Influence of the geopolymer formulation on the endogeneous shrinkage

The shrinkage of geopolymer materials is an important parameter to control for several applications. In this topic, this study aims to investigate the role of used precursors (metakaolin and alkaline solution) and different additives (sand, normalized sand, wollastonite, and glass fibers) on the shrinkage. For this, different samples were synthesized and stored at 20 degrees C and 100% RH. Then, shrinkage measurements were carried out at different consolidation times. The results have shown that the measured shrinkage is endogenous because it is not accompanied by mass variation and is only due to self-desiccation. It was also evidenced that the used alkaline solution controls the shrinkage. In fact, the increase of alkalinity or the use of sodium instead of potassium solutions permits to decrease the shrinkage values to less than 400 mm/m at 28 days. This result is explained by different depolymerization degrees of the solution inducing different geopolymerization rates and kinetics and affecting the pore distribution size. The metakaolin seems to have a lower impact on the shrinkage. The effect of additives was also studied. All additives reduce the shrinkage with different extents in the following order: normalized sand > sand > glass fiber > wollastonite. The normalized sand has shown the highest reduction of shrinkage (about 142 mm/m at 7 days) which can be explained by a modification of the granular skeleton and the porosity. A correlation was proven to show the increase of the shrinkage with the increase of liquid to solid ratio. (c) 2021 Elsevier Ltd. All rights reserved.