Mechanical property and microstructure of quaternary phase paste blended with metakaolin

Abstract Calcium aluminate cement (CAC) shows some superior properties compared to Portland cement. However, the conversion of metastable hydrates limits the application of traditional CAC. This study reports a low cost and high strength quarternary phase (Q phase, Ca20Al26Mg3Si3O68)-metakaolin (MK) composite. The mechanical property and microstructure of this composite hydrated at ambient temperature were investigated. The results indicate that incorporating 15% MK into Q phase paste leads to a highly dense and homogeneous microstructure and the resulting composite achieves a continuous strength development within 120 days and gains a high ultimate compressive strength up to 110 MPa, without experiencing a strength loss, attributed to the continuous formation of nanoscale CAH10 and AH3. In contrast, the control paste made without MK shows an obvious decrease in strength due to the formation of a large amount of pores in outer products caused by the conversion of initially formed hydrates. The results of XRD, BSE/EDS and 29Si and 27Al NMR indicate that the beneficial effect of MK on the mechanical property and microstructure of Q phase paste mainly arises from its pentahedral Al.