Concrete sulphates attack

Taylor, H.F.W. In Marchand, J., Skalny, J. (eds.) Discussion in Materials Science of Concrete Sulphate Attack Mechanisms, p. 46. The American Ceramic Society, Westerville... 

Air-entraining admixtures, therefore, produce concrete which is more durable to conditions of freezing and thawing, particularly in the presence of de-icing salts, more resistance to sulphate attack, provides better protection to embedded reinforcement and is more tolerant of poor curing conditions. There appears to be no... 

Fig. 12.7 SEM (backscattered electron image) of a concrete showing massixe deposits of ettringite (one marked E ), which through sulphate attack has grown on aggregate surfaces, with associated cracking. Courtesy WHD Microanalysis Consultants Ltd, Ipswich, UK (H64).

Corrosion is not the only deterioration mechanism in reinforced concrete. Alkali-silica reactivity (ASR), sulphate attack, thurmasite attack, delayed ettringite formation, freeze thaw, thermal movement, settlement and other movement can all lead to concrete damage and their assessment must be included in the surveys. 

Plasticizers and superplasticizers improve radically the pore stmcture (effect of w/c) and concrete becomes less permeable to air and water [159], Collepardi and Massida [159] found a capillary porosity and pore size lowering with decreasing permeability of concretes in which water reducers were used. The resistance to the sulphate attack was also improved [159]. 

The sulphate attack has been known from a long time, and already in 1858 Vicat [247] studied the chemical causes of hydraulie eompounds corrosion in sea water [247]. Bied [248] invented the teehnology and developed the production of calcium aluminate cement, as a remedy for rapid destmetion of concrete in France, caused by the sulphate ground water attaek, from the dissolution of gypsum and anhydrite. 

In spite of numerous research there are still many umesolved and controversial problems inspiring the intensive discussions. A good example is the problem of ettringite and the mechanism of its expansive action. There are maity hypotheses concerning this problem, but they all are at least doubtful [176], On the basis of considerations presented by Scherer [173], Brown and Taylor [176] are willing to accept the crystalhzation pressure of micrometric particles as a main cause of ettringite expansive effect. Simultaneously, as a rule the expansion of concrete due to the sulphate attack is always linked with the formation of ettringite. 

Similarly, as in the case of the other types of corrosion, the inner and outer concrete corrosion can be distinguish, as it has been mentioned earlier, and, in the case of sulphate attack, this classification can be considered as a classic one. 

The sulphate attack is more severe as the concrete is subjected to cyclic wetting and drying. Therefore the laboratory expansion measurements do not reflect completely the field conditions. Mehta [251], basing on the observations of various concrete structures, foimd that the decrease of adhesion and strength, as well as the... 

The thaumasite CaSO CaSiOj CaCOj 15H2O [257, 258] appears often, apart from gypsum and aluminum hydroxide or silica gel, in the deteriorated concrete. The formation of this phase takes place when the sulphate attack occurs at low temperature and together with an intensive carbonation process. 

As it has been mentioned earUer, during the sulphate attack, as in the case of the other corrosion processes, the zonal phase composition changes of the paste, with the decreasing concentration of sulphate ions, in the direction to the concrete interior along the diffusion path, is observed. Gollob and Taylor [256] studied the... 

Fig. 6.63 Appearance of normal features not subjected to sulphate attack A—unhydrated cement, B—dense inner C-S-H gel surrounding unhydrated cement, C—inner C-S-H gel constituting fully hydrated cement grain, D— region of small hollow shell hydration grains, E—groundmass or outer product C-S-H gel, F—Ca(OH)j surrounding a sand grain chip, G—deposit of calcium hydroxide within the groundmass (After [261]) Diamond S., Lee R.J. in Materials Science and Concrete, Special volume Sulfate Attack Mechanisms (J. Marchand amd J. Skalny eds.), p. 138, Fig. 1, 1999, published by The American Ceramic Society, 735 Ceramic Place, Westerville, Ohio 43081, 2001, reproduced with the permission of The American Ceramic Society...

