Cement is the world’s second-most consumed material and its production contributes substantially to global CO2 emissions. Replacing clinker with supplementary cementitious materials (SCMs), such as calcined clay, can lower the carbon footprint. Calcined clay, available in abundance and processed at 800–900C, can replace up to 50% of clinker. This review critically examines the rheological behavior of clay-based cement systems, focusing on their yield stress (YS), viscosity, and thixotropy. Compared to OPC, clay-based systems show a 20–70% increase in YS with >30% calcined clay, and nano-metakaolin enhances thixotropy by up to 60%. Herschel–Bulkley’s model (R2 > 0.98) describes best their non-linear shear-thinning behavior. While shear-thinning improves pumpability in applications like 3D printing, it may reduce formwork stability. The review explores how SCM type, water-to-cement ratio, and superplasticizer dosage affect flowability. It uniquely links rheological properties to practical engineering demands, aiding in the development of high-performance, sustainable cement systems.