What is heat of hydration and how does it affect mass concrete?

Heat of hydration is heat released during cement-water reaction, causing temperature rise in concrete. In mass concrete (large pours like dams, foundations), this heat cannot dissipate quickly, causing internal temperatures up to 70-80C. The resulting thermal gradients create differential expansion/contraction, leading to thermal cracking. Mitigation includes: using low-heat cement (Type IV), replacing cement with pozzolans, pre-cooling aggregates, post-cooling with embedded pipes, and limiting pour lifts.