Standard construction grade cement in powder form is easily ruined by moisture, high humidity and fog will quickly ruin a good bag of cement. To preserve cement for a long period, remove it from the bag and place it in a plastic bucket with an air tight top.
A plastic trash bag cut to fit over the top of the bucket is quickly secured with cotton rope. It is best to cut the trash bag so that two layers of plastic cover the bucket.
A piece of the plastic liner used in cement sacks is shown below. Notice that it is manufactured full of holes which let moisture out of the freshly baked hot cement but allow moisture in at a later date, either at the retail sales outlet or the final purchaser's storage area.
Why is there moisture left in the hot powder that must be released from the bag? The cement manufacturing corporation is logically concerned with maximizing profit, huge amounts of energy are require to mine, crush, and then bake the chemically bound water out of limestone, the raw material of cement. Maximum profit is gained by turning the heat off before all the moisture is driven from the crushed limestone and allowing residual moisture and toxic gasses to escape from the bag.
There is no reason why the cement manufacturing corporation is concerned about how well cement is stored after it is bagged and sold. From the corporation's perspective, if a bag is ruined by contact with moisture then it has the opportunity to sell another bag to replace the ruined one.
Carbon Dioxide and Mercury vapors are among the major environmental costs of cement manufacture. Mining, packaging and transport are other environmental costs which must be incurred again to replace cement which penetrating moisture has turned back into stone while it's still in the bag.
One bag of cement weighs 94 pounds, each five gallon bucket holds 47 pounds, 21.3 Kilograms. Standarc construction cement will harden in humid or foggy climates, even when the bags are under a roof. Avoid paying for cement twice by packaging it to stay useable for years.