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A Test for "unsoundness" of cements containing magnesium oxide
Ramachandran, V. S.
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N21 d
QD. 1261 National Research Conseil national
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Council Canada de recherches CanadaBLDG
A TEST FOR "UNSOUNDNESS" OF CEMENTS CONTAINING MAGNESIUM OXIDE
by V.S. Ramachandran
Presented at
VTT Symposium 50
Third international conference on the durability of building materials and components, Espoo, Finland
VOI. 3, 1984, p. 46
-
54Reprinted with permission k
DBR Paper No. 1261
Division of Building Research
T h i s p a p e r , w h i l e b e i n g d i s t r i b u t e d i n r e p r i n t form by t h e D i v i s i o n of B u i l d i n g Research, remains t h e c o p y r i g h t of t h e o r i g i n a l p u b l i s h e r . It s h o u l d n o t be reproduced i n whole o r i n p a r t w i t h o u t t h e p e r m i s s i o n of t h e p u b l i s h e r . A l i s t of a l l p u b l i c a t i o n s a v a i l a b l e from t h e D i v i s i o n may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , D i v i s i o n of B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a , O t t a w a , O n t a r i o , K1A 0R6.
A TEST FOR "UNSOUNDNESS" OF CEMENTS CONTAINING MAGNESIUM OXIDE
V.S. Ramachandran
Division of Building Research National Research Council Canada
Ot t awa
,
CanadaABSTRACT
Two normal pottland cements, each mixed with 0-25% Si02 and 0-52 MgO, were subjected to autoclaving, atmospheric steam, or water-curing at 50 or 100°C and their degree of hydration, microhardness, expansion and porosity determined. Maximum
expansion, hydration and porosity occurred in autoclaved cements containing MgO. A microhardness value of 66-71 kg/mm2 in
cements cured at 25°C for 28 days could be obtained by autoclaving cements containing 20-25% Si02 for 3 h. The cement-Si02 mixture exhibited lower autoclave expansion than normal cements and may be used for a realistic assessment of unsoundness.
Deux ciments Portland ordinaires ont chacun Et6 mElangEs B 0-25% de Si02 et 3 0-5% de MgO; ils ont ensuite Et.5 soumis B un
traitement en autoclave, 3 la vapeur atmosphgrique ou 3 un
traitement de cure B 50 ou 100°C. Leurs degrEs d'hydratation, de microduretg, de stabilits et de porosits ont ensuite 6t6
dEtermin6s. Les degrgs de dilatation, d'hydratation et de porositg les plus Qlevgs ont 6tS mesur6s dans les ciments contenant du MgO et soumis au traitement en autoclave. Une microduretg de 66
B
71 kg/mm2 dans les ciments ayant subi un traitement de cure B 25°C pendant 28 jours pourrait &reatteinte en autoclave au bout de 3 h avec les ciments contenant de 20 B 25% de Si02. Le m6lange ciment-Si02 a dEmontrE une plus grande stabilits en autoclave que les ciments ordinaires et peut Etre utilis6 pour faire une gvaluation rEaliste de
1 INTRODUCTION
In the chemical analysis of portland cement, MgO does not exceed 6%. Except for a small amount in the crystalline lattice of cement compounds, MgO exists predominantly in the free form. At the clinkering temperature of 1400-1500°C the free MgO is in a "dead-burnt state" in the form of periclase crystals. Under normal exposure to ambient conditions of humidity and
temperature it may take several years for periclase to hydrate. As the conversion of MgO to ~ g ( 0 H ) ~ involves a molar volume expansion of 117%, the presence of excessive amounts of MgO in hardened concrete may lead to unsoundness in the form of
expansion and crack development.
