Calcium chloride is a chemical that is commonly used as an accelerator in the process of cement hydration. Its main purpose is to speed up the setting of concrete, resulting in high initial strength concrete. However, it is important to note that the maximum amount of calcium chloride that can be added to the mix is 2% in flake form. This limitation is crucial for ensuring that the concrete retains its structural integrity and does not become compromised. By adhering to this guideline, builders can create durable and reliable structures that will stand the test of time.
Methods of Adding Calcium Chloride
Calcium chloride is a chemical compound that can be obtained in different forms such as pellets, granules, flakes, or solution. The regular flake form contains at least 77 percent calcium chloride while the pellet and other granular forms have a minimum of 94 percent.
One important characteristic of calcium chloride is its solubility in water, which means that it can dissolve in water and form a solution. Due to this property, it is recommended to use calcium chloride in solution form to ensure its effective use.
In summary, calcium chloride is a versatile compound that can be acquired in different forms, each with varying concentrations of calcium chloride. Its solubility in water makes the solution form the recommended choice for its usage.
When dealing with a particular solution, it is important to exercise caution in its handling. Direct contact with cement should be avoided as this can result in a rapid setting of the cement. It is advised to instead dilute the solution in water and then mix it with the aggregate. This method is recommended as a safer alternative. By diluting the solution, the likelihood of it coming into direct contact with the cement is minimized, thus reducing the risk of rapid cement setting. Mixing the diluted solution with the aggregate further ensures that the cement is not affected and maintains its intended properties. Therefore, it is essential to follow these guidelines to ensure that the cement is not compromised in any way during the mixing process.
Effect of Calcium Chloride on Properties of Concrete
1. Effect on Physical Properties
1. Setting Time
Calcium chloride is a commonly used accelerator in concrete that has the ability to significantly decrease the initial and final setting time of concrete. This attribute makes it a popular choice for use in low temperatures, as it enables faster finishing and earlier use of slabs.
However, it is important to note that the use of calcium chloride as an accelerator is not recommended in hot weather. This is because it causes the concrete to set very rapidly, which makes it difficult to place and finish properly.
In accordance with both the CSA A266.2-1973 and ASTM C494-1971 standards, the use of calcium chloride as an accelerator requires that the initial setting time of the concrete should occur at least one hour earlier, but no more than three hours (as per CSA) or three and a half hours (as per ASTM), in comparison to the reference concrete. This ensures that the concrete achieves the desired level of setting time and strength, while also maintaining its overall quality and durability.
2. Water Cement Ratio
Calcium chloride, despite its common use as an accelerator in concrete, does not have a significant impact on the amount of water needed to achieve a desired slump. Therefore, its use is not expected to have a major influence on the overall strength of the concrete produced. However, it can have an effect on the early stiffening of the concrete, which may in turn reduce bleeding.
It is important to note that the primary function of calcium chloride as an accelerator is to speed up the setting and hardening process of the concrete. This can be useful in cold weather conditions where the slower setting time of concrete can be a hindrance. However, it is not a replacement for proper mix design and curing practices, which remain critical factors in achieving high-quality concrete.
While some may believe that the use of calcium chloride can enhance the strength of concrete, research suggests that its impact on strength is minimal at best. Rather, the use of other additives such as fly ash, silica fume, and superplasticizers may have a greater impact on the strength and durability of concrete.
Overall, while calcium chloride can be a useful tool in concrete production, it is important to understand its limitations and not rely solely on its use for achieving optimal results. Proper mix design, curing practices, and the use of other additives should all be considered in producing high-quality concrete.
3. Air Entrainment
The addition of calcium chloride to concrete does not lead to the entrapment of air. However, when used in conjunction with air-entraining agents, it helps to increase the air content with less air-entraining agent required. In other words, calcium chloride does not interfere with the air-entraining process in concrete. Instead, it enhances the effectiveness of air-entraining agents, allowing for greater air content with less agent needed. This can result in a more efficient and cost-effective approach to achieving the desired air content in concrete.
4. Freezing and Thawing
Concrete that includes calcium chloride as an ingredient has the advantage of rapid hardening and early resistance to damage caused by freezing and thawing. This characteristic can be particularly valuable during winter construction when the concrete is exposed to early application of de-icing salts. However, as the concrete ages, it may become less resistant to frost attack, even if it has already matured.
The inclusion of calcium chloride in concrete can accelerate the hardening process, which allows the material to achieve its desired strength much faster. This benefit can be especially beneficial in situations where the concrete needs to be used quickly or when rapid construction is necessary.
Another advantage of using calcium chloride in concrete is that it can provide early resistance to damage caused by freezing and thawing. This characteristic is particularly important during winter months when de-icing salts are often applied to surfaces, as these salts can cause damage to concrete that is not resistant to frost attack.
However, as concrete containing calcium chloride ages, its resistance to frost attack may diminish, even if the material has already reached maturity. It is important to keep this in mind when considering the long-term durability of structures built using this type of concrete.
5. Dry Shrinkage
The addition of calcium chloride is associated with an increase in drying shrinkage. The extent of this increase is dependent on several factors, including the amount of calcium chloride that is added, the specific type of cement being used, the duration of the curing process, and the prevailing environmental conditions.
