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Mechanism of cement paste degradation due to chemical and physical
processes = Mécanisme de dégradation de la pâte de ciment causée
par des processus chimiques et physiques
Litvan, G. G.
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Mechanism of Cement Paste Degradation
Due to Chemical and Physical Processes
by
G.G. Litvan
ANALYZED
Reprinted from
8th International Congress
on the Chemistry of Cement
Rio de Janeiro, Brazil, September 22-27,1986
p. 1-3
(IRC Paper No. 1509)
Price $3.00
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C.C. l i t v a n , Mvimion of Building Research, National Research Council Canada
FCllANISU
OF
q N TPASTE
D E C I W , ~ T I D U DUE TO CHENI+LAND PHYSICAL PROCESSES~ E C A N I ~ ~ E DE DEGRADATION DE
LA
PATB DE CIWZNT CAUSER PAR DRS IYOCESSUS c H I n I Q u t s ET PHYSIQUESSUPMARY: -0 area. of research a r e discueeed; namely, c l a r i f y i n g the reaeon. f o r volume i n a t a b i l i t y t h a t accompanies a11 degradation procaseas aId e o t a b l i r b i n g the r e l a t i o n s h i p between t h e c h . r a ~ t e r i s t i c e of t h e pore system and f r ~ e z e - t h a v resistance of concrete. Studies i n theae a r e u a r e needed, and promise t o y i e l d
r i g a i f i c a n t prograae i n improving d u r a b i l i t y of concrete.
SHo discutidas duas breas de pesquisa: ou seja, esclarecimento das raz6es para a in2
tabilidade de volume qua acompanha todos os.processos de degradacio, e estabelecinento da re-
laciio entre as caracteristicas do sistema de poro e a resistincia ao congelamento-descongela-
mento do concreto. Sio necessbrios estudos nesta irea, e prometem resultar em progress0 sig-
nificativo na melhora da durabilidade do concreto.
C.C. Litvan, Division of Building Research, National Research Council Canada
INTRODUCTION
The durability of c e e n t paste is a m e t topical field now that economic restraint and conservation are, and will remain, the rule of the day. My task as special reporter is to discuss future research trends in cement paste degradation; that is. what will probably be developed, and what has not but should be the subject of future
research. The very nature of the task thus compels me to go beyond the scientifically proven facts and engage in an expression of personal views and in some speculation which, I hope, will prompt some discussion.
The degradation process has s variety of causes :
-
undesirable reactions between a constituent of the cement paste and a substance originating from the surrounding environment (CO,
SO,,-, ,'LCseawater, various chemicals! ;
-
undesirable reactions between s constituent of the cement paste and the aggregate (alkali-aggregate reaction);-
freezing and thawing;-
ahrasion.The term "degradation" means that the paste fails to serve the intended purpose for the length of time expected. It should be noted that. normally, cement paste or concrete can and often does have an essentially indefinite life span, in the absence of degradation processes.
Volume Instability
The performance of cement paste is deemed to be unsatisfactory usually because of loss of structural integrity. Concrete as a construction material is used almost solely for structural purposes.
Why are volume instability and loss of
mechanical strength associated with certain chemical and physical processes in cement paste? This is then the central question in research on degradation to which we are seeking an answer. Most theories are in agreement concerning the primary causes of the various deterioration processes; for example, the reaction of lime and calcium silicate hydrate with carbon dioxide causes carbonation shrinkage, the reaction of sodium and potassium hydroxide with silica or carbonate causes major cracking in alkali- aggregate reaction. and exposure to below-freezing temperatures causes freeze-thaw danege. The reasons for the volume instability, cracking and loss of strength, however, are lesa well understood, as evidenced by the multitude of explanations existing for almost every process.
The possibility that all or many degradation processes have a common mechanism has not been considered. to my knowledge; yet we know that in nature valid explenatione are usually simple and universal.
Chemical degradation processes usually consist of dissolution of matter and formation of a new phase; for example, in alkaliaggregate reactions the dissolution of silica and the formation of silica gel. These reactions involve breakage of
bonds; in the cited example, those between the aggregate and the paste.
Cement paste is a composite system with built-in #tresses caused by drying shrinkage, moisture and temperature gradients. and chemical reactions, such as hydrations or ageing. The process of degradation thus results in a change of the level and distribution of stresses that will manifest themselves in volume changes leading, in extreme cases, to cracking and destruction of the entire solid.
It has been shorn that removing a constituent of the hardened paete; for example, leaching lime, results in expansion (1). It was observed that partial dissolution of the paste by HCL causes an increase of the volume of the remainder of the paste
(1). Sulfate attack and rusting of reinforcing steel are a variation of the same mechanism.
Carbonation of liov in the paste in situ does not involve breakage of the bond between the lime granules and the rest of the paste. In this procees, the addition of C02 onto the Ca(0H) exposed on the surface and the elimination oz H20 occur, vhich may even increase bond strength. Addition reactions or adsorption occurring on the surface have been found to result in contraction (2) and that, of course, is the volume change
accompanying carbonation.
Exploring the relationship between bond breakage, surface compound formation and volume changes appears to be a promising area of research. Freezing and Thawing
Air entrainment has made possible the use of concrete in northern countries. The still
unacceptably high rate of failure due to freeze-thaw action clearly indicates, however, that in practice the problem has not yet been solved satisfactorily. Sensitivity of the airlntraining process to a host of material, process and environmental factors seems to render the method far less reliable than is required.
