The microstructure and mechanical properties of recycled metakaolinite-based geopolymer: Dependence of recycled powder replacement ratio

Recycled geopolymer powder (RGP) is an environmentally friendly substitute for precursors in geopolymer preparation. In this study, the influence of RGP with different compressive strengths (10 and 30 MPa, labeled RGP10 and RGP30, respectively) and substitution ratios (5%, 10%, 30%, 50%, and 100%) on the microstructure and properties of metakaolinite-based geopolymers was investigated. The chemical structure and composition of the recycled geopolymer were characterized by X-ray diffraction and Fourier-transform infrared spectroscopy. The microstructure was characterized by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and mercury intrusion porosimetry. The results showed that the incorporation of RGP10 at a low substitution ratio (≤10%) destroyed the microstructure of the resulting geopolymer, thereby decreasing its compressive strength. The geopolymer formed by substituting RGP30 in ratios of ≤10% exhibited a dense microstructure due to the filling effect, and the 28-d compressive strength increased by 9.5% when the substitution ratio was 10%. At high substitution ratios (≥10% and ≤50%), the compressive strength of all geopolymers decreased due to the low reactivity of RGP. However, the decrease in the compressive strength of geopolymer induced by RGP10 was less notable than that of RGP30, as the presence of a larger amount of unreacted metakaolinite in the RGP10 scenario let to the formation of more geopolymer during alkali activation. Notably, the alkali-activated RGP exhibited a low compressive strength (approximately 3.0 MPa) regardless of its original strength. These observations indicate that both the substitution ratio and original strength of RGP significantly influence the microstructure and mechanical properties of metakaolinite-based geopolymers. Additionally, the incorporation of an appropriate amount of RGP can help enhance the properties of metakaolinite-based geopolymers. •Effect of recycled geopolymer powder (RGP) on the metakaolinite (MK)-based geopolymer is studied.•Adding proper amount of RGP is helpful to improve the properties of MK-based geopolymer.•At high replacement ratios, the RGP decreased the compressive strength.•The replacement and properties of parent geopolymer influence the microstructure.