Standard specifications for the unsoundness test of portland cement containing MgO prescribe limitations based on physical and chemical requirements. The MgO content should not exceed a particular value and the cement paste should not undergo volume change above a specified limit when hydrated under prescribed conditions. In the Le Chatelier test the expansion is measured by subjecting the cement paste to boiling in water, but in this case only a small percentage of the total MgO may be hydrated. In the autoclave test method the length change in the cement paste bar is determined after exposure to high steam pressure at a temperature of 216°C for 3 h. All free MgO is expected to hydrate. One of the major objections to the use of the autoclave method is that under high-pressure steam-curing
conditions the cement paste forms a weaker matrix than it would under ambient conditions of curing. Consequently, exaggerated values for expansion can result /1,2/. Ramachandran and Sereda
1 3 1 found that two portland cements containing similar amounts
of MgO showed different autoclave expansion values because of differences in the mechanical properties of the cement matrix.
Previous studies have shown that by adding Si02 to cement under autoclaving conditions the mixture produces a hydrated product with characteristics similar to those of cements cured at
ambient temperature 141. It was thought that it might be more realistic to use cement-silica mixtures rather than cement alone in the autoclave test method. At first, in the present
investigation, different amounts of Si02 were added to normal portland cement and the mixtures were autoclaved to produce a matrix of a strength equal to that of neat cement paste cured at ambient temperature for 28 days. The applicability of this approach was examined by adding different amounts of MgO to cement-Si02 mixtures and determining their expansion,
microhardness, porosity and hydration characteristics before and after autoclaving.
2 EXPERIMENTAL
2.1 Materials
The composition of two normal type I portland cements is shown in Table 1. Standard Ottawa sand passing a 100-mesh sieve was the source of Si02. Magnesium oxide supplied by Baker Chemical Co. was calcined at 1400°C for 6 h to obtain dead-burnt MgO.
Cement-Si02 pastes containing 0, 5 , 10, 15, 20 and 25% Si02 were mixed with 0, 3, 4, 4.5 and 5% MgO at a waterlsolid ratio of 0.25 and cured at 25, 50 and 100°C, steam-cured at atmospheric pressure or at a pressure of 2 MPa. The pastes were prepared as follows: powders were mixed, water was added and the mixture transferred to a cylinder 2.5 cm in diameter. The pastes were kept under vapour-sealed conditions and rotated until set in order to prevent segregation. At the required time, the cylindrical samples were then cut in the form of discs and subjected to the tests.
2.2 Technique
A Leitz miniload hardness tester was used to determine micro- hardness. The test method employed a Vicker's pyramid indentor, the indentation value being the average of ten determinations. A Differential Scanning Calorimeter (DuPont model 1090) was used to estimate amount of Si02, Ca(OH)2 and Mg(OH)2 in the samples.
A s t a n d a r d a u t o c l a v e w i t h a cage housed t h e d i s c s , and a d i a l micrometer was used t o measure e x p a n s i o n a f t e r t r e a t m e n t . T o t a l p o r o s i t y was d e t e r m i n e d w i t h a mercury p o r o s i m e t e r s u p p l i e d by Aminco-Winslow Co. D e t a i l s of t h e s e t e c h n i q u e s a r e a v a i l a b l e / 5 , 6 / .
3 RESULTS AND DISCUSSION
F i g u r e 1 shows a u t o c l a v e e x p a n s i o n p l o t t e d as a f u n c t i o n of t h e amount of MgO p r e s e n t i n cements. Both cements A and B e x h i b i t almost no expansion up t o a b o u t 2%, b u t above 3-3.5% t h e c u r v e s become s t e e p . S m a l l e r e x p a n s i o n s a t lower c o n c e n t r a t i o n s of MgO may be a t t r i b u t e d t o t h e accommodation of i n t h e p o r e s of t h e p a s t e and t o t h e a b i l i t y of t h e p a s t e t o c o u n t e r a c t
s m a l l e r e x p a n s i v e p r e s s u r e s . A t any p a r t i c u l a r c o n c e n t r a t i o n of MgO cement B showed h i g h e r e x p a n s i o n t h a n cement A. C o n t a i n i n g h i g h e r amounts of C3S, cement A e x h i b i t e d h i g h e r s t r e n g t h t h a n cement R on a u t o c l a v i n g and may t h e r e f o r e be a b l e t o r e s i s t e x p a n s i v e f o r c e s more e f f e c t i v e l y / 3 / ( s e e F i g u r e 2 ) .
