Sulfate Attack in Concrete
1. Introduction
Sulfate attack is a significant problem which locally affects concrete and other construction materials. Aqueous solutions of sulfate salts present in certain soils and groundwater attack Portland cement-based concrete. The speed and severity of the attack depends the amount of (soluble) sulfate available, the presence of water, the composition of the cement, and certain characteristics of concrete such as permeability. As progresses this attack, the hardened cement paste gradually loses its resistance, the process ends with the disintegration of the concrete.
Among the elements built in concrete likely to suffer from the attack of the sulphates present in certain terrains and in certain aquifers appear the footings, the walls of foundation, retaining walls, pillars, piles, culverts, pipes and slabs of surface.
The most serious attack occurs on elements with one face in contact with sulphate solutions, while the other side allows evaporation. This is the case, by example, for retaining walls.
This reaction has been the subject of research for some fifty years. A large part of early work on the question was carried out , where the problem it constitutes has been known for a long time and is continually under study.
2. What’s sulfate attack
Sulfate attack typically happens to ground floor slabs in contact with soils containing a source of sulfates. Sulfates dissolved by ground moisture migrate into the concrete of the slab where they react with different mineral phases of the hardened cement paste.
3. Source on sulfate attack in concrete
The causes of sulfate attacks on the concrete are three in number which are: soil,ground waters, Seawater,result of atmospherique or industrial. Soils and aquifers frequently contain sulfates extremely concentrated solubles. Their presence is believed to result from the relatively dryness and the "pumping" action it exerts on the sulfates present in the strata rich in lower level salts. In many countries there are surface deposits of this nature, consisting principally of sodium sulfate.
It has been found, especially in some countries, highly concentrated magnesium sulphate. To the soil surface, or in its vicinity, the compounds are generally not concentrated enough to interfere with the growth of crops and vegetation; they can however be enough to damage the concrete.
With a few exceptions, precipitation is more important because it keeps the level of sulphate concentrations at the depths low at which construction work is normally carried out.
In countries that contain gypsum deposits, concentrations may be high; It's not about however only localized phenomena. Calcium sulphates of natural origin such as gypsum are poorly soluble; they do not, by themselves, give rise to significant reactions noticeable. Sometimes, however, they are transformed, over time, into more soluble salts, such as sodium and magnesium sulphates.
High concentrations of soluble sulphates may be encountered in places which have served for a few years in coal storage. Marshlands have
Sometimes they too have high levels of such salts. In the case of deposits
minerals containing sulfides - such is the case for the pyrites of certain deposits of shales - oxidation, even due to simple bacteriological action, may be sufficient to cause transformation into sulfates. Sewage and some waste sometimes give off sulfur-containing gases that oxidize rapidly to form sulfates. Seawater attack of concrete has traditionally been treated as a problem entirely distinct from sulfate attacks that occur in soils and waters underground. The concentration of sulphates in seawater (in the soluble and free state) is extremely high; it is of the order of those which have proved harmful to concrete in groundwater. In seawater, a large proportion of the concrete is only partially submerged; it is therefore susceptible to acquiring high moisture contents and therefore subject to attack by sulphates. Frost action, erosion, corrosion of steel reinforcement, property damage due to ice and other causes of impact and abrasion,
further complicate the problem. The same complications can arise on the surface soils where the danger of sulphate attack comes from the nature of the soil or water underground.
4. Mechanism of the reaction
It has been demonstrated that the attack of concrete by sulfates results from a chemical reaction which produced in the presence of water between the sulfate ion and the hydrated calcium aluminate, and the calcium hydrate building blocks of hardened cement paste. The products that resulting from these reactions are ettringite and gypsum. The volumes of these solids are much larger than those of the bodies which gave rise to them; it results from stresses that can deteriorate the paste and eventually disintegrate the concrete.
