Abstract :
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.
