Effect of MgO on Heat of Hydration and Set Time - Zheng

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CEMENTmadCONC RETE RESEAR CH. Vol. 22, pp. 1-5, 1992. Printed in the USA.0008-8846/92 5.00+.00. Copyright © 1991 Pergamon Press plc.

HYDRATION AND SETTING TIME OF MgO-TYPE EXPANSIVE CEMENT

Liu Zheng Cu i Xuehua and Tang MingshuDepar tm ent o f S i l ica te Engineer ing

Nan j ing Ins ti tu te o f Chem ica l TechnologyNa nj ing J iangsu 210009 P.R. Ch ina

Refereed)(Received M ay 21, 19 90; in final form Sept. 3, 1991)

ABSTRACT

The hydration and setting time of MgO-type expansive cement was investigated. The resultsshow that the addition of MgO retards the initial hydration of the cements and increases thesetting time. The causes for the retardation of hydration can be attributed to: (1) since thesolubility product constant of Mg(OH) 2 is far less than that o f Ca(OH) 2, and when MgO isadded and Mg(OH) 2 is formed, the ionic product for calcium and hydroxyl ions [Ca2÷][OH'] 2,

or Ca(OH) 2 saturation ratio, the ratio of Ca(OH) 2 concentrat ion to saturated Ca(OH) 2concentration under the same ionic strength is reduced in the liquid phase of the hydratingcement. And the time when the Ca(OH) 2 saturation ratio reaches the maximum is increased,the initiation of the second peak on the heat evolution curve of cement, i.e. the end of theinduction period, is delayed; (2) Mg(OH) 2, which is formed by MgO hydration in a high alkalimedium such as the liquid phase of hydrating cements, precipitates with a tiny crystallite size onthe cement grains and forms a protective layer, which retards further hydration of thecements.

Int roduct ion

MgO-type expansive cement was developed to compensate the thermal shrinkage in mass

concrete, and simplify or eliminate the costly temperature-controlling measures. The effects ofMgO on the properties of cement were investigated in another paper.O) Here, the effects of MgOon the initial hydration and setting time of cement were discussed in detail.

Many factors can influence hydration and setting time of cement, and the influences of variouschemical admixtures, slags, fly ashes, and silica fumes, etc. have been comprehensivelystudied.(2,3, 4) According to the different theories of cement hydration, these admixtures change thecomposition and concentration of the liquid phase of the hydrating cement, or react with somecomponents in cement to accelerate the hydration, or form a protective hydrate layer to retard thehydration. However, the effects of MgO on cement hydration are little understood.(2) Therefore, itis significant to investigate the effects.

Experimental Procedures

1) Materials and Samvles

Portland cement and commercial MgO, calcined at a certain temperature, were used in the work.


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2 Liu Zheng et al. Vol. 22 No. 1

Their chemical compositions were as follows:

Materials SiO2 A1203 Fe203 CaO MgO SO 3 I.L.

Cement 21.72 5.00 5.40 65.41 0.99 2.10MgO 0.35 0.22 1.34 1 .43 95.66 - 1.00

By adding different amounts of MgO into portland cement and mixing them to ensurehomogeneity, MgO-type expansive cements with diffrent contents of MgO were obtained. Thenumber of the cement samples vs. the content of MgO relations were as follows:

Sample No. F-0 F-2 F-3 F-4 F-5

MgO content ( ) 0 2 3 4 5

2) Test Methods

The setting time of the cements was determined by Vicat Needle (ASTM C191-79). The heatevolution curves o f the cements and MgO were measured with Microcalorimeter (Model 204D2,made in Japan) with 0.8 water/cement ratio and were performed for 2 days.

Results

1) Time of Setting

The setting time of MgO-type expansive cements is shown in Table 1.

Table 1The setting time of MgO-type expansive cement (hr:min)

Sample No. F-0 F-2 F-3 F-4 F-5

Initial setting time 2:43 3:11 4:00 5:35 6:00Final setting time 4:46 5:00 5:26 6:45 7:44

The setting time (the initial and the final) o f the cements is increased with the increase of MgOcontents. This indicates that the addition of MgO retards, to a certain extent, the initial hydration ofthe cements, increasing the setting time.

