Quantification of chloride diffusion in fly ash–slag-based geopolymers by X-ray fluorescence (XRF)

• XRF technique is reliable in quantifying Cl− concentration in geopolymer.
• Fly ash-only geopolymer usually has lower resistance to Cl− ingress than PC.
• Slag substitution can reduce chloride penetration rate.
• Rethink about the durability of reinforced fly ash-based concrete.

Chloride diffusion in cementitious materials is one of the key factors affecting the durability performance of concretes. This paper reports an investigation on the quantification of chloride diffusion in fly ash–slag-based geopolymer pastes. X-ray fluorescence (XRF) technique was used to determine the chemical compositions of the samples after short-term ponding test (contact with the 3% NaCl solution for 72 h). The effect of the coexistence of Al-substituted calcium silicate hydrate (C-A-S-H) gel and aluminosilicate geopolymeric gel on the ingress of chloride was evaluated. The results showed that incorporation of slag as a secondary precursor in fly ash-based geopolymers led to the refinement of the pore structure, reducing both sorptivity of the pastes and the ingress of chloride ions. More compact C-A-S-H gel formed in geopolymer pastes with partial slag substitution led to lower permeability compared with the porous aluminosilicate gel that formed in the full fly ash-based geopolymer paste, contributing to the slower transportation of chloride in the fly ash–slag blended geopolymer pastes. The fly ash–slag-based geopolymer paste with 50% of slag performed better than the Portland cement paste in terms of resistance to chloride ingress.