Fresh and hardened properties of ternary geopolymer cementitious materials based on terracotta clay, fly ash and GGBS

Fly ash-based geopolymers are considered sustainable substitutes for traditional Portland cementitious systems. In this study, workability, rheological behaviour, mechanical performance, and microstructural evolution of ternary geopolymer blends comprising fly ash (FA), ground granulated blast-furnace slag (GGBS), and terracotta clay (TC) were examined. The TC dosage was adjusted to achieve improved compressive strength without sacrificing workability. Following uniaxial compressive strength testing, an investigation was performed using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and thermogravimetric analysis (TGA) to confirm the formation of an aluminosilicate gel network with tetrahedrally coordinated Al 3+ and charge-balancing K⁺ ions. Results showed that the addition of TC reduced the loosely bound fly ash particles and helped to densify the matrix. The rheological behaviour of geopolymer blends confirmed Bingham-type fluid characteristics. The crucial role of TC in modifying gel formation and structural development was highlighted by establishing relationships between rheological parameters, microstructural characteristics, and compressive strength. The proposed ternary geopolymer mixes with thermally processed TC confirm the feasibility as a low-carbon precursor in FA-based binders.