Concrete is the most versatile and widely used construction material available. It is inexpensive and its components are manufactured from raw materials abundant the world over. Concrete is a composite material which, in its simplest form, is produced from coarse and fine aggregate, portland cement, and water. The function of the cement is to react with water to form the cement paste that glues the aggregate particles together into an artificial stone: concrete.

Just as cement formation involves a series of chemical processes, so does cement deterioration. Examples of deterioration mechanisms involving chemical processes are acid attack (e.g., in concrete exposed to agricultural or industrial environments), alkali-silica reaction (in concrete made with alkali-reactive aggregate), and sulfate attack. The chemical nature of concrete deterioration is often unappreciated; this lack of understanding results in the violation of some basic chemical principles that, in turn, lead to concrete deterioration and loss of its service life.

Concrete, Sulfates, and Sulfate Attack
Sulfate attack on concrete is a relatively rare but complex damage phenomenon caused by exposure of concrete products or structures to an excessive amount of sulfate from internal or external sources.

External sulfate from internal sources is more rare but originates from such concrete-making materials as hydraulic cements, fly ash, aggregate, and admixtures. For example, portland cement might be over-sulfated (contain more calcium sulfate than allowed by standards). Another possibility is the presence of natural gypsum or pyrite in the aggregate; this can, under adverse processing and environmental conditions, lead to damaging interactions with concrete components, thus causing durability problems. Admixtures also can contain small amounts of sulfates, but these amounts are typically well below dangerous levels. Proper quality control of concrete components will eliminate all of these potential problems completely. Cases of internal sulfate attack, especially in improperly cured concrete, have been reported in Europe, Canada, the United States, and elsewhere.

External sources of sulfate are more common and usually are a result of high-sulfate soils and ground waters, or can be the result of atmospheric or industrial water pollution. In some arid areas, for example, the soil may contain excessive amounts of gypsum or other sulfate-bearing deposits intimately intermixed with clays and other soil components or present in the form of veins. When exposed to ground water, these sulfates can dissolve and be transported to the concrete foundations, retaining walls, and other underground structures. In highly industrialized areas, concrete can be exposed to solid, liquid, or gaseous waste inadvertently released into the environment. Attack by external sulfates often occurs in arid areas with highly sulfated soils or might be caused by industrial waste waters.

Depending on the chemical form of the sulfate and the atmospheric environment to which the concrete is exposed, sulfate attack might show itself in different forms: it can lead to expansion and cracking of concrete, or to surface efflorescence (formation of salt deposits), spalling, and delamination, or to softening and complete loss of mechanical integrity. The severity of the attack depends on concrete quality, chemical nature and concentration of the sulfates, and environmental conditions.

Concrete quality is the most important issue in sulfate attack. The negative effects of environment, sulfate concentration, and composition are most noticeable in concrete of lower quality. High-quality concrete– concrete that has been properly designed (low water cementitious ratio, adequate cement content), produced (properly mixed and cured to reach high density and low permeability), and placed and finished–will not suffer from sulfate attack to the same degree as an inadequately cured, higher water-to-cement ratio concrete. High-quality, well-cured, and properly placed concrete might not suffer any damage even in the harshest sulfate environment.

The compounds responsible for sulfate attack are water-soluble sulfate-containing salts, such as alkali-earth (calcium, magnesium) and alkali (sodium, potassium) sulfates, that are capable of chemically reacting with components of concrete. Their adverse effect will depend on their chemical form (calcium sulfate, magnesium sulfate, sodium sulfate, etc.), their solubility in water, concentration in the water exposed to concrete, and duration of exposure.

The atmospheric conditions that might influence the severity of the concrete damage caused by sulfate are the fluctuations of temperature and humidity. Typically, under otherwise constant conditions, for concrete placed in a dry environment and at reasonably constant external temperatures, and in the absence of flowing water, the probability of sulfate attack is much lower than for concrete exposed to sulfates under highly variable moisture and temperature conditions.

