Influence of agricultural waste ash on the mechanical strength of alkali-activated slag recycled aggregate concrete from a microstructural perspective

Using cement-based natural aggregate concretes to meet the increasing infrastructural demands has determinantal effects on the environment, such as CO2 emissions due to cement manufacturing and degradation of natural resources. The present study aims to find the optimum replacement content of two agricultural wastes i.e., sugarcane bagasse ash (SBA), and rice husk ash (RHA) to be incorporated in alkali-activated slag concrete developed from ground granulated blast furnace slag (GGBS), along with 100% recycled coarse aggregates (RCA). Four concretes each for SBA and RHA were prepared by replacing them with GGBS from 5% to 20% at the steps of 5%. Compressive strength and water absorption parameters were used to evaluate the influence of SBA and RHA in alkali-activated slag recycled aggregate concrete (AASRAC) for all eight concretes. These macro properties were justified at the micro-level by SEM-EDS and XRD analysis. From the present study, it was found that 10% SBA (S10) and 10% RHA (R10) replaced with GGBS exhibited a maximum strength of 69 MPa and 66 MPa and minimum water absorption of 1.97% and 2.12% respectively, compared to other concretes developed in the study. The formation of highly polymerized and condensed products due to enhanced reactivity of the precursor and the highest Ca/Si and Ca/(Si + Al) for both S10 and R10 mixes resulted in the increased structural buildup of the cross-linked tobermorite-like structures, leading to the formation of a condensed C-(A)-S-H gel structure. Thus, it was concluded from the study that the optimum replacement % of SBA and RHA in AASRAC is 10%.