Capillary pores in cement and concrete are small voids that can form within the matrix during the hydration process or due to improper compaction and curing. These pores range in diameter from a few nanometers to several micrometers and are critical in determining the mechanical properties and durability of concrete structures. Capillary pores are interconnected, creating pathways that can allow the ingress of harmful substances like water, salts, and other chemicals, which can be especially detrimental under severe environmental conditions.