The growing demand for sustainable solutions in the construction sector has driven research into the use of industrial waste as alternatives to conventional materials. This study investigated the utilization of kaolin industry residues in the production of geopolymers, focusing on the substitution of soft metakaolin with flint metakaolin and conventional sand with sandy kaolin residue in geopolymer mortars. The research was conducted in three stages: first, a physicochemical characterization of the raw materials was performed, including tests to determine mineralogical, chemical, granulometric compositions, and specific mass. In the second stage, the substitution of soft metakaolin with flint metakaolin at different percentages (0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%) was evaluated, and the compressive strength of geopolymer pastes was analyzed. Although a linear relationship between the increase in flint metakaolin substitution and compressive strength was not observed, the highest strength (52 MPa) was achieved with the full substitution of soft metakaolin by flint metakaolin. In the third stage, geopolymer mortars were produced by combining geopolymer paste with aggregates, testing substitution percentages of conventional sand with sandy kaolin residue (0%, 25%, 50%, 75%, and 100%). Physical tests revealed that substituting 50% of the conventional aggregate with sandy residue resulted in a more compact matrix with lower water absorption, higher specific mass, and reduced porosity. The results showed that replacing 50% of conventional sand with sandy residue led to the highest compressive strength (46 MPa), suggesting that this proportion enhanced matrix densification and improved interaction between the residue and the paste. Morphological analysis using SEM confirmed that the formulation with 50% sand substitution presented a densely compacted matrix with good cohesion between the paste and aggregates and an adequate distribution of N-A-S-H and C-A-S-H gels. These findings highlight that partial replacement of sand with sandy kaolin residue improves the densification and strength of geopolymeric mortars while demonstrating the potential of flint metakaolin and sandy residue as sustainable alternatives in construction.
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