Properties of MgO–SiO2–K2HPO4 cement modified with ground blast furnace slag

In this article, a new chemically-bonded ceramic composite, called magnesium silicon potassium phosphate cement (MSPPC), without retarders, is prepared from dead-burnt magnesium oxide, silica fume and dipotassium hydrogen phosphate (K 2 HPO 4 or DHP). MSPPC has excellent workability and mechanical properties, but the water resistance still needs to be improved (its strength retention rate is only 78.41% after 90 days of water immersion). Notably, in contrast to the acidic or near-neutral pH internal environment of traditional magnesium potassium phosphate cement (MKPC), the MSPPC system has a unique high-alkaline internal environment, which appears to have the natural alkali-activated conditions. In order to improve the water resistance, the effect of slag on MSPPC is studied. The results show that with the increase of slag content, the fluidity decreases, the setting time increases and the pH value of the hydration process rises. After 90 days of air curing, the specimens containing 10 wt % of slag can reach a maximum flexural and compressive strength of approximately 27.2 MPa and 124.8 MPa, respectively. The best water resistance is achieved when the slag content is 20 wt %, with a strength retention rate of 90.42% after 90 days of water immersion. The hydration mechanism of slag in MSPPC system is also different from that in traditional MKPC system. The hydraulic nature of slag can be effectively activated by KOH, K 2 SiO 3 and MgSiO 3 (the hydration products of MSPPC). The BSE-EDS, 29 Si MAS NMR and SEM-EDS tests show that Ca, Al and Si elements in the slag are released into solution and combined with Si elements in the MSPPC system to produce C-(A)-S-H gels. Such gels wrap around the surface of the soluble substance and, together with the physical filling effect of the slag, contribute to the mechanical strength and water resistance of MSPPC.