Abstract
The impact of adding two types of layered double hydroxides (LDHs), commercial hydrotalcite (HT) and its thermally treated form (CLDH), on the reaction kinetics and phase assemblage development of a sodium silicate-activated slag cement was investigated. The reaction kinetics of LDH-modified cements was assessed by isothermal calorimetry, and the results were correlated with in situ attenuated total reflection Fourier transform infrared spectroscopy results collected over the first days of reaction, to identify the structural evolution of the main binding phase forming in these cements: a sodium-containing calcium aluminosilicate hydrate (C-(N)-A-S-H)-type gel. The addition of either HT or CLDH into sodium silicate-activated slag paste accelerates the precipitation of reaction products and increases the formation of HT in these cements, without causing significant changes to the C-(N)-A-S-H binding phase. This is extremely relevant in terms of the durability of alkali-activated slag cements, as a higher content of the HT-like phase has the potential to reduce their chloride permeability and enhance carbonation resistance.
Highlights
Hydrotalcite (HT)-like (Mg,Al)-Layered double hydroxides (LDHs) are commonly identified as a reaction product which forms in these cements when the magnesium oxide (MgO) content in the raw material is higher than 5 wt.%, being generated along with the sodium-rich calcium aluminosilicate hydrate (C-(N)-A-S-H)-type gels that dominate the structure of these cements.[14,15,16]
There is a linear trend in the data for all samples, yielding a magnesium/aluminium ratio of around 2. This matches the magnesium/aluminium ratio of the LDH phase that is normally observed in long-term-cured alkali-activated slag samples, which is around 2·1.20,53 This suggests that the addition of either Calcined layered double hydroxides (CLDHs) or HT does not seem to have any significant impact on changing the layer cation compositions of the HT-like phases forming in silicate-activated AAS, consistent with the observations by X-ray diffraction (XRD) (Figure 4) for these cements. Both HT and CLDH additives have a similar effect in accelerating the kinetics of the reaction of AAS cement, shortening the induction periods and accelerating the formation of C-(N)-A-S-Htype gels
The incorporation of HT did not seem to affect the total heat release compared with the reference mix, which suggests that the changes in the kinetics of the reaction induced by this additive are mainly associated with a filler effect
Summary
Alkali-activated cements are gaining interest as a sustainable and technically sound alternative to traditional Portland cement for the production of concretes.[12,13] Alkali-activated slag cements are formed by the reaction between blast-furnace slag and an alkaline activator. Hydrotalcite (HT)-like (Mg,Al)-LDHs are commonly identified as a reaction product which forms in these cements when the magnesium oxide (MgO) content in the raw material is higher than 5 wt.%, being generated along with the sodium-rich calcium aluminosilicate hydrate (C-(N)-A-S-H)-type gels that dominate the structure of these cements.[14,15,16]
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