Abstract
The structural evolution of a sodium carbonate activated slag cement blended with varying quantities of Mg(OH)2 was assessed. The main reaction products of these blended cements were a calcium-sodium aluminosilicate hydrate type gel, an Mg-Al layered double hydroxide with a hydrotalcite type structure, calcite, and a hydrous calcium aluminate phase (tentatively identified as a carbonate-containing AFm structure), in proportions which varied with Na2O/slag ratios. Particles of Mg(OH)2 do not chemically react within these cements. Instead, Mg(OH)2 acts as a filler accelerating the hardening of sodium carbonate activated slags. Although increased Mg(OH)2 replacement reduced the compressive strength of these cements, pastes with 50 wt% Mg(OH)2 still reached strengths of ∼21 MPa. The chemical and mechanical characteristics of sodium carbonate activated slag/Mg(OH)2 cements makes them a potentially suitable matrix for encapsulation of high loadings of Mg(OH)2-bearing wastes such as Magnox sludge.
Highlights
Alkali-activated slag cements are Portland cement-free binders produced through the chemical reaction between an alkaline activator and ground granulated blast furnace slag (GGBS)
The effects of GGBS replacement by Mg(OH)[2] on the reaction kinetics of a sodium carbonate-activated slag cement were determined by isothermal calorimetry, Fig. 1 and 2, along with X-ray diffraction (XRD) analysis (Fig. 3) of the pastes during the rst week of curing
Calcite was identi ed in samples with higher contents of Mg(OH)[2] (Fig. 3C and D). These results are consistent with what has been reported for sodium carbonate activated slag cements, where the Ca2+ released from the dissolving slag reacts with CO32À from the activator to form carbonate salts such as calcite and gaylussite, increasing the pH through the release of hydroxide ions.[4,6]
Summary
Alkali-activated slag cements are Portland cement-free binders produced through the chemical reaction between an alkaline activator and ground granulated blast furnace slag (GGBS). The effect of the addition of Mg(OH)[2] to alkali-activated binders has not been reported in the open literature, Collier et al.[25] identi ed that the addition of Mg(OH)[2] to Portland cement/slag cement composites potentially promoted the formation of higher contents of hydrotalcite. If this were to hold true for Mg(OH)[2] addition to alternative binders, this could interact with the cement, enhancing hydration and forming part of the binding phase, increasing the achievable waste loadings in an immobilisation context. Particular emphasis is laid on exploring and understanding the binding gels within this system, and the degree to which Mg(OH)[2] might have in uenced their chemistries, to assess the level of structural substitution of Mg-containing phases that is possible
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