Alkali activation of metakaolins: parameters affecting mechanical, structural and microstructural properties

The composition, structure and properties of the reaction product resulting from the alkali activation of metakaolin (MK) are directly impacted by the specific surface and composition of the initial kaolin and the type, concentration and relative amount of alkali activator used. This study aimed to analyze the effect of these parameters on the flexural strength, degree of reaction, porosity and chemical and mineralogical composition of alkali-activated metakaolin pastes. Two types of metakaolin with different specific surfaces were activated under hydrothermal conditions (85 °C, 2 h) using solutions consisting of waterglass and Na(OH) as activators (Na concentrations = 6, 8, 10, 12, 15, 18, 20 M) and two metakaolin/solution ratios. Regression analysis was used to quantify the effect of the parameters tested (activator concentration and MK/solution ratio) on flexural strength. Mathematical models were likewise built to relate the degree of reaction and amount of sodium fixed in the polymer structure to the synthesis parameters. According to ²⁹Si and ²⁷Al MAS NMR, XRD, FTIR, DTA/TG data and chemical analysis, the material obtained by activating two MKs with waterglass plus NaOH was an amorphous hydrated sodium aluminosilicate in which a Q⁴ Si (3Al) type three-dimensional structure predominated, i.e., a structure where three Al atoms are connected to SiO₄ tetrahedra. The alkaline ions in the structure maintain the electrical balance. The general formula obtained for this inorganic polymer was Na₂O·3SiO₂·Al₂O₃·3H₂O.