Polysialate matrixes from Al-rich and Si-rich metakaolins: Polycondensation and physico-chemical properties

Two metakaolins from Al-rich and Si-rich kaolinitic clays were used to design polysialate matrices with different Si/Al and NaK/Al ratios. The aim was to investigate the influence of oligomers formed during dissolution and hydrolysis on the polycondensation and transformation to hard and stable matrices. Products of geopolymerization of the different matrices were subjected to mechanical testing considering various loading configurations. The geopolymer matrices showed compressive strength from 51 ± 5 MPa (Si/Al=1.23) to 61 ± 2 MPa (Si/Al = 2.42) and bi-axial four-point strength from 11 ± 2 MPa to 16 ± 1.1 MPa respectively. These results were consistent with density, leaching ability and microstructure. It is proposed that the mechanical properties and the stability of the products of reactions can be discussed as the important parameters for the evaluation of the quality of geopolymer matrices. Moreover, polycondensation and the final performance of the product are greatly influenced by the unreacted crystalline or semi-crystalline phases that act as fillers and contribute to increased stability and mechanical properties. A good geopolymer material resulting from our study will be defined as an amorphous matrix of polysialates in which various unreacted or partly reacted crystalline phases are embedded. It is suggested that Si-rich metakaolin will present the more hardening and stable matrix with shorter setting time leading to small pores and an enhanced interlocked microstructure.