Aluminum Sulfate Hydration Retarders for High-Calcium Fly Ash Used in Highway Construction
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ALUMINUM SULFATE HYDRATION RETARDERS FOR HIGH-CALCIUM FLY ASH USED IN HIGHWAY CONSTRUCTION M. TOHIDIAN* and JOAKIM G. LAGUROS** *Standard Testing and Engineering, Oklahoma City, Oklahoma **School of Civil Engineering and Environmental Science, The University of Oklahoma, Norman Oklahoma 73019 USA Received 20 November,
1986; refereed
ABSTRACT The rapid hydration and setting associated with the use of high-calcium fly ash as an additive in soil and aggregate base stabilization in highway construction imposes certain limitations in regards to operational time and volume of work executed. Aluminum sulfate and its ammonium salt were evaluated as hydration reaction retarders. Mixtures of Ottawa sand and Class C high lime fly ash in a 1:1 weight ratio were used for the evaluations. These additives minimized the adverse effects of delayed compaction by recovering some of the compressive strength lost to the rapid hydration, although in all cases the density of the mixes decreased. The recovery of strength was related to the heat of hydration, wherein the peak temperature was reduced from 90OF to the range of 86-78 0 F at 2 hours; further temperature decreases were observed as reaction time increased. The availability of the sulfate ions, as manifested by the presence of ettringite, helps the hydration process continue, minimizes the adverse effects of delayed compaction and assists positively in the reduction of the void area of mixes and in stratlingite formation, which contributes to a strong crystalline framework. INTRODUCTION Fly ash containing free lime is an effective stabilizing agent in highway construction. Such high-lime fly ashes are used for improving the strength characteristics of substandard subgrade soil material [1] and for providing adequate cementation in coarse and fine granular base courses [2]. However, fly ash mixtures lose their workability and strength if not shaped and densified immediately [1,3], and become more time sensitive as the percentage of fly ash is increased [2]. These characteristics are reflected in the dual problems of loss of strength due to delayed compaction and rapid setting. In soil stabilization the amount of fly ash used is less than 20% [1], and thus only the effects of delayed compaction are of concern. Aggregate base course mixes, however, require high fly ash contents, in excess of 25%, and thus the maximization of strength gain in actuality is not attained because rapid setting interferes. Studies in delayed compaction are very few [5], but they all point to the fact that density and strength are, at times, substantially reduced. Retarders have rarely been used with fly ash mixtures [5]. Some of the critical studies of retardation mechanisms [2,5-10] with other cementitious materials present the dominant conclusion that retardation is caused by the precipitation and adsorption of insoluble materials on surfaces. This study introduces aluminum sulfate chemicals which, as additives, retard the process of hydration and provide adequate operational time before the fly ash m
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