Influence of foaming agent on the properties of calcium sulfate mineral foams
Résumé
Mineral foams appear to be a relevant alternative solution to insulate buildings. The mineral foam product will be used as a partial load-bearing insulating material, so a minimum compressive strength of 2 MPa is required, and a 0.2 W/(m.K) maximum thermal conductivity is acceptable. The concentrated mineral suspension is obtained by mixing of water, mineral binder (calcium sulfate) and surfactants. The mix-then-foam process is then applied before the setting of the mineral matrix.
Mineral pastes are realized with a small water to binder weight ratio, leading to highly concentrated suspensions. Some superplasticizer is added to obtain a sufficient flowability. As calcium sulfate quickly reacts, a retarder is added to master the setting time. Previous experiments reveal that the presence of a foaming agent is compulsory to maintain foam stability until setting by changing of the surface tension of the fluid. Five different molecules have been tested.
In order to characterize the foaming agents, critical micelle concentration (CMC) tests are performed. Foaming agent can be divided in two groups, one with CMC between 900 and 1320 mg/l and intermediate surface tension from 34 to 38 mN/m, the other contains foaming agents with various behaviors.
Fresh paste density is controlled. By increasing surfactant content, the first group leads to a sharp decrease of the suspension density while other fails. Rheological behavior of the paste is investigated using spread tests, yield stress ranges between 1 to 5 Pa whatever the type or content of foaming agent. Such values are small for a mineral suspension.
Foaming agent nature and content deeply changes the paste properties and the foams porous structure. Analyzing lightweight mineral foams performances versus density, the first group of foaming agent gives the best compromise between thermal conductivity and compressive strength. The optimization of calcium sulfate mineral suspension is then possible, identifying an optimal content of each foaming agent.