Cement is the most exceptional construction materials without which concrete can not be made. China is top cement producing countries in the world. China produced 2,500 million metric tons of cement and India produced 280 million metric tons in 2014. Cement is very important material which binds aggregates when mixed with water. It is mostly used to make cement mortar and concrete. After mixing water, cement paste has a property to get hardened and to achieve strength within a short period of a time. It is therefore very important for us to know, in how much time cement will set, and in how much time we will have to use the cement.
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Setting Time of Cement
Cement has many important properties which are gained by adding required amount of water to start the reaction between them. When water is mixed with cement, the paste is formed and it remains in the plastic state for a first few minutes. During these first few minutes the paste can be moulded in any required shape due to its plasticity. However, within this specific time, reaction continues between cement and water. During this reaction, paste loses its plasticity and gets hardened. Afterwards, moulding the hardened paste in the desired shape becomes difficult. This early time of the hardening of cement paste is known as “Setting Time of Cement”.
Setting time is defined as a specified time required for concrete or mortar to change from liquid state to plastic state and plastic state to solid state so that surface becomes sufficiently rigid to withstand a definite amount of pressure. It is necessary to place and consolidate the concrete or the mortar before initial setting start and do not disturb till the final setting of mortar or concrete has taken place.
Setting time of cement usually depends on various factors, such as the type of cement, fineness of cement, chemicals, sand, ambient temperature during casting, presence of certain type of salts, the percentage of water etc. For example, setting time of cement is less during winter or in cold area due to low temperature.
Setting time is usually differentiated as initial and final setting time of cement.
Initial Setting Time of Cement
Initial setting time of cement is the time when the paste (cement + water) starts losing its plasticity. This mean that if the initial setting time of cement is 30 minutes, the cement mortar or concrete must be placed in position within 30 minutes of adding water. If delayed, mortar or concrete will lose strength. If delayed due to some unavoidable circumstances, either you have to reject cement/concrete or add additional cement depending upon delay.
Final Setting Time of Cement
Final setting time of cement is the time when the paste (cement + water) completely loses its plasticity. Within final setting time, concrete hardens sufficiently to attain the predefine shape in which it is castes. After the final setting time of concrete, you can remove the form or scaffolding. To know the correct time of form or scaffolding removal, read removal of formwork and precautions.
Control the Initial Setting Time and Final Setting Time
You can control the initial setting time and final setting time by adding admixtures in the concrete or mortar. There are many types of admixtures available in the market. To know more, read the concrete admixtures. Sometimes, 30 minutes of initial setting time of cement is too less to work with it. In such cases, admixture helps to prolong the initial setting time and worker gets enough time to pour concrete in the form or to apply the mortar for plastering the wall. By controlling the initial setting time and final setting time of cement, you can control the hydration process or hardening process of concrete/mortar. To carry out good quality of construction work, concrete/mortar should be transported, placed, compacted with vibrators or finished within initial setting time. To know more, read concrete compaction and types of vibrators.
Test to Determine Setting Time of Cement
It is very necessary to evaluate the setting time of cement. We have already discussed the test procedure to calculate the initial setting time and final setting time of cement which is based on IS:4031 (Part 5) 1988. Read the procedure of test which help to determine the Setting Time of Cement.
Conclusion
Initial and final setting time of cement are very important properties of cement, which are responsible for the quality of concrete and mortar. Hence it is advisable to check the initial setting time and final setting time of cement before you use it for the construction work. You can also use admixtures to control setting time of cement. Normally the initial setting time for 43 or 53 grade of OPC or PPC is 30 minutes and final setting time is 10 Hours.