Question

Which chemical process is responsible for the long-term strength gain of concrete?

• A. Hydration
• B. Carbonation
• C. Sulphate attack
• D. Alkali-aggregate reaction

Hint:

Hydration is the essential chemical reaction for concrete's strength. It's a continuous process that allows concrete to gain strength over weeks, months, and even years, provided sufficient moisture is available. The other options describe processes that are typically detrimental to concrete's long-term performance and durability.

Answer 1

The Correct Option is A

Step 1: Understand the composition and setting of concrete.
Concrete is primarily a mixture of cement, aggregates (sand and gravel), and water. The strength of concrete develops as the cement reacts chemically with water.
Step 2: Identify the key chemical process for strength gain.
The long-term strength gain of concrete is primarily due to the chemical reaction between cement and water, which is known as hydration. During hydration, various calcium silicate hydrates (C-S-H gel) and calcium hydroxide are formed. The C-S-H gel is the main component responsible for the strength and durability of hardened concrete. This process continues for a long time, contributing to the progressive strength gain.
Step 3: Evaluate the given options. Option 1: Hydration. This is the correct process. The reaction of cement with water forms products that bind the aggregates together, leading to hardening and strength development over time.
Option 2: Carbonation. Carbonation is a process where carbon dioxide from the air reacts with calcium hydroxide in the hardened concrete to form calcium carbonate. This process can lead to a slight increase in surface hardness but primarily reduces the alkalinity of concrete, which can lead to corrosion of steel reinforcement, thus generally considered a detrimental process rather than a strength gain mechanism.
Option 3: Sulphate attack. Sulphate attack is a chemical deterioration process where sulphates from external sources (e.g., soil, groundwater) react with components of the hardened cement paste, leading to expansion, cracking, and loss of strength. It is a destructive process.
Option 4: Alkali-aggregate reaction. This is a deleterious chemical reaction between certain reactive silica in aggregates and the alkali hydroxides in the cement paste. It causes expansion and cracking of the concrete, leading to deterioration, not strength gain. The final answer is $\boxed{\text{1}}$.