Rule of Thumb

 

1. “rul “rule e of th thum umb” b” th that at ap appr pro oxi xima mate tely ly 2% of th the e to tota tall Mg MgO O content in a cement clinker will remain in solid solution in ot othe herr clin clink ker ph phas ases es an and d th the e rem emai aind nder er of the the Mg MgO O is expected to be present in the form of periclase crystals 2. “!he fre free e MgO will de decre crease ase a as s" "e e2O# increases in the mixture. $&" is an excellent stabili'er for MgO and is capable of a transformation of considerable amounts of MgO into a non( expan ex pandin ding g com compou pound nd but it was con conced ceded ed tha thatt the rap rapid id cool co olin ing g of cl clin ink ker al also so ha has s a )e )ery ry be bene ne*c *cia iall e+ e+ec ectt on the the stabili'ation of MgO #. Mix design guideline to avoid expansion

Alumina + MgO / Fe2O3 < 2.7 MgO,"e2O# - 1. 1.# # plan plantt in tr trou oubl ble/ e/ au auto tocl cla) a)e e ex expa pans nsio ion n pre)alent MgO,"e2O# - 1.0 danger 'one/ high probability of  expansion/any appreciable appreciable free $aO gi)es excess expansion MgO,"e2O#  1.0 probability of expansion failure decreases rapidly MgO,"e2O#  1.20 standard control target maximum )alue3 failure due to MgO )ery unlikely . 4n a 5O# rich en en)ir )ironm onment ent the sil silicates icates fo form rmed ed are $2 $25 5 with a high le)el of 5ulphur substitution or a sulphosilicate phase. 6oth 6ot h ar are e )er )ery y di7 di7cul cultt to con con)er )ertt to $#5 and re re8ui 8uire re har hard d burning. . !he pr presence esence of calc calcium ium lang langbeinit beinite e and prop propably ably spur spurrite rite a carbonated calcium silicate3 indicates that the build up was formed at about 900:$ in the presence of $O2 and therefore while the limestone was undergoing calcination. ;. 4f redu reducing cing cond condition itions s pre)a pre)ail il within the kil kiln n burning burning 'one th the e oxi xide de blen blend d of th the e ra raw w mi mix x de desi sign gn is up upse set. t.5p 5pec eci* i*ca call lly y tri)alent tri)a lent iro iron n ox oxide ide "e2O# "e2O#33 is red reduced uced to di)ale di)alent nt "eO eO." ."eO eO replaces $aO in the $#5 crystal lattice.!here is thus more $#5 formed and less li8uid phase."ree lime is unable to combine witth $25 be wi bec cas ase e th the e "eO is more mobi bile le so free li lim mes to 7=> of t!e critical speed. -3" is muc! more reactive t!an -2" and under $standard% temperature 22. conditions of 26@- approximatel !alf of t!e -3" present in a tpical cement ill #e !drated # 3 das and 6> # 2 das. 0!e -B"BC produced # #ot! -3" and -2" !as a tpical -a to "i ratio of approximatel &.7. 0!is is considera#l loer t!an t!e 39& ratio in -3" and t!e excess -a is precipitated dioxide   is slol a#sor#ed to as calcium !droxide (-C crstals. -ar#on dioxide convert t!e portlandite t!e portlandite (-a(OC  (-a(OC2 into insolu#le  insolu#le calcium car#onate 21.

-3" + *.3C ⇒ -&.7"C3 + &.3-C -2" + 3.3C ⇒ -&.7"C3 + 6.3-C 1n t!e a#sence solu#le !ic! calciumsu#se5uentl sulfate -3Aconvert reacts to rapidl to form t!e p!ases -2AC andof -*AC&;' -2AC?. 0!is is a rapi rapidd and and !ig! !ig!l l exot exot!e !erm rmic ic reac reacti tion on.. 1f fi fine nel l grou ground nd gps gpsum um (-a"O*D2C2O or !emi!drate (-a"O*D6.=C2O is #lended it! t!e -3A  prior to mixing it! ater t!en t!e initial reactions are controlled # t!e formation of a protective laer of ettringite on t!e surface of t!e -3A crstals.

2#.

-3A + 3- + 3"O* + 32C ⇒ -3A.3-"O*. 32C -3A + dissolved calcium (-a2+ + dissolved sulfate ("O*2B + ater ⇒ ettringite E!en t!e availa#le sulfate !as #een consumed t!e ettringite reacts it! -3A to form a p!ase it! a loer "O3 content knon as monosulfate. -3A.3-"O*.32C + 2-3A + *C ⇒ 3(-3A.-"O*.&2C Man studies !ave s!on t!at t!e !dration of -*AF (or more correctl t!e -2A B -2F solid solution is analogous to t!at of -3A #ut proceeds more slol. 0!e iron enters into solid solution in t!e crstal structures of ettringite and monosulfate su#stituting for aluminium. 1n order to reflect t!e varia#le composition of ettringite and monosulfate formed # mixtures of -3A and -*AFF t!e -*A t!e are refe referred rred to res respec pectiv tivel el as AFt AFt (alumi (alumino4 no4ferr ferrite ite tri trisul sulfat fatee !drateand AFm (alumino4ferrite monosulfate !drate p!ases. E!en cement is first mixed it! ater some of t!e added calcium 2. sulfate (particularl if de!drated forms are present' and most of t!e alkali sulfates present' dissolve rapidl. 1f calcium lang#einite is present t!en it ill  provide #ot! calcium and sulfate ions in solution' !ic! are availa#le for  ettringite formation.  "ulp!ur com#ined in t!e clinker p!ases as reported to !ave an accelerating on t!eeffect. cement setting !ile t!e sulp!ate com#ined as alkali sulp!ate !as effect a retarding

 

apidd sett api settin ingg # pota potass ssiu ium m sulp sulp!a !ate te is due due to t! t!ee sng sngen enit itee (,2(-a (,2 (-a"O* "O*2C 2C2O 2O format formation ion it it!! a red reduct uction ion of t!e sulp!a sulp!ate te from from t!e solution !ic! prevents satisfactor retardation of -3A !dration. ,2"O* causes a decrease of t!e flo value of fres! mortar specimen and loers t!e  #leeding and slump values of concrete concrete specimens "etting time is a function of clinker mineralog (particularl free lime 2;. level' clinker c!emistr and fineness. 0!e finer t!e cement and t!e !ig!er t!e free lime level' t!e s!orter t!e setting time in general. -ement paste setting time is ar#itraril defined as t!e time !en a pat of cement paste offers a certain resistance to penetration # a pro#e of standard cross4section and eig!t. 2