High-volume, high-value usage of flue gas desulfurization (FGD) by-products in underground mines. Quarterly report, October--December 1994

Research under Subtask 2.2, Chemical and Mineralogical Characterization, included further refinement of mineralogical transformation and the initiation of a kinetic study. The expansion of the FGD materials during moisturizing is attributable to three reactions: the hydration of portlandite to slaked lime; the formation of ettringite from fly ash and anhydrite, and; the formation of gypsum from anhydrite. The sequence of these reactions are being examined in a kinetic study. Completion of the first 15 days of study finds the steady decrease in anhydrite with concomitant formation of ettringite (on fly ash surfaces) and gypsum (pore and crack in-fillings). Geotechnical characterization (Subtask 2.3) focused on swell experiments which will model in situ emplacement. Specimens of FGD material have been stored in 3-inch diameter pipe and, after 39 days, 0.5% of axial swell has been recorded with material strengths of 600 to 1,000 psi. Experiments to determine the amount of moisture loss due to the heat of hydration indicate about 9 to 10% of the water is lost. Confined swell tests are also underway with pressures of 15 to 20 psi recorded at 25 days. Work performed under Task 4 (Background for Phase 11) included determination of the compressive strengths for the experimental mine roof rock. Values in the 5,000 to 7,500 psi range were found, which is typical for this type of strata in the region. Work on the hydrologic monitoring program (Subtask 4.2) included completion of the hydraulic conductivity assessment of the strata, as well as completion of the monitoring well plan. The highest hydraulic conductivity was found for the Princes No. 3 coal seam with values of 1x10{sup -3} feet/min. The weathered sandstone over the coal had conductivities in the 10{sup -4} to 10{sup -5} feet/min range