Synthesis, characterization and mechanisms of one-part geopolymeric cement by calcining low-quality kaolin with alkali

Precursors for the preparation of one-part geopolymers were synthesized by calcining low-quality kaolin, which containing a lot of quartz, with NaOH or Na2CO3, then the cement pastes of ground precursor powders were synthesized by only adding water and cured at 80 °C for 3 days followed by curing at ambient environment for 25 days. The geopolymeric cement pastes obtained reasonable dry compressive strengths, though they were not quite resistant to water. Alkalis seriously alter the thermal decomposition course of kaolin, and the octahedral-coordinated Al in kaolin also turns into tetrahedral-coordinated Al according to Fourier transform infrared spectroscopy analysis. By using X-ray diffractions, it is discovered that nearly all minerals convert into X-ray amorphous substances and nepheline when the low-quality kaolin is thermally treated with alkalis at 950 °C though most kinds of minerals except kaolinite are stable after the thermal treatment without alkali at 750 °C. P-zeolite is produced during the hydration of the precursors, and the optimized precursor shows partly dissolving characteristics in water under scanning electronic microscope. It is supposed that the hydrolysis of sodium silicates and glassy substances induces the crystallization of P-zeolite and facilitates the hydrated pastes cementing, on the other hand, the delayed hydrolysis of sodium silicates with high SiO2/Na2O ratio after solidification might make the one-part geopolymers expand and reduce the softening coefficients.