I am not an expert in Egyptian pyramids but it seems that the angle of Egyptian pyramids is large enough to keep the pyramid not covered by a sand from sandstorms for centuries. For a free pile of sand the angle of repose is about 30-35 degrees (approximated from a friction coefficient). For a pile of sand growing near a faces of a pyramid stones this angle seems to be greater. Thus, it is reasonable to set up the angle of a pyramid as more than 40 degrees to keep it resistant to the sand storms (and eternal, in fact).