Hydraulic Pastes of Alkali-Activated Waste Glass and Limestone Cement Using in Situ Caustification

One-part waste glass (WG) and limestone (LS) alkali-activated cements were investigated in pastes using powdered activators CaO+Na2CO3. 9 formulations of pastes were selected from a full factorial experimental design with %LS, %Na2O-Na2CO3 and %CaO as factors. The activators promoted an in situ caustification that dissolved the WG amorphous structure leading to the formation of cementitious reaction products. The pastes developed 1-year strengths up to 35 MPa and were stable underwater. The characterization by SEM, EDS and Si-29-NMR indicated that while using a Na2CO3:CaO ratio close to 1:1, the main reaction products were polymerized C-S-H, silica gel and modified silica gel, intimately intermixed and crosslinked through Q(3) bonds. Such coexistence of reaction products was better in terms of strength and underwater stability than those coexisting in pastes of WG+LS systems activated with solutions of only NaOH reported in literature. Moreover, curing underwater promoted the formation of shorter silicon-oxygen chains, suggesting more formation of C-S-H; nonetheless, the considerable presence of Q(3) and Q(4) signals along with a 1-year compressive strength of 32MPa in pastes cured underwater evidenced that most of the silica gel was stable. The results indicated that in situ caustification is a promising one-part activation for SiO2-rich precursors such as waste glass to develop hydraulic alkali activated cements with low cost and CO2 emissions.