The effects of microencapsulated phase-change materials (MPCM) on the rheological properties of a geopolymer paste (GPP) were investigated. In order to quantify the time-dependent viscosity increase of the geopolymer paste containing MPCM (GPP-MPCM), a new rheological model was successfully developed. Three different MPCMs were compared in order to examine the effect of the hygroscopic nature of the microcapsule shells and the size distribution of the microcapsules. In addition, the effect of microcapsule concentration was investigated. It was found that microcapsules with polar functional groups on the shells affect the viscosity and the geopolymerization reaction of the geopolymer paste much more than microcapsules with hydrophobic shells. In addition, aggregated microcapsules influence the viscosities less than unaggregated micro capsules.