Improving the Performance of Fly Ash–Portland Cement with Limestone Calcined Clay

Fly ash (FA) increases the performance of cement concrete through pozzolanic reaction and microstructure development at later ages. FA is usually associated with lower early age compressive strength especially at higher replacement levels. This study attempts at improving the early age strength of FA portland pozzolana cement with limestone calcined clay pozzolan (${\mathrm{LC}}^2$). Different proportions of limestone and calcined clay ($1:1$, $1:2$, and $2:1$) in tandem with FA at various replacements of cement clinker (30%, 40%, and 50%) were tested. The combined effect of using ${\mathrm{LC}}^2$ with FA has shown to enhance the early age hydration of cement along with improved compressive strength. Heat of hydration results showed increased heat release in FA mixes blended with ${\mathrm{LC}}^2$. Thermodynamic modeling analysis was used to study the change in phase assemblage of these mixtures. A higher solid volume observed in the thermodynamic simulations in the combined blends of FA and ${\mathrm{LC}}^2$ is hypothesized to be the cause of improved performance compared with only FA blended cement. Mercury intrusion porosimetry (MIP) results further confirmed an improvement in pore structure characteristics of FA cement blended with ${\mathrm{LC}}^2$. The possibility of reducing associated carbon emissions without compromising in strength are highlighted by using FA and ${\mathrm{LC}}^2$.