Sulfate Attack Resistant Cement
Concrete used for environmental applications will most likely be prone to abuse from components of the environment. Concrete that will encounter some of these harsh conditions must therefore be somewhat resistant to them. Many concrete applications of civil and environmental engineering will experience sulfate attack.
Sulfate attack is the term used to describe the series of chemical reactions between sulfate ions and the components of hardened concrete. Many of the components of the hardened concrete that react with the sulfate ions come from cement paste. Sulfates will cause the most damage when they are in the gas or liquid state. It is important to note that not all sulfate attacks will be the same. The type of environmental and physical conditions that the sulfate reacts with governs how much and how strenuous the damage will be.
There are two mechanisms that can be considered a sulfate attack. These two mechanisms are:
1. Formation of gypsum
2. Formation of ettringite
Both of the mechanisms listed above will damage concrete. The damage is caused by an increase in overall solid volume. They will also cause a loss of strength and loss of mass. This is caused by the deterioration of the cohesiveness of the cement hydration products. Once again, it is important to note that the amount of damage relies heavily on the surrounding environmental conditions.
Sulfate attack is broken up into two parts. They are categorized as “chemical vs. physical” and “internal vs. external.” The chemical sulfate attack is caused by chemical reactions involving the sulfate anion (SO42-). This reaction causes an increase in solid volume and may lead to an expansion in the concrete.
The physical sulfate attack is caused by the formation from the solution of sodium sulfate decahydrate, followed by its repeated recrystallization into sodium sulfate anhydrite. This process is temperature dependent. It leads to repeated increase in volume, which can lead to fatigue and a loss of cohesion.
Internal sulfate attack refers to attack where the source of sulfate is internal to the concrete. The internal source of sulfate can come from the cement, aggregate, chemical admixtures, water, or supplementary materials.
External sulfate attack refers to attack where the source of sulfate is external to the concrete. The external source of sulfate can come from groundwater, soil, solid industrial waste, fertilizers, etc.
Sulfate Resistant Cements
There are many different types of cement that can sustain sulfate attack. “The three main types can be categorized as follows:
A high percentage of concrete uses Portland cement in its mixture. Ordinary Portland cement contains tricalcium aluminate (C3A). The presence of this clinker makes the sulfate resistance of ordinary Portland cement limited. This is because in the first phase of hydration ettringite is yielded. After the calcium sulfate within the mixture has been consumed, it is converted to monosulfate. Once the concrete has hardened, it will be exposed to sulfates from an external source and the monosulfate converts back to ettringite. This will lead to scaling, cracking, and loss of cohesion.
A sulfate resistant Portland cement exists. In this version the amount of Al2O3 in the clinker is reduced. The oxidation of this material mainly occurs within the ferrite phase. The amount of C3A is decreased or may not be included. The amount of ettringite formed in the hydration of this cement is therefore notably reduced. Due to this, the amount of monosulfate available for reaction is also reduced. This will help reduce the amount of scaling, cracking, and loss of cohesion.
Although many measures can be taken to resist the amount of sulfate attacks, sulfate resistant Portland cement cannot eliminate all attacks. This is due to the hydration of the ferrite phase is very slow and some ferrite may be present in mature pastes. When this ferrite reacts with sulfate ions, ettringite can be formed. Expansion will result from this. Although expansion will occur, it will progress much more slowly and less severe than if normal Portland cement was used. The main reason that sulfate resistant Portland cement is efficient is due to the fact that the amount of monosulfate present in the mature paste has been greatly reduced. It is important to note that the use of sulfate resistant Portland cement helps minimize expansion. “It does not apply to its ability to resist degradation of the Calcium Silicate Hydrate (C-S-H) phase, a damage mechanism typical or magnesium sulfate attack.” (Marchand p.108).
Another type of Portland cements that resist sulfate attack better than normal Portland cement is “Fly ash-Portland Cements.” A cement is to be considered a “Fly ash-Portland Cement” when 30% of the clinker is replaced by fly ash. When using Fly ash, one must be careful with the class used. Certain classes of fly ash are better than others. For example, Class F ashes are more effective than Class C ashes.
The combination of clinker with natural pozzolanas or with silica fume can have a similar effect as with the fly ash-Portland cement combination. A benefit to using pozzolanic materials is that a lower porosity and reduced permeability of the hardened cement paste is achieved. This helps prevent the sulfate solution from penetrating deeper regions of the concrete. A reduced amount of C3A will make this combination even more efficient.
The only drawback is that cement with pozzolanic additives are more susceptible to magnesium sulfate attack. This can be corrected by adding high ash contents (such as 70%).
Yet another type of Portland cement that is resistant to sulfate attack is “Portland-slag cement.” This type of Portland cement occurs when there are high amounts of slag (60%) present within the mix. The efficient performance of this cement is due to the reduced amount of C3A. A fraction of this clinker is replaced with granulated blast furnace slag. Once again, the amount of monosulfate within the cement is reduced. “Portland-slag cement” can be used in systems where there is alkali or calcium sulfate attack. This cement performs poorly with magnesium sulfate attack and cannot be recommended.
