Application of Central Composite Design for Optimizing Mechanical Performance of Geopolymer Paste from Fly Ash Using the Mechanosynthesis Method: Structural and Microstructural Analysis

This research aims to optimize the mechanical performance of a geopolymer paste derived from fly ash (FA) using the Central Composite Design (CCD) method. The study also explores mechanosynthesis as a modern technique to create a pre-geopolymer powder, which is then used to develop the paste. Key factors considered include grinding speed and duration, curing time and temperature, and NaOH concentration. Twenty-nine geopolymer pastes were prepared based on the CCD experimental matrix, and their compressive strength (MPa) and bulk density (g/cm³) were measured after 28 days of ambient solidification. The structural properties of the raw materials and resulting geopolymer samples were analyzed using X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. Morphological characteristics were examined with Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) spectroscopy. The compressive strength of the samples ranged from 11.22 to 32.41 MPa, while bulk density varied from 1.31 to 1.62 g/cm³. The optimized conditions for the highest-performing geopolymer paste (46.47 MPa and 1.64 g/cm³) were identified as a grinding speed of 300 rpm, grinding time of 15 min, curing time of 24 h, curing temperature of 80 °C, and a NaOH concentration of 10 M. The performant geopolymer paste demonstrated a low-porosity structure primarily composed of dense amorphous sodium aluminosilicate gel. Future research could explore the application of different raw materials and additives to enhance the properties of geopolymer pastes further. Additionally, investigating the long-term durability and environmental impact of these materials can provide deeper insights into their potential for sustainable construction applications.