Influence of C-(N)-A-S-H gel formation and microstructural evolution on thermo-electrical transport behaviour of GGBS–copper slag alkali-activated cementitious powder derived from paver blocks

The valorisation of industrial by-products into sustainable construction materials has attracted considerable research interest; however, the relationship between alkali-activated gel formation, microstructural evolution, and thermo-electrical transport behaviour remains insufficiently understood. In this study, alkali-activated cementitious paver blocks were developed using ground granulated blast-furnace slag (GGBS) as the primary aluminosilicate precursor and 100% copper slag (CS100%) as the fine aggregate under ambient curing with a low-alkalinity activator (3 M NaOH). The intrinsic thermo-electrical transport properties were evaluated using powdered specimens (<600 μm) prepared from hardened paver blocks, whereas the compressive strength and microstructural characterisation were performed on intact paver-blocks. The GGBS + CS100% system was compared to conventional ordinary Portland cement (OPC) + manufactured sand (MS) system. Microstructural analyses revealed the formation of a dense, predominantly amorphous C-(N)-A-S-H-rich aluminosilicate gel network with refined pore architecture and enhanced particle connectivity. Consequently, the GGBS + CS100% composite achieved a 60-day compressive strength of 72.35 MPa, representing an approximately 24% improvement over the OPC + MS (58.45 MPa). The corresponding powdered composite exhibited lower thermal conductivity (0.209 W/mK) and electrical conductivity (0.7516 mS/cm) than the OPC + MS counterpart (0.225 W/mK and 1.044 mS/cm, respectively). The reduced transport trends were attributed to the formation of highly polymerised aluminosilicate gel network, reduced ionic mobility, and increased resistance to heat and ionic transport. These findings establish a clear structure–property relationship and demonstrate that alkali-activated GGBS–copper slag composites are promising sustainable paving materials that combine high mechanical strength, improved electrical insulation, and acceptable heat-transfer properties.