High quartz content in clay substantially impacts the grinding of raw mix, quality of clinker and final cement produced. The presence of quartz requires very fine grinding and a long sintering time to react significantly, all of which are very expensive. This study assessed an extensive plant sample which involved a vertical roller mill to determine the correlation between quartz content in clay to the 90 µm residue, feed rate, clinker microstructure and mill power. The characterisation study performed on raw materials revealed three clay categories based on quartz content, namely low, normal and high quartz at (42.4%–48.8%), (57.1%–64.9%) and (81.5%–89.3%), respectively. The grinding test showed an increment of 90 µm residue for high-quartz clay. Meanwhile, high, medium and low feed rates generated the most coarse, coarse and fine products, respectively. Based on the distribution curve of mill products, high quartz content in clay significantly reduced the particles passing percentage. Coarser particles were observed to be present in mill products for the high-quartz category compared to the low-quartz clay and normal-quartz clay, signifying inadequate size reduction for the high-quartz clay in the vertical roller miller. The power consumption was higher (7.2 kWh/t–9.0 kWh/t) for grinding raw mixes that contained high-quartz clay than low-quartz clay. This study provided a significant relationship between grindability and the amount of quartz content in clay with the purpose of optimising the grinding process when dealing with high-quartz content in clay.