The action of changing mixed cement from a fluid state to a solid state is called setting of cement and time required for it to set is called setting time of cement. Setting time of cement is same as setting time of concrete.
Stages of setting of cement, setting times of cement and factors affecting and initial and final setting time of different cements and various processes involved are discussed.
When water is mixed with cement to form a paste, reaction starts. In its pure form, the finely ground cement is extremely sensitive to water. Out of the three main compounds, viz. C3A, C3S and C2S, reacts quickly with water to produce a jelly-like compound which starts solidifying and setting of cement takes place.
Stages of Setting of CementThe setting process of cement starts as it is mixed with water. The chemical phenomenon that takes place are divided into 3 stages,
1. Hydrolysis and Hydration StageThe process of setting starts after the addition of water. In this process the four compounds of cement (C3S, C2S, 3CA1, 4CAFe) get hydrated. C3S compound of cement gets hydrated and form a complex hydro silicates.
2. Colloidisation StageThe products formed from the above stage seperates out in the form of a gel which gets gradually thickened and acts as glue around aggregates. Thereby initiating the setting of the cement. During this stage, the mortar (cement-water-sand paste) becomes fully saturated and can take no more water.
3. Crystallisation StageAs the name of the stage indicates, most of the components of gel or colloidal state forms into crystalline state. Compounds which are least stable such as tri-calcium hydro aluminate and calcium hydroxide are the one to undergo onto stable crystalline phase. Calcium hydro silicate gel also hardens almost simultaneously. This nearly simultaneous development of crystals and hardening of gel results into a strong and inter-grown mass of crystals and gels.
Setting Time of Cement
1. Initial Setting TimeInitial Setting Time is defined as the period elapsing between the time when water is added to the cement and the time at which the needle of 1 mm square section fails to pierce the test block to a depth of about 5 mm from the bottom of the mould. A period of 30 minutes is the minimum initial setting time, specified by ISI for ordinary and rapid hardening Portland cements and 60 minutes for low heat cement.
2. Final Setting TimeFinal Setting Time is defined as the period elapsing between the time when water is added to cement and the time at which the needle of 1 mm square section with 5 mm diameter attachment makes an impression on the test block . 600 minutes is the maximum time specified for the final set for all the above mentioned Portland cements. IS: 269-1976 specifies the strengths in compression on the standard mortar-cube.
Factors Affecting the Setting Time Of CementMany factors affect the setting time of cement after mixed with water, they are
- Composition of cement.
- Amount of gypsum in cement
- Fineness of cement
- Water cement ratio
- Type of admixture used
- Storage of cement
Initial and Final Setting time of different type of cement.Table 1: Initial and Final Setting time of different type of cement.
|Type/Name of Cement||Initial Setting Time in min||Final Setting Time in min|
|Sulphate Resisting cement||30||600|
|Portland Pozzolana Cement||30||600|
|Rapid hardening cement||30||600|
|Portland Slag Cement||30||600|