There are lots of fancy words in the cement field, so I will elaborate on the MOST COMMON acceptions of these phrases.

Flash setting : this is due to a lack (or absence) of calcium sulfate (gypsum/anhydrite) addition to clinker. This addition is useful to 'divert' the natural hydration route of aluminate phases from hydrogarnets and other hydroaluminates towards ettringite. Hydroaluminates induce flash setting: a rapid and exothermic hardening in a matter of minutes. Ettringite and counterparts induce a much softer hydration route, easier to manage in terms of workability.

False setting: this is usually due to the nature of calcium sulfates added, specifically too much calcium sulfate hemihydrate (bassanite, or more commonly plaster). Calcium sulfates come in various degrees of hydration: dihydrate (gypsum), hemihydrate (bassanite), anhydrous (anhydrite). When too high an amount of hemihydrate is present, it simply follows its own hydration route towards gypsum precipitation, leading to an early setting of cement. Hemihydrate may form in hot spots of the mill during clinker-gypsum co-grinding. 120-140°C is enough.

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Dear Jehan Hassanin,

False set :  

False set occurs when too much of gypsum dehydrates in the cement Manufacturing plant . This leads to stiffening due to the rapid reformation of gypsum with interlocking needle-like crystals. Additional mixing without added water restore workability. Ettringite  (Ca6Al2(SO4)3(OH) 12· 26H2O ) precipitation can also contribute to false set.

Flash set

Flash Set is an early loss of work-ability in paste, mortar, usually accompanied by the evolution of considerable heat resulting primarily from the rapid aluminates. If the proper amount or form of calcium sulfate is not available to control the ca hydration, rapid stiffening takes place. Flash set cannot be dispelled , it can the plasticity mixing without the addition of water.

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Unlike false setting (setting of solids) when intensive mixing

may restore the situation, fash setting is a rapid develop-

ment of gel in freshly mixed cement and there is no way

to recover the situation