2) Heat Evolution of Hvdration

At age 2 days, the hydrat ion heat for MgO is 65.29 Cal/g, for the cement containing 4 MgO(F-4) 45.03 Cal/g, and for portland cement (F-0) 43.47 Cal/g. Their heat evolution curves are

shown in Figure 1.The rate of heat evolution for MgO is very large within 5 hours at an earlier time of hydration,

and then reduces gradually. The shape of the heat evolution curve for portland cement (F-0) issimilar to that of the expansive cement containing 4 MgO (F-4), but the second peak on the heatevolution curve for F-4 occurs later than F-0 by 1.2 hours. This suggests that the addition of MgO


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Vol. 22, No. I EXPAN SIVE CEME NT, MgO , HYD RATION, SETI ING 3

F-0• ~ F 4

11.__33 ~ . ) 2 . 5 h r s v~o

1 2 3 4O 5

tim (hours)

FIG. 1Heat evolution curves of the cements and MgO

delays the initiation and end of the acceleration period of cement hydration, i.e. prolongs theinduction period, and hence, to a certain degree, retards the initial hydration. According to LiChengmu's investigation,(5) for the cement mortars containing different amounts o f MgO, the timeof the second peak on the heat evolution curve increases with the increase of MgO contents, asshown in Table 2. These results of heat evolution of the cements and the mortars correspond to thatof the setting time.

Table 2The time of the second peak on heat evolution curve of the

mortars containing different amounts of MgO(5)

MgO content ( ) 0 3 6 9

Peak time (hours) 16 17 19 20

D i s c u s s i o n

Two main hypotheses have been advanced to explain the induction period of C3S and cement:protective hydrate theories and Ca(OH)2 nucleation theories.(2) The former suggests that the grainsof C3S or cement come into contact with water, an initial C-S-H (the first hyration product) isformed around the grains as a protective hydrate layer or a semi-permeable silicate hydrate to retardfurther hydration of the grains, thus initiating the induction period. While the latter proposes that[Ca 2÷] become supersaturated in the liquid phase o f hydrat ing C3S or cement, and only when thesupersaturation with Ca(OH)2 reaches a certain limit (1.5-2.0 times the saturation), Ca(OH) 2crystals nucleate and precipitate rapidly from solution. This means the end of the induction period.The ionic product for calcium and hydroxyl ions, [Ca2+][OH']2, changes with time and correlateswith the heat evolution of hydration. When [Ca2+][OH-] 2 reaches the maximum, the inductionperiod ends, which is closely related to the second peak on the heat evolution curve.(6) Therefore,what can influence the ionic product for calcium and hydroxyl ions, [Ca2+][OH-]2, can also affectthe rate of cement hydration. Uchikawa et al.(7) found that the addition of CaO increased the


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4 Liu Zh eng e t a l . Vol . 22 No. 1

Ca(OH) 2 saturation ratio, the ratio of Ca(OH) 2 concentration to saturated Ca(OH)2 concentrationunder the same ionic strength in the liquid phase o f hydrating cement, and shortened the time whenthe ratio reaches the maximum, hence accelerating the initiation of the second peak on the heatevolution curve and decreasing the setting time of cement. The precipitation of insolublehydroxides formed by cations from admixtures retards the hydration of C3S or cement, which isattributed by Kondo,(8) Thomas, et al.,(9) and Arliguie, et al.(1°) to the formation o f an impermeablelayer of insoluble hydrated products around the grains of C3S or cement. Whereas, Young(11)considered that the precipitation kept the contact solution from becoming supersaturated withCa(OH) 2, thus retarding the hydration.

Thermodynamically, the solubility product constants of Ca(OH)2 and Mg(OH)2 are 5.5 x 10 -6and 1.8 x 10 -11, respectively. (12) In solution, the following equations can be written

Ca2+ + 2OH- = Ca(OH) 2 $ (1)

K(sp) = [Ca 2+] [OH-] 2 = 5.5 x 10 -6

Mg 2+ = 2OH- = Mg(OH) 2 $ (2)

K(sp) = [Mg 2+] [OH-] 2 = 1.8 x 10 -11

As K(sp) for equation (1) is larger by 5 powers of ten than K(sp) for equation (2), Mg(OH)2precipitates in the liquid phase of hydrating cement before Ca(OH)2 does, thus the OH-concentration is lowerd. The ionic product for Ca 2+ and OH-, [Ca 2+] [OH-] 2, or the Ca(OH) 2saturation ratio, is reduced, and more time is needed to reach the maximum Ca(OH)2 saturationratio. Therefore, the addition of MgO delays the initiation of the second peak on the heat evolutioncurve, and prolongs the induction period and retards the acceleration period, which leads to the

increase of the setting time of the cements. Dynamically, the much slower hydration speed of MgOthan CaO causes further retardation of cement hydration.

