Early-age tensile strength and calcium hydroxide content of concrete containing low-calcium fly ash

Abstract

The effects of using fly ash in concrete on the early-age tensile strength and the effectiveness of reactive bond breakers are concerns for applications such as tilt-up panels and lift-slabs, however, experimental data is scarce in literature. Fifteen concrete mixtures with up to 40% low-calcium fly ash were tested. The early-age tensile strength of fly ash concrete was found to correlate well with compressive strength. The calcium hydroxide (CH) content on concrete surface was found to vary in a wide range because of the varying distribution of aggregates. Generally, the CH content decreased with the increase of fly ash. The CH content increased with age in the control concrete while the change was relatively small for the fly ash concretes up to the age of 7 days. Thus, the reduction of CH content in concrete containing high volume low-calcium fly ash may have significant effect on the performance of bond breakers.