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Reports the results from a laboratory investigation of the effect of fly ash on the temperature rise and early-age tensile strain capacity of concrete. Twelve different fly ashes, with a wide range of chemical compositions, were used in various proportions (25, 40, and 56 percent) in the study. The results of conduction calorimeter tests show that the rate of heat development was strongly influenced by the composition of the ash. Generally, the rate and quantity of heat evolved increased with the calcium level of the fly ash. High-calcium ashes (>20 percent CaO) did not significantly reduce the seven-day heat of hydration when used at a replacement level of 25 percent. However, the heat of hydration decreased as the level of replacement was increased for all ashes tested, regardless of composition. Consequently, even high-calcium ashes may be effective in reducing the temperature rise in concrete, provided they are used at a sufficient level of replacement. Flexural tests were carried out on concrete prisms at early ages; the tensile strain capacity was determined as the strain (in the tensile fibers) at 90 percent of the flexural strength. The flexural strength decreased with higher levels of replacement; however, the strain capacity was similar or slightly higher in fly ash concretes (compared with control specimens) at three and seven days. These results imply that the beneficial effect of reduced temperature rise in fly ash concrete is not necessarily offset by a reduced capacity to resist thermal strains.

The authors have examined the lime-reactivity strength data of 14 samples of fly ash obtained from different thermal power plants of India. The sand-lime-fly ash mortars cured at 50 C and relative humidity of 90 percent were tested in compression at different ages up to 90 days. It was found that lime reactivity is best correlated with combined parameters of fineness and soluble silica content, rather than with each parameter considered individually. Also examined were the strength of concrete mixtures in which part of the cement is replaced by a low reactive fly ash. Fineness of fly ash and testing ages for strength were the variables. It is concluded that the soluble silica content was related to later-age strengths, while the early-age strength correlated better with fineness of fly ash. The mechanism for the latter may not be chemical, but physical, such as dispersion of cement particles or micro-filler effect.

Classified fly ash (CFA) is produced by separating the fine components of fly ash by means of air classification. CFA is made of fine particles of micro-meter size and spherical shape and is expected to improve the consistency of fresh concrete and the durability of hardened concrete. The use of CFA in roller compacted concrete (RCC) pavement has the effect of reducing the water content of RCC mixtures and, therefore, the drying shrinkage and number of joints in pavement. RCC pavements have become popular for roads and streets in Japan. The maximum thickness of RCC slabs that may be placed in one layer is limited to 25 cm, because of limitations in the compactibility of the concrete and control of the pavement surface profile. To increase the slab thickness of RCC placeable in one layer, an improved concrete that requires minimal energy for obtaining a high filled-volume ratio is desirable. In this paper, the effects of CFA additions to cement on compactibility and water content of RCC mixture were studied.

The most common way of estimating the degree of hydration of cement in practice has been to measure the Nonevaporable water content (W n) using the relation W n = 0.25 * C. However, when silica fume is incorporated in a mixture, some of the nonevaporable water is converted to evaporable water. More specifically, the water originally bound in the Ca(OH) 2, which reacts with the amorphous silica, is released in a polymerization process. This was shown in a study of mature cement-silica fume systems in which a new quantitative thermogravimetric method was used. Thus, for the same degree of hydration of the cement, the total nonevaporable water content is lower in a mixture with reacted silica fume than in one without. Moreover, the degree of hydration of the cement is slightly increased when moderate amounts of silica fume are added, provided sufficient water is present in the pore system, either by water curing or a high initial water-cement ratio. In mixtures with low w/c, the faster self-desiccation and the reduced permeability caused by silica additions affect the moisture state in a larger specimen (even if it is water cured) and, consequently, the degree of hydration of the cement.

The efficiency of finely ground blast furnace slags (BFS) was studied in relation to the fundamental physico-mechanical and structural properties of cement mortars. A positive effect of BFS fineness on the workability of fresh mortars was proved. Due to a small water content (20 percent by mass of the cementitious material) when combined with a high-range, water-reducing admixture (HRWRA), the compressive strengths of mortars ranging between 80 and 100 MPa are ascertained at normal curing conditions. Special curing conditions, such as autoclaving or hot water curing, produce specimens with compressive strengths in the range of 100 to 130 MPa, depending on the grading of BFS and the composition of the binder. With additional heat curing, the compressive strength of the mortars increase, in general, by about 10 to 50 percent over that of either autoclaved or hot water cured mortars. In this paper, some other properties of high-strength mortars incorporating finely ground BFS are discussed, including porosity and durability investigations. The efficiency of BFS addition is compared with other fine mineral powders, such as silica fume and fine silica powder, with special attention paid to binder compositions and curing conditions.