Sulfate Attack Q & A
Which internal components of precast concrete might be unstable in the presence of sulfates?
When cement reacts with water to develop the desired chemical and mechanical stability (durability) and the required strength, certain important chemical compounds are formed within the cement paste. The most important component is calcium silicate hydrate (C-S-H) which, because of its importance, is sometimes referred to as the ?heart of concrete.? Some of these crucial cement paste compounds are unstable in the presence of sulfates; they might decompose or re-crystallize into different, useless compounds, causing the concrete to expand, lose its compactness and strength, and ultimately decrease its service life. For example, calcium hydroxide can be consumed in the production of ettringite and calcium within C-S-H can be removed or replaced by magnesium.

Can sulfates damage precast concrete? What form of sulfate-related damage might occur?
Yes, sulfate can damage precast concrete. Such deterioration of precast concrete is easily preventable by intelligent adaptation of the known best concrete-making practices. However, if the best concrete production practices are not followed, serious damage could occur.

Precast concrete can be exposed to a sulfate-bearing environment, such as ground water or atmospheric pollution. Depending on the other factors mentioned, concrete might lose its aesthetic qualities (surface cracking, exposure of aggregate), become more permeable, or even lose its desired minimum acceptable strength. The observed damage often is the consequence of several simultaneous damage mechanisms (e.g., sulfate attack and alkali-silica reaction or freezing-thawing); therefore, evaluation of the damage should be performed by a highly qualified laboratory.

Can sulfate attack be minimized by the use of special cements?
Yes and no. About 50 years ago, a special sulfate-resisting cement was developed for concrete used in sulfate-containing environments; today it is known in the United States as ASTM Type V cement (Type 50 in Canada). Sulfate-resisting cements have a low content of tricalcium aluminate, also referred to as C3A. During cement hydration, C3A reacts with gypsum to form certain compounds composed of aluminate, sulfate, calcium, and water. These calcium sulfo-aluminate hydrates are stable under normal conditions of concrete use, but could undergo chemical and physical changes upon concrete exposure to sulfates. The idea behind the Type V cement (Type 50) is to minimize the problem by decreasing the amount of aluminate (present in C3A), thus minimizing the possibility of sulfate-to-aluminate interaction.

Sulfate-resisting cements are not a cure-all against sulfate attack unless the concrete is of good quality. In other words, low-quality concrete made with sulfate-resisting Type V cement will not necessarily show resistance against sulfates. This is because sulfate-resisting cements do not prevent, but only decrease, the probability or the rate of concrete deterioration. In addition, such cements can be helpful only against some sulfate compounds. An example of such a situation is magnesium sulfate, where sulfate-resistant cement might help with respect to sulfate ions, but does not protect concrete against magnesium.

Can concrete degradation be caused by sulfate attack?
Yes, it can. The most typical violations include: wrong mix design (e.g., high water-to-cement ratio), improper handling (adding extra water, inadequate compaction), and improper curing.

Excessive water-to-cement ratio leads to high cement paste porosity which, especially if concrete is inadequately cured, results in easy movement of water and the chemical species dissolved within it through the pores (high permeability). At water-to cement ratios above approximately 0.4 to 0.45, the individual pores are connected, allowing water to permeate through it. In contrast, at lower water-to-cement ratios, the pores are separated from each other, resulting in a negligible rate of water permeation through the concrete.

Adding water at the job site to ease mixing and finishing is another way to increase the effective water-to-cement ratio. Inadequate compaction can result in high proportion of entrapped air, forming pathways for water to penetrate concrete.

Improper curing is one of the most common reasons for deterioration of precast concrete. Wrong heat treatment of concrete, especially at high temperatures and low relative humidities, can be most damaging. Excessive curing temperatures and a high rate of heating and/or cooling could lead to the formation of microcracks. Such microcracking can result in the decline of mechanical properties (such as modulus of elasticity and strength) and lead to increased water permeability of concrete, allowing otherwise stable concrete to undergo chemical deterioration due to penetration of sulfates, chlorides, carbonates, and other potentially harmful species.

Another form of heat-induced damage is directly sulfate-related and is sometimes referred to as delayed ettringite formation (DEF). The phenomenon is uncommon, but potentially costly to precast concrete producers.

