Hardening Accelerator for Fly Ash Blended Cement

Abstract

Using fly ash blended cement (OPC-FA) in construction results in benefits in many aspects of technology, environment impact as well as economic efficiency. However, the use of OPC-FA with higher replacement level of OPC by FA is limited by the low rate of reaction at early age of fly ash, particularly at low temperature. The low reactivity leads to low early compressive strength affecting to the time of construction.

Several chemicals, which potentially can be used to accelerate the early strength development of OPC-FA, were tested alone and in combination. The European standard EN-934-2 was used to evaluate the effect of the tested chemicals on the strength development. A combination of small dosages (total less than 0.5%) of sodium thiocyanate, diethanolamine, glycerol (“3-component admixture”) appeared to be effective in enhancing the early strength of OPC-FA.

The accelerating effect of the 3-component admixture on the hydration of OPC-FA was investigated and the results indicate that the admixture accelerates the hydration of OPC, while the pozzolanic reaction of FA is not promoted. In the presence of the 3-component admixture the dissolution of interstitial phases in OPC was increased. Consequently, gypsum was consumed more rapidly, which lead to a more rapid and larger formation of hemicarboaluminate. The formation of hemicarboaluminate resulted in consumption of CH originating from the hydration of silicate phases in OPC. The 3-component admixture also promoted the dissolution of silicate phases leading to an acceleration of the hydration of these phases.

A synergistic effect on the acceleration of the hydration of OPC-FA was observed when applying the 3-component admixture. The sample using the 3-component accelerating admixture had a higher rate of hydration compared to samples using the chemicals individually. The increase in maximum rate of heat evolution in the sample using the 3-component admixture was higher than the sum of increases in maximum rate of heat evolution released from samples using the three chemicals individually. In addition, the 3-component admixture had an increased accelerating effect on the hydration OPC-FA system compared to that of plain OPC at both low and normal curing temperature.