One solution to the problem is to entrain air using pre-formed voids; that is, to add granules of solids having pores of the required size (3). The reason for discussing this approach here is the finding made in the course of its development that, while in conventional air entrainment only air voids larger than 10 um in diameter are believed to be beneficial, particulates which had smaller pores, mainly 0.3 to 2 um in diameter. were the most effective in protecting cement from freeze-thaw damage. In fact, granules with large pores
primarily between 7 and 10 um in diameter proved to be ineffective. It vas also found that air- entrained cement-paste specimens containing a high-range water reducer had very good freeze-thew
resistance if mainly smaller pores (0.3 to 2 ym in d i a m t e r ) were present (4). These observatione suggest that a review of the relationship batween pore characteristics and freeze-thaw durability of concrete in general, and the role of the large entrained air voids in particular, could be rewarding. A number of workers investigated the relationship between the pore characteristics and
C.C. Litvan, Division of Building Besearch, National Gsearch Council Canada
the frost resistance of aggregates (5-8). It is probable that the relationships between pore characteristics and frost resistance will prove to be the same for most porous solids.
Once determined, the critical pore characteristics can serve as the basis for
predicting frost resistance, making it possible to use material properties instead of performance under accelerated conditions that are seldom
representative of the real environment. N s o , the loss in mechanical properties caused by conventional air entrainment csn be reduced, or avoided
altogether. It may also be possible to find a chemical addxture that will entrain pores of only the required small sizes.
Prediction of Performance
Our understanding of the mechanism of the various degradation processes has been insufficient to solve problem such as prediction of performance of cement paste under specified conditions.
Assessment of the resistance of cement paste to carbonation, chemical attack, alkali-aggregate reaction and freeze-thaw resistance is currently based on performance in accelerated tests. This approach suffers from the inherent contradiction of the objectives: to speed up testing in order to obtain results as soon as possible, and to test for a long enough time to ensure the creation of representative conditions. Host of the current tests are not very reliable, presumably because the conditions of the accelerated test differ in a qualitative sense from those in the field. k
mentioned earlier, the best solution is to base prediction on material characteristics, but to do so the mechanism of degradation needs to be better understood.
Prediction of performance ie also hampered by the lack of knowledge about the effect of various factors and the combination of such factors on the overall degradation process. For example, it makes little sense to talk about frost-resistant cement paste without specifying the environment in which such a paste is expected to pcrforn without degradation. Although moisture content, rate of cooling. and wall thickness are recognized to be factors, their effect is seldom known in
quantitative terms. Performance can obviously not be predicted on the basis of only one side of the equation; that is, the material properties, without kqowledge of the characteristics of the environment and their influence on the process. Perhaps more than ever before, this knowledge is now required. For example, cemnt is being umed for the
construction of drilling platform, liquefied-gas storage tanks and chemical plants, often located in hostile environments.
Blended cements are already vldely used, but
their utilization will coatinue on a larger r u l e
along with new types of additives. The era of pure ordinary portland cement concrete will come to an end. The effect of composition on durability is fsr from being satisfactorily understood. and knowledge of long-term behaviour of pastes made of blended coments is even more uncertain.
Although many questions remain to be answered, there is little doubt that the know-how to produce cement paste with good durability will be
developed.
This paper is a contribution of the Division of Building Research, National Research Council of Canada.
REFERENCES
1. C.C. Litvm, "Volume Instability aE Porous Solids 1.' 7th International Congress on the Chemistry of Cemnt, Vol. 111, Paris. 1980, pp. VII. 46-VII. 50.
2. C.C. Litvan, "Volume Instability of Porous Solids'II. Mesolution of Porous Silica Glass in Sodium Mydroxide*. J. Materials Science
19,
No. 8, 1984, pp. 2473-2780.
3. C.C. Utvan and P.J. Sereda, "Particulate Admixture for Enhanced Freeze-Thaw Resistance of Concrete". Cement and Concrete Research
i,
No. 1, 1978, pp. 53-60.
4. C.C. Utvan, 'Air Entrainment in the Presence of Superplasticizers'. J. American Concrete Institute, July/August 1983, pp. 326-331.
5. R.D. Walker and Tai-Chou Hsieh, 'Relationship between Aggregate Pore Characteristics and the Durability of Concrete Exposed to Freezing and Thawing". Highvay Research Record No. 226, Highway Research Board, Washington, D.C., 1968. pp. 41-49.
6. Y. Koh and E. Knmada, "The Influence of Pore Structure of Concrete Made with Absorptive Aggregates on the Frost Durability of- Concrete". Proceedings RLLEU/IUPAC International Symposium on Pore Structure and Properties of ~ t h r i a l s , Prague, Sept. 1973, ARTIA, Prague, 1973, Vole 11, pp. F-45-F-62.
7. H. L n g e and S. &dry, 'Determination of the Frost Resistance of Limestone Aggregates in the Light of Poroqity fnvestigation'. Proceedings RILEM Symposium on hrability of Concreta, Prague, Sept. 1969, Building hsearch Institute 1969, pp. 8-129-B-138.
8. M. kneuji,