6. Efflorescence
In certain cases, the use of calcium chloride in concrete results in the formation of a white deposit on the surface of the cured concrete. However, this occurrence does not typically cause efflorescence like other types of salts do, as it tends to attract water under normal exposure conditions. The white deposit is insoluble in water, and thus requires the use of dilute hydrochloric acid for removal.
2. Effects on Chemical Properties
1. Sulphate Attack
When exposed to sulphate solutions, the presence of calcium chloride can have a negative impact on concrete. This is because sulphates react with calcium and aluminium ions found in the cement paste, resulting in the formation of calcium sulphate and calcium sulphoaluminate hydrates. As a result of this reaction, the concrete becomes disrupted.
Additionally, there is evidence to suggest that the presence of calcium chloride further reduces the concrete’s resistance to sulphate attack. This means that the negative effects of sulphate exposure on the concrete are compounded when calcium chloride is present. Therefore, it is important to avoid exposing concrete to solutions of sulphates when calcium chloride is present in order to minimize damage to the concrete.
2. Heat of Hydration
The presence of calcium chloride in the hydration process leads to a faster rate of heat generation. This effect is particularly noticeable during the first 10 to 12 hours of the hydration process. Although the total amount of heat generated is not significantly affected, the early development of the heat may have practical benefits in situations where concrete is being poured during the winter months.
In summary, the addition of calcium chloride to the hydration process has been found to accelerate the rate at which heat is generated. This effect is most pronounced during the early stages of hydration and may be beneficial in certain contexts, such as winter concreting. While the overall amount of heat generated is not significantly altered, the early development of heat can be advantageous in promoting the proper curing and setting of concrete.
3. Alkali-Aggregate Reaction
The use of high alkali cement in conjunction with certain types of aggregates can result in concrete deterioration due to the swelling of the aggregates. This reaction is further exacerbated by the presence of calcium chloride in the concrete, which aggravates the alkali-aggregate reaction. However, in some cases, it may be necessary to use calcium chloride in concrete. In such situations, expansion can be controlled by utilizing low alkali cement, pozzolan, or a non-reactive aggregate. By taking these steps, the negative effects of the alkali-aggregate reaction can be mitigated, and the concrete can remain structurally sound.
4. Corrosion of Reinforcement Steel
Concrete that contains calcium chloride may have a decreased ability to maintain a protective film that shields steel from the external environment, potentially leading to corrosion. This effect is particularly concerning in prestressed concretes where the wires’ larger surface area and greater stress differences increase the risk of corrosion. As a result, the use of calcium chloride is prohibited in prestressed concrete. Additionally, it is not recommended for steam-cured concreting.
3. Effects on Mechanical Behaviour
1. Compression Strength
Calcium chloride is often used as an accelerator in concrete, as it can speed up the hardening process. According to ASTM C-494 standards, the concrete must demonstrate at least a 125% increase in hardening rate compared to a control concrete after three days. However, after six months or one year, the requirement decreases to only 90% of the control specimen.
When compared to conventional concrete, calcium chloride concrete can experience strength gains ranging from 30% to 100% within the first three days. It’s important to note that excessive amounts of calcium chloride can actually lead to lower strength gains. Additionally, the strength gains are greater for richer mixes when the same amount of chloride is used.
Interestingly, the impact of calcium chloride on strength gain is particularly significant at lower temperatures.
2. Flexural strength
The addition of calcium chloride to concrete does not result in a significant increase in flexural strength, unlike its effect on compressive strength. The ASTM C-494 standard mandates that the flexural strength of concrete with calcium chloride should be a minimum of 110 percent of the control specimen after three days of curing. However, over extended periods of curing, the flexural strength of concrete with calcium chloride may even be lower than that of the control specimen.
3. Shrinkage and Creep
When calcium chloride is added to concrete, it leads to an increase in the shrinkage of the material. As a result of this increased shrinkage, there is a subsequent increase in the creep of the concrete. This phenomenon occurs because as the concrete shrinks, it is subjected to internal stresses that cause the material to deform over time. This deformation is known as creep and can cause structural issues if not properly addressed. Therefore, it is important to carefully consider the use of calcium chloride in concrete and to take steps to mitigate any negative effects that may result from its use.
Advantages of Using Calcium Chloride in Concrete
The use of a high-performance concrete mix can offer numerous benefits for construction projects. One of the key advantages is its high initial strength, which allows for quicker removal of formwork and faster construction progress. Additionally, high-performance concrete can have a reduced final set time, meaning that it reaches its full strength more quickly than traditional concrete mixes.
Another benefit of using high-performance concrete is that it can exhibit reduced bleeding, which can help to improve the overall appearance and quality of the finished product. Additionally, the improved workability of high-performance concrete can make it easier to use and manipulate during the construction process, which can result in a smoother and more efficient construction process.
High-performance concrete can also offer fast formwork turnaround, as it can be removed more quickly than traditional concrete mixes. This can help to speed up the construction process and reduce labor costs.
Furthermore, the use of high-performance concrete can be cost-effective in the long run, as it can lead to reduced maintenance and repair costs due to its superior durability and strength. Finally, high-performance concrete can be especially beneficial when used in conjunction with fly ash, a byproduct of coal-fired power plants, as it can help to improve the sustainability of construction projects by reducing waste and utilizing a recycled material.