PhD Offer: Additive construction with cement foam
Two different techniques can be used for the 3D printing (additive construction) of cement materials: (i) concrete/mortar extrusion or (ii) selective fluid bonding of a particle bed. The former is by far the most used at present, the latter can be broken down into two variants: (iia) the application of a solution composed of water and admixtures on a mixture of cement powder and aggregates (selective activation of cement), (iib) by application of cement paste on aggregates (selective paste intrusion).
It is this second variant that interests us here. The principle is simple: a fluid material is deposited by the nozzle of the 3D printer and then penetrates the particle bed, allowing the production of very complex structures without the need for supporting structures. However, there are several problems with this technique: to achieve high mechanical strength, the applied aggregate layer must be completely penetrated by the cement paste to bond with the layers below. The shape of the injected area must be controlled. This results from the combination of the hydraulic properties of the granular bed porosity, the rheological properties of the cement slurry, and the effects of capillary pressure.
The injection of cementitious foam, instead of cement slurry, can significantly improve the control of the deposit in the granular bed. Indeed, the presence of air in high proportion would allow to reduce the required driving pressure; if it is well chosen, the surfactant used for the foaming of the cement grout can play an equivalent role to the superplasticizer generally used to reduce the effective viscosity of cement paste; the microstructure of the liquid foam in the porosity optimizes the distribution of the grout at the contacts between the grains (bridging); the capillary cohesion intrinsic to the liquid foam can help to control the shape of the injected zone; the presence of air would limit the effects of drying shrinkage, which could, at least in part, compensate for the smaller quantity of cement injected with the foam compared to the traditional method.