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Dredge Material Stabilization Using the Pozzolanic or Sulfo-Pozzolanic Reaction of Lime By-Products to Make an Engineered Structural Fill

Large contracts are being awarded to remove and find beneficial uses for dredge spoil. An example is the Delaware Deepening Project which encompasses over 100 miles of the Delaware River. One of the challenges is to find economical and environmentally suitable means to stabilize and solidify (S/S) the solids so they can be reused for structural fill and/or cover soil. This lab study demonstrates the methodology and results of using three sustainable industrial by-products that can compete with Portland cement. They are lime kiln dust (LKD), Class F coal fly ash, and spray dryer ash (SDA). SDA is the residue from spray dryer absorbers, a more common type of advanced sulfur dioxide gas scrubber that uses lime. There are existing spray dryer installations in the Middle Atlantic States and several new units are coming on-line in Massachusetts and New Jersey. These spray dryers could provide an inexpensive S/S additive and an opportunity for CO2 credits. These industrial by-products were investigated to determine their potential for stabilizing and solidifying the dredge solids from the Cox Creek confined disposal facility (CDF) for Baltimore, MD with the objective of making a structural fill material. Another lab study utilized harbor dredged material from the USCOE Ft. Mifflin Confined Disposal Facility near Philadelphia, PA. The performance of the spray dryer ash is compared with a blend of LKD and Class F fly ash. Adequate lime alkalinity needs to be added to take advantage of the pozzolanic and sulfo-pozzolanic, cementitious reaction potential. Raising the pH levels to 9-11 for pozzolanic hydration reactivity coincide with pH levels that stabilize leachable heavy metals, including oxyanions. Furthermore, in order to produce structural fill the moisture of the dredge spoil must be reduced as close as possible to the optimum moisture content resulting in compaction at or near maximum dry density. The addition of these by-products will add dry bulk solids and in addition are known to chemically reduce the free moisture through several types of hydration reactions. A “mellowing” period before compaction may help prevent swelling from Ettringite precipitation. Strength measurements with several curing times are presented.