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As the reactivity of C3A in dierent cements may vary, the availability of SO4-

4 must be adjusted to the rate at which Al 3+ ions enter the liuid !hase" #his

may be attained by combinin$ the slowly soluble CaS%4 &anhydrite' with the

readily soluble CaS%4()*)O &$y!sum' as sources of calcium sulfate" nder

conditions eistin$ in industrial cement mills &ball mills', the latter

com!ound decom!oses !artly or com!letely to CaSO4(.)*)O

&hemihydrate', a form of calcium sulfate that dissolves even faster" /f the

ratio of anhydrite to dihydrate is not !ro!erly adjusted and the dissolution of 

calcium sulfate is too low, due to an insu0cient amount of hemihydrate

!resent, a !late-li1e calcium alum2nate hydrate and.or monosulfate &Am' is

formed instead of ettrin$ite and a ash set of the !aste may be observed

even if the overall amount of hemihydrate !resent is adeuate"5)

/n cements with C3A of low reactivity or those with very little or no C3A at all,

!art of the hemihydrate may !reci!itate to yield calcium sulfate dihydrate

&6secondary $y!sum7' crystals which may cause a stienin$ or even a 6false

set of the !aste"5) /n a false set, unli1e the situation in a 6ash set8, the

!lasticity of the mi may be restored by intensive miin$" 9oreover, the

settin$ !rocess is not associated with abnormally hi$h heat evolution and

the subseuent develo!ment of stren$th is not adversely aected"

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An accelerated set may also be observed in cements $round in a hi$h-

!ressure roller mill rather than in a ball mill, mainly due to the absence of 

conversion of dehydrate to hemihydrate under these conditions"  :3,:4 alse

or even ash settin$ may also occur in cements hi$h in ; )% due to the

!reci!itation of syn$enite, ; )Ca&S%4') &i$ure :"%' < :

inally, false set was observed in cements hi$h in C3A due to ecessive

formation of At < 3"

 #he overall S%3 content of =ortland cement must not eceed >3-4 !er cent

to avoid 6sulfate e!ansion8 in the later course of hydration due to ecessive

ettrin$ite formation"

6.6 Hydrated Portland cement paste

6.6.1 CONSTITUENTS OF THE PASTE

 #he followin$ are the constituents of a hydrated =ortland cement !aste"

 #he C-S-* !hase is the main !roduct of =ortland cement hydration" /ts

structure is similar to that of the C-S-* !hase formed in the hydration of 

!ure dicalcium and tricalcium silicate" #he material ehibits a very low

crystallinity" /ts mean CaO.Si%)  molar ratio may vary de!endin$ on the

cement com!osition, water.cement ratio and hydration tem!erature" ?ithin

a sin$le !aste the CaO.Si%)  ratio may vary from one area to another"

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 #y!ically, this ratio may vary between about ") and )"3 if determined by

transmission electron microsco!y<@  or electron !robe microanalysis<<-"

 Boun$ !astes a!!ear to show a bimodal distribution"<@-%  #here is no

relationshi! between the avera$e Ca.Si ratio of   the C-S-* !hase and the

maturity of the !aste" #his material may also incor!orate limited amounts of 

forei$n ions as 9$)+, Al3+, ; + , a+ or S%)-4 in its structure"

=ortlandite, i"e" crystalline calcium hydroide, is the second most abundant

!roduct in hydrated =ortland cement !astes" /t is !resent in the form of 

relatively lar$e crystalline a$$re$ates and may even be distin$uished easily

from the rest of the !aste by o!tical microsco!y"