An investigation on pozzolanic activity of Iranian kaolins obtained by thermal treatment

Use of metakaolin as a cementitious supplementary material has found interesting applications during recent years. This interest is in part due to the technical advantages and related to environmental and energy issues, particularly in CO2 gas emission challenges in another part. However, kaolins that have effective pozzolanic activity and meet the desired requirements are still being studied. Current research aims to investigate calcination of two types (from Zonouz and Khorasan districts, Iran) of kaolin at temperatures of 600-850 °C for production of metakaolin with pozzolanic activity. The samples were analyzed using Particle Size Distribution (PSD), BET, DTA/TG, XRD, SEM, and FTIR techniques to specify the mineralogical composition and extent of de-hydroxylation. Pozzolanic activity of the resulting calcined samples were evaluated by determination of the residual quantity of Ca(OH)2 via STA method on hydrated mixtures of 50% metakaolin and 50% calcium hydroxide. Then, optimal parameters in calcination of kaolin to metakaolin were calculated for each of the two samples. The calcination temperature and time for conversion of kaolin to metakaolin in the Zonouz and Khorasan samples were found to be 700 °C and 800 °C, respectively, with a holding time of 3 h. Content of the pozzolanic activity for Zonouz metakaolin was obtained as 77.5%, while that for the Khorasan metakaolin could even hardly reach 55.0%.