Calcium Chloride Role in Concrete

This article is about Calcium Chloride Role in Concrete. Calcium chloride serves as an accelerator in the cement hydration process, promoting rapid concrete setting and the attainment of high initial strength. Its usage is limited to a maximum of 2% when added in flake form.

Calcium Chloride Adding Methods in Concrete

Calcium chloride is offered in various forms such as pellets, granules, flakes, or as a solution. The flake variant contains at least 77% calcium chloride, while the pellet and other granular forms have a minimum of 94%. Since all types of calcium chloride readily dissolve in water, it is advisable to utilize the solution form for optimal results.

To prevent rapid cement setting, it is important to avoid direct contact between the solution and cement. Thus, a recommended approach is to dilute the solution in water and combine it with aggregate.

Read Also: How Water Impurities Effects on Concrete Strength, Durability and Other Properties

Effect of Calcium Chloride on Properties of Concrete

1. Effect on Physical Properties

1. Setting Time

Calcium chloride is commonly used as a concrete accelerator to speed up the initial and final setting time. It is particularly beneficial in low temperatures as it allows for faster finishing and earlier use of slabs. However, caution is advised when using this accelerator in hot weather as it causes the concrete to set too quickly, making it challenging to place and finish properly.

Both the CSA A266.2-1973 and ASTM C494-1971 standards specify that when calcium chloride is used, the initial setting time of the concrete should be at least 1 hour earlier (but not more than 3 hours for CSA or 3½ hours for ASTM) compared to the reference concrete.

2. Water Cement Ratio

Calcium chloride’s impact on water reduction for achieving a specific slump in concrete is not significant, and it is not expected to have a dominant role in strengthening the concrete. Its primary function as an accelerator may lead to early stiffening, potentially reducing bleeding.

3. Air Entrainment

Calcium chloride in concrete does not cause air entrainment. However, when combined with air entraining agents, it enhances the production of air content with a smaller amount of entraining agent.

4. Freezing and Thawing

Concrete with calcium chloride hardens quickly and gains early resistance to freezing and thawing damage, which is beneficial during winter construction when de-icing salts are used early on. However, as the concrete matures, it may become less resilient to frost damage.

5. Dry Shrinkage

The addition of calcium chloride can cause an increase in drying shrinkage. The extent of this increase is influenced by factors such as the amount of calcium chloride used, the type of cement being used, the duration of the curing period, and the surrounding environmental conditions.

6. Efflorescence

In certain cases, the use of calcium chloride in concrete can lead to the formation of a white deposit on the cured concrete surface. However, under typical exposure conditions, it has a water-attracting property and is unlikely to cause efflorescence like other salts. These whitish deposits are insoluble in water, requiring the use of diluted hydrochloric acid for their removal.

Read Also: Flexural Test on Concrete, Its Significance, Procedures and Applications

2. Effects on Chemical Properties

1. Heat of Hydration

Calcium chloride accelerates the rate of heat of hydration and enhances the rapidity of the hydration process, particularly within the initial 10 to 12 hours. Although the overall amount of heat developed remains largely unchanged, its early progression can be advantageous for winter concreting.

2. Sulphate Attack

Exposure of concrete to sulphate solutions leads to the deterioration of the material. Sulphates react with calcium and aluminium ions present in the cement paste, resulting in the formation of calcium sulphate and calcium sulphoaluminate hydrates. This reaction disrupts the integrity of the concrete. The presence of calcium chloride has been observed to decrease the resistance of the concrete to sulphate attack.

3. Corrosion of Reinforcement Steel

The presence of calcium chloride in concrete reduces the effectiveness of the protective film that shields the steel from the external environment. This increases the risk of corrosion. Calcium chloride is not allowed in prestressed concrete, as it leads to higher corrosion rates due to larger wire surface areas and greater stress differences. Additionally, using calcium chloride in steam-cured concrete is not recommended.

4. Alkali-Aggregate Reaction

High alkali cement, when combined with specific aggregates, can lead to concrete deterioration caused by aggregate swelling. The presence of calcium chloride in the concrete worsens the alkali-aggregate reaction. However, in cases where calcium chloride must be used, expansion can be managed by employing low alkali cement, pozzolan, or a non-reactive aggregate.

3. Effects on Mechanical Behaviour

1. Compression Strength

Calcium chloride is commonly used as an accelerator in concrete, effectively speeding up the hardening process. According to ASTM C-494 guidelines, the concrete with calcium chloride should exhibit at least a 125% increase in hardness compared to the control concrete after 3 days.

However, after 6 months or one year, the requirement decreases to 90% of the control specimen.When comparing conventional concrete to calcium chloride concrete, the strength gain in the first three days can vary from 30% to 100%. Excessive amounts of calcium chloride beyond accepted standards can actually result in lower strengths.

Additionally, richer mixes tend to experience larger increases in strength for the same amount of chloride. The impact of calcium chloride on the percentage gain in strength is especially significant at lower temperatures.

2. Flexural strength

The addition of calcium chloride to concrete does not significantly increase its flexural strength compared to its compressive strength. According to ASTM C-494, the flexural strength at 3 days should be at least 110% of the control specimen. However, over extended curing periods, concrete containing calcium chloride may exhibit lower flexural strength than the control specimen.

3. Shrinkage and Creep

The addition of calcium chloride to concrete increases both shrinkage and creep.

Advantages:

The benefits of using this particular material include high initial strength, shorter final set time, reduced bleeding, improved workability, faster form work turnaround, cost-effectiveness, and compatibility with fly ash.

Read Also: What are Durability Properties of Concrete with GGBFS?

FAQs about Calcium Chloride used in Concrete