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19. THEORIES OF SETTINGTheories of Settingof Gypsum products o!!oid"! Theory

Hydr"tion Theory #isso!ution precipit"tion Theory or ryst"!!ine Theory

$%. o!!oid"! Theory& 'hen mi(ed )ith )"ter * p!"ster enters into the co!!oid"! st"te throughso!+ ge! mech"nism . In the so! st"te *hemihydr"te p"rtic!es "re hydr"ted to form the dihydr"te*

there,y entering into "n "cti-e st"te . s the me"sured "mount of )"ter isconsumed* the m"ss

con-erts into " so!id ge!. $1. Hydr"tion Theory This theory suggests th"t rehydr"ted p!"ster p"rtic!es /oin together

through hydrogen ,onding to the su!f"te groups to form the set m"teri"!.#isso!ution precipit"tion

Theory& ,y 0ouis h"te!ier in 12 This theory is ,"sed on disso!ution of p!"ster "nd inst"ntrecrysti!i3"tion of gypsum* fo!!o)ed ,y inter!oc4ing of the cryst"!s to form the set product

19. TEORI SETTINGTheories produk Settingof Gypsum koloid Teori Hydrtion Teori!em"u"rn presipitsi Teori tu #rystlline Teori $% koloid Teori&. 'etik di(mpur dengn ir) plester msuk ke negr koloid melluimeknisme sol*gel. +lm kedn sol) prtikel hemihydrte yng terhidrsi untuk mem"entukdihydrte) sehingg msuk ke dlm kedn ktif. Se"gi ,umlh yng diukur dri iris(onsumed) mss mengkon-ersi men,di gel pdt.

. $1 Hydrtion Teori Teori ini menun,ukkn "h prtikel plester direhidrsi "erg"ung"ersm mellui iktn hidrogen dengn kelompok sulft untuk mem"entuk setmteril.+issolution presipitsi Teori& oleh /ouis #htelier pd 100 Teori ini didsrkn pdpem"u"rn plester dn re(rystili2tion instn gipsum) diikuti dengn sling kristl untukmem"entuk produk set

51. 1. 0oss of g!oss test for initi"! set. occurs hen e(cess "ter in the mi( is t"4en up in forming

the dihydr"te* so the mi( !oses its g!oss. This occurs "ppro(. "t mins."nd the m"ss h"s no

me"sur",!e compressi-e strength. . Initi"! i!more test for initi"! set This occurs "ppro( "t 15minsthis time is m"r4ed ,y definite incre"se in strength.

6/i susut g!oss untu4 set ")"! 7 51. 1.. ter/"di hen 4e!e,ih"n "ter d"!"m c"mpur"n di"m,i! d"!"m

mem,entu4 dihydr"te* sehingg" c"mpur"n 4ehi!"ng"n g!oss ny". H"! ini ter/"di 4ir"+4ir". dimins."nd m"ss" tid"4 memi!i4i 4u"t te4"n teru4ur. . 6/i i!more ")"! untu4 set ")"! ini ter/"di4ir"+4ir" p"d" 15mins s""t ini dit"nd"i deng"n pening4"t"n y"ng p"sti d"!"m 4e4u"t"n.

8. ONTRO0 OF SETTING E:;NSION0ess the '&; r"tio o S.E. o mi(ing time oS.E.>Ho)e-er the most effecti-e method for contro! of S.E. is the "ddition of chemic"!s.

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89. HGROSO;I S.E.>So f"r )e h"-e "ssumed th"t the p!"ster or stone is "!!o)ed to set

in "ir.>If the setting process is "!!o)ed to occur under )"ter* the setting e(p"nsion is more th"n

dou,!e in m"gnitude. 2%. St"ge I & Initi"! mi( is represented,y three round p"rtic!es of hemihydr"tesurrounded ,y

)"ter s the cryst"!s of dihydr"te gro)* they cont"ct e"ch other* "nd the S.E. ,egins

21. St"ge II & Re"ction st"rted "nd cryst"! of dihydr"te st"rt forming>In !eft* )"ter "roundp"rtic!es is reduced ,y hydr"tion "nd p"rtic!es "re dr")n more c!ose!y together ,y surf"ce

tension "ction of )"ter.>In right* ,ec"use the setting is t"4ing p!"ce under )"ter* the )"ter of

hydr"tion is rep!"ced "nd dist"nce ,et)een the p"rtic!es rem"ins the s"me. 2$. St"ge III &>The )"ter "round p"rtic!es is "g"in decre"sed in the eg. on !eft.>The p"rtic!es

)ith their "tt"ched cryst"!s tend to ,e dr")n together "s ,efore ,ut the contr"ction is opposed ,y

the out)"rd thrust of gro)ing cryst"!s.>On the other h"nd the cryst"!s in right di"gr"m "re not so

inhi,ited* "s )"ter is "g"in rep!enished. 25. St"ge I St"ge & The effect ,ecome more m"r4ed. The cryst"!s ,eing inhi,ited on the

!eft ,ecome intermeshed "nd ent"ng!ed much sooner th"n those on right* )hich gro) much more

free!y during e"r!y st"ges ,efore the intermeshing fin"!!y pre-ents the further e(p"nsion.

28. >The ,"sic mech"nism of cryst"! gro)th is s"me in ,oth inst"nces* "nd ,oth phenomenon"re true setting e(p"nsion.>To distinguish ,et)een them* S.E. )ithout )"ter immersion is often

termed "s Norm"! setting e(p"nsion