The sulphate attack with C-S-H gel decomposition and thaumasite formation is particularly harmful for the durability of concrete, because it occurs with the destraction of the most important binding constituent of concrete and leads to the collapse of this material. Because of the relatively low temperatures of thaumasite formation, the concrete foundations, and elements of the underground sewage systems, as well as the concrete road elements are especially susceptible for this type of destructioa In order to prevent thaumasite formation the classic approach should be applied, first of all the permeability of concrete should be reduced, and the use of cement with mineral additions should be considered. Bensted [273] suggested the lowering the C3A and alite content in cement. 

It should be mentioned that in a warm, dry climate the chlorides improve the sulphate attack [280], Presumably, it is the reason, apart from the known effect of temperature on the rate of chemical reactions (Arrhenius mle), that in a warm climate the concrete corrosion occurs more rapidly than for example in the North Sea. The examples of disastrous quick concrete corrosion in the warm seas in the Middle East are known, as well as the excellent durability of drilling platforms situated on the North Sea [278]. The conditions governing in different geographic zones temperature, erosion, biological environment, are highly variable and therefore the mechanisms of concrete deterioration are greatly modified [61]. 

Mehta, P.K. Sulphate Attack on Conrete A Critical Review. In Skalny, J. (ed.) Sulphate Resistance of Concrete III. The American Ceramic Society, Westerville (1992)... 

Piasta, W. et al. (1997) Influence of limestone powder filler on microstracture and mechanical properties of concrete under sulphate attack, in Proceedings 10th ICCC, Goteborg, paper 4iv 018. 

Mortars and concretes made with high alumina cement are resistant against sulphate attack and also are better at resisting CO2 from ordinary drinking or mineral water. In comparison, the resistance against alkalis is lower than that of Portland cements. 

The sensitivity of concrete structures to sulphate attack is strongly related to the exposure conditions. Structures in an environment of high sulphate content in the air or in w ater, for example sewage tunnels, are particularly vulnerable. After sulphate ions penetrate the pore system of cement paste, complex reactions start with C3 A leading principally to two kinds of processes gypsum corrosion and sulphoaluminate corrosion (Mindess etal. 2003). The products of sulphate reactions with cement expand and can cause cracking and destruction. The permeability of the material s structure and the quality of cement decide upon the rate of these processes. Special Portland cements as well as high alumina cements may be used for elements exposed to sulphates (cf. Section 4.1.1). 

Matrix problems, such as sulphate attack or freeze-thaw cycling are not unique to FRC composites, and they can be tested by the procedures developed for conventional mortars and concretes. Generally, the presence of fibres will have little effect on these chemical and physical processes. 

This cement is very resistant to attack by sulphates, sea water and acid waters. It is also used with crushed firebrick to produce refractory concrete. 

Sulphate in general appears to behave very similarly Hatch and Rice have shown that small concentrations in distilled water increase corrosion more than similar concentrations of chloride". In practice, high-sulphate waters may attack concrete, and the performance of some inhibitors appears to be adversely affected by the presence of sulphate. Sulphates have also a special role in bacterial corrosion under anaerobic conditions. Both sulphates and nitrates are acceptable in low-pressure boiler feed water as they are believed to be of value in controlling caustic cracking. 

By consuming the Ca(OH)2 the pozzolan makes it unavailable for sulphation and carbonation reactions and increases the density of the concrete, which lowers its permeability and reduces the risk of chloride attack. 

Destructive expansion from reaction with sulphates can occur not only if the latter are present in excessive proportion in the cement, but also from attack on concrete by sulphate solutions. The reaction involves the Al,0,-containing phases in the hardened cement, and in sulphate-resisting Portland cements, its effects are reduced by decreasing the proportion of the aluminate phase, sometimes to zero. This is achieved by decreasing the ratio of AljOj to Fe203 in the raw materials. In the USA, sulphate-resisting Portland cements are called Type V cements. 

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