Cement is chemically active component and it forms a compounds / gel / silicates / bonds when it comes in contact with H[SUB]2[/SUB]O (Water).
Humidity / moisture / steams in the air is also a form of water.
When the moisture from air makes itself available for cement bags in Store Room, it starts the process of hydration in it.
As by looking at the cement, it is difficult to determine either cement is till fresh or partly hydrated and its effect on the strength, doing the testing also do require longer time and my give a little idea about properties in 24 hours and 3 days test results of compressive strength.
As the hydration process is not visible to our open eyes and cannot be controlled once it is started, it is better to set the guidelines for storage and usage of the cement, so that it is always assured that, concrete produced with that cement will always yield to desired properties without any issues.
By doing trials and errors, experiments and data's provided by many, finally its decided to term cement as good as fresh till 3 months when stored as per the guidelines - it formed a expiry date in your terms.
Because many do suggest to not to use cement after 3 months of its manufacturing (not storing).
Cement can be used further base on your experience and knowledge about its properties, but due to absence of knowledge and abilities to understand in many - it becomes hard and fast rule to discard the cement and making it label as not fit.
Below article could help you in understanding basic guidelines for storage of cement.
Guidelines for Storage of Cement IS 4082-1996 laid down the stipulation for storage of cement as follows: The height of stack shall not be more than 10 bags to prevent the possibility of lumping up under pressure. The width of the stack shall be not more than four bags length or 3 metres. In...
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As we cannot quantify the effect of hydration on each bag of cement after storing it for 3 month from the date of manufacturing, it is not only difficult to predict which bags are good or bad or consuming bags by collecting there parts, normally that cement is not used for structural concrete purpose.
Cement can be utilised for finishing works base on your experience and knowledge about degree of infection (hydration) happened on cement bags.
After analysing the degree on infection, you can safely tag it for using either for plastering works, brick works or other finishing works or just for patch works.
I hope this information is useful for you.