Scanning electron microscopy (SEM)-assisted mechanistic study: pH-driven compressive strength and setting time behavior in geopolymer concrete

This research study investigates the influence of pH on the setting time, compressive strength (CS), and surface roughness of different waste materials, namely fly ash, rice husk ash (RHA), red mud, and ground granulated blast furnace slag (GGBS). The average pH values for these waste materials were found to be 8.77, 9.3, 8.3, and 11, respectively. The initial setting time (IST) and final setting time (FST) varied among the materials, with red mud having maximum IST and FST with a value of 180 min and 24 h. CS (MPa) measurements revealed that GGBS exhibited the highest value of 36 MPa, followed by RHA (28 MPa), red mud (22 MPa), and fly ash (24 MPa). Surface roughness analysis showed that red mud had the highest roughness value of 69.71, followed by fly ash (62.81), RHA (53.14), and GGBS (49.08). The findings indicate a positive correlation between pH levels and CS of 97%, and setting time surface roughness has a negative correlation with CS when analyzed with a heatmap. Higher pH values were associated with increased CS, particularly after 28 days of curing which is shown by the heatmap and sub-plots. Additionally, microscopic analysis provided insights into particle size, orientation, and clustering, aiding in understanding the bonding and reactivity patterns contributing to the observed variations in CS.