2H2O and CaSO4 on the initial hydration of clinker having different burning degrees”

CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 737-739, 1985. Printed in the USA. 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.

A REPLY TO A DISCUSSION BY J. BENSTED OF THE PAPER "INFLUENCE OF CaSO4.2H20, CaSO4.1/2H20 and CaSO4 ON THE INITIAL HYDRATION OF CLINKER HAVING DIFFERENT BURNING DEGREES"*

H. UCHIKAWA, S. UCHIDA, K. OGAWAand S. HANEHARA Central Research Laboratory, ONODACement Co., Ltd. I - I - 7 , Toyosu, Koto-ku, Tokyo 135, Japan

Thank you very much for your appreciation on our paper ( I ) . I t is our pleasure that our opinion, the formation of C-S-H causes the s e t t i n g of fresh cement paste and the formation of e t t r i n g i t e by the hydration of calcium alumina~e and calcium aluminoferrite phase has a l i t t l e contribution to s e t t i n g , coincides well with yours(2). Your paper, however, d i d n ' t allude to the f a c t that the v i v i d hydration of a l i t e starts as soon as the Ca(OH)~ saturation r a t i o in l i q u i d phase attained to maximum level and the hydration of a l i t e is controlled by Ca(OH)p and CaSOa saturation r a t i o in l i q u i d phase rather than the hydraulic r e a c t i v i t y of cl~nker minerals. One of the authors had already reported that the addition of gypsum accelerates the hydration of CRS and a l i t e (3). Based on t h i s f a c t and the data reported by other researcMers, we agree with you in t h i s point.~,One of the reasons for t h i s acceleration is considered to be increase in CaL. in l i q u i d phase as pointed out by Wu et al (4). I t was confirmed by Tamas et al (5), Uchikawa et al (6), Mohan et al (7) and Parrot et al (8) that C-S-H formed from pure CRS and a l i t e in early stage of hydration consists mainly of dimer of s i l i c a t e s ~ I t is probable that sulphate ions might substitutes s i l i c a t e ion to form solid solution because both of them have s i m i l a r structure. But r e l a t i o n s h i p between the formation of sulphate-bearing C-S-H solid solution and the acceleration of C-S-H formation is not clear. I t might be unsuitable to suppose that the amounts of sulphate ions incorporated i n t o C-S-H structure during even in early stage of hydration u n t i l 3-4 hours influences the rate of formation of C-S-H. I f the assumption that the hydration of CRS is accelerated by incorporated sulphate ions into C-S-H is v a l i d , how can w~ explain the acceleration e f f e c t of Ca(OH)~ and CaCI~ in the hydration of CRS and a l i t e in which Cl and OH are s t r u c t ~ r a l l y quite d i f f e r e n t from s i l i c a t e ion ? Our results on heat evolution during early stage of hydration, change in Ca(OH) 2 saturation r a t i o in l i q u i d phase with time and setting time of the cement prepared from same c l i n k e r and phospho-gypsum, phospho-gypsum CCR 14, 645 (1984). 737

738

Vol. 15, No. 4 H. Uchikawa, et al.

containing Ca(OH)p and natural gypsum, are shown in Fig. I, 2, and Table I. Retarded hydration of alite by the phospho-gypsum was recovered by the addition of 2% of Ca(OH)2 to phospho-gypsum. Retardation of alite hydration and setting time occured in phospho-gypsum containing cement is considered to be caused by i n i t i a l low concentration of Ca(OH)2 and delayed increase in Ca~H)p saturation ratio in the liquid phase due to the consumption of Ca~- ahd OH" by acidic component in the gypsum. Premature stiffening is sometimes observed in the cement containing phospho-gypsum. C~A hydrated under low Ca(OH)~ concentration precipitates slender acicular ~ttringite crystals with high aspect ratio which increase yield value of fresh cement paste. This might be one of the reasons for premature stiffening of phosphogypsum containing cement. The most important factor controlling the hydration of alite, that is setting time of cement, is the level of Ca(OH)~ saturation ratio in liquid phase at i n i t i a l stage under utilization temperature and its time dependency (9). Table I. Setting time of cement kinds and amounts of gypsum added

setting time(hr.:min.) I.S. F.S.

natural gypsum (5%) phospho-gypsum (5%) phospho-gypsum containing 2% of Ca(OH)2 (5%)

2:49 4:41 3:01

4:09 6:06 4:15

composition of clinker: HM=2.10, IM=l.8, f.CaO=O.3%

9

6

• Natural gypsum

8

L

D Phospho-gypsum 3

.

• Phospho-gypsum containing 2% o f Ca(OH)2

C~ :IC)

2

2

I

0 0

I

10 Time ( h r . )

20

I

2

I

I

I

4

6

Time ( h r . )

FIG. l Heat evolution curves of cements Fig. 2 Ca(OH)p saturation ratio in liqui~ phase of cement paste (20°C, W/C=O.40) (20°C, W/C=4, Symbol is same as that in Fig. l) Your discussion touched the essentials of alite hydration and was very informative and useful for us.

Vol. 15, No. 4

739 DISCUSSIONS REFERENCES

1 H. Uchikawa, S. Uchida, K. Ogawa and S. Hanehara, CEMENTAND CONCRETE RESEARCH, Vol. 14 [5] pp. 645-656 (1984) 2 J. Bensted, Engineering Foundation Conference on Characterization and Performance Prediction of Cement and Concrete, Henniker, New Hampshire, 25-30th July 1982. (Editor: J. F. Young) pp. 69-86, Engineering Trustees Inc, Washington DC (1983) 3 G. Yamaguchi, K. Takemoto, H. Uchikawa and S. Takagi, Fourth I n t ' l . on the Chemistry of Cement, Washington, I, pp. 495-499 (1960) 4 Z. Wu and J. F. Young; J. Amer. Cer. Soc., Vol. 67 I l l

Symp.

pp. 48-51 (1984)

5 F. D. Tamas, A. K. Sarkar and D. M. Roy, Hydraulic Cement Pastes: Their Structures and Properties, Sheffield, 8-9th April 1976; pp. 55-72, Cement and Concrete Association, Slough (1977) 6 H. Uchikawa and R. Furuta, CEMENTAND CONCRETERESEARCH, Vol. II I l l 65-78 (1981)

pp.

7 K. Mohan and H. F. W. Taylor, CEMENTAND CONCRETERESEARCH, Vol. 12 i l l pp. 25-31 (1982) 8

L. J. Parrot and J. F. Young, CEMENTAND CONCRETERESEARCH, Vol. II [ I ] pp. 11-17 (1981)

9

H. Uchikawa, Ceramic Bulletin, Vol. 63 [9] pp. 1143-1146 (1984)