Mechanical properties and reaction products of reactive magnesia and CFB slag/silica fume pastes

Reactive magnesium oxide (RMO) and circulating fluidised bed combustion (CFB) slags were used to prepare magnesium silicate cements using sodium hexametaphosphate (NaHMP) as a water reducer. The effects of curing condition and the initial levels of RMO and silica fume (SF) were studied for up to 90 d. The mechanical properties of the prepared pastes were evaluated through measurements of compressive strength. Mercury intrusion porosimetry (MIP) was employed to investigate the pore structure. X-ray diffraction, thermogravimetric analysis, mercury penetration analysis and scanning electron microscopy (SEM) were carried out to investigate the reaction products and final products. The results indicated that the final products were mainly magnesium silicate hydrate, hydrotalcite phases and hydromagnesite. Steam curing yielded higher strength, increased reaction products and closure of the macropores. SEM analysis showed that the product after steam curing exhibited abundant nanolattice structures (length ≤100 nm). MIP analysis showed that steam curing led to lower porosity and fewer macropores. The specimen prepared with 64 wt% CFB slag, 16 wt% SF and 20 wt% RMO exhibited the highest compressive strength (105 MPa).