A comprehensive investigation is carried out on the effects of compressive strengths, air-void and microstructure characterizations of lightweight foamed concretes with 0%, 25% and 50% Portland cement replacements of ground granulated blast furnace slag (GGBFS). The specimens were cured for 14, 28, 90, and 180 days under two curing regimes, i.e. water and natural tropical weathering. The results indicated that the GGBFS increases the water cement ratio of the foamed concrete mixtures to obtain an equivalent consistency with the control mixtures. The early strengths of lightweight foamed concretes with GGBFS replacements were lower than those of controlled mixtures. However, higher compressive strengths were observed beyond 28 days. Air-void size and distribution of the foamed concrete have a direct correlation with its compressive strength. Lightweight foamed concrete with 50% of GGBFS replacement has the narrowest distribution of air-void size, the densest microstructure, and hence the highest compressive strength.