by adding admixture in place of water can reduce water to certain extent in cement

you can cover structure after construction with jute bag or sheet of plastic,so that evaporation loss may be less and if water sprinkle will not get evaporated,drainage should be proper so that water collected in pit may be used again for construction work.

several activities go on at construction sites  that make use of water: mixing of concrete, wetting of dry surfaces, washing of equipment etc. to reduce water wastage during mixing of construction materials, proper equipment should be used such as mixers with which only the required volume of water is used. To also reduce wastage and recycling at construction sites, washing and cleaning of equipment should be done in a reservoir. water storage and delivery  facilities should be safe and leakage free to reduce water wastage. 

you can reduce water consumption by using curing compound agent  which is a material  used for curing concrete instead of water .

You can use some waxy material to reduce evaporation from the concrete.

If you mean to prevent excessive evaporation from concrete surface, then  you can use plastic materials to cover you concrete.