Micro-characterizations and geopolymerization mechanism of ternary cementless composite with reactive ultra-fine fly ash, red mud and recycled powder

To find cementitious composites that can be suitable for superseding Portland cement and alleviate the demerits of it for the environment, this study develops newly cementless ternary geopolymer composites of red mud (RM)-reactive ultra-fine fly ash (RUFA)-recycled powder (RP) activated by sodium hydroxide and sodium silicate, and using different mixing ratios as well as curing conditions. Then, the microstructural properties, pore structures, mechanical properties and geopolymerization reactions of them were studied systematically. Herein, we successfully synthesize cementless ternary geopolymers with the excellent workabilities and mechanical properties, which could be comparable to traditional cement. Their compressive strength can reach up to 46 MPa and the strength enhancements were mainly attributed to gel products such as N-A-S-H, C-S-H and (N, C)-A-S-H being found in specimens, which were interwoven to form a dense microstructure. Besides, the disparities of RM and RP contents in matrix would result in the transformation of gel products: C-A-S-H and Fe-silicate gels were mainly presented in specimens with high RM, N-A-S-H gels in specimens with high RUFA, and (N, C)-A-S-H gels specimens in specimens with high RP. These gels facilitated to reduce of dry shrinkage by refining the pore size. All these results contribute to a better understanding of the chemical properties of alkali-activated materials and facilitate their applications.