Microwave curing of alkali-activated binder using hwangtoh without calcination

• Microwave dramatically shortened curing period of AAHP compared to the conventional heat.
• Microwave promoted formation of more stable product than the conventional heat.
• Temperature profile inside the microwave cured specimen was opposite to that under the conventional heat.
• XRD analysis indicated kaolinite and halloysite in microwave-cured AAHP transformed to hydroxysodalite.
• 29Si NMR also indicated that the prominent Q4 Si phase in microwave-cured AAHP is hydroxysodalite.

Microwaves were used to accelerate the curing of alkali activated hwangtoh paste (AAHP); the resulting compressive strength, porosity and composition were then compared against conventional heat-cured specimens. This found that a compressive strength of ∼20 MPa can be developed within 4 h, versus 72 h with conventional heat. While the 29Si NMR analysis showed the similar trends in the chemical shift with the curing time, the MIP results showed that the microwave-cured AAHP contained a greater cumulative pore volume and mean pore size than the conventional thermal-cured AAHP of the equal strength. The compressive strength of microwave-cued AAHP was attributed to the formation of a well-crystallized hydroxysodalite phase and an internal temperature gradient generated during curing.