Thermodynamic modelling of phase evolution in alkali-activated slag cements exposed to carbon dioxide

Carbonation of cementitious materials induced by their interaction with atmospheric CO2 is one of the main degradation mechanisms threatening their durability. In this study, a novel thermodynamic model to predict the phase evolution of alkali-activated slags exposed to an accelerated carbonation environment is presented. This model predicts the phase assemblages of carbonated alkali-activated slag cements, as a function of CO2 uptake under 1 v/v % CO2 conditions, considering the bulk slag chemistry and activators used. The changes taking place during the carbonation process regarding the physicochemical properties of the main binding gel, an alkali calcium aluminosilicate hydrate (C-(N)-A-S-H), the secondary reaction products CaAl and MgAl layered double hydroxides, and amorphous aluminosilicate gels, were simulated and discussed. The predictions of the thermodynamic model are in good agreement with experimental data retrieved from the literature, demonstrating that this is a valuable tool for predicting long-term performance of alkali-activated slag cements

Estimating the thermodynamic properties (Δ Gof and Δ Hof ) of silicate minerals at 298 K from the sum of polyhedral contributions

Many physical properties of silicate minerals can be modeled as a combination of basic polyhedral units (Hazen, 1985, 1988). It follows that their thermodynamic properties could be modeled as the sum of polyhedral contributions. We have determined, by multiple regression, the contribution of the [4]A12O3,[6]A12O3, [6]Al(OH)3, [4]SiO2, [6]MgO, [6]Mg(OH)2,[6]CaO, [8-z]CaO, [6−8]Na2O, [8−12]K2O, H2O, [6]FeO, [6]Fe(OH)2, and [6]Fe2O3 components to the total ΔG0f and ΔH0f of a selected group of silicate minerals. Using these data we can estimate the ΔG0f and ΔH0f of other silicate minerals from a weighted sum of the contribution of each oxide and hydroxide component: ΔG0f = Σnigi ,  and ΔH0f = Σni hi, where ni is the number of moles of the oxide or hydroxide per formula unit and gi and hi, are the respective molar free energy and enthalpy contribution of 1 mol of each oxide or hydroxide component. The technique outlined here can be used to estimate the thermodynamic properties of many silicate phases that are too complex or too impure to give reliable calorimetric measurements. Experimentally measure ΔG0f and ΔH0f vs. predicted ΔG0f and ΔH0f for the minerals used in the model have associateda verager esiduals of 0.26% and 0.24% respectively. Thermodynamic properties of minerals not used in the model but for which there are experimentally determined calorimetric data have average differences between measured and predicted values of 0.25% for ΔG0f or 18 minerals and 0.22% for ΔH0f for 20 minerals