Physical and mechanical properties of fly ash and slag geopolymer concrete containing different types of micro-encapsulated phase change materials

A mix design procedure for geopolymer concrete (GPC) was developed in order to maintain a high compressive strength after adding micro-encapsulated phase change materials (MPCM). The most relevant factors which affect the properties of fly ash/slag based GPC containing MPCM are considered. Class F fly ash and slag, sodium hydroxide and sodium silicates were chosen as binder and alkaline solution, respectively. Two types of MPCM were used for a better understanding the effect of different MPCMs on the properties of the GPC. The setting time of geopolymer pastes was found to depend on both the amount of water adsorbed by the microcapsules, the viscosities of the samples, and possibly the latent heat. Accordingly, the initial setting time increased and the final setting time decreased with MPCM concentration. A slump test and compressive strength measurements have been utilized to examine the workability and mechanical properties of the new mix design. It was observed that the addition of MPCM reduces the slump and the compressive strength of GPC. These effects were more pronounced for the MPCM that form agglomerated structures and has a surface containing some polar groups, than for the more spherically shaped and less agglomerated MPCM with a hydrophobic surface. Although the addition of MPCM reduced the compressive strength of geopolymer concrete, the mechanical performance was higher than that of Portland cement concrete after 28 days of curing. A combination of SEM imaging and X-ray-tomography suggested that MPCM agglomeration, gaps between MPCM and the concrete matrix, an increased amount of entrapped air, and microcapsules that break under stress might contribute to the reduced compressive strength of GPC. (C) 2018 The Authors. Published by Elsevier Ltd.