Abstract In the system C 3AC 4AFSiO 2MgO, the glass formation region that is closely similar in composition to the interstitial material of portland cement clinker has been determined. The interstitial melt that exists in equilibrium with the silicates at the clinkering temperature contains a little silica. This forms clusters, which increase the viscosity of the melt and cause the latter to supercool appreciably. Crystallization of C 3A and C 4AF increases the concentration of clusters, which progressively interlock in the residual liquid, thus giving a glass on quenching. The presence of SiO 2 in the interstitial melt also enables the metastable formation of the orthorhombic aluminate and the oriented overgrowth of the ferrite on the aluminate to occur through encouraging supercooling of the melt.