Sand is used in cement, and as a combination works well with cement. The same is not true for lime, it cannot bind with sand. The strength of the resulting mix will not be sufficient and it may crumble because the mix is not homogenous. There is no mel (union) between lime and sand. With surkhi, lime becomes one, but with sand, the particles of the materials are still separate from each other after mixing. Surkhi also helps with its absorption properties, to set and create a stronger bond. Sand is more of an inert material like a filler, whereas surkhi has a relationship with lime there is a connection, hence I feel lime and Surkhi are better partners, more compatible.