Effect of elevated temperature on concrete: Considering hydration kinetics and microstructure, how does sustained high temperature during early age and curing influence the final (28-day and beyond) compressive strength of ordinary Portland cement concrete?

Introduction / Context:
Temperature strongly influences cement hydration and the pore structure of concrete. While contractors may observe faster stiffening on hot days, the long-term mechanical performance is governed by the microstructure that develops during curing. This question asks how high temperature exposure (especially at early age) affects the eventual compressive strength of ordinary Portland cement concrete.

Given Data / Assumptions:

• Ordinary Portland cement, normal aggregates, conventional mixes.
• “High temperature” refers to curing/early-age heat that accelerates hydration (e.g., hot weather concreting or high internal temperatures in mass concrete).
• Strength evaluation is at 28 days and beyond.

Concept / Approach:
Elevated temperature speeds early hydration, giving higher early strength but coarser capillary pore structure due to rapid precipitation of hydration products. This coarser, more discontinuous C-S-H network increases permeability and microcracking risk, which reduces long-term strength and durability. The general trend is: higher early strength, lower ultimate strength compared with identical concrete cured at moderate temperature.

Step-by-Step Solution:
Recognize that heat accelerates reaction rates → early strength rises quickly.Rapid hydration causes non-uniform product deposition → coarser pores and internal microcracks.Coarser microstructure translates into lower 28-day and ultimate strength compared with cool, moist curing.Therefore, the net long-term effect is a decrease in strength.

Verification / Alternative check:
Hot-weather concreting guides recommend temperature control, slowed setting (admixtures), and extended moist curing to mitigate strength loss and shrinkage cracking. Mass concrete specifications limit temperature rise and gradients for the same reason.

Why Other Options Are Wrong:
Increases strength/No effect: contradicts observed long-term microstructural outcomes.“Early strength unchanged”: incorrect; early strength often increases, but the question asks about final strength.

Common Pitfalls:
Confusing faster set with higher long-term strength; neglecting additional curing needs in hot weather.

Final Answer:
Decreases the strength of concrete.