Long-Term Mechanical Properties of Blended Fly Ash-Rice Husk Ash Alkali-Activated Concrete

This paper presents an experimental investigation on the long-term mechanical properties of alkali-activated concrete prepared with low-calcium fly ash and rice husk ash (RHA). Detailed chemical, microstructural, and pore structure analysis were evaluated to elucidate the variation of the long-term mechanical properties. The addition of 10% RHA leads to a reduction in all mechanical properties—that is, compressive strength, flexural strength splitting, tensile strength, and elastic modulus—for all ages up to 365 days. Lower workability and higher density resulted from the addition of 10% RHA attributed to its higher unburnt carbon content and specific surface area. From 7 to 28 days, both concrete (100% fly ash and 10% RHA-90% fly ash concrete) showed an incremental increase in strength for all mechanical properties. However, all properties subsequently decreased from 28 to 365 days. Crack propagation was evident after 28 days in both concrete, which is identified as negatively influencing the microstructure, and is the cause of the observed deterioration in the mechanical properties after 28 days. It is postulated that the combined effects of the degree of microcracking and the homogeneity in structure are the primary factors that govern the long-term mechanical strength properties for blended fly ash-RHA alkali-activated concrete.