A f t e r c u r i n g a t 25OC f o r 28 d a y s , cements A and B showed microhardness v a l u e s of 7 1 and 66 kg/mm2, r e s p e c t i v e l y . I n a u t o c l a v e d cement-Si02 m i x t u r e s c o n t a i n i n g d i f f e r e n t amounts of S i 0 2 , t h o s e c o n t a i n i n g 20-25% S i 0 2 had s t r e n g t h s of 69-72 kg/mm2 ( T a b l e 2 ) .
Cement A c o n t a i n i n g 5% MgO and cement B c o n t a i n i n g 4% MgO showed a u t o c l a v i n g e x p a n s i o n s of 4.7 and 5.7%, r e s p e c t i v e l y , and t h e c o r r e s p o n d i n g v a l u e s f o r cement-Si02 m i x t u r e s were 1.3 and 0.5%. The reduced e x p a n s i o n i n cement-Si02 m i x t u r e s may be a s c r i b e d t o t h e h i g h e r s t r e n g t h s t h e y developed. I n b o t h t h e r e i s e v i d e n c e t h a t a l a r g e amount of f r e e Ca(OH)2 formed d u r i n g h y d r a t i o n and t h a t i t i n t e r a c t e d w i t h S i 0 2 t o d e v e l o p a s t r o n g e r body
( T a b l e 2). These r e s u l t s show why, i n t h e s t a n d a r d a u t o c l a v e t e s t , e x a g g e r a t e d expansion v a l u e s r e s u l t .
0 0 1 2 3 4 5 MgO, % MgO, % Figure 1. Effect of different amounts of MgO on the autoclave expansion of cements Figure 2. Effect of different amounts of MgO on the microhardness of autoclaved cements
T a b l e 1. Composition of p o r t l a n d cements
Samples Oxide A n a l y s i s , % M i n e r a l o g i c a l Composition, % Surf a c e
Area,
S i 0 2 A1203 Fe203 CaO MgO SOj A l k a l i C3S C2S C 3 A C,AF m2 /kg
Cement A 21.93 4.59 2.56 64.03 2.10 2.41 0.53 52.63 23.14 7.83 7.79 338 Cement B 21.16 5.87 2.21 63.07 1.56 3.29 0.79 43.96 27.47 11.82 6.73 332 T a b l e 2. I n f l u e n c e of S i 0 2 on s t r e n g t h and h y d r a t i o n e f f e c t s i n a u t o c l a v e d cement-Si02 m i x t u r e s Cement A Cement B 2
i n m i x t u r e Microhardness % S i 0 2 % Ca(OH)2 Microhardness % S i 0 2 % Ca(OH)2 kg/mm2 r e a c t e d formed k g / m 2 r e a c t e d formed
Table 3 compares the porosity of autoclaved cement with that of cement-Si02 mixtures containing 4 or 5% MgO. In cement A
containing 5% MgO, the addition of Si02 decreases porosity from 33.1 to 24.8%. In cement B containing 4% MgO, porosity
decreases from 33.5 to 24%. Lower porosity values in the mixtures may be explained by the lower expansions they
exhibited. Strengths in the mixtures were also higher, being greater than 50 kg/mm2 compared with 20 kg/mm2 for cements containing no Si02.
That the autoclave treatment converts almost all added MgO can be concluded from the results tabulated in Table 4. Results for Mg(OH)2 are based on the weight of the hydrated material.
