Precipitation of salts in confined space is the key mechanism for rock weathering and damage to building materials. If a repulsive force or disjoining pressure acts between the growing crystal in a pore and the pore wall, a thin layer of aqueous solution remains between crystal and pore wall. Supersaturation of this confined solution is responsible for the crystallization pressure that leads to tensile stresses in the material and eventually to material failure. With an extended surface forces apparatus, we have studied the surface forces acting between two mineral surfaces (mica) across potassium nitrate solution up to concentrations close to bulk saturation. The measured surface forces in the selected model system are related to the disjoining pressure between mineral and crystal surface, which is the upper limit for the crystallization pressure. Besides, these force measurements reveal new insights into in-pore crystallization concerning the properties of the confined solution between the crystal and the pore surface that influence the crystallization pressure.