Strength performance of mechanically activated Ca-rich CFBC fly ash pastes with added silica

Abstract

As the old PC boilers are being replaced by CFBC boilers, more of CFBC ash waste is produced in the Estonian energy sector. Due to the lower cementitious and different chemical properties of the CFBC ash, new methods of recycling should be considered as depositing the ash in landfills has structural integrity concerns but also limits the reuse of the ash materials. Using CFBC ash mixtures as alternative cement binders might be one of the solutions. Paaver, et al. (2020) has showed that mechanical activation significantly improves the compressive strength of the CFBC ash. This thesis studied the effect of added silica as milled glass or glass wool to improve the compressive strength of raw and mechanically activated ash. The uniaxial compressive strength of raw ash pastes with added bottle glass or glass wool yielded low values of only 0.74 to 3.13 MPa after 28 days, therefore compared to the strength of ca 50 MPa for both milled ash with added glass or glass wool after 90-days, the raw ash has minor value in recycling as a cement-type product. Milled ash with added glass or glass wool did not show any significant improvement in uniaxial compressive strength. Milled ash with 1 wt% of added glass wool showed the highest compressive strength which was 1MPa higher than the ash mixture with no added silica, after 90 days, therefore some Si and Al might have reacted in the mixture. Cement hardening accelerator, Ca(NO3)2, showed the best results after 28 days of curing and reached to similar values after 90 days as did the 1 wt% glass wool mixture and no added silica mixture. NaOH as alkali activator yielded similar values after 28 days as the other ash mixtures but after 90 days of curing, the compressive strength was significantly lower. Due to the poor reactivity of glass and glass wool, the soluble Si and Al were not sufficiently released though glass wool mixture with slightly higher compressive strength values show signs of ash matrix and glass wool fiber reactions. As ettringite is the main mineral that increases the cementitious properties by filling the pores in the cement matrix, less content yielded to lower compressive strength.