Incorporation of oxyanionic B, Cr, Mo, and Se into hydrocalumite and ettringite, application to cementitious systems

Abstract

B, Cr, Mo, and Se are often enriched in solid wastes such as fly ash and spent oil shale, and occur at high concentrations in their leachates. These elements are usually present as oxyanions, and are mobile at the near neutral to alkaline pH values of most natural waters. Therefore, mechanisms for removing these oxyanions from wastewaters are of great importance. Hydrocalumite (Ca4Al2(OH)12X6H2)) and ettringite (Ca6Al2(OH)12X326H20) are commonly formed as secondary phases during the hydration of fly ash in alkaline solutions. They also comprise 10 to 20% of the hydration phases formed in Portland cement. Previous research has shown that these phases incorporate substantial amounts of a wide variety of anoins from solution. Precipitation of these mineral phases could be an effective mechanism for removing oxyanions from wastewaters.

Secondary phases formed from the interaction of fly ash and lime water were characterized as a function of reaction time by direct examination of the solid phases using optical microscopy, X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). Solution compositions were also monitored during leaching, to establish the correlation between the solution composition and the development of secondary phases. Ettringite formed at early reaction times in all the fly ashes examined (two Class F and one Class C fly ash). The ettringite crystals contained a high percentage of Si, indicating that they are solid solutions between ettringite (Ca6Al2(OH)12(SO4)326H20) and thaumastite (Ca6Si2(OH)12(SO4)2(CO3)224H20). With increasing reaction time, ettringite transformed to hydrocalumite in the Class F fly ashes, and to stratlingite (Ca2Al(OH)6[AlSiO3(OH)2]4H20) in the Class C fly ash. This is the first time that the transformation of ettringite to hydrocalumite has been directly observed by examination of the minerals. The observations suggest that under the experimental conditions, this transformation is a thorough-solution process. However, solid-state conversion from ettringite to hydrocalumite may also occur locally within a hydrocalumite crystal.

A substantial decrease in the solution concentrations of SO4, B, Mo, and Se occurred when lime was added to the fly ashes. At early reaction times, ettringite incorporated these anions into its structure. With increased reaction time, ettringite converted to hydrocalumite , and a further reduction in the concentrations of the anions was observd. Thi implies that incorporation of these anions by hydrocalumite could lead to lower residual solution concentrations than incorporation by ettringite. However, in the Class C fly ash, the lowest B concentration was coincident with the maximum formation of ettringite. The increase in the B solution concentrations observed at later reaction times is likely related to the transformation of ettringite to stratlingite, and indicates that ettringite incorporates more B than stratlingite.

The uptake behavior of B, Cr, Mo, and Se in pure ettringite-water and hydrocalumite-water systems was studied in order to clarify the relationship between the decrease in anion concentration and the precipitation of these phases in the lime-leached fly ashes. Hydrocalumite and ettringite were precipitated from solutions containing 10 ppm B, Cr, Mo, and Se. The residual solution concentrations of Cr, Mo, and Se were below detection after incorporation into hydrocalumite, and the C concentrations were below detection after incorporation into ettringite. The anion preference by ettringite was in the order: B(OH)4>SeO4^2- > CrO4^2- > MoO4^2-. In ettringite, anion size and electronic configuration are the key factors that influence the extent of anion uptake. It was not possible to establish an order of preference of hydrocalumite for these anions, except that borate was least preferred. The low uptake of B appears to be caused by a change in its coordination from B(OH)4 to HBO3^2- during incorporation.

The stability and solubility of the hydrocalumite phases were determined by synthesizing solid solutions with OH as one endmember and B, Cr, Mo, Se and SO4 as the other endmembers. After 9 to 18 months of reaction, the solid phases were characterized and the solution compositions determined. In the ternary CaO-Al2O3-H2) system, hydrogarnet was the dominant mineral phase. OH-hydrocalumite was also present, and a free energy of formation (LlG 0 r. 298) of -7336.14 ± 1.14 kJ·mor1 was calculated for this phase. At oxyanion contents greater than I 0%, hydrocalumite became the dominant phase in all the quaternary systems CaO-Al2O3-XO3/Y2O3-H2O (X = Cr6 \ Mo6-, Se6 -, and S6 -; Y = B3 -). With an increase in the molar percentage of the anion, three different phase assemblages were generally observed: I) hydrocalumite, hydrogamet, and portlandite; 2) hydrocalumite and hydrogamet; and 3) hydrocalumite and ettringite at the high end. However, ettringite was not observed in either the chromate or molybdate solid solution series. A free energy of formation of-17408.22 ± 4.34 kJ·mor1 was calculated for the borate ettringite (C~Ali(OH)dB(OH).i].i [OH]:?· 24H2O).