Final answer:
Calcium silicate hydrate (C-S-H) is the compound in Portland cement that makes up about a quarter of its volume and is essential for the long-term strength gain of the cement.
Step-by-step explanation:
The compound that constitutes about one quarter of the volume of Portland cement and is responsible for the aging or long term gain in strength of the cement is calcium silicate hydrate (C-S-H). Portland cement contains phases such as tricalcium silicate (C3S) and dicalcium silicate (C2S), which hydrate and harden to form C-S-H and calcium hydroxide when mixed with water. This hydration process results in the development of strength over time. As the C-S-H structure forms and becomes more intricate, the concrete progressively becomes stronger, contributing to the material's long-term durability and load-bearing capacity. The slow, continuous formation of C-S-H is crucial for the longevity and mechanical properties of concrete structures.