A type of cement that exhibits high sulfate resistance is Supersulfated Cement. The reason for this cements excellent efficiency is that it prevents Al2O3 from reacting with ettringite. These cement pastes have a tendency to be incompletely hydrated years after hydration. Although this is true, they still display an excellent sulfate resistance. This is due to there is a very low free calcium hydroxide content. This prevents significant ettringite formation in the presence of sulfates.
Another type of cement used for sulfate resistance is calcium aluminate cement. This cement performs very well when exposed to sulfate solutions and even better when the water to cement ratio is low. This is so because the cement exhibits very low permeability of the surface layer. An important observation with this cement is that magnesium sulfate solutions are less aggressive to calcium aluminate cement based concrete than alkali sulfate solutions.
Certain measures can be taken to prevent sulfate attack. One way to do this is to protect the concrete against composition-induced internal sulfate attack. The cement used to make concrete can be a source of sulfate attack. Due to this, standards and requirements of ASTM (see ASTM C150; ASTM C1157; BS 5328) and other organizations on cement and clinker composition should be followed very strictly. If this is done, then proper concentrations and ratios of the clinker materials to give sulfate levels that will lead to excessive expansion will be prevented.
Aside from clinker materials, aggregates and mineral additives are other potential sources of excessive sulfate. Therefore, aggregates and intermixed mineral admixtures should not contain sulfate-bearing compounds that will be allowed to react with cement components of concrete mixture in the future.
Quality control is the best method to use in preventing composition-induced internal sulfate attack. Continuous monitoring and proper records of the clinker material and other sources of sulfates should be kept in order to allow Quality control to work in its most efficient manner.
Another measure taken is the protection of concrete against heat-induced internal sulfate attack. Proper mixture design is an efficient way of protecting concrete from degradation by heat-induced internal sulfate attack. The materials used in designing concrete mixtures must pass specifications and have a history of satisfactory performance. An important note is that the lowest possible water to cementitious material (w/cm) ratio is recommended.
A very important time during the production of concrete that should be taken special care of is during casting and curing. The formwork material and the thickness of it can affect the heat transfer of the concrete. This must be taken into consideration when designing for homogeneous heat and humidity distribution within the concrete. Exposed concrete surfaces should be kept wet. An even distribution of heat and humidity should be maintained when inside the curing chamber. Another important consideration that should be taken note of is the preset time. A preset time should be accurate to allow the cement to set properly.
The heating rate can effect to concrete. The heating rate should be kept steady at approximately 15-208 C (25-358 F) per hour. The temperature rise should be evenly distributed within the concrete specimen and within the curing chamber. When heating the specimen, it should not be heated to the extent that the external surfaces are dried out. While heating the specimen, the difference in temperature between the inside and outside surfaces should be monitored. The difference between the external and maximum internal temperature of a specimen should never exceed 208 C (358 F). The maximum temperature of the specimen should not rise above 658 C (1508 F).
Once again, quality control must be followed to produce effective results. The control of the time-temperature regime is crucial to heat-cured process and must be followed closely.
A third measure to be taken is the protection of concrete against external sulfate attack. This measure is extremely important because the external surfaces of a structure are in contact with their surrounding environment. Therefore a good understanding of this environment should be taken note of, especially the sulfate-containing species.
An important procedure for this measure involves the variation of sulfate concentration. If the sulfate concentration varies, the concrete should be designed for the highest observed sulfate level. The environmental and atmospheric conditions should also be taken into consideration when properties such as temperature and humidity vary.
“The three main strategies for improving resistance to sulfate solutions are:
Problems can be experienced if sulfate or other aggressive chemicals penetrate hardened concrete. This is the reason why the concrete used should be dense and have a low porosity. As shown in the table below, the maximum w/cm ratio should be 0.5 and the minimum should be 0.4.
Proposed requirements to protect against damage to concrete by sulfate attack by external sources of sulfate:
In severe sulfate rich environments, the use of appropriate mineral admixtures may be used. Special notice should be taken to use of fly ash and/or slag. This is due to the fact that research shows performance of concrete using these materials can vary significantly.
It is always important to remember that sulfate resistant cements are not a substitute for proper concrete making. The use of sulfate resistant cements is to be used in addition to protection provided from a low w/cm ratio, adequate cement content, good mix design, etc.
Corrosion of Concrete in Sanitary Systems
The concrete used in sanitary systems (i.e. pipes, tanks, etc.) can be subject to harsh conditions. These conditions can lead to deterioration or even disintegration of the concrete. This generally occurs when a noticeable amount of hydrogen sulfide is present within the system. Typically the process begins with the dissolution of cement paste over time. In spots where the bacteriogenic degradation has taken place, the cement is completely broken down leaving it in a soft mass. Flowing water enhances this chemical action by transporting the products of dissolution to different locations. The cross sections of the pipes (or concrete materials effected) are gradually reduced.
“The bacteriogenic degradation of concrete is in essence a combined sulfate-acid attack” (Marchand p.113). The rate of bacteriogenic degradation also influences the development of corrosion. This corrosion is also effected by the quality of the cementitious binder used, acid solubility of the aggregate, and flow conditions of the pipe (or system).
It has been shown that calcium aluminate cement is more effective than Portland cement for this application. This is because the calcium aluminate cement has better stability at low pH values and better neutralization capacity than does Portland cement. The efficiency of the calcium aluminate cement can be further advanced by combining it with a synthetic calcium aluminate based aggregate rather than a non-reactive silicate material.