This work aims at studying the effect of alkali concentration and metakaolin content on the resistivity of ground granulated blast-furnace slag (GGBFS) to strength regression during ageing. GGBFS was activated by 6 and 10 wt.% containing sodium hydroxide and liquid sodium silicate at a ratio of 1:1, respectively (namely GGBFS6 and GGBFS10). GGBFS10 was replaced by 10, 20, 30, 40 and 50 wt.% metakaolin, respectively. All hardened specimens were cured in 100% relative humidity (RH) at 37 ± 2 °C for 28 days (zero time), then aged at 95 °C for 56 days. The results showed that the pH value of GGBFS10 decreased as the amount of metakaolin increased. Also, the pH of GGBFS10 was higher than that of GGBFS6. The strength regression resistivity (SRR) during accelerated ageing enhanced with the decrease in alkali concentration. The compressive strength for GGBFS6, GGBFS10 and GGBFS10/metakaolin was evaluated. Some of the selected samples were analysed using infrared spectroscopy (FTIR), thermogravimetric analysis (TG/DTG) and X-ray diffraction (XRD) techniques to determine the mineralogical phase transition. The results are in a good harmony with those of FTIR, TG/DTG and XRD techniques and confirm that the SRR increases with metakaolin content.