USE OF COAL COMBUSTION BYPRODUCTS IN THE PRODUCTION OF CALCIUM SULFOALUMINATE-BELITE CEMENT

Calcium sulfoaluminate/belite (CSAB) cement hardens rapidly, which has advantages over Portland cement (OPC) in some construction applications. The cement-forming phases in CSAB are yeelimite ("Klein's Compound" or Ca₄Al₆O₁₂SO₄) and belite (larnite or Beta-Ca₂SiO₄). Traditionally, CSAB cement is produced by heating mixtures of gypsum, limestone, bauxite, and shale or clay at 1300 - 1350^oC. The use of coal combustion flyash, synthetic gypsum, and/or fluidized bed combustion (FBC) spent bed material in the preparation of the cement can substantially lower CO₂ emissions and reduce energy costs when compared with OPC production. In this laboratory study, a mixture consisting of approximately 15 wt% Class F flyash, 15 wt% FBC spent bed material, 17 wt% bauxite, and 53 wt% limestone was heated at 1250^oC for one hour to produce CSAB clinker. The CSAB clinker was interground with synthetic gypsum at 20 wt% of the clinker and mixed with water and sand to produce mortar. The mortars were moist cured and tested for compressive strength at 1, 7, 28, 56, and 112 days. These data were compared with those from commercial CSAB cement. During curing, yeelimite reacted with gypsum and water to form ettringite and poorly crystalline Al(OH)₃. Strength development of the laboratory CSAB cement mortars was not as high as commercial CSAB mortars, probably because the laboratory cement contained less yeelimite and more of the slow-reacting belite. The slow cooling rate of the laboratory CSAB cement likely resulted in larger crystals and thus could also have contributed to the slower strength gain.