Geopolymer technology for application-oriented dense and lightened materials. Elaboration and characterization.

In this paper, dense and lightened geopolymer materials based on calcined kaolin and different alkali activators were prepared and submitted to a physical and mechanical characterization: density measurement, mechanical tests (flexural and compressive tests) and thermal conductivity analysis were carried out. First, pastes were prepared by using both sodium and potassium silicate solutions. It was found that the chemical composition of the alkali solution has a minor effect on the flexural strength, while it significantly affects compressive strength and stiffness in three-point-bending, being the best performance obtained with the sodium silicate solution. Then, starting from this paste formulation, different dense materials were prepared, including mortars and composite samples containing a commercial acrylate emulsion (Primal B60A), used to improve the workability of the pastes and the mechanical properties of the cured samples. The Primal-free dense specimens showed good compressive strengths (20–30 MPa) in line with literature for metakaolin-based materials. However, Primal addition was effective in enhancing the compressive strength up to 46 MPa. Finally, different sets of low-density materials were prepared: lightweight samples, produced with expanded glass spheres (Poraver®) and macroporous samples, foamed with hydrogen peroxide in different amounts. All these samples appeared homogenous with very limited segregation phenomena. The thermal conductivity ranged from 0.12 to 0.78 W/m K, in function of the density of the samples: the lowest values, presented by the macroporous materials, were comparable with those of most commonly used building insulation materials. As a whole, this work has shown the versatility of geopolymers towards the elaboration of a variety of materials with different macroscopic features: from pastes, to mortars, to lightweight samples, to macroporous samples. Finally, the feasibility of producing bilayer materials has been assessed: samples made by superimposing a lightened layer over a millimetric layer of geopolymer paste or mortars after its partial curing were successfully prepared.