When water is mixed with cement to form a paste, reaction starts. In its pure pore, the finely ground cement is extremely sensitive to water. Out of the three main compounds viz. C3 A, C3 S and C2 S, the C3 A reacts quickly with water to produce jelly-like compound which starts solidifying. This action of changing from a fluid state to a solid state is called âSettingâ and should not be confused âHardeningâ
During the next stage of hydration, cement paste state hardening owing to the reaction of C2 S and C2 S and the paste gains strength. In the first few minutes, the setting action is more preforming and after some time hardening action becomes rapid.
In practice, such solidifying action or loss of plasticity is required to be delayed, because time is needed for mixing transporting and placing of concrete into final position before the mix loses its plasticity due to setting action.
It is usually specified that the plastic concrete should be placed and consolidated before initial set has occurred, it should not then be disturbed until concrete has hardened. This initial setting should not be too small and therefore, the standards specify minimum initial setting time.
Once initial stiffening of concrete has taken place it is desirable that it should harden or strength as rapidly as possible, so that there is minimum of delay before shuttering can be removed and risk of frost-damage minimized. Standards, therefore, specify maximum value of final se time.
It is not possible, however, in practice to exactly locate the initial setting time and final se time. The Indian standards have selected two arbitrary points which relate strength of cement time from adding water.
âInitial setting timeâ is defined as the period elapsing between the time when the water is added to the cement and the time at which the needle of 1 mm square section fails to the test block to a depth of about 5 mm from the bottom of mould. 30 minutes is the minimum setting time specified by ISI for ordinary, hardening cement and other cements and 60 mm low heat cement (see Table).
The âfinal setting timeâ is defined as the period elapsing between the time when the water is to the cement and the time at which the needle of 1 mm square whit 5 mm dia attachment (the Fig. 8.1) makes an impression on the test block, while the attachment fails to make impression on the test block. 600 minutes is the maximum time specified for final set for all the mentioned Portland cement.
Aim: To determine initial and final setting time of Cement.
Significance: This test given a judgment about the available time for transportation placing of concrete, commencement of curing and stripping.
|Weight of Cement||Water||Initial setting time minute||Final setting|
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