What is the best way to store bags of cement on site?
The primary concern for storing portland cement is to prevent exposure to moisture. Cement stored in damp air or moisture sets more slowly and has less strength than cement that is kept dry.
Bagged cement stored on a job site should be stacked closely together on pallets to reduce air circulation and to raise the cement above any moisture sources; it should also be covered with tarpaulins or other water-proof covers for further protection from moisture.
If possible, store cement inside a structure to further reduce the potential for moisture exposure, but never place the cement directly next to an exterior wall. The relative humidity within the structure should be reduced as much as possible.
These recommendations are from Design and Control of Concrete Mixtures, EB001
Question and answer courtesy of Portland Cement Association (PCA). More information is available at: http://www.cement.org/tech/cct_faqs.asp.