Formulating Low-Energy Cement Products

The study examined several formulations that may serve as a green substitute for traditional portland cement. The primary objective of the project was to produce a durable, low-energy cementitious material from flue gas desulfurization (FGD) gypsum that was converted to hemihydrate. The study also included spent ash from circulating fluidized bed combustion (CFBC), and Class F fly ash. Hemihydrate would give the by-product cement early strength development, and the spent-bed/ultrafine ash blend would provide the by-product cement with long-term strength (gaining slowly at first) and decrease solublity. A spent-bed/ultrafine ash ratio of $40/60$ produced the best compressive-strength results of the preliminary clinkerless cement blends produced in the study. The expansion of these clinkerless cements was caused by the formation of ettringite shown by X-ray diffraction (XRD). The system stopped expanding when calcium hydroxide was largely consumed. Substituting 50% of the clinkerless cement blends with hemihydrate increased short-term compressive strength 200% and reduced longer-term expansion up to 90%, enabling the production of low-energy 100% by-product cement.