Development of near-white surface of Ca(OH)2-Na2CO3-activated coal bottom ash

Abstract

This study demonstrated that the Ca(OH)2-Na2CO3 activation produced a near-white surface of hardened samples having a high lightness index using dark coal-fired bottom ash. In this study, the lightness index (L*) (index of black (0) to white (100)) of raw dark bottom ash was 36.73, but after the activation, the hardened sample showed a significantly increased L* (77.93) (i.e., 112.2% L* increase) with a near-white surface. The Ca(OH)2-Na2CO3-activation also decreased the values of a* (index of green to red) and b*(index of blue to yellow), implying the removal of chromatic colors. However, this whitening effect significantly depended on the particle size of raw bottom ash, controlled by the milling process, and the produced amounts of white reaction products (particularly, C-S-H (gel) and CaCO3), affected by the milling process and substituted ratios with fly ash and ground granulated blast furnace slag (GGBFS). The compressive strength of the 100 wt% bottom ash sample was only 5.3 MPa at three days, but it was enhanced to have enough strength over 7.4–10.3 MPa (adequate for structural bricks) without losing the surface brightness noticeably (L* > 70) by the milling process or by substituting bottom ash with fly ash or GGBFS.