Fabrication and Testing of CSAB Cements in Mortar and Concrete that Utilize Circulating Fluidized Bed Combustion Byproducts

摘要

The Gilbert fluidized bed combustion material has potential for use in the production ofcalcium sulfoaluminate belite cements.The utilization of the Gilbert CFBC spent bedmaterial in CSAB cement shows potential as a large-volume use for the material.Heating FBC bottom ash,PCC fly ash,limestone,and bauxite at 1250°C,produced alarge quantity of Klein’s compound and belite.The Gilbert FBC ash provides neededcalcium sulfate and,particularly,calcium oxide.The calcium oxide within the ash is aneffective substitute for limestone,which is required as a raw material for CSAB cementclinker.In fact,if changes in the Gilbert FBC combustion process were to result insubstantially less lime in the spent bed material,its’value as a CSAB clinker rawmaterial would be limited since FGD gypsum would provide a more concentrated andrefined source of calcium sulfate.The synthesized CSAB clinkers were soft and readily milled to cement fineness.Millingthe clinker with FGD gypsum was effective in provide the additional calcium and sulfaterequired to“activate”the clinker to form ettringite.The compressive strength of thecommercial and laboratory CSAB cements produced high-early strengths that exceededthose of ordinary Portland cement.Additional long-term strength was possibly providedby hydration of dicalcium silicate(C2S)within the clinker.The durability of the laboratoryCSAB cements was similar to that of commercially available CSAB cements.The testsin which the CSAB cements performed well were resistance to deicer chemicals,dryingshrinkage and resistance to freezing and thawing.However,all of the CSAB cementscarbonated more rapidly than OPC and tended to undergo strength regression ascarbonation became more extensive.Thus,the CSAB cements would likely provide littleprotection for reinforcing steel within concrete.Milling the laboratory CSAB clinker with Class F fly ash,in addition to FGD gypsum,appeared to improve the dimensional stability of CSAB mortar.In every cement thatcontained fly ash addition,destructive expansion did not occur and drying shrinkageimproved.However,fly ash addition generally decreased the compressive strength,although the water reduction achieved with the fly ash helped to offset this(see Table3).Future work will focus on optimizing the quantity of fly ash addition to providemaximum water reduction benefits and minimize the strength loss.A major issueregarding the production of CSAB cement is one of cost.Because CSAB clinkerproduction requires substantial quantities of bauxite,the cost of these cements is high.In order to minimize or eliminate bauxite,alternatives to this raw material need to bepursued.One approach is to formulate high-ferroaluminate CSAB cements using feedmaterials that contain high percentages of iron.High-iron calcium aluminate cementsare currently produced where rapid strength gain and fire resistance is desired.Thereplacement of some bauxite with high-iron raw materials will have the net effect ofreplacing some of the aluminum with iron,which is considerably less expensive.Thefeed materials could include coal fly ash and/or red mud,which is a byproduct of bauxiteprocessing.The production of high-ferroaluminate CSAB cements will thus be pursuedin future research.