Yes, generally you can use rainwater to mix cement, provided it is clean and free from significant contaminants.
Is Rainwater Suitable for Concrete Mixing?
According to the information available, as rainwater is considered potable, concrete produced using it should give similar strength as the concrete made with potable water gives. This suggests that rainwater, in its pure form, is chemically suitable for the hydration process required for cement to set and harden effectively. Potable water, or drinking water, is the standard benchmark for water quality in concrete mixing because it is typically free from harmful impurities.
Potential Issues: The First Minutes Matter
While rainwater itself can be pure, its journey from the sky to your collection container is where problems can arise. The reference notes, "But surely during the first few minutes of a rainfall, the water carries more contamination which may affect the concrete." This initial 'wash' of the atmosphere and collection surfaces can pick up dust, dirt, leaves, bird droppings, and airborne pollutants.
What Contaminants Can Be Present?
Water used for mixing concrete should be relatively clean. Impurities can interfere with the chemical reactions of hydration, affecting setting time, strength development, durability, and potentially causing efflorescence or corrosion of embedded steel. Potential contaminants in collected rainwater include:
- Dirt and sediment
- Organic matter (leaves, twigs)
- Dissolved salts (from air pollution or roofs)
- Acids (from acid rain)
- Algae or bacterial growth (in storage)
Practical Considerations for Using Rainwater
To minimize risks and ensure the quality of your concrete mix when using rainwater:
Collection Method
- Avoid the First Flush: Collect water after the initial few minutes of rainfall have washed away surface contaminants from roofs and gutters. Diverting the first flow is a common practice.
- Clean Surfaces: Ensure the collection surfaces (roofs, tarps) and gutters are clean and free of debris.
- Use Clean Storage: Store collected water in clean tanks or containers that are covered to prevent further contamination.
Water Quality
- Visual Check: The water should appear clear and free from visible dirt or debris.
- Smell Test: It should not have a strong or unusual odor.
- pH Level: While not strictly necessary for small projects, extremely acidic or alkaline water can be problematic. Normal rainwater pH is slightly acidic (around 5.6) due to dissolved CO2, which is usually acceptable.
Project Scale
For small, non-critical projects like garden paths or fence posts, using collected rainwater after the initial flush is generally acceptable. For structural concrete or projects requiring specific strength and durability, using potable water or testing the rainwater quality might be advisable.
Comparing Rainwater and Potable Water for Concrete
Here's a simple comparison:
| Feature | Potable Water | Clean Rainwater (after first flush) | First Flush Rainwater |
|---|---|---|---|
| Purity | High, treated | High (chemically) | Potentially low |
| Contaminants | Minimal | Minimal (if collected properly) | High (dirt, pollutants, etc.) |
| Suitability | Standard, reliable | Generally suitable | Not recommended |
| Strength | Good, consistent results | Similar to potable water | Potentially compromised |
| Availability | Tap/well dependent | Depends on rainfall and collection | Depends on rainfall |
In summary, clean rainwater, particularly that collected after the initial period of rain, can be effectively used to mix cement and produce concrete with properties similar to those made with potable water. The key is to ensure the water is as free from physical and chemical contaminants as possible.
