AbstractThis study was designed to evaluate the performance and microstructure of eco-cements containing rice husk ash (RHA), ground granulated blast furnace slag (GGBFS), and circulating fluidized bed combustion (CFBC) fly ash (CFA). Eco-cement samples with a water to (GGBFS+RHA+CFA) mass ratio of 0.40 were prepared with four RHA/(GGBFS+RHA) mass percentages of 0%, 15%, 30%, and 45% and three CFA/(GGBFS+RHA) mass percentages of 10%, 20%, and 30%. The compressive strength performance and overall quality [tested using ultrasonic pulse velocity (UPV)] of the eco-cements were found to be significantly and differently affected by the percentages of RHA, GGBFS, and CFA used. Although CFA was found to reduce compressive strength and UPV at early ages of curing, CFA enhanced these values at later ages and promoted the development of the eco-cements due to the increased formation of hydration products [ettringite (AFt), C─ S─ H, and C─ A─ S─ H], as identified using energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) analyses. The incorporation of more RHA was found to reduce the compressive strength and UPV values of the eco-cements. The compressive strength values of the eco-cements at 28 and 91 days were in the ranges of 20.4–39.1 and 24.1–45.3 MPa, respectively, whereas the UPV values of all eco-cements at 28 and 91 days were in the ranges of 2,897–3,368 and 3,062–3,462 m/s, respectively. The RHA-free eco-cement activated by 20% CFA had the highest compressive strength and UPV values at all curing ages. The substitution of GGBFS with 15%–45% RHA can be used to produce eco-cements used in mortar/concrete with various requirements such as low to medium strength and low cost.