This study assesses the effect of exposure to high temperature on physical properties and microstructure of metakaolin (MK) based geopolymer paste activated by NaOH solution 12M. The MK was partially substituted by different types of calcium rich additions; namely calcium carbide residue (CCR), quick lime (LMA), slaked lime (LME), and ordinary Portland cement (OPC). The paste were cured at ambient temperature (30°C) for 90 days. After curing, the geopolymer paste were exposed to high temperatures in oven at a temperature of 350 °C for 2 hours, before testing their physical and microstructural properties. The results showed that all geopolymer pastes exhibit at least 35% mass loss after exposure to 350°C.  The substitution of MK by 15% of the addition increases the mass loss from 35 to 36.4% with LMA; from 35 to 37.6% with LME from 35 to 37.4% with CCR and from 35 to 36.8% with OPC. Moreover, the paste made with the LMA and OPC tend to undergo more cracks than those made with the LME, CCR and control. The XRD analysis shows that the geopolymeric products of zeolite x was not stable and tend to disappear at 350°C; while the zeolite A was stable. The FTIR essentially showed the disappearance of bending vibrations corresponding to H-O-H at 1600 cm-1 and O-H at 3350 cm-1 when samples are heated at 350°C. This study shows that the thermal stability of geopolymer pastes is differently affected depending on the type of calcium addition; given the different reaction of geopolymer products to the temperature.