Synthesis and mechanical properties of metakaolinite-based geopolymer

Metakaolinite-based geopolymer has been synthesized at about 20 °C from metakaolinite under activation of NaOH solution (4-12 mol/L) and sodium silicate solution. The reaction product was compressed at 4 MPa in mould and then cured at 65 °C in oven for 10 h. Flexural strength, compressive strength and apparent density of the material were systematically measured. The effect of concentration of NaOH solution on the mechanical and chemical properties of the geopolymer was investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared spectrometry (IR). Results show flexural strength, compressive strength and apparent density of the geopolymer increase along with the increase of concentration of NaOH solution within 4-12 mol/L. Storage time in air has slight effect on flexural strength while almost has no effect on the compressive strength and apparent density. XRD and IR analyses indicate the geopolymer material is composed of amorphous phase and semi-crystalline phase, containing adsorbed atmospheric water and small amount of inert ingredients originated from the metakaolinite. Content of the amorphous phase in the geopolymer increased along with the increase of NaOH concentration. SEM results show the synthesized geopolymer maintained layer structure of metakaolinite particulates, thus it was thought that the geopolymeric reaction mainly occurred at the surface of the microflake of metakaolinite particulates. But this presumption need further study. Strength data of the material show such a colloids reaction based on metakaolinite could make building materials with good quality.