Sulphates are chemical salts, which dissolve in water forming solution. Sulphates are found in most of the soils as calcium, potassium, sodium and magnesium sulphates. Out of these sulphates the solubility of calcium sulphate is low. Due to less solubility of calcium sulphate in water, ground contains less calcium sulphate than others. Ammonium sulphate usually is present in agricultural soil and water due to the use of fertilizers and in sewage and industrial effluents.
Decay of organic matter in shallow lakes and marshy land often form hydrogen sulphide (H2S) gas, having a very pungent smell. It can be transformed into sulphuric acid by bacterial action. Water used in concrete cooling towers also can be a source of sulphate attack on concrete. Thus natural as well as industrial water is a source of sulphate attack.
Solid sulphates do not react with concrete severely, but when in water solution they penetrate into the porous mass of concrete and react with the hydrated products of cement. Magnesium sulphate is more reactive and causes maximum damage to the concrete than others. Magnesium sulphate decomposes the hydrated calcium silicates as well as Ca(OH)2 and hydrated tricalcium aluminate (C3A). Eventually it forms hydrated magnesium silicate, which has no binding properties.
The sulphate attack or reaction is indicated by the characteristic whitish appearance on the surface. Usually the sulphate attack starts at the edges and corners, and after wards cracks and spalling of concrete start. The reaction between the hydrated products of cements and water solution of sulphate causes increase in volume of cement paste in concrete or mortar. In the hardened concrete calcium aluminate hydrate (C3A) can react with sulphate salt from outside through pores.
ADVERTISEMENTS:
The product of reaction is calcium sulpho aluminate, forming within frame work of hydrated cement paste. Due to this reaction the volume of solid phase increases upto 227%, causing gradual disintegration of concrete. The extent of sulphate attack depends on the concentration of sulphate solution and permeability of concrete.
The chemical equation of reaction of sulphates with hardened paste of cement is shown below:
(i) Reaction between sodium sulphate and calcium hydroxide:
(ii) Reaction with calcium aluminate hydrate:
Calcium sulphate reacts with only calcium aluminate producing calcium sulpho aluminate (3 CaO. Al2O3. 3CaSO4.31H2O). This product calcium sulpho aluminate is called as Ettringite. Molecules of water may be 31 or 32.
The magnesium sulphate not only reacts with calcium hydroxide and hydrated calcium aluminate like other sulphates, but also decomposes the hydrated calcium silicates completely and makes it breakable into pieces easily i.e. friable mass. Thus it is more harmful.
The rate of sulphate reaction (attack) increases with the strength of sulphate solution. A saturated solution of magnesium sulphate can cause serious damage to concrete of high w/c ratio with in a very short period where as concrete made with low w/c ratio can withstand the reaction of magnesium sulphate for 2 to 3 years. The strength of the solution is expressed as concentration, as the number of parts of mass of sulphur trioxide (SO3) per million parts of water (PPM). 1000 ppm is considered as moderately severe and 2000 PPm very severe especially if magnesium sulphate is the prominent constituent.
ADVERTISEMENTS:
Another factor which influences the rate of reaction is the speed of replenishment of sulphate consumed in the reaction. From this, it can be concluded that when concrete is subjected to pressure of sulphate containing water on one side, the rate of reaction is higher. Similarly, alternate wetting and drying due to tidal variation or spraying causes rapid reaction or attack.
Methods of Controlling Sulphate Effects:
For controlling the sulphate effects following measures may be adopted:
(a) Use of Sulphate Resistant Cement:
ADVERTISEMENTS:
To control or resist the sulphate effect on concrete the most effective and efficient method is the use of low calcium aluminate (C3A) cement. In general it can be said that 7% C3A content is a rough division between cement of good and poor performance in sulphate waters.
(b) Quality of Concrete:
A well designed, prepared dense and impermeable concrete exhibits better resistance to sulphur attack. Similarly a concrete prepared with small water-cement ratio exhibits a higher resistance to sulphur attack.
