The cementitious and microstructural characteristics of an innovative waste-based eco-cement consisting of ground granulated blast-furnace slag (GGBFS), circulating fluidized bed combustion ash (CFA), and rice husk ash (RHA) were investigated in this study. Locally sourced CFA was employed as a crucial sulfate-rich activator, and its effect on the properties of the eco-cements was studied. Five eco-cement samples were prepared using CFA/(GGBFS + RHA) ratios of 10, 15, 20, 25, and 30%, with a constant ratio of RHA/GGBFS. Results showed that increasing CFA prolonged setting time in the eco-cement. At later curing ages, the eco-cements activated by 15% CFA (CFA15) recorded higher compressive strength, dynamic moduli, ultrasonic pulse velocity, and lower drying shrinkage values than the other eco-cements. Moreover, increasing CFA to 30% decreased compressive strength by about 16%. Besides, a significant difference was found between the microstructure of CFA15 and those of the other eco-cements, which comprised C–S–H/C–A–S–H gels, ettringite, Ca(OH)2, and CaCO3 as their principal hydration products. Interestingly, the smallest distance between individual C–S–H and C–A–S–H layers and the intergranular fracture surface was observed in CFA15. Consequently, the X-ray diffraction patterns of CFA15 verified the increase in C–A–S–H, which is responsible for mechanical strength. This study provides further evidence for the promising potential of using CFA, GGBFS, and RHA to produce sustainable eco-cements.