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One cannot think of concrete without cement. Cement is the most basic materials which is used to make the concrete/mortar. Cement is a common construction material mostly used in the modern construction world. Concrete/mortar is the mixture of cement, aggregates, chemical admixture and water. During the mixing, cement reacts with water and generates heat of hydration. This process is also known as hydration of cement and it plays important role in gaining strength. The strength of cement is directly related to the strength of concrete as well as life of the structure. To know more, read cement.
The proportion of the cement for a particular concrete is determined by the concrete mix design. The concrete is designed for the particular strength requirement of the house/building components. Generally minimum amount of cement is determined by carrying out concrete mix design to get the required strength of the building/house components.
But fresh cement is a very hygroscopic fine powder having a property of absorbing moisture from the air quickly. And when the cement bags absorb moisture from the air, the hydration process will start and cement will start to lose its strength gradually. It means, some little amount of moisture content is always present in the air which continue hydration process inside the cement bags and slowly little amount of initial setting always take place.With this process, over a period of time, the strength of cement reduces.
The fresh cement when stored for longer period of time, loses its strength gradually. The cement when used after one year of its production, loses its strength approximately by 40-50% as compared to freshly produced cement. It is the property of hydration which is responsible for strength. And if such hydration take place due to non-use or due to atmospheric moisture, it means, it is already gaining strength and hence it will not serve the purpose for which it is used.
The strength of the cement, which has been stored for a long period of time, should always be checked before it has been taken in use for important works.
Generally, cement not more than three months old can be used without testing. It is recommended that always use the cement bags within three months from the date of manufacturing of cement.
How to check whether the cement is fresh or not?
Cement is very fine material packed in jute bags. When it gets older, the lumps are observed in the bags which develop due to the reaction between the fine cement and the atmospheric moisture.
In cement godown, fresh cement bags are stored or placed in such a way that the old bags are used first and then the fresh one. Storing cement bags in bulk in the godown has a rule called ‘first in first out’. It simply means the older cement bags should go out of the godown first for its use.
Cement bags is stored in a sound godown, have lesser chances to react with moisture of air, so they stay fresh for longer time and do not reduce the strength over a longer period of time. Hence cement godown should be well prepared to store the cement. Read the tips which helps to make a good cement godown.
However, if you use one-year old cement bags, you may never achieve designed strength. Hence, such old cement should be rejected and disposed of from the site or it may be used for non-structural purposes (like in plain cement concrete (P.C.C.), mortar for flooring, plaster etc.) with slightly higher proportion and of course with permission of Engineer-in-charge or Architect and may be after due testing.
In conclusion, never use one-year old cement to construct a house. Use lumps free and fresh cement to get the good quality of construction work. Good cement leads to the safe and longer service life of the house. It will also save your money towards repair and maintenance.