stated that the high concentration of hydroxyl ions (i.e. high pH values) due to higher alkali content of the solution results in the expansive type of ettringite. According to Heinz and Ludwig [51], several factors affect the formation of ettringite such as sulfate content, pH of the solution and availability of the calcium hydroxide. Higher alkali content increased the solubility of sulfate ions in solution which being absorbed by CSH resulted in formation of expansive type of ettringite. The XRD results (Figs. 6 and 7) shows increased intensity of CS (21, 26, 29 degree 2h) and nonexpansive ettringite (7, 17 and 34 degree 2h) in bacterial treated CKD concrete (5% and 10%) responsible for the strength development in concrete where as in 15% bacterial treated CKD concrete, reduction in cement content reduced the required alkalinity (needed for hydration reaction) which in turn decreased the CSH and thus reduced the strength compared to control concrete.