The impact of calcium hydroxide addition on HVFA mortar and concrete properties

Abstract

Fly ash, a by-product of coal combustion, can be used as a cement substitute. High-volume fly ash (HVFA) concrete refers to fly ash substituting more than 50% of the cement. Previous research has indicated a decrease in compressive strength with an increase in fly ash content. Moreover, HVFA concrete exhibits low early strength due to the limited availability of calcium hydroxide resulting from cement hydration, which is insufficient to drive the pozzolanic reaction. This study, therefore, introduces additional calcium hydroxide to the mixture to react with class C and F fly ash in HVFA concrete. The fly ash replaces 50% and 60% of the Portland cement by mass, and the study investigates the impact of adding calcium hydroxide at 10%, 20%, and 30% of the fly ash content by mass. The addition of calcium hydroxide to the HVFA concrete mixture resulted in decreased workability and accelerated initial setting time. However, this addition did not influence the early strength of the mortar and concrete. Interestingly, it was found that the compressive strength at 28 and 56 days increased with the increase in calcium hydroxide content. The increase in later-age strength was more pronounced in HVFA concrete with class F fly ash, which can be attributed to the enhanced availability of calcium hydroxide, thereby facilitating the pozzolanic reaction.