度;粉煤灰再生混凝土中氯离子浓度随着氯盐溶液侵蚀时间的延长逐渐增大,随着向粉煤灰再生混凝土内部的深入,逐渐降低。     

Microstructure,dielectric and piezoelectric properties of 0–3 lead free barium zirconate titanate ceramic-Portland fly ash cement composites

Piezoelectric ceramic – Portland cement composites have been developed for sensor application in concrete structures to overcome the acoustic matching problem that may occur for piezoelectric ceramic or polymers with concrete. Pozzolanic materials such as fly ash are commonly used in concrete to enhance durability. The objectives of this research were to investigate the effects of fly ash addition on the physical properties, dielectric properties and piezoelectric properties of 0–3 barium zirconate titanate ceramic– Portland cement composites. The results showed that the dielectric constant of these composites decreased when the fly ash content in the composite increases. However, the piezoelectric coefficient (d₃₃) value of BZT–PC composite with fly ash 10% by volume was found to be similar to that of BZT–PC composites.