How does the water-to-cement ratio primarily affect the capillary porosity of hardened concrete?

The water-to-cement ratio (w/c ratio) is the primary factor influencing the capillary porosity of hardened concrete. The w/c ratio is the ratio of the mass of water to the mass of cement used in a concrete mix. Capillary pores are microscopic spaces within the hardened cement paste that remain after the cement hydration process. A higher w/c ratio results in a higher capillary porosity. During hydration, cement particles react with water to form hydration products like calcium silicate hydrate (C-S-H) gel. However, not all the water added in the mix is consumed during this hydration process. The excess water remains in the concrete, creating pores. When the concrete hardens and this excess water evaporates, it leaves behind interconnected capillary pores. A higher w/c ratio means there is more excess water, leading to larger and more numerous capillary pores. These pores increase the permeability of the concrete, making it more susceptible to the ingress of water, chlorides, sulfates, and other aggressive substances that can cause deterioration. Conversely, a lower w/c ratio results in a lower capillary porosity. With less water in the mix, the hydration products fill more of the space, reducing the number and size of capillary pores. This results in a denser, less permeable concrete with improved strength and durability. For example, a concrete mix with a w/c ratio of 0.6 will have significantly higher capillary porosity and lower strength compared to a mix with a w/c ratio of 0.4, assuming all other factors are constant. Therefore, controlling the w/c ratio is crucial for achieving the desired performance characteristics of concrete.