Properties of hydroxyapatite-based geopolymer synthesized from calcined kaolin

This research synthesized the geopolymer from calcined kaolin (600°C for 2 h in a
muffle furnace) using sodium hydroxide (NaOH) and potassium hydroxide (KOH) as alkali
activators. Calcium phosphate (CP) and heat treatment were introduced to study their effects on
the unconfined compressive strength (USC) and crystalline phases formation of kaolin-based
geopolymer. Results showed that the highest USC strength at 28 days of geopolymer was
obtained from 0.4 moles of Na2O/SiO2 and K2O/SiO2 and gave strength of 12.95 and 11.56 MPa,
respectively. The USC of geopolymer was observed to decrease with the addition of CP. The
amounts of 5 wt.% of CP gave a 28-day strength of kaolin-based geopolymer containing NaOH
and KOH activators at 9.95 and 8.88 MPa, respectively. The CP-doped geopolymers were then
heated at temperatures of 500, 600, and 700°C for 2 h Results indicated that heat treatment at
600°C gave the highest USC of CP-doped geopolymer containing NaOH and KOH equal to 7.61
and 4.61 MPa, respectively with the appearance of hydroxyapatite, muscovite, and quartz in the
XRD diffractograms.