DEVELOPMENT OF EXPANSIVE CEMENTS USING DRY FLUE GAS DESULFURIZATION SOLID WASTES

DRY FLUE GAS DESULFURIZATION (DFGD) IS AN EMERGING ADVANCED SULFUR DIOXIDE (SO2) CONTROL SYSTEM FOR PULVERIZED COAL-FIRED POWER PLANTS WITH AN ESTIMATED 20,000-MW GENERATING CAPACITY WORLDWIDE. IN THIS METHOD, EXCESSIVE AMOUNTS OF CALCIUM HYDROXIDE ARE USED TO CAPTURE THE SO2 IN THE FLUE GAS. THE FLY ASH FROM THESE FACILITIES DIFFERS FROM CONVENTIONAL PULVERIZED COAL FLY ASH IN THAT IT CONTAINS SUBSTANTIAL AMOUNTS OF SULFUR COMPOUNDS AND UNREACTED CALCIUM HYDROXIDE IN ADDITION TO THE COAL ASH-DERIVED FLY ASH. THE RELATIVELY HIGH RATIO OF CALCIUM HYDROXIDE TO FLY ASH IS THEORETICALLY VERY PROMISING SINCE BOTH ARE KNOWN TO FORM POZZOLANIC REACTION PRODUCTS SIMILAR TO CEMENT HYDRATION, SUGGESTING THAT THE ECONOMICS OF USING THESE FLY ASHES IN BLENDED CEMENT WOULD BE MORE FAVORABLE THAN THOSE FOR CONVENTIONAL FLY ASHES. HOWEVER, THIS POTENTIAL HAS NOT BEEN REALIZED TO DATE BECAUSE THE PROPORTION OF SULFUR COMPOUNDS IN THE DFGD WASTES IS TOO HIGH. THE PROPOSED RESEARCH WILL INVESTIGATE THE POTENTIAL FOR PRODUCING A SPECIAL CLASS OF CEMENTS, TERMED EXPANSIVE CEMENTS, USING A HIGH PROPORTION OF DFGD WASTES BLENDED WITH PORTLAND CEMENT AND ANOTHER SPECIFIC ADDITIVE. THIS ADDITIVE IS DESIGNED TO REACT WITH THE SULFUR COMPOUNDS IN THE DFGD ASHES AND RESULT IN A SMALL CONTROLLED EXPANSION OF THE CONCRETE BEFORE IT ACHIEVES A HARDENED STATE. THIS EARLY EXPANSION WOULD COMPENSATE FOR THE EVENTUAL DRYING SHRINKAGE THAT OCCURS IN ALL PORTLAND CEMENT CONCRETES, THEREBY REDUCING THE TENSILE STRESSES IN THE AGED CONCRETE THAT CAN LEAD TO CRACKING. DURING PHASE I, THE EFFECTS OF THE FLY ASH COMPOSITION (RANK OF COAL FROM WHICH IT ORIGINATED) AND THE RATIO OF THE EXPANSION ADDITIVE TO THE SULFUR CONCENTRATION IN THE DFGD ASHES WILL BE INVESTIGATED. BASED ON THESE TESTS, THE PRELIMINARY ECONOMICS OF PRODUCING A DFGD EXPANSIVE CEMENT WILL BE DEVELOPED AND MARKETING STRATEGIES FORMULATED. IF THE RESULTS OF PHASE I ARE ENCOURAGING, A PHASE II PROGRAM WILL BE DEVELOPED TO QUANTIFY THE ENGINEERING PROPERTIES OF CONCRETES PRODUCED FROM DFGD FLY ASHES AND DEMONSTRATE THAT ALL SPECIFICATIONS FOR EXPANSIVE CEMENTS REQUIRED BY APPLICABLE ASTM STANDARDS ARE SATISFIED.