Maybe you could use natural building materials? Do you have access to lots of clay and straw/other fibres? If so, then maybe cob would be an option?
Look around, see what sort of natural materials are in your environment, and add them to your question. Might spark a few ideas. Also describe your rainfall situation (especially whether you have to deal with torrential monsoon rain events). Some solutions that are fine for a hot dry season may fail completely in a wet season.
Do you have (or are you willing to develop) bricklaying skills? If so, I find vault construction like this quite inspirational:
Video at youtube.com/watch?v=PB8TWMKHHMQ
(I realise you want to use less sand, but mortar and concrete use vastly different amounts, so clay bricks might be an option.)
You could then lime-wash, render, or paint the outer surface white — which should reflect up to 40% of the incoming heat from the sun and help moderate the temperature.