Recommended Practice for Stabilization of Sulfate-Rich Subgrade Soils (2009)

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3 also detailed in the report. As will be discussed later, it is possible that providing high concentrations of soluble silica from fly ash or, for example, ground granulated blast furnace slag (GGBFS) can provide a ready source of soluble silica which will alter the thermodynamic mechanisms to favor the formation of cementitious products rather than ettringite and/or rapidly lower the pH of the system so that additional ettringite will not form (17). The chemical composition among fly ashes varies considerably, and therefore whether or not a specific ash will impact ettringite formation is a complex issue. CHEMISTRY OF ETTRINGITE AND THAUMASITE Ettringite is a hydrous calcium alumino-sulfate mineral that precipitates under alkaline (high pH) conditions in soil and concrete systems with high sulfate activity (4, 11, 14). A categorical explanation of the mechanism of formation of these minerals was first provided by Hunter (2) and was based on the geological, geochemical, and mineralogical reasons for their precipitation. Chemistry of ettringite formation is given in equation [1]. SOOHAlCaOHSOOHOHAlCa 342662 2 44 2 26.).(])([26)(34)(26 →++++ −−−+ ...(1) Resources needed to form ettringite are made available partly by the additive and partly by minerals present in soil. For 1 mole of ettringite to form 6 moles of CaO, 1 mole of Al2O3, 3 moles of SO4, and 32 moles of water are required. Calcium ions are provided by lime, Portland cement, or fly ash; alumina is supplied by dissolution of oxyhydroxides and phyllosilicates; and sulfates are supplied by dissolution of gypsum, oxidation of sulfide, or ion migration of sulfate ions as water diffuses through the matrix (2-5). Even though these components may be available, thermodynamic favorability and concentration of limiting reagents control the precipitation of ettringite and thaumasite (3). Although several calcium-aluminum-sulfate hydrates can form during stabilization, only mono-sulfate hydrate and tri-sulfate hydrate forms are stable in solution (18). Formation or hydration of the mono-sulfate phase does not result in expansion, whereas the formation and/or hydration of the tri-sulfate phase can cause significant volume changes in stabilized layers. The silica-bearing analog of ettringite, thaumasite, is a complex calcium carbonate silicate sulfate hydrate mineral represented by the structural formula ]24)()(])([[ 22423266 OHSOCOOHSiCa • which, as previously stated, probably proceeds from an isostructural alteration of ettringite in the presence of carbonates and soluble silica as described in equation [2]. OHOHSOOHAlOHCOSOOHSiCa OCOSiOHOHSOOHAlCa 2 2 4422324266 2 2 3 2 42234266 24)(224)()(])([ 2)(226)(])([ ++++••• →+++•• −−− −− …(2) A low temperature, generally considered to be 15°C, and intensive carbonation are required to form and maintain thaumasite as a stable phase in the matrix (2, 13). Along with carbonates and sulfate ions, thaumasite consists of silicon, which is generally considered to be primarily derived from the decomposition of C-S-H formed during cementing reactions or possibly pozzolanic reactions in treated soils. A lesser concentration of reactive alumina combined with a higher calcium and sulfate content favor the decomposition of cementitious materials. Remnants of