It has been proven experimentally on many occasions, with mortars and concretes, the action destruction of this mechanism. Preventive measures have been developed whose effectiveness has been checked both in the laboratory and on site. They are based on the reduction or elimination of at least one of the four bodies which intervene in the preceding reactions. The principle that wants that, to stop the reaction, it is enough to neutralize only one of the bodies is, indeed, well established.
Although preventive measures based on the specific reaction with sulphates described above have proven effective in most cases, there are examples- where these measures are shown to be ineffective. Magnesium sulfate has been shown to be more corrosive than sodium sulphate; this finding has led to the assumption that the magnesium ion is involved in a separate reaction that could be destructive.
This second hypothetical reaction would explain the cases of deterioration occurring despite the use of a cement resistant to sulfates (low in calcium aluminate) and although the quality of the concrete cannot be questioned.
5. Effect of sulfate attack in concrete
Sulfate attack can lead to expansion, cracking, strength loss, and disintegration of the concrete. Sulfate attack is generally attributed to the reaction of sulfate ions with calcium hydroxide and calcium aluminate hydrate to form gypsum and ettringite.
But the presence of water-soluble sulfates in cement, aggregates, or from the intrusion of the environment penetrates in the concrete. They react with aluminates and the products formed occupy about 227% more volume.
6. Basic preventive measures
Because one can reduce in importance or prevent the reaction which tends to occur with sulfates by totally or partially eliminating at least one of the four reactants described above, the preventive measures are easily recognizable.
Sulfate ion if present in hazardous quantities (concentration values are
indicated below) and in soluble form, may even be prevented from entering the concrete by employing methods involving impermeable liners or drainage exclusion. On special installations such as underground water pipes, it has been used with some success with bituminous coatings. Some new coating materials have not yet demonstrated their effectiveness given their still limited use.
Obviously necessary to allow the sulphates to attack the concrete, water constitutes besides the sulfate ion carrier. It easily penetrates the concrete by capillarity. The waterproof coatings and drainage are, in this case again, the best preventive measures. One should not neglect, when drawing up the plans of the structures of foundations, to take into account the need to reduce or prevent the ingress of water in the concrete.
The reacting body of calcium aluminate hydrate comes from cement. It is possible,when making Portland cement, to reduce the amounts of calcium aluminate normally present, which will give the concrete sufficient resistance to attack by sulfates. Cements of this type, referred to as "sulfate resistant cements", have been manufactured and used for many years.
Their strength gains may be slightly less than those of Portland cements
normal. They are, moreover, essentially cements with low heat release,
advantageous characteristic in the case of massive concrete constructions. The specifications currently assign a maximum of 5 percent CA (calcium aluminate) to sulfate-resistant cements.
The fourth reactant, the calcium ion, is present as calcium hydrate and
is an inevitable product of cement hydration. Some special processes of
manufacture, however, make it possible to regulate its production. Such is the case during the treatment of concrete pipes under high pressure steam, or when using active pozzolan as a concrete admixture.
7. Sampling and testing of soils and underground forgeries
Groundwater samples can be taken by means of boreholes and, during
excavations, in seeps. Care should be taken not to dilute samples with
surface waters. Soil samples can be obtained during the surveys normally
carried out to examine the places intended for construction. They will be taken from various points laid out in a grid; at places with various stratifications, one will obtain additional samples.
To determine the sulphate content of groundwater, one can use in a
any chemical laboratory one of the classical methods of analysis. When analyzing the soils, the total sulfate content should be determined by extraction with hydrochloric acid hot diluted. To extract the water-soluble sulphates, a weight of water equal to the soil sample weight. These two operations are necessary to separate sulfate from calcium (usually gypsum), the solubility of which is low, the sulphates of sodium and
high solubility magnesium.
