Effectiveness of fly ash on suppressing ASR expansion and its evaluation

The use of mineral admixture as partial replacement for portland cement in concrete is an effective means for controlling expansion due to the alkali silica reaction (ASR). However, a reliable standard method for testing the effectiveness of mineral admixtures to prevent ASR has not been established. In order to develop a reliable method for testing the effectiveness of fly ash in controlling ASR, the suppression effects of low calcium fly ash on the ASR expansion which occurred in siliceous gravel, zeolite pearlite and silica glass at the curing conditions of 40°C, 60°C and 80°C were studied respectively based on the Chinese accelerated mortar bar method. The effects of the reactivity of aggregate and curing temperatures on the effectiveness of fly ash on suppressing the ASR expansion and the technique for assessing the effectiveness of fly ash and other mineral admixtures are discussed. Results show that the suppressing effect of fly ash on the ASR expansion is strongly dependent on the alkali reactivity of aggregate and the conserving temperature. The higher the alkali reactivity and the curing temperature are, the lower the suppressing effect is to become. The dependence of the suppressing effect of fly ash on the alkali reactivity and the conserving temperature is attributed to different ASR behaviors for various aggregates and at various temperatures. The suppressing effect of fly ash on a certain reactive aggregate at a certain conserving condition can not be simply extended to other aggregates and conditions. Methods for testing the effectiveness of fly ash and other mineral admixtures against ASR should be established according to application objectives. The correlation of the result based on the assessing method and the long-term performance of concrete needs further study.