Adding certain calcium compounds, particularly calcium chloride, to concrete is done primarily to accelerate the rate at which the cement hydrates and sets, leading to faster concrete strength development.
The Role of Added Calcium in Concrete
While calcium is a fundamental component of cement itself (calcium silicates are the main reactive compounds), the question "Why add calcium to concrete?" typically refers to adding calcium compounds as admixtures. The most common calcium-containing admixture used for affecting concrete setting is calcium chloride (CaCl₂).
According to the provided reference, Calcium chloride is a preferred additive for accelerating cement hydration and reducing set time for concrete applications, particularly in colder weather.
How Calcium Chloride Works as an Accelerator
Calcium chloride speeds up the chemical reactions between cement and water (hydration). This acceleration happens because calcium chloride acts as a catalyst, influencing the formation of hydration products. The presence of chloride ions and calcium ions from CaCl₂ affects the solubility and reaction rates of the cement compounds, leading to quicker formation of the solid structure.
- Increased Reaction Rate: It increases the rate at which the main cement compounds, like tricalcium silicate (C₃S), react with water.
- Faster Heat Generation: The hydration process releases heat (exothermic reaction). Accelerating this process generates heat more quickly, which is particularly beneficial in cold temperatures where hydration naturally slows down.
- Quicker Set Time: By speeding up hydration, calcium chloride significantly reduces the time it takes for the concrete to lose its plasticity and begin hardening.
Key Benefits of Adding Calcium Chloride
Adding calcium chloride provides several practical advantages:
- Reduced Set Time: As stated in the reference, it reduces the time concrete needs to reach its initial set.
- Faster Strength Development: This leads to quicker attainment of necessary strength for formwork removal or loading.
- Improved Performance in Cold Weather: This is a critical application. In cold conditions, hydration slows considerably. Adding calcium chloride helps counteract this, allowing concrete to set and gain strength more effectively before freezing damage can occur.
- Increased Productivity: It can enable concrete to be produced faster and allow jobs to be completed as quickly and economically as possible. This is valuable in projects with tight deadlines.
Practical Applications
Adding calcium chloride is common in situations requiring accelerated setting and strength gain, such as:
- Paving repairs needing fast reopening to traffic.
- Cold weather construction to mitigate freezing risks.
- Precast concrete production for faster mold turnaround.
- Applications where early formwork removal is desired.
| Feature | Concrete Without Accelerator | Concrete With Calcium Chloride |
|---|---|---|
| Initial Set Time | Longer | Shorter |
| Early Strength Gain | Slower | Faster |
| Cold Weather Performance | Slower/Riskier | Improved |
| Speed of Job Completion | Slower | Faster/More Economical |
Note: While beneficial for acceleration, excessive calcium chloride can have negative long-term effects, such as increasing the risk of steel reinforcement corrosion. Modern construction often uses alternative non-chloride accelerators when corrosion is a concern.
In summary, adding calcium in the form of calcium chloride to concrete is done to accelerate the hydration process, reduce set time, speed up early strength development, and improve performance, especially in cold weather, ultimately making construction faster and potentially more economical.