Groundwater concentrations are expressed in parts per million of SO4 or SO3. Concentrations in soil samples can be expressed as weight percent, or in grams per liter (of the liquid extracted by the above method) of SO4 or SO3 sea has a relatively fixed concentration of sulphate. No standard method for testing concrete has been developed. We may, however, make assessments by means of outdoor exposures and tests in
laboratory. The Building Research Division and some other agencies are in
able to provide all useful information and suggestions in each particular case.
8. How to treat sulphate attack on concrete?
Here are some of them
· Adequate concrete thickness
· Use a cement type that is resistant to sulfate attack
· Use low-permeability concrete
· Use low water/cement ratio (<0.45)
· Apply good compaction and curing
· Remove all salts from the mix constituents.
9. General Practical Recommendations
For a given severity severity category, determined from the concentrations of sulfates and mitigating influences, a distinction should be made between non-structural elements and structural reinforced concrete. More rigorous precautions are obviously necessary in the last case.
In cases where only moderate attacks are anticipated, cement may be used
Standard Portland. However, the water-cement ratio should not exceed 0.50. We will specify in addition to a minimum cement content, for example 550 pounds per cubic yard for concrete of frame. If sulphate-resistant cement is used, the minimum cement content maybe lower; one could, for example, use about 475 pounds per cubic yard.
When a very serious attack is anticipated, a sulfate-resistant cement should be used. The maximum water-cement ratio will be 0.50 and the minimum cement content will be of the order of 550 pounds per cubic yard.
The use of sulphate-resistant cement will be mandatory when an attack is anticipated extremely severe. The water-cement ratio should not exceed 0.45 and the cement content should be in the range of 600 to 625 pounds per cubic yard.
Placement and compaction techniques will be used to ensure the minimum
porosity and permeability. They will include in particular the use of air entrainers.This precaution is, moreover, generally speaking, always indispensable in the case of concrete placed in soils with a high sulphate content. While increasing maneuverability, and as a result of the density, the adjuvants are however not sufficient to ensure resistance to
sulfate attacks.
Impermeable coatings are recommended where existing conditions are
particularly dangerous or when other measures cannot be applied in full
of prevention. It should be noted, however, that the service life of coatings of this nature applied many years ago turned out to be quite short. We dispose today of bitumens, epoxies and other organic coatings whose behavior might be more satisfying. They can, in any case, ensure a good
protection when, for example, a fall in the level of the aquifer is expected
during and after construction. They can also protect concrete until it
becomes denser and less permeable. This last point can present a great
importance in the case of elements such as culverts and underground conduits.
Provision must be made, for concrete elements and frameworks at risk of being attacked by sulfates, interceptor drainage. We should also, in general, always replace high sulfate soils with granulated, well-drained backfill drained.
When structural elements have already suffered fairly serious attacks by sulfates, it is often possible to extend their service life by employing either one of the two following methods, or both methods simultaneously. When sulfates are brought in contact with the concrete by groundwater, it is sometimes possible to install systems of drainage to reduce or eliminate reactions. We can also often, example in the case of support pillars partially embedded in the ground, expose the damaged part of the concrete, eliminate the deteriorated materials and restore the construction by
using the "shotcrete" process, or by following other methods, We will pose, after this operation, a waterproof coating.
It is recognized that cements other than Portland cements have properties of sulfate resistance comparable to or better than that of Portland cements resistant to sulfates. The best known of these, and perhaps the only one generally available, is high alumina cement. Its advantages and disadvantages will not be studied in this digest.
When metal reinforcement is used, it must be placed in the concrete at a distance of at least 3 inches from the surface and at least 4 inches from the corners. For a given set of conditions, thin sections are more affected than solid elements.
10. Conclusion
The speed and severity of sulfate attacks depend on the concentration of the latter on the places, the type and degree of freedom of the sulfate ions, the ease of access to water, the type of cement and quality of concrete. Sulfate resistant cements, low ratios water-cement, minimum cement content, air entrainers, coatings waterproofing, installation of drainage devices, and special attention made to the covering of the reinforcement, make it possible, among other things, to fight against the action of sulfates.
