Comparative analysis of cold reaction sintered mixtures with fly ash, BF slag, brick, copper slag, and metakaolin

The construction industry faces increasing pressure to reduce its carbon footprint by adopting sustainable alternatives to ordinary Portland cement. This thesis investigates the synthesis and characterization of geopolymer binders consolidated via Cold Reaction Sintering (CRS), a low-energy processing route that combines alkali activation with pressure-assisted densification at moderate temperatures (120 °C). Five aluminosilicate precursors were systematically evaluated: Metakaolin (MK) as a reference material, and four industrial by-products including Fly Ash (FA), Blast Furnace Slag (BFS), Copper Slag (CS), and Brick Waste (BR). Potassium silicate-activated powders were consolidated under uniaxial pressures of 35–65 bar for 10–15 minutes. The influence of precursor mineralogy, initial water content, and compaction pressure on the phase evolution, microstructure, and mechanical properties was examined using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and mechanical testing. Results indicate that the efficacy of the CRS process is fundamentally governed by the intrinsic reactivity of the precursor. Blast Furnace Slag achieved the highest mechanical performance (flexural strength ~59 MPa, compressive strength >200 MPa) due to the formation of a dense, hybrid calcium-aluminosilicate-hydrate (C–A–S–H) gel coexisting with hydrotalcite. Metakaolin demonstrated superior geopolymerization, forming a defect-free, monolithic K–A–S–H network with a flexural strength of ~29 MPa. Conversely, Brick Waste acted as a largely inert filler, resulting in high porosity (19%) and poor hydrothermal stability due to the lack of chemical bonding between quartz/mullite grains. Fly Ash exhibited a "core-shell" consolidation mechanism, where reactive glass shells fused to form a binder while preserving rigid cenospheres, yielding high stiffness (E ~39 GPa). Copper Slag showed intermediate behavior, where the activation of iron-rich glass phases induced limited crystallization but insufficient particle fusion. This study demonstrates that Cold Reaction Sintering can successfully valorize reactive industrial wastes into high-performance structural ceramics without high-temperature firing, provided the precursor chemistry and water balance are optimized.