Effect of SO3 and MgO on Portland cement clinker: Formation of clinker phases and alite polymorphism

• SO3 hindered the formation of C3S by increasing the formation temperature by ∼100 °C.
• SO3 could promote the formation of C2S rims, indicating the dissolution of C2S.
• MgO enhanced the formation of C3S in both the blank and SO3-bearing clinkers.
• MgO favored the formation of M3 type alite, whereas SO3 could stabilize M1 alite.
• Seven characteristic windows in XRD were confirmed to identify the M1 and M3 types of alite.

The effects of SO3 and MgO on the formation of cement clinker and the polymorphism of alite were thoroughly investigated. Quantitative analysis of the mineral composition was performed using the Rietveld method based on powder X-ray diffraction patterns. Clinkerization of the raw meal was observed by ex-situ experiments. The C3S and C2S contents were linearly related to the SO3/MgO ratio. Adding SO3 hindered the formation of C3S, the formation temperature of which increased by approximately 100 °C. MgO enhanced the formation of C3S by both accelerating the clinkerization reactions and lowering the formation temperature of C3S. The C4AF content increased when MgO was added. Microscopically, MgO led to the formation of small C3S crystals, whereas SO3 enlarged the size of C3S crystals and promoted the formation of C2S rims. M3 alite was the primary component of the MgO-doped clinkers, whereas the M1 type was stabilized by SO3; these results were confirmed using seven characteristic windows of high-angle-resolution XRD patterns.