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The influence of a number of calcium salts on the total chemical shrinkage (used as a measure of cement hydration) of different portland cement pastes was followed during the first 48 hours. All calcium salts (acetate, chloride, formate, nitrate and nitrite) were added in an equimolar dosage of Ca’-corresponding to 1.5 % calcium nitrate by cement weight. An automatic Vicat-apparatus was used to monitor the setting time of the cement pastes. Experiments conducted at 5 OC, 13 OC and 23 OC revealed that calcium nitrate was the most effective set accelerator at lower temperatures and even more effective than calcium chloride at the lowest temperature. The anions of the different calcium salts were also found to influence the setting and the efficiency of each accelerator strongly depended on the cement type.

The investigation presented in this study shows an example of the improvements of fly ash fineness achieved by a physical process, air-classification and a mechanical process, grinding. To make high-performance concrete, three different types of fly ash namely, original, air-classified, and ground fly ash, with different finenesses was used as cement replacement. The percentage replacement of cement by each type of fly ash was used as 0, 10, 15 and 20% by weight of cementing materials. Finally, the results were compared with silica fume concrete. The results showed that substitution of part of the cement with original or classified fly ash produces concrete mixtures with greater workability than the control as measured by slump and slump-flow. On the contrary, it was found that the ground fly ash, having more or less the same degree of fineness as classified fly ash resulted in a lower workability due to the loss of its spherical shape and lubricant effect. The inclusion of original fly ash reduced the early strength and this reduction was more significant with the increase of percentage replacements. Classified and ground fly ash improved the early strength. The long-term strength development of classified and ground fly ash concrete was found to be considerably higher than that of control concrete for all the percentage replacements.

This paper describes a five-year Electric Power Research Institute (EPRI) program directed toward increasing ash utilization in the cement and concrete market within the United States, in the face of the impacts on ash quality due to more aggressive Nox controls. EPRI is undertaking this program to provide the technical basis for protecting the bulk sale of coal ash in high-volume applications in cement and concrete and other high volume civil engineering applications. In addition to higher carbon levels in ash from NOx control systems, problems associated with ammoniated ash have become a major concern for coal-fired facilities in recent years as a result of the increased use of ammonia-based environmental control technologies. Many coal-fired power producers have become concerned that post-combustion Nox controls could lead to fly ash containing high levels of ammonia. Therefore, EPRI conducted a research program designed to assist power producers evaluate and mitigate the impacts of high carbon and ammoniated ash.

Effect of rice husk ash addition on the chloride diffusivity of concrete is investigated in the present paper. The concrete specimens, having water-cementitious materials ratio of 0.30, 0.36 and 0.53, with and without rice husk ash are subjected to accelerated chloride penetration using the following two methods: i) Immersion in saturated NaCl at 20°C. ii) Exposure to alternate cycles of 3days immersion in 3% NaCl solution at 20°C and 4days drying at 50% R.H. After the accelerated chloride penetration, the total chloride content of concrete specimens at various depths was determined. Consequently, using the chloride content distributions, the chloride diffusion coefficient of concrete is evaluated by applying Fick’s second law. In addition, the pore size distribution of concrete is determined in order to assess the effect of concrete microstructure on chloride diffusivity. From the experimental results, the total chloride content of concrete incorporating rice husk ash was shown to be lower than that of the control concrete after accelerated chloride penetration. The concrete specimens incorporating rice husk ash had chloride diffusion coefficients 57% to 25% lower than the control concrete. The effect of rice husk ash on pore refinement in concrete was observed, especially in the pore radii larger than 50nm. The pore size distribution of concrete tended to shift towards the smaller pores with the addition of rice husk ash. The decrease of the chloride diffusion coefficient of concrete incorporating RHA may therefore be attributed to the pore-refinement effect.

Normal portland cement concrete is the most widely used material in the construction industry in Singapore. Under normal curing conditions, it can take one day or more before the concrete can be safely handled without damage. In precast production, it is desirable for the concrete to attain sufficient strength within a short period of time so that moulds and other resources can be used more efficiently. A revolutionary method of curing using microwave heating utilizes the internal energy dissipation associated with the excitation of molecular dipoles in electromagnetic fields. This method enables faster and more uniform heating and has been found to shorten the process time necessary to achieve high early strength. At present, the use of microwave curing on an industrial scale is still in its infancy. The Prefabrication Technology Centre in Singapore has developed the first prototype mechanised industrial microwave curing system in this region. This paper will discuss the use of this system for the production of ferrocement secondary roofing slabs on an industrial scale. It is believed that this technology has great potential to revolutionalise curing of precast concrete.