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Effects of aluminosilicate minerals in clay soil fractions on pore water hydroxide ion concentrations in soil/cement matrices
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Cementitious solidification/stabilization processes are often appropriate for treatment of soils contaminated with metallic constituents. However, thorough knowledge of potential impacts of soil properties on solid and solution phase characteristics and, consequently, metal attenuation characteristics of soiUcement/waste matrices is necessary to accurately translate bench-scale test results to full-scale applications. Research on soil stabilization indicates that pozzolanic reactions can occur between clay minerals and calcium hydroxide. Pozzolanic reactions between aluminosilicate minerals in soils and portiandite or calcium silicate hydrate, generated by cement hydration, may significantly reduce the hydroxide ion concentration in soiUcement/waste matrices. Research described herein was undertaken 1) to ascertain the pH decrement in soil/cement matrices as a function of clay:cement ratio and 2) to develop a methodology to predict hydroxide ion concentrations in soil/cement matrices. To assess effects of pozzolanic reactions in soil/cement matrices, synthetic soil/cement mixtures with a range of clay:cement ratios were cast and cured for various time periods up to about 280 days. Mixtures of sand and commercially available sodium bentonite, a form of montmorillonite, with 0. 5, 10, 20, and 40 percent bentonite by total, air-dry weight were mixed with Type I Portland cement at 10, 20, and 30 percent cement by weight of air-dry soil. Pore water was expressed and analyzed for hydroxide, calcium, sodium, potassium, and silica. Pore water data were input to SOLTEQ-B, a chemical equilibrium speciation model, to ascertain pH levels and composite solid calcium:silicon ratios. Results indicate that pozzolanic reactions reduce pore water pH levels in montmorillonite/cement matrices by one unit when clay:cement ratios exceed 0.5. Pozzolanic reactions are nearly complete within 32 days. However, a slow, ancillary reaction, possibly sorption of hydroxide ions onto low calcium:silicon ratio calcium silicate hydrate in conjunction with potassium and sodium, may require up to 137 days to attain metastable equilibrium. Composite calcium:silicon ratios can be adequately estimated assuming that montmorillonite reacts with portlandite and calcium silicate hydrate to a calcium:silicon ratio of 0.83.
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Includes bibliographical references: p.43-47.
Issued also on microfiche from Lange Micrographics.
Cook, Evan Russell (1998). Effects of aluminosilicate minerals in clay soil fractions on pore water hydroxide ion concentrations in soil/cement matrices. Master's thesis, Texas A&M University. Available electronically from
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