Compatibility studies between N-A-S-H and C-A-S-H gels. Study in the ternary diagram Na2O–CaO–Al2O3–SiO2–H2O Review Article

Sodium aluminosilicate hydrate (N-A-S-H) gel, the main reaction product of the alkali-activated aluminosilicates, differs of the aluminium-modified calcium silicate hydrate (C-A-S-H) gel of PC pastes. Increasing the level of SCM to reduce PC content of binders are being considered to address reduction in CO2 emissions, activation of the additional SCM content by alkali activation represents a possible environmentally sustainable solution. Therefore, mixtures of C-A-S-H and N-A-S-H gels might be anticipated and the present study assesses the compatibility relationships between them. Compositional diagrams are provided to indicate phase compositional ranges and the phase assemblages obtained under equilibrium conditions. In calcium-rich formulations (pH in excess of 12), C-A-S-H and C2ASH8 form as stable phases. However, in the lime poor part of the diagram an amorphous gel (N,C)-A-S-H precipitates but its stability is dependent on system pH and available Ca. (N,C)-A-S-H gels are de-stabilised by Ca to give C-A-S-H gels in suitable systems.