Response Surface Optimization and Microstructural Characterization of OPC–Fly Ash–GGBS Stabilized Pond Ash for Sustainable Geotechnical Applications

The environmental and land management issues associated with disposal of large quantity of pond ash (PA) from coal-fired thermal power plants are substantial, necessitating sustainable reuse strategies. The aim of this study is to investigate the performance of a ternary binder system of ordinary Portland cement (OPC), fly ash (FA) and ground granulated blast furnace slag (GGBS) for the engineering properties of PA for geotechnical applications. The Box–Behnken Design (BBD) approach in Response Surface Methodology (RSM) was used for the evaluation of the effect of binder composition on the unconfined compressive strength (UCS), California Bearing Ratio (CBR), and maximum dry density (MDD). The proportions of OPC, FA and GGBS were varied within predetermined limits, and 15 experimental mixtures were prepared. The experimental results indicated that the UCS was between 760 to 1260 kPa, soaked CBR was between 5.92% to 12.36%, and MDD ranged from 14.32 to 15.88 kN/m³. The developed quadratic models showed high statistical adequacy, with p-values below 0.0001 and coefficients of determination (R²) greater than 0.99 for all responses. The optimum combination of binders was obtained using multi-response optimization which yielded 2.45% OPC, 10.84% FA and 19.58% GGBS with a desirability index of 1.0, with a predicted UCS of 1272.81 kPa, soaked CBR of 12.63%, and MDD of 15.90 kN/m³. The validation experiments showed excellent agreement with the predicted results, with small deviations (less than 1%). SEM and EDS observations revealed a denser matrix with Ca–Si–Al-rich cementitious products. These features may indicate the development of C–S–H- and C–A–S–H-type phases, along with fewer visible voids and stronger bonding between the particles. The results demonstrate that the addition of OPC, FA, and GGBS is an effective and environmentally beneficial approach for improving the mechanical and compaction characteristics of PA, supporting its potential use in pavement foundations and other geotechnical engineering applications.