Fresh Properties and Early Compressive Strength of Alkali-Activated High Calcium Fly Ash Paste

Abstract

This paper reports the influence of high calcium content in alkali-activated fly ash (AAFA) paste mixtures on the compressive strength, setting time, and workability. Class C fly ash was used to prepare alkali-activated paste mixtures. Four different sodium silicate to sodium hydroxide ratios of 2.5, 1.5, 1.0, and 0.5 were used as alkaline activators with a constant sodium hydroxide concentration of 10 M. Two curing regimes were also applied, oven curing at (70 °C) for 24 h, and ambient temperature at (23 ± 2 °C). The delay time between mixing and starting curing the specimens in the oven regime is 2 h. The compressive strength of the specimens was tested at 7 days for both curing regimes. The results showed that the fresh AAFA pastes had shorter setting time and lower workability with increasing the ratio of alkaline activators to fly ash ratio. However, the compressive strength of hardened AAFA was increased when the ratio of alkaline activators to fly ash ratio increased. A water to fly ash ratio of 0.3 with alkaline activators to fly ash ratio of 0.25, which displayed a balance between the fresh and hardened AAFA properties. The optimum sodium silicate to sodium hydroxide ratios were 1.0 for ambient curing and 1.5 for oven curing. The compressive strength of AAFA has reached up to 8 ksi after 7 days for ambient curing and 10 ksi for oven curing with initial setting time of approximately 180 min.