Thermogravimetric analysis and microstructure of alkali-activated metakaolin cement pastes

This study reports the thermogravimetric analysis and microstructure characteristic of alkali-activated metakaolin cement pastes using Na2SiO3-to-NaOH ratio of 1.5. Metakaolin was used at 70, 80, 90, 95 and 100% by total mass of binder, and Portland cement (ASTM Type I) was used at 0–30% by total mass. Alkali/binder ratio of 1.5 and water curing at 23 °C was used. Thermogravimetric analysis and scanning electron micrographs showed that at 70% metakaolin the major phase found is calcium silicate hydrate (C–S–H) which is normally found in normal water-based cement paste mixes with a slight shoulder peak of N–A–S–H. At higher metakaolin content (80% and higher), sodium aluminium silicate hydrate (N–A–S–H) phase was detected as found in geopolymer, and this is the case in 100% metakaolin mix where essentially only N–A–S–H was found. The compressive strength results were found in the region of 43–48 MPa with higher strength found at higher metakaolin content. Therefore, the results of alkali-activated metakaolin cement paste mixes showed interesting change in the major phase of C–S–H to N–A–S–H depending on amount of Portland cement and metakaolin.