What is DEF?
Delayed ettringite formation is a form of internal sulfate attack caused by heat-induced decomposition of ettringite formed during the initial hydration of cement in concrete. Ettringite is unstable at temperatures above approximately 165 (F) and will decompose upon heating. Cooling and subsequent moist curing of concrete ettringite could, under certain conditions, reform and cause expansion of the cement paste. This expansion, in turn, could lead to cracking of concrete, which would result in the loss of strength, high water permeability, and decreased service life.

The exact reasons for this kind of internal sulfate damage are complex and not completely understood. However, all evidence leads to the conclusion that reactive cements, especially under conditions of accelerated, high-temperature curing, are more susceptible to this kind of damage than cements that hydrate slowly under ambient temperature conditions. In other words, a finely ground Type III cement cured at above 165 (F) degrees, will almost certainly lead to DEF. In contrast, less reactive cements cured at temperatures below 165 (F) degrees will probably not develop this internal sulfate damage.

What can a precast concrete producer do to prevent sulfate attack?
The simplest answer is: follow the known best practices of concrete making. Thus, before a decision is made with respect to the use of concrete materials and appropriate curing, the conditions of concrete making and use must be properly assessed. The following rules represent a summary of precautions to be taken.

Concrete mix composition:
The materials used in the production of concrete must pass the existing specifications and have a proven history of satisfactory performance. The lowest possible water-cementitious ratio is recommended. Based on the atmospheric and environmental conditions of the projected concrete use, the reactivity of the to-be-used cement should be seriously considered and evaluated. Use of Type V (Type 50) or Type II cements, with or without supplementary materials, should be considered, but always taking into consideration the environment of use. Using some, but not all, mineral admixtures can be beneficial; therefore, prior testing is recommended. Note that fly ashes differ, thus not all of them lead to decreased permeability. The aggregate gradation should enable production of dense (well-packed) concrete resulting in low permeability.

Formwork:
The formwork material and its thickness affect the heat transfer; this fact must be taken into consideration when designing for homogeneous heat and humidity distribution within the concrete. Exposed concrete surfaces should be kept wet, but condensed water should not drip on them. When concrete products are stacked within the curing chamber, even distribution of heat and humidity should be maintained by proper circulation.

Curing of Concrete:
Precuring or preset time must be adequate to allow the cement to set properly. Depending on the type of cement used, this might be between two and four hours. Note that prematurely heated fresh concrete will develop lower strength, which could lead to decreased durability.

The heating rate of concrete should be steady (limited to about 25 to 35 (F) degrees maximum per hour) and the rise in temperature rise uniformly distributed for the whole product as well as within the curing chamber. The curing procedure will prevent formation of microstructural faults and cracks that might adversely affect long-term durability of the treated concrete member. Heat treatment must not lead to drying out of exposed surfaces, as drying and heating can result in pore coarsening; this is best achieved by maintaining adequately high relative humidity and its homogeneous distribution within the curing chamber.

When designing concrete to be exposed to heat treatment, the hydration heat of the cement should always be taken into consideration: it is the total heat input (heat of the ambient temperature plus heat of hydration plus heat added during curing) that controls the quality of the product. It is best to limit the maximum curing temperature to below 150 (F) degrees. Because heat dissipation is an important aspect of curing, the size of the concrete product has to be taken into account.

Post-curing:
Measures must be established allowing controlled cooling and prevention of premature dry-out; such measures allow elimination or reduction of crack formation and lead to improved durability by decreasing the permeability. The difference between the external and maximum internal temperature of a member should never exceed 35 (F) degrees. Curing and post-curing practices described above not only help in minimizing danger of sulfate attack but help in production of strong, durable concrete.

Quality Control:
Good quality control by knowledgeable technical personnel is essential. The most important aspects of proper heat-cured concrete production are control of the mix design and of the time-temperature regime described above (temperature, humidity). The temperature and its homogeneous distribution in the curing chamber should be monitored continuously and, more specifically, a good correlation between the curing chamber temperature and the temperature of the concrete should be maintained. Proper understanding of the possible damage mechanisms and of the beneficial aspects of proper mixing, placing, densification, and heat treatment, followed by correct application of such knowledge, will result in quality concrete.