Interaction Between Drying Shrinkage and Hardening of Geopolymers

The geopolymers are amorphous mineral materials manufactured from alumino-silicates and a strongly alkaline solution. One of their advantages is a lower carbon footprint than conventional cementitious binders. However, they are subject to significant drying shrinkage. In this study, geopolymer samples are produced from metakaolin, silica fume and a potash solution. The mix compounding is selected to reach the molar ratios: Si/Al = 1.8, K/Al = 1.15 and H2O/K = 5.3. Cylindrical samples are exposed to various curing conditions. The curing temperature reaches 40 ℃ for 0, 6 or 24 h, then 23 ℃. The relative humidity is maintained at 100% RH for 0, 1, 3 or 5 days then at 65% RH. A protocol including periodic weighing, 3D scanning and instrumented macroindentation is used to monitor the drying shrinkage and hardening kinetics. Samples volume measurement and macroindentation are performed periodically. The 3D shrinkage, referring to volume measured just after mold removal, is estimated at each term. The hardness and stiffness of the tested material are deduced from the macroindentation curve. It appears that the hardest samples are also the most cracked. Covering the sample for 5 days at 23 ℃ or 24 h at 40 ℃ limits the shrinkage to ~ 1% but leads to large decrease of the hardness compared to hardest samples. Optimal curing conditions are identified: covering the sample for 3 days at 23 ℃. This optimization allows to limit the shrinkage to 3% without cracking, while reaching satisfactory mechanical properties.