This study describes the fabrication of green composite pastes with multifunctional properties that can be utilized in special applications. Low-cost mesoporous hematite nanoparticles (HNPs) were prepared using a sol-gel technique. Different doses of HNPs (0.25, 0.5, and 1%) were added to alkali-activated slag (AAS) using an aqueous solution of NaOH (5 wt % of slag) as an activator. All composite pastes, AAS and AAS-HNPs, were kept under high humidity at normal temperature for 90 days. After 28 days of curing, a group of specimens was fired at elevated temperatures in a range of 200–900 °C, and another group was exposed to 1000- 3000 KGy gamma radiation. The formed phases and morphological features of some selected composites were characterized using X-ray diffraction (XRD), thermal analyses (TGA/DTG), Fourier transformation infrared (FTIR), and scanning electron microscope with energy dispersive X-ray (SEM/EDX). The outcomes of the compressive strength test emphasized that AAS-0.25 HNPs composite paste has a superior thermal resistivity at all firing temperatures and the best impedance to radiation till 3000 KGy. Agar diffusion test (ASTM D4300-1) revealed that AAS containing 1% HNPs composite possessed an excellent antimicrobial activity with various microorganisms; two bacterial strains (Escherichia Coli ATCC 8739 and Staphylococcus aureus ATCC 6538) and two fungal strains (Aspergillus flavus and Mucor circinelloide).