Briefly, clay minerals have a particular layered crystalline structure (phyllosilicates). There are various types (kaolinite, illite, montmorillonite, etc.) but generally the negative charge prevails on the surface of the individual lamellar crystals. Charge balancing is generally achieved by adding water or cations. The cation exchange properties too depend on this structure. The minerals constituting silt and sand do not have this lamellar structure.

I agree with Stefano De Angelis. Do a Web search for "pH and surface charge on clay particles" to get many detailed answers. Sand does not have the platelet structure of clay, so its behaviour is less complex, so do a similar search for sand instead of clay.