Abstract
Hydrated aluminosilicates were synthesized with and without aqueous heavy metals (Me), such as cobalt (Co), chromium (Cr), and zinc (Zn), by a sol–gel process at different initial molar ratios of Ca/(Si + Al) (0.6–1.6) and Me/Si (0.0–2.0), and constant Al/Si ratio (0.05) using equilibrium-approaching experiments. The chemical composition of the reactive solutions during aluminosilicate precipitation and maturation was monitored by ICP-OES. The mineralogy, nanostructure, and chemical composition of the precipitates were studied by XRD and high-resolution TEM. At Me/Si ratios ≤ 0.2, calcium–aluminium–silicate–hydrates (C–A–S–H) with a defect 14 Å tobermorite-like structure formed, whereas at a Me/Si ratio of 2.0, either trioctahedral Co- and Zn-smectite or amorphous Cr gels precipitated, independent of the initial Ca/(Si + Al) ratio used for gel synthesis. The immobilization capacities for {text{Co}}^{2 + } , {text{Cr}}^{3 + } , and {text{Zn}}^{2 + } by C–A–S–H, Cr gel, and trioctahedral smectite are 3–40 mg/g, 30–152 mg/g, and 122–141 mg/g, respectively. The immobilization mechanism of heavy metals is based on a combination of isomorphous substitution, interlayer cation exchange, surface (ad)sorption, and surface precipitation. In engineered systems, such as underground concrete structures and nuclear waste disposal sites, hydrated aluminosilicates should exhibit a high detoxication potential for aqueous heavy metals.
Highlights
Importance of calcium–aluminium–silicate– hydrate phasesAluminium-substituted calcium-silicate-hydrates (C– A–S–H) are of great technological importance, because they represent the main hydration product in ordinary Portland cement (OPC) and significantly contribute to the physicochemical characteristics, mechanical properties, and durability of hardened cementitious materials, such as concrete and shotcrete [1,2,3,4,5,6,7]
This paper reports on the synthesis and on the characterization of C–A–S–H and trioctahedral smectite precipitated at different molar ratios of Ca/(Si ? Al) (0.6 to 1.6) and Me/Si (0.0 to 2.0), and at a molar Al/Si ratio of 0.05
Temporal changes in the chemical composition of the reactive fluids are displayed in Fig. 2 for experiments conducted at initial molar Me/Si ratios of 0.2 and Ca/(Si ? Al) ratios of 1.0
Summary
Aluminium-substituted calcium-silicate-hydrates (C– A–S–H) are of great technological importance, because they represent the main hydration product in ordinary Portland cement (OPC) and significantly contribute to the physicochemical characteristics, mechanical properties, and durability of hardened cementitious materials, such as concrete and shotcrete [1,2,3,4,5,6,7]. Besides the pronounced positive effects of new cement blends containing SCMs on the technical and durability properties, and on ecological aspects, i.e. reduction of the global CO2 emissions from the cement industry sector [20], their increasing production may pose an environmental risk: SCMs usually contain higher contents of heavy metals than pure OPC [21]. It is important to know how these critical elements are fixed in the hardened cement paste
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