How does Na, K alkali metal concentration change the early age structural characteristic of kaolin-based geopolymers

Abstract The present work aims to investigate the influence of the alkaline activator concentration of kaolin-based geopolymers on the compressive strength at the curing early age regarding the structural characteristics. We used alkaline activators (Na, K) at molar concentrations from 4 M to 12 M, while the volume ratio sodium silicate/MOH was kept equal to 0.20 for all compositions. Samples were consolidated by curing at 70 °C for 24 h, and the mechanical and structural characterizations were obtained after 3 days of aging. Results show that the type and the concentration of the alkali-MOH solutions changed both the amount of reacted-kaolinite, the structural characteristics of gel phases, the formation of new crystalline phases, and the compressive strengths at the early age. For all compositions, the increase of MOH concentration led to an improvement of the compressive strength at the early age, attaining 88–165% of the standard value of 4 M MOH geopolymers. Optimized compressive strengths were 25 MPa and 32 MPa for [NaOH]=10 M and [KOH]=12 M respectively. We propose that an optimized alkali concentration is found with an adequate charge-balance of the Si–Al tetrahedral substitution, which prevents the formation of carbonated species from the alkaline excess.