Abstract
MgO contained in cementitious materials is experimentally observed to precipitate as poorly crystalline hydrotalcite (Mg-Al LDH). However, the geochemical modelling of hydrotalcite is challenging due to the lack of consistent thermodynamic dataset for this phase. Hydrotalcites with Mg/Al = 2 were synthesised in the presence of NaHCO3, NaCl, Na2SO4, NaNO3, and NaOH. Mass balance, XRD and FT-IR indicated the incorporation of the anions in the interlayer together with some carbonate. The crystallinity of the phase increased depending on the anion: SO42− < Cl− < NO3− < OH− < CO32−. An in-situ increase of temperature monitored by XRD and TGA showed that the stability of the hydrotalcite structure with temperature also depended on the incorporated anion. The solubility products were calculated based on the solution analysis of samples re-equilibrated at different temperatures, while the entropy and heat capacity were obtained from the additivity method or the molar volume. A simple solid-solution model for hydrotalcite containing CO3, OH, SO4, Cl and NO3 is suggested.
Highlights
In the context of lowering the 5 to 8% anthropogenic CO2 emissions from cement industry [1], from the Portland cement manufacture, alternative binders or partial replacement of the clinkers by so called supplementary cementitious materials (SCMs) are studied
The main layer can have a variable Mg/Al ratio [10,11], Al can be replaced by iron [12,13], and different interlayer anions can be contained in the interlayer [10]
The X-ray powder diffraction (XRD) data of the hydrotalcite samples (i.e. Ht-CO3, Ht-Cl, Ht-SO4, Ht-NO3, Ht-OH) synthesised at 80 ◦C are shown in Fig. 2; the main reflection peaks of hydrotalcite are indexed in Table 2 based on [41]
Summary
In the context of lowering the 5 to 8% anthropogenic CO2 emissions from cement industry [1], from the Portland cement manufacture, alternative binders or partial replacement of the clinkers by so called supplementary cementitious materials (SCMs) are studied. The use of reactive aluminosilicate containing SCMs, such as fly ash, blast furnace slag or calcined clays, increases the aluminium and silicate content, which can affect the phase composition in the hydrated cement and increase the Al and Si incorporation into the calcium (alumino-) silicate phases (C-(A-)S-H) [2]. The positive charge of the main layer is compensated by anions and water in the interlayer region [14]. Mg-AlLDH can be written as [Mgl-xAl x(OH)2]x+ [Ax/nn− mH2O]x− , with 0 < x < 0.33 [14]; “A” indicates the presence of anions such as OH− , Cl− , NO3− , CO32− , SO42− in the interlayer. The affinity is CO32− > SO42− for divalent anions and OH− > F− > Cl− > Br− > NO3− > I− for monovalent anions [14]
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