Why is the sand in my cement mixes so thirsty?
The cement/sand ratio for mixing mortar or mixed aggregate varies depending on what you are rendering or making. In the building industry different qualities of mortar/concrete are required. The usual standard is that water and cement are measured by volume and are the same amount.
However in real life, proportions are not so exact.
If for example you are laying aggregate on a dry base, for example earth, sand the water will be sucked out of the mix, and the mix will be weakened.
Laying on dry earth or sand must be avoided, and the area must be soaked before laying, or the area should be covered in plastic to keep the water in the mix.
As mentioned elsewhere, sand can be bone dry, or in any state up to totally water logged - you need to adjust the amount of water added to the mix to allow for this.