Effect of inorganic retarders on setting, mechanical properties, and microstructure of fly ash-slag geopolymer mortars under hot-weather conditions.

Under hot-weather construction conditions, the rapid setting problem of geopolymer mortar as a sustainable material severely restricts its practical application. This study systematically investigated the retarding behavior of representative inorganic salts (ZnCl2, Na2SO4, BaCl2) in fly ash and slag-based geopolymer mortar at 30-50°C using multi-scale characterization. The results revealed three distinct retardation pathways: ion competition depletion (ZnCl2), precipitation equilibrium regulation (Na2SO4), and interfacial barrier-phase separation (BaCl2). Among the three, BaCl2 exhibited the strongest retarding effect, extending the final setting time from 12.09 min to 139.57 min at 6% content. Notably, all three salts can be sourced from industrial by-product streams or recovered from wastewaters, rather than from primary production, which underscores their circular-economy potential. Life cycle assessment shows that geopolymer mortars containing retarders achieve 24-26% lower embodied carbon and 8-13% lower material costs than OPC mortar. These findings establish a resource-efficient design framework for inorganic retarders, with the key implication that future research and practice should prioritize direct procurement from industrial wastes to maximize both workability control and environmental benefits.