Design of inorganic polymer cements: Effects of matrix strengthening on microstructure

The effects of bulk composition and microstructure on strengthening mechanisms and fracture resistance were investigated for metakaolin based geopolymers. Crack initiation and propagation under mechanical and environmental stresses were correlated to pore size distribution and bulk composition. Based on IR spectra and porosity analysis, it was observed that capillary pores and the nature of alkali-aluminosilicate hydrated named polysialates (H–M–A–S) determine cracks initiation site in the Al-rich geopolymer samples with low Si/Al molar ratio (range 1.23–1.5). These samples exhibited low flexural strength, Young modulus and Impact toughness as well as delayed failure under environmental stresses. Increasing the Si/Al molar ratio (range 1.79–2.42) improved the flexural strength and Impact toughness. Formation of more polymerized H–M–A–S phases contributed to strengthening the matrices and hinder axial cracks; consequence of toughening mechanisms developed by the coarsening on molecular scale of H–M–A–S.