According to Deng Min's studies,03) MgO hydrates in solution of higher pH value to formMg(OH) 2 with tiny crystals. From this, it can be inferred that as the MgO grains are very fine andare mixed homogeneously within cement, tiny crystals of Mg(OH) 2 formed by the hydration ofMgO may precipitate on the surface of the cement grains to form a protective layer, which, to acertain degree, retards further hydration of the cement grains, so as to delay the initiation of thesecond peak on the heat evolution curve and increase the setting time of the cements.

In brief, the retardation of cement hydration and the increase of the setting time by adding MgOmay be ascribed to both the decrease of Ca(OH) 2 saturation ratio due to the formation of Mg(OH) 2

and the precipitation of Mg(OH) 2 on the surface o f the cement grains to form a protective layer.

.

o n c l u s i o n s

The addition of MgO retards the initial hydration of cement and increases the setting time ofcement.

2. Two causes for the retardation of hydration are proposed:

(i) As the solubil ity product constant of Mg(OH) 2 is far smaller than Ca(OH)2, Mg(OH)2precipitates earlier than Ca(OH) 2. The formation of Mg(OH) 2 reduces the Ca(OH)2 saturationratio, thus delaying the initiation of the maximum of Ca(OH) 2 saturation ratio.

(ii) When MgO hydrates in high-alkali medium such as the liquid phase o f hydrating cement,Mg(OH) 2 with tiny crystals precipitates around the cement grains to form a protective layer,hence retarding further hydration of the cement grains.


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Vol. 22, No. 1 EXPAN SIVE CEMEN T, MgO , HYD RATION, SETI ING 5

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11-1/3 (198 0).3 . Ta y l o r, H . F. W. , P r o f . o f t h e 8 th I nt . C o n g r e s s C h e m i s t ry o f C e m e n t , R i o d e J a n e ir o , Vo l .

87 (1986) .4 . U ch ikaw a , H . , i b id , Vo l . I, 249 (1986) .5 . L i C h e n g m u , T h e R e s e a r ch o n t h e M e c h a n i c a l P r o p e rt ie s o f M g O - Ty p e D e l y e d E x p a n s i v

Con cre te , unp ub l i she d re su lt s , 1989 ( in Ch inese ) .6 . Yo ung , J .F., H .S . To ng and R .L . Berge r, J . Am er. Ceram. Soc . 60 , 193 (1977) .7 . U c h i k a w a , H . , S . U c h i d a , K . O g a w a a n d T. O k a m u r a , J. C e r a m . S o c . J a p a n 9 3 , 20 1 ( 1 9 8

( in Japanese) .8 . K o n d o , R . , M . D a i m o n , E . S a k a i a n d H . U c h i k a w a , J. A p p l . C h e m . B i o t e c h n o l . 2 7 , 1 9

(1977) .9 . Th om as , N .L . , D .A . Jam eson and D.D. Dou b le , Ce m. Concr. Res . 11 , 143 (1981) .10 . Ar l igu ie , G . , J .P. Ol l iv ie r and J. Grande t , C em . Concr. R es . 12 , 79 (1982) .11 . Y oun g , J .F. , S i l ica te Ind . 43 , 209 (1978) .1 2 . D e a n , J . A . , L a n g e ' s H a n d b o o k o f C h e m i s tr y, 1 3 th ed . M c G r a w - H i l l B o o k C o . , N e w Yo r

1985, pp . 5-8 .1 3 . D e n g M i n , M e c h a n i s m o f M g O E x p a n s i o n i n C e m e n t , M a s t e r' s T h e s i s, N a n j i n g I n s t i tu t e

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Effect of MgO on Heat of Hydration and Set Time - Zheng