The relative effects of various treatments on the conversion of MgO to Mg(OH)2 are shown in Table 5. Curing at 50°C was
ineffective in hydrating MgO. Conditions other than autoclaving do not completely hydrate MgO unless curing is extended to
several days. Expansion values tend to be lower in pastes cured under non-autoclaved conditions. In cement-Si02 mixtures
containing 5% MgO, autoclaving (3 h), boiling (2 days), steam curing (1 day) and curing at 50°C (2 days) produced length changes of 1.3, 0.3, 0.09 and 0.3%, respectively. The
corresponding values for cement B-Si02
+
4% MgO were 0.52, 0.3, -0.13 and -1.10%, respectively.4 CONCLUSIONS
Normal type I portland cements containing equal amounts of MgO may exhibit different autoclave expansions. Under autoclave treatment there appears to be a critical amount of MgO beyond which a steep increase in expansion occurs; and cements with 20-25% Si02 show strengths similar to those of cements cured at 25°C for 28 days.
Cement-Si02 mixtures containing different amounts of MgO show lower expansion than cements in autoclaving because the mixtures
-
5 3-
T a b l e 3. P o r o s i t y v a l u e s of cement and cement-Si02 m i x t u r e s c o n t a i n i n g MgO P o r o s i t y , % No. M a t e r i a l Cement A Cement B 1 Cement, 25OC c u r e d 28 d a y s 13.5 14.8 2 Cement, a u t o c l a v e d 22.2 22.5 3 Cement+
4% MgO 25.5 33.5 4 Cement+
5% MgO 33.1-
5 Cement+
20% S i 0 2 a u t o c l a v e d 20.5 22.4 6 Cement+
20% S i 0 2+
4% MgO 25 .O 24 .O 7 Cement+
20% S i 0 2+
5% MgO 24.8-
T a b l e 4. Amount of Mg(OH)2 i n a u t o c l a v e d cements c o n t a i n i n g d i f f e r e n t amounts of MgOMg(OH)2, % % MgO added Cement A Cement B 0 0 0 3 4
.o
3.7 4 5.2 4.9 4.5 5.7-
5 6.5-
T a b l e 5. E f f e c t of c u r i n g p r o c e d u r e s on amounts of Mg(OH), produced i n cement-Si02 (20%) m i x t u r e sCuring p r o c e d u r e Cement A
+
20% S i 0 2 Cement B+
20% S i 0 2+
5% Mgo+
4% Mgo A u t o c l a v i n g 2 MPa, 3 h 4.9 4.3 R o i l i n g , 8 h 2 .O 1 .O B o i l i n g , 2 d a y s 4.6 3.4 Steam c u r i n g , 1 day 3.1 2.4 A t 50°C, 2-3 d a y s 0 0develop higher strengths. The autoclave method hydrates almost all added MgO, whereas treatments such as boiling, curing at
50°C or with steam at atmospheric pressure result in only partial hydration and lower expansion.
The standard autoclave test method is too severe, and attempts should be made to modify it. One solution would be to use cement-Si02 mixtures rather than cement alone.
5 ACKNOWLEDGEMENT
Grateful thanks are due to G.M. Polomark for experimental assistance. This paper is a contribution from the Division of Building Research, National Research Council Canada, and is published with the approval of the Director of the Division. 6 REFERENCES
1. Ramachandran, V.S., Unsoundness of cements containing MgO and CaO. 11 Cemento 77(1980)3, pp. 159-168.
2. Ramachandran, V.S., Unsoundness of cements containing MgO and CaO iscu cuss ion). I1 Cemento 78(1981)3, pp. 145-149.
3. Ramachandran, V.S. and Sereda, P.J., The role of cement in autoclave expansion of cement-lime-limestone mixtures. World Cement Technology 9(1978)1, pp. 6-8.
4. Ramachandran, V.S., Feldman, R.F. and Beaudoin, J.J., Concrete Science, London, Heyden & Son, 1981, 427 p. 5. Ramachandran, V.S. and Beaudoin, J.J., Significance of
water:solid ratio and temperature on the physico-mechanical characteristics of hydrating 4Ca0.A1203.Fe203. Journal of Materials Science 11(1976)10, pp. 1893-1910.
6. Ramachandran, V.S., Feldman, R.F. and Sereda, P.J., An unsoundness test for limes without cement. Materials Research and Standards 5(1965)10, pp. 510-515.
