Using incinerated sewage sludge ash to improve the water resistance of magnesium oxychloride cement (MOC)

Cement pastes were prepared to evaluate the compressive strength and microstructure of the blended cements. Besides, the volume stability of cement mixture during air curing and water immersion was tested by measuring the length change of mortar bar specimens. In addition to cement paste and mortar, Na2SiO3·9H2O and Al(NO3)3·9H2O were used to react with MOC as the replacement of ISSA to synthesize pure hydration products. Paste studies revealed that ISSA additions of 10–30% significantly improve the water resistance of MOC. Mortar studies showed that ISSA significantly reduced expansion of mortar bars immersed in water. The improved water resistance and reduced expansion is directly related to the decrease in MgO content of pastes and an improved stability of Phase 3 (3Mg(OH)2·MgCl2·8H2O) and Phase 5 (5Mg(OH)2·MgCl2·8H2O) in water. Results from sol-gel experiments suggest that ISSA can act as a source of soluble Al and Si which results in the formation of an amorphous M-Cl-A-S-H type cementitious gel. The gel formed could help improve Phase 3 and Phase 5 stability in water by interlocking mechanisms which impede the access of water to Phase 3 and Phase 5.