Microcrystalline cast stone is a silicate inorganic nonmetallic material prepared via a melting–casting–crystallization process using slag as the main raw material and appropriate nucleating, quenching, and tempering agents. However, crystallization of the microcrystalline cast stone causes overall shrinkage, producing defects such as holes on the surface and inside and affecting the quality and yield. In this work, the effect of quartz-sand addition on the formation of voids in microcrystalline cast stone prepared from blast-furnace slag is studied. The theoretical composition, phase transformation, and microstructure of microcrystalline cast stone are analyzed through FactSage thermodynamic calculations, X-ray diffraction, and scanning electron microscopy. The results show that the main crystal phases in the microcrystalline cast stone are gehlenite and akermanite for 5, 7, 8, and 10 % quartz-sand addition. When the amount of quartz sand added is 8 %, the overall shrinkage rate of the ordinary augite phase is 15 %. The maximum amount of dicalcium silicate is formed, filling the holes on the surface and inside and inhibiting the formation of holes. The crystal morphology is snowflake-like and the crystal connections are dense. The flexural strength reaches a maximum value of 40.08 MPa, which improves the overall performance of the microcrystalline cast stone. The results of this study can provide technical support for improving the yield and quality of microcrystalline cast stone prepared by melting.