• Several tests were carried out to obtain heavyweight concrete's physical, mechanical and microstructural properties after being exposed to 20, 150, 300, 500 and 800 ℃. • The thermal stability of MG concrete is relatively high, up to 500 ℃, compared to QU, and the number of cracks developed for the high w/b ratio concretes. • Due to the good thermal stability of magnetite concrete up to 800 ℃ and the high progression of cracks; due to the high evaporation and dehydration ratio as well as decomposition of paste in quartz concretes. • Correspondingly, after temperature loading with 800 ℃, QU-III and MG-III showed the lowest declination values of flexural strength, thus explaining the slow disintegration of C S H in QU and postponement of the second phase of C S H decomposition in MG with high w/c ratio. Nuclear buildings are exposed to hazardous radiation and high temperature during their operation and in the case of fire. Heavyweight concrete is used in constructing nuclear buildings for the advantages of strength, durability, heat resistance, and shielding against radiation. The physical, mechanical and microstructure properties of the heavyweight magnetite concrete (MG) were investigated and compared to the reference normal weight quartz concrete (QU) as the concrete. Each type of concrete has been studied for 42, 47 and 52% water-cement ratios. Results indicated that the thermal stability of MG is relatively high, up to 500 ℃, compared to QU concrete. The loss in MG's mass was relatively lower than the QU concrete. Changes in the porosity were quite similar in QU and MG, but the effect of the growth in the porosity on the compressive strength of MG was relatively lower than in QU. At 20 ℃, the compressive and flexural strength values for the (QU) mixes decreased with raising the water-cement ratio, while it developed in the magnetite (MG) concrete as the w/c percentage increased. The result showed that the residual mechanical properties of ordinary QU were improved as the w/c ratio increased. The microstructure investigations of QU and MG before and after the heat treatment identified some changes in the concrete structure. These changes represented the appearance of microcracks, decomposition of C S H, and voids at 500 and 800 ℃. Therefore, it affected the physical and mechanical properties of QU and MG.