The cement in concrete and mortar hardens by a chemical reaction with water so if there is no water present then essentially no further strength development will occur. It is thus vital that the mix is kept in a moist condition for the first few days after it has set. Common causes of moisture loss from exposed concrete or mortar surfaces are: high ambient temperatures, exposure to direct sunlight, drying winds and excessive suction of moisture by surrounding porous materials. Common prevention methods are to cover with damp hessian and/or plastic sheeting (being sure that this does not mark the concrete) to prevent evaporation, prewetting of surrounding porous materials, misting (for use especially with mortars and renders) with light water spray at regular intervals and pooling water on horizontal surfaces.