Study of effect of sodium sulfate amount on compressive strength of cement pastes with high fly ash volume

Abstract

This study focuses on evaluating effect of sodium sulfate (Na₂SO₄) as an activator on compressive strength of the cement pastes with high fly ash volume. The replacements of cement by fly ash in the paste were ​​85, 88, 90, 93, 95, and 97% by mass of binder. The Na₂SO₄ contents added to the mixture were 0, 1.5, 2.0, 2.5, and 3.0% by mass of the binder. The ratio of water-to-binder for all mixture proportions was 0.20. After 24 hours of casting, the cubic specimens with dimensions of 50×50×50 mm were immersed in water at temperature of 27±2^oC until the designated test ages. The experimental results showed that the specimens using an activator had compressive strength 2 to 4 times higher than the control specimens without Na₂SO₄ at an early age of 3 days. At the ages of 7 and 14 days, the compressive strength of the specimens containing Na₂SO₄ continued to be 1.5 to 2.0 times higher than that of the control specimens. Up to the age of 28 days, the compressive strength of the control specimens was approximately equal to that of the specimens having the activator. As a consequence, the content of Na₂SO₄ as the activator added to the mixture helped promote the strength development at an early age for the pastes containing a high volume of fly ash and the optimum Na₂SO₄ content was 2.5% by mass of binder.