CO2 binding capacity of alkali-activated fly ash and slag pastes

Abstract

Quantification of the CO2 binding capacity of reinforced concrete is of high importance for predicting the carbonation potential and service life of these structures. Such information is still not available for alkali activated materials that have received extensive attention as a sustainable substitute for ordinary Portland cement (OPC)-based concrete. To address this gap, this paper evaluates the CO2 binding capacity of ground powders of alkali activated fly ash (FA) and ground granulated blast furnace slag (GBFS) pastes under accelerated carbonation conditions (1 % v/v CO2, 60% RH, 20°C) for up to 180 days. The CO2 binding capacity, the gel phase changes, and the carbonate phases are investigated with complementary TG-DTG-MS, FT-IR and QXRD techniques.Five mixtures with different FA/GBFS ratio are considered. CEM I and CEM III/B pastes are also studied to provide a baseline for comparisons. The results showed that the alkaliactivated pastes have a lower CO2 binding capacity in comparison to cement-based pastes. Furthermore, alkali-activated pastes have similar CO2 binding capacity regardless of the FA/GBFS ratio. It was observed that the silicate functional groups corresponding to the reaction products in the pastes were progressively changing during the first 7 days, after which only carbonate groups changed. It was also found that the CO2 bound in the alkali-activated pastes occurs to a substantial extent in amorphous form.

Effect of curing conditions on the pore solution and carbonation resistance of alkali-activated fly ash and slag pastes

“…Beside alkalis (Na), calcium ions may also contribute to maintaining a high pH of the pore solution. This is supported by the authors' previous study [37], where it was shown that not all the CaO was consumed by carbonation of studied alkali activated pastes. This implies that, after NaOH carbonation, the remaining CaO can act as a buffering agent for the pH of the pore solution.…”

Properties of Alkali-Activated Slag Paste Using New Colloidal Nano-Silica Mixing Method

“…The reason is that unlike OPC, AAC has no portlandite [30], so the carbonate process is affected by various factors. To date, the carbonation of AAC has been associated with the pH reduction of the pore solution, the precipitation of Na-rich carbonates and the decalcification of Ca-rich phases (C-S-H gel) [56,57]. The CaCO 3 , which is formed as a result of carbonation, is classified into crystalline phases and an amorphous phase [57].…”

“…To date, the carbonation of AAC has been associated with the pH reduction of the pore solution, the precipitation of Na-rich carbonates and the decalcification of Ca-rich phases (C-S-H gel) [56,57]. The CaCO 3 , which is formed as a result of carbonation, is classified into crystalline phases and an amorphous phase [57]. Therefore, CaCO 3 , which is distinguished by calcite, vaterite and aragonite, may be different from XRD and TG/DTG [57].…”

“…The CaCO 3 , which is formed as a result of carbonation, is classified into crystalline phases and an amorphous phase [57]. Therefore, CaCO 3 , which is distinguished by calcite, vaterite and aragonite, may be different from XRD and TG/DTG [57]. The weight loss in the region of 665–800 °C observed in Figure 8 is estimated to be the weight loss due to the carbonate, but the analysis of causes and trends is not clear in the scope of this study.…”

Effects of CO2 Curing on Alkali-Activated Slag Paste Cured in Different Curing Conditions

“…This weight loss is likely due to the decomposition of amorphous calcium carbonate (CaCO 3 ∙xH 2 O [36]) rather than vaterite, which is because the peaks for vaterite was not detected in the XRD result as shown in Figure 5f. Amorphous CaCO 3 was reportedly decomposed at temperatures between 245 °C and 645 °C [37].…”