Bi-axial four points flexural and compressive strength of geopolymer materials based Na2O-K2O-Al2O 3-SiO2 systems

Bi-axial four point flexural and compressive strength tests were used for the assessment of mechanical properties of geopolymer material-based, calcined kaolin and kaolinitic clays. Various activating solutions which consist in the mixture of potassium and sodium hydroxide, water and sodium silicate were designed and tested. Six specimens, over a wide number of investigated compositions, were selected with SiO2/Al2O3 varying from 1:1 to 3:1, The compositions 1:1 and 2:1 were obtained by using two different grades of kaolin as raw materials (standard and sand-rich), while the 3:1 was obtained by adding required amounts of silica to either kaolin or kaolinitic clay, All the samples were prepared by slip casting, using density (≈1,5 g/cm3) as indicator of the optimum viscosity for shaping geopolymer pastes, and cured at room temperature for different periods. The bi-axial four points flexural strength values vary from 13 to 21 MPa while the compressive strength vary from 45 to 67 MPa, being essentially influenced by curing time, SiO2/Al2O3 and K 2O/Na2O/H2O ratios, Increasing the SiO 2/Al2O3 ratio from 1:1 to 3:1, the mechanical properties increase but longer setting and curing times were required, Low SiO2/Al2O3 ratio results the appearance of micro cracks and deformations during curing. The flexural and compressive behaviour of the specimens studied were directly correlated to the porosity, density and the final product microstructure. The chemical behaviour of the six compositions is discussed with respect to basic dissolution-hydrolysis- polycondensation processes that occur in Na2O-K2O-Al 2O3-SiO2 systems.