Mixed systems based on an initial HCBS of quartz sand and a suspension of refractory clay are described. It is established that the introduction of a small amount (2–5%) of clay has a substantial positive effect on the properties of quartz sand HCBS and materials based on them. The sedimentation stability, density, casting (binder) strength, and mechanical strength of the material a heat treatment at 800–1400°C are shown to increase. The content of the colloidal component (particles less than 0.1 μm in diameter) in the molded system is shown to depend on the porosity, strength, and drying shrinkage of the binder.