Alkaline activation of high-crystalline low-Al2O3 Construction and Demolition Wastes to obtain geopolymers

Abstract Construction and Demolition Wastes (CDW) with low contents in Al2O3 (4–6%) and amorphous phase (16–27%) have been investigated for geopolymerization. The proportion of alkaline activators (NaOH and sodium silicate (SS)), the temperature and time of initial treatment, the curing conditions (dry, wet or sealed) and the addition of Portland cement were assessed. The low reactivity of the samples has been especially evident in those rich in carbonate aggregate, something that is not only reflected in the low compressive strength obtained (6–8 MPa), but also in the appearance of efflorescence. Temperature application (60°C–90 °C) and dry/sealed curing have generally improved results. In the case of the siliceous CDW, 28-day compressive strengths of 11–15 MPa have been reached in 20 mm-side cubic specimens and 21.5 MPa in brick prototypes. The optimum was estimated at SS/NaOH = 3.6 and 37.9% activator. FTIR, SEM and Rietveld-refined XRD analyses have corroborated the geopolymer gel formation.