(c) Use of Air Entrainment:
ADVERTISEMENTS:
The use of air entrainment upto 6% of the mass of cement has been observed to have beneficial effect on the sulphate resisting qualities of concrete. This effect may be due to improvement of impermeability of concrete which controls the segregation, bleeding, and workability of concrete.
(d) Use of Pozzolana:
Replacement a part of cement by some pozzolanic material reduces the sulphate attack. Mixing pozzolanic material into the concrete converts the leachable calcium hydroxide Ca(OH)2 into insoluble non leachable cementitious product. The pozzolanic action improves the impermeability of concrete. The reduction in calcium hydroxide reduces the possibility of magnesium sulphate attack on concrete.
(e) High Pressure Steam Curing:
High pressure steam curing improves the resistance of concrete to sulphate attack. It is due to the fact that when high pressure steam curing is adopted, a certain percentage of cement is replaced by pozzolanic material which reacts with Ca(OH)2 improving the sulphate resistance property of the concrete.
(f) Use of High Alumina Cement:
The use of high alumina cement has been found to check the sulphate attack partly due to the absence of any free calcium hydroxide (CaOH2) in the set concrete in contrast to Portland cement. High alumina cement contains approximately 40% alumina. The high percentage of alumina when present in ordinary Portland cement it is very susceptible to sulphur attack.
But this high percentage of alumina present in high alumina cement behaves in a different way. The main reason of high resistance to sulphur attack is attributed to the formation of a thin protective film which checks the diffusion of sulphate ions into the interior of the concrete. It is observed that high alumina cement does not show higher resistance to sulphate attack at higher temperatures.
Miller and Manson on the basis of their research on the effect of sulphate on concrete for a period of about 25 years have concluded as follows:
(a) There is a definite correlation between the sulphate resistance of Portland cement and the amount of tri-calcium aluminate C3A cement contained. High resistance has been observed in Portland cement containing not more than 5.5% C3A.
(b) Finer grinded cements showed no effect on sulphate attack.
(c) The resistance of different Portland pozzolana cements varied very much.
(d) Four non Portland types of calcium aluminate cements consistently showed a very high resistance to the sulphate bearing water. However there were indications that these cements are not completely stable at temperatures above 21°C to 38°C.
(e) Steam cured specimen at 100°C and more specially at 176°C were highly sulphate resistant. The degree of improvement was greater for those cements which were not highly sulphate resistant originally, that is those cements having relatively high C3A.
(f) Out of the 40 admixtures tried, only few of them gave markedly improved resistance. The most effective were linseed, soyabean and tung oils.
When hardened Portland cement is exposed to ground water containing sulphate compounds, it is necessary to limit the permeability of concrete, the use of high, resistant sulphate cement and higher quantity of cement content. IS 456-2000 has given recommendations for the type of cement, max free water-cement ratio and minimum cement content.
These recommendations are reproduced in Table 17.8 below:
Note:
1. Cement content given in above table is irrespective of grades of cement.
2. Use of super sulphated cement is generally restricted where the prevailing temperature is above 40°C.
3. Super Sulphated cement gives an acceptable life provided that the concrete is dense and prepared with a w/c ratio of 0.4 or less, in mineral acids down to pH 3.5.
4. The cement content given in these tables are minimum recommended. For SO3 contents near the upper limit of any class, the cement content more than the values shown are recommended.
5. For severe conditions, such as thin sections under hydrostatic pressure on one side and sections partly immersed, the water cement ratio may further be reduced.
6. Portland slag cement conforming to IS 455 with slag content more than 50% exhibits better sulphate resisting properties.
7. Where chloride is encountered along with Sulphate in soil or ground water, ordinary Portland cement with C3A content from 5 to 8% shall be desirable to be used in concrete instead of sulphate resistant cement. Alternatively Portland slag cement conforming to IS 455 having more than 50% slag or a blend of ordinary Portland cement and slag cement may be used provided sufficient information is available on performance of such blended cements in these conditions.