Compensation of the negative effects of micro-encapsulated phase change materials by incorporating metakaolin in geopolymers based on blast furnace slag

Abstract This study aims to use NASH (sodium alumina silicate hydrate) gel and CASH (calcium alumina silicate hydrate) gel to overcome the negative effects of incorporating microencapsulated phase change materials (MPCM) on the mechanical strength and thermal conductivity of blast furnace slag-based geopolymer mortars. The coexistence of CASH and NASH gels was obtained by using an inclusion of a small amount of metakaolin (MK) in a geopolymer matrix based on blast furnace slag (GBFS); this results in a geopolymer with high mechanical strength. Several tests were performed to characterize different mortars (with and without MK) such as workability, microstructural properties, water porosity, mechanical properties (compressive strength, dynamic Young modulus) and thermal properties (thermal conductivity, the specific heat capacity). The results obtained showed that the coexistence of NASH and CASH gel brought improvements in terms of mechanical properties and thermal conductivity compared to GBFS-based geopolymer-MPCM only. Indeed, the addition of 10 and 20% of MK was sufficient to obtain this coexistence. With a concentration of MPCM up to 10% in the geopolymer mortars, the compressive strength was increased by about 10 MPA and the thermal conductivity was increased by about 31%, which led to an improvement in the specific heat capacity of up to 1280 J/Kg.K. These improvements were due to the high reactivity of MK under the activation conditions used. This favored the good dissolution of silica and aluminum in the MK, which participated well with the calcium in the GBFS to create the NASH and CASH gels. In fact, it was felt that these two gels filled the small pores caused by the MPCM and that this compensated well for their negative effects on the geopolymer matrix. Geopolymer-MPCM based mortars after their optimizations with NASH and CASH gel coexistence showed good workability, compressive strength, and thermal performance. On the other hand, a reduced water porosity compared to Portland cement-MPCM based mortars.