Estimating long-term durability of geopolymer concrete containing waste shells in different sulfate environments towards sustainable development

The mechanical properties of geopolymer shell concrete (GPSC) with half of river sand replaced by oyster shells (OS) were observed to be similar to that of ordinary concrete. Further investigation is required about the performance of GPSC subjected to aggressive environments. This study examined the influences of salt solutions (Na 2 SO 4 and Na 2 SO 4 +NaCl), combined with wet-dry cycles, on the mechanical properties and SO 4 2- transport behavior of GPSC. The phase composition and pore characteristics were also investigated by XRD, FTIR, MIP, and SEM. Regardless of the exposure environment, the continuous polymerization reaction induced an increasing C,N-A-S-H gel, hence enhancing compressive strength and refining the pore structure. In the early stage of exposure to Na 2 SO 4 solution, the promotion of Na 2 SO 4 on polymerization and gel crystallization, as well as the inhibition of high Na + concentration on alkali leaching, contributed to further enhancing the improvements. In the later stage of exposure, the Na 2 SO 4 crystals, caused by the coupling effect of diffusion and water evaporation, led to microcracks formation, resulting in the peeling and damage of the GPSC. The SO 4 2- diffusion coefficient first increased and then decreased with increasing exposure time. The presence of NaCl significantly improved the performance of GPSC after exposure, resulting from the additional positive effect on C,N-A-S-H gel. The Cl - with mall size and greater migration rate disrupted SO 4 2- diffusion, alleviating the performance degradation of GPSC. The influence of SO 4 2- on Cl - attack GPSC was attributed to its impact on the gel and microstructure. Due to the similar adsorption mechanism, there was competitive adsorption between SO 4 2- and Cl - . Furthermore, the introduction of OS could alleviate the degradation of GPSC performance during the later stage of exposure, and GPSC with a larger strength grade exhibited greater performance stability after exposure.