Effect of oil shale ash, Al, Zn and H2O2 foaming agents on formation of NASH and CSH cementitious gels in alkali activated reactions of kaolin and metakaolin

Although waste left after combustion of oil shale is a rich in silicon, calcium and to less extent aluminum, alkali activation of the ash was of limited success in previous studies. The present work attempts to overcome this problem by blending with Al-rich kaolin /metakaolin and further studied the effect of Al, Zn (powder) and H2O2 foaming agents. Two sets of FTIR absorption bands were identified and their frequencies and relative intensities were followed. The first belongs to formation of sodium aluminosilicate (NASH) at 536 cm−1 (Si–O–Al bending), 796 cm−1 (Al–O stretching and 3695 cm−1 (Free O–H stretching) and lower frequency shift of Si–O–Si stretching bands. The second set for calcium silicate hydrate (CSH) at 800, 1000 and 1200 cm−1 assigned to Si–O–Si stretching (Q1, Q2 and Q3, respectively). In accordance with XRD analysis, alkali activation of oil shale ash mixed with kaolin/metakaolin mixture indicated synergistic effect of dissolution of anhydrite (CaSO4), silicates from oil shale ash and aluminosilicate from kaolin/metakaolin to form CSH and NASH, respectively. The formation of CSH was interrupted almost completely by addition of foaming agents while that of NASH was reduced in the order: Al > Zn > H2O2, an order reflecting the order of reactivity of foaming agent with NaOH. Lowering efficiency of alkali activation by foaming agents may be due to alkali and Ca dilution arises from volume expansion. Introducing 50 wt. % of the ash into the alkali activated kaolin/metakaolin was found to increase compressive strength (18.3 MPa) in comparison with alkali activated ash alone (fragile) and alkali activated metakaolin (12.5 MPa).