Properties and environmental assessment of eco-friendly brick powder geopolymer binders with varied alkali dosage

This paper presents the effect of alkali dosage on the mechanical properties and water resistance of alkali-activated brick powder geopolymers (BPG) by measurements of compressive and flexural strengths, bulk density, water absorption and softening coefficient. To reveal the micromechanism of BPG under varied alkali dosage, the mineralogical phases, reaction degree, micromorphology, and pore structure of BPG are analyzed by techniques of XRD, TGA, SEM and MIP, respectively. Furthermore, the environmental impacts of BPG are evaluated by considering CO2-e emission and energy consumption. Results show that the BPG prepared with low alkali dosage (2% or 4% Na2O) exhibits the loose microstructure with few amorphous gels and relatively high porosity (including high proportion of capillary pores) resulting in the relatively poor mechanical properties and water resistance. However, a higher alkali dosage (6% or 8% Na2O) promotes the geopolymerization reaction resulting in the denser microstructure with more amorphous gels and relatively lower porosity, thereby greatly enhancing the mechanical properties and water resistance with maximal flexural strength, compressive strength and softening coefficient of 2.2 MPa, 31.1 MPa and 0.77 respectively. Water immersion may cause Na+ and OH to dissolve from BPG matrix resulting in a charge imbalance of three-dimensional network structures, which will further lead to strength degradation. In comparison to ordinary Portland cement paste, BPG can reduce CO2-e emission by about 40%-70% and energy consumption by about 20%-50%. The optimal mixing parameters have an alkali dosage of 6%, a silicate modulus of 1.6 and a water-to-binder of 0.3 by considering the compressive strength and environmental impacts.