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Technical Cement Forum / IST and FST

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Dastgir 07/05/2009 02:02:59

IST and FST

Dears,

I am confused regarding the concept of IST and FST.

Can anyone tell exactly what happens when there two points are arrived at.

I have read that at IST, the stiffening of cement paste start and at FST hardening of

this paste start.

Chemically what is stiffening and what is hardening?

Is start of gel formation is stiffening and after completion of gel formation,

start of its hardening is FST?

All these are my raw surmise.

Pl. someone explain the exact sequence of events.

Kind regards,

Gulam Dastgir

Michael Clark 07/05/2009 09:57:11

1. Initial set. Occurs when the paste begins to stiffen considerably.

2. Final set. Occurs when the cement has hardened to the point at which it can sustain some load.

These particular times are just arbitrary points used to characterize cement, they do not have any fundamental chemical significance. Both common setting time tests, the Vicat needle and the Gillmore needle, define initial set and final set based on the time at which a needle of particular size and weight either penetrates a cement paste sample to a given depth or fails to penetrate a cement paste sample. The Vicat needle test is more common and tends to give shorter times than the Gillmore needle test.

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