*IDS O R P T I O S - I S D REa4CTIOS. I The Setting of Plaster of Paris* BY H.1RVET A . S E V I L L E
The setting of plaster of Paris is accelerated or retarded by various inorganic and organic substanceq vhich apparently act as catalysts in the reaction. The effect of this catalysis is .uch that a plaster which ordinarily sets in rhirty-five minutes may be caused t o set in five minutes by the addition of an accelerator. or delayed for an hour or more by a retarder. I n attempting to account for thiq effect, Rohlanti' statetl that any substance which increases
FIG
I
The Katc of ,setting of Plaster of Pniia, and t h e >oluhility of Calcium Sulfate as influenced I)>- lminoniiim Sulfate.
the solubility of calcium sulfate will accelerate the setting of plaster of Paris, and that substances which decrease the solubility of calcium sulfate must retard the setting. That this explanation is unsatisfactory iq shown in Fig. I , in which the effect of increasing concentration of aininoniiini sulfate on the rate of setting of plaster of Paris is compared with the effect of ammonium sulfate upon the solubility of calcium qiilfate?. -A complete absence of any relation is demonstrated. Furthermore, the solubility of calcium sulfate is decreased by small amounts of practically all soluble sulfates. yet these exert a marked accelerating effect upon the setting of plaster of Paris.
* Contrilnitioii from t h r Department
of Chcmistr\- of the t-niversity of Illinois. P. Rohland: Z . Elelitrochem., 14, 4 2 1 ( 1 9 0 8I . T h e soluhility d a t a are from Seidell's Tables of Solubilities; the data for rate of setting of plaster of Paris are from experiments described in this paper.
The explanation offered 1)y I~ohlnntlTI based upon the assumption that the mechani,sm of the setting of plaster of Paris is: (a) The heniihytlrate. (*:!SOi . +FI?O, tlissolves in Trater to form a saturated solution. (1)) Hydration of the hcn~ih~-tlraic t o the tlihytlrate, C'aS04 . z H & , occurs. 01iil)lc than the hemihyclrate, the solution i c ) Since the dihytlrtrte is 1 is non- supersaturatctl and gypsmi ci t:tllizcs, forming a compact nia!:s of interlacing needle-shap+xl cry,stnl~. If this n-ere thc true mechanism of the process. the effect of aaltletl sillstance n-oi!ltl, no tlouht. he 2 s Iiohl:i11tl nniicipntctl. More recent results, hon.ever, intlicate tha,t colloitlal p1ienonien:i :UT concernetl i n tmhcsctting of plaster of Paris, as in the setting of lime-silicn ccments. ('nyazzi' nncl Ti,aiilie'. for int1el)cntlciit r('ason!:, siiggestccl that tlie setting of plaster of Paris consisted in the forixition of a gel which gradually changetl into the needle-shaped cry3t:ils of gypiiin. Trniilie studied the effect of accelerators upon the tinw of setting. 1iie:wiiring the tinie required for the plaster t o attain that state of hnrtlncs:.; v-liere it T K ~ S not tlisplacetl by B consitierahle pressure of his finger. IIe shon-et1that the cation!: are the important factors in tlie acceleration. antl concliitletl that their activity here agrees with the order in which they precipit:ite colloitls from solution. kinetic iiiethod of studying the problem \vas clevelopecl 11y KO.Ostn-alcl and \VolsIi3 Their procetlure was t o tiie:tsure. at intervals, the viscosity of sions of plaster of Paris ( z t o j per cent) antl t o consider the inosity an index of t'lie progress of the action. The 1-iscosity-time curyes are found t o he 8-~hnpetl:the viscosity rises slon-ly a t first and t'hen rnpitllp to a constant at it!: niasiniiuii xilue. By this niethod they were able to study the effect of yaryiiig the temperntiire, t'he concentration of the suspension, the age and fineness of the inaterial, arid the effect of added solutes. As a result of these esperiment. they conclude that the setting of plaster of Paris is a colloidal process. antl they point out the similarity of its viscosity curves to those of gelatin. A\
Experimental Part J l e t h o d a n d Jlaterz'als.-Since the hydration of plaster of Paris is an exothermal reaction. completely espressed, C'aS04 . +H?O+ I+HJI =C'aS04 . 2HpO+3,900 calories. it was thought that t,he reaction could best be followed thermometrically. By relating rise in temperature to tinie, a curve may be drawn which indicates the course of the reaction. By this method it. is possilAe t o use mixtures containing a much larger proportion of the solid t'han could be studied by the viscosity measurements, since the viscosity of mixtures which actually set quickly becomes infinite.
3
Cavazzi: Gazz. chim ital., 42 11. 626 (1912). Tmube: Kolloid-Z., 2 5 , 62 (19191. Ostwald a n d Kolski: Kolloid-Z , 27, 78 (1920).
I039
.iDSORPTIOS A S D R E l C T I O S
-4 high-grade commercial plaster of Paris \vas used. In each experinlent used n-ith 2 0 to 1 2 0 cc. of water or a solution. These materials were stirred for a tlefinite length of time in a paraffined paper cup. the cup \vas then placed within mother cup in i: u-ell insulated receptacle; a thermometer \vas placed in the mixture through a hole in the ashestos plate which covered the cup. The rise in temperature was noted at short intervals as the process of setting took place. Thp time-temperature curves showed a definite niaxiniiuiii n-hich K R S reprotlucil~le within 3 0 seconds. The time corresponding t o this peak vas considered the setting time for the particular mixture. and coniparatiw value$ for different mixtures were thus ohtained. 40 p. of the plast,er were
TABLE I 10 g. of plaster of P u i s stirretl $ minute with j o cc. of 11-ater or a solution. Time corresponding t o maxiniuni rise in temperature is given.
Tim(. i n M i n u t w
Pure Tvater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. HC'1.. . . . . . . . . . . . . . . . . . . ......................... . LlC1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . :. S u ( ' 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .' SHAC-1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ,' KC'1 ........................................... kBr. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
