Yes, concrete sand can absorb water.
While sand might appear solid, it's composed of individual particles with spaces between them. This allows water to be held in two primary ways:
- Surface Moisture: Water adheres to the surface of the sand particles due to surface tension and intermolecular forces. This is the most significant way sand "holds" water.
- Capillary Action: In finer sands, capillary action can draw water upwards into the spaces between particles, further contributing to water absorption.
The amount of water absorbed by concrete sand depends on several factors:
- Particle Size and Gradation: Finer sands, with smaller particles and a wider range of particle sizes (good gradation), tend to absorb more water because of the increased surface area and capillary action.
- Mineral Composition: The mineralogy of the sand particles can also affect water absorption. Some minerals are more porous than others.
- Cleanliness: Clay and silt particles present in "dirty" sand increase water absorption because these materials are more porous and have higher surface areas than sand particles themselves.
Why is water absorption important in concrete?
The water content in concrete mixes significantly affects the workability, strength, and durability of the final product. If the sand used in the mix has already absorbed water, less water will be available for the hydration of the cement. This can lead to:
- Reduced Workability: A drier mix is harder to work with.
- Lower Strength: Insufficient water for cement hydration results in a weaker concrete.
- Cracking: Improper hydration can lead to cracking and reduced durability.
To account for the water already present in the sand, concrete mix designs must consider the moisture content of the aggregates (sand and gravel). This involves testing the sand and adjusting the amount of water added to the mix to achieve the desired water-cement ratio.
