A building constructed from mortar becomes stronger on long standing. Give a reason.

Explanation

**Step-by-Step Solution:**

1. **Understanding Mortar Composition**: Mortar is a mixture typically made from cement, sand, and water. The cement component is primarily calcium silicate.

2. **Chemical Reaction with Carbon Dioxide**: Over time, the mortar in a building undergoes a chemical reaction with carbon dioxide (CO2) present in the air. This process is known as carbonation.

3. **Formation of Calcium Carbonate**: During carbonation, the calcium silicate in the mortar reacts with CO2 and water to form calcium carbonate (CaCO3). The chemical reaction can be summarized as:
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