Thermochemical energy storage seems promising to meet new energy challenges in our society. However, the hygroscopic salts used by these systems must be associated with a host material as porous and resistant as possible. Geopolymers are good candidates due to their affordability, eco-friendly character, their good mechanical properties and thermal resistance. To optimise their porosity, we have chosen to design them by Direct Ink Writing.
To make the initial geopolymer ink printable, polyethylene glycol was added to the formulation in order to get the proper rheology (shear thinning behaviour with high enough moduli).

As a result, structures have been successfully printed and show good fidelity to the model at two different scales. Porosity has been increased up to 20% thanks to space between the filaments. But as curing is also a critical step after 3D printing, extruded filaments were subjected to different curing conditions to optimize their mechanical properties. Three curing temperatures (22°C, 50°C and 70°C) under two different humidity conditions (100% and <30% relative humidity) were tested.
The mechanical properties have been evaluated over time by three-point bending test.

Results show that by increasing the temperature, the filaments reach their maximum bending strength more quickly. Filaments cured at high relative humidity have a flexural strength of approximately 25 MPa with a Young's modulus of approximately 5 GPa stable over time and relative humidity conditions. Filaments cured at low relative humidity have a flexural strength that can be 8 times higher but are not stable depending on humidity conditions. This behaviour is probably due to the remoistening of polyethylene glycol or silicates. Further tests must be carried out to determine exactly their respective roles.