4.6 : Setting Time of Cement

The setting time of cement refers to the process of cement paste transitioning from a plastic state to a solid state. This process is crucial in construction as it dictates the timeframe for concrete placement, compaction, and finishing. The onset of this solidification is termed the initial set, indicating when the paste becomes unworkable. The final set is when the paste has solidified completely, and further handling or manipulation can no longer affect its shape. The cement strength is minimal at this stage as the hydration of tricalcium silicate is merely initiated.

The initial and final setting times are typically measured using the Vicat apparatus. This instrument evaluates the resistance of a standardized cement paste to needle penetration.

If cement is stored in humid conditions, it may undergo a false set. This is a situation where the cement rapidly stiffens within minutes after mixing without generating significant heat. Similarly, when not enough gypsum is added to ground Portland cement clinker, the hydration of tricalcium occurs swiftly upon water addition, leading to an almost immediate set known as a flash set characterized by the rapid evolution of heat.