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 liftslabs, 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 signifi cant effect on the performance of bond breakers.