No! Pure sand cannot be used as we must compact the mixture. We use prescribed dosage of cement ratios to bind the soil. To achieve the compaction necessary for the strength to produce a strong block, we need a clay soil of 10%>. If the insitu soil is too sandy, then most definitely some ‘clayish’ soil has to be mixed in with the existing insitu soil. A compaction test will give you a clear idea of the proper mix to work with.