Mechanical and microstructural behavior of earthen materials treated with ceramic waste powder and volcanic ash-based geopolymer binders for compressed brick production

This study proposes an environmentally sustainable approach for stabilizing high-plasticity clay through a hybrid geopolymer binder produced from ceramic waste powder (CWP) and volcanic ash (VA), using an alkaline activator composed of sodium hydroxide and sodium silicate at a NaOH/Na₂SiO₃ ratio of 2.0. The main innovation of this study is the systematic comparison of single-precursor and hybrid CWP–VA binders at identical overall additive contents of 4%, 8%, and 12%, enabling the individual and synergistic contributions of the two precursors to be clearly distinguished. This approach provides a direct assessment of whether precursor hybridization can deliver greater stabilization efficiency than the use of individual materials. A multi-scale testing program was employed to establish relationships between mechanical performance, bearing capacity, durability, and internal structural quality using unconfined compressive strength (UCS), California Bearing Ratio (CBR), water absorption, and ultrasonic pulse velocity (UPV), complemented by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analyses. The results demonstrate that combining CWP and VA provides substantially greater stabilization efficiency than using either precursor independently at the same total dosage. The optimum mixture, containing 6% CWP and 6% VA, achieved a 28-day UCS of 5.26 MPa, corresponding to an 1182% improvement relative to untreated soil. The mixture also exhibited unsoaked and soaked CBR values of 41.11% and 27.96%, respectively, while maintaining a water absorption of only 6.8%. The measured UPV of 2130 m/s further indicated improved internal densification and structural continuity. SEM analysis revealed that stabilization transformed the initially loose and porous clay structure into a denser matrix characterized by interconnected geopolymer gel bridges. EDS analysis further confirmed the development of a Ca-, Si-, and Al-rich reaction matrix, supporting the formation of aluminosilicate-based geopolymer products. Consistent with these microstructural observations, XRD analysis indicated the development of predominantly amorphous sodium aluminosilicate hydrate (N-A-S-H) and calcium aluminosilicate hydrate (C-A-S-H) gel phases within the broad 20°–40° 2θ region, accompanied by partial dissolution of the original crystalline minerals. Overall, the findings demonstrate the synergistic effectiveness of the cement-free CWP–VA hybrid geopolymer and highlight its potential as a high-performance, waste-derived alternative for sustainable clay stabilization and compressed earth brick applications.