Characterization and Physical Properties of Concrete Blocks Developed with Hongsa Fly Ash as a Cement Replacement

This study investigates the utilization of local Hongsa Fly Ash (HFA) as a sustainable supplementary cementitious material (SCM) to fabricate non-load-bearing concrete blocks. The material characterization via X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and scanning electron microscopy (SEM) confirmed that HFA is a Class C fly ash. It features an amorphous aluminosilicate glassy phase, crystalline quartz, hematite, free lime (CaO), and fine spherical morphology (2-5?m). Concrete block specimens were developed with a constant aggregate-to-binder ratio of 90:10, where Ordinary Portland Cement (OPC) was partially replaced by HFA at 0%, 10%, 20%, 30%, 40%, and 50% by weight. The mechanical and microstructural properties were evaluated at curing ages of 7, 14, and 28 days.

The experimental results demonstrate that the compressive strength was successfully maintained within a low-to-medium replacement threshold of 10%–30%. At 28 days, the optimal mix containing 30% HFA achieved a peak compressive strength of 10.55 MPa, performing equivalently to the control mix (10.54 MPa). Quantitative EDS mapping and SEM observations validated that this strength retention was driven by the activation of a secondary pozzolanic reaction. The amorphous silica from HFA consumed the free lime to generate additional, pore-filling calcium silicate hydrate (C-S-H) gels, reducing the calcium-to-silicon (Ca/Si) ratio and densifying the matrix. However, higher replacement levels (40%–50%) induced a distinct cement dilution effect, limiting the available calcium ions and slowing down hydration, which caused the strength to decrease to 8.80 MPa at 50% substitution.

Nevertheless, all developed formulations exceeded the standard structural specifications for non-load-bearing masonry units. This research establishes a viable path for the valorization of industrial by-products, offering a cost-effective and eco-friendly binder solution for the local construction industry.