7.3 : Porosity in Cement Paste

The porosity of concrete is a measure of the void spaces within its structure. These spaces impact its strength and durability significantly. When water and cement interact, a chemical reaction called hydration creates a semi-solid paste. This paste includes combined water, making up approximately 23% of the cement's dry mass, and gel water, which fills minuscule voids known as gel pores, accounting for about 28% of the cement gel volume.

The balance of water to cement in the mix is critical—it influences the extent of hydration and, thereby, the porosity and strength of the cured concrete. In an ideal hydration process, extra space not occupied by solid hydration products forms capillary pores, representing around 18.5% of the original cement volume. As the concrete cures, continuous hydration can fill the capillary pores with hydration products, increasing the density of the material.

For complete hydration and optimal strength, the curing process must be managed carefully. It allows for additional hydration, reducing porosity by filling the capillary spaces with more stable hydration products and, as a result, densifying the paste. This meticulous process is crucial for achieving durable, high-quality concrete that is less susceptible to environmental and mechanical degradation.