Question

State whether the following statements are true or false. Setting of mortar is the result of reaction between slaked lime and \(\mathrm{CO}_{2}\) of air.

Short answer

Answer: True.

Step-by-step solution

Understanding the Setting of Mortar

The process of setting of mortar involves the hardening of a mixture of cementitious materials, such as cement or lime, with sand and water to form a binding material used in construction. In this specific statement, we are concerned with the role of slaked lime in the process.

Identifying Slaked Lime

Slaked lime, also known as calcium hydroxide or \(\mathrm{Ca(OH)}_{2}\), is a white powder produced by mixing quicklime (\(\mathrm{CaO}\)) with water. This slaked lime can be used as the cementitious material in mortar.

Carbonation Process

The carbonation process is a chemical reaction in which carbon dioxide gas from the air reacts with the calcium hydroxide present in the mortar mixture. The carbonation process can be represented by the following chemical equation: \(\mathrm{Ca(OH)}_2 + \mathrm{CO}_{2} \rightarrow \mathrm{CaCO}_{3} + \mathrm{H}_{2}\mathrm{O}\) In this reaction, the carbon dioxide from the air reacts with slaked lime (\(\mathrm{Ca(OH)}_{2}\)) in the mortar to form calcium carbonate (\(\mathrm{CaCO}_{3}\)) and water (\(\mathrm{H}_{2}\mathrm{O}\)). Calcium carbonate is a solid compound that contributes to the hardening of the mortar.

Evaluating the Statement

Based on the information we have gathered, the statement "Setting of mortar is the result of reaction between slaked lime and \(\mathrm{CO}_{2}\) of air" is true. The hardening of mortar composed of slaked lime is due to the carbonation process, which involves the reaction between slaked lime and carbon dioxide present in the air to form calcium carbonate.

Key concepts

Slaked Lime

Slaked lime, also known scientifically as calcium hydroxide \( \mathrm{Ca(OH)}_2 \), is a soft, white powder formed when quicklime (calcium oxide, \( \mathrm{CaO} \)) is mixed with water. This process is called slaking. The resulting compound serves as a crucial component in various construction uses, notably in the preparation of mortar.

• It is often preferred in construction due to its ability to combine with carbon dioxide.
• Furthermore, slaked lime is more stable than pure lime, making it easier to handle.

The slaking process is exothermic, meaning it releases heat as it occurs, which can easily be observed as the steam rising from the reaction mixture.

Carbonation Process

The carbonation process is pivotal in the setting and hardening of mortar. When slaked lime, present in the mortar, comes into contact with the carbon dioxide in the air, a chemical reaction occurs.
This results in the formation of calcium carbonate (\( \mathrm{CaCO}_3 \)), which is a solid and integrates into the mortar, enhancing its solidity and durability.

• The carbonation process can be described with the formula:
• \( \mathrm{Ca(OH)}_2 + \mathrm{CO}_2 \rightarrow \mathrm{CaCO}_3 + \mathrm{H}_2\mathrm{O} \)

This process takes time as the carbon dioxide slowly permeates through the mortar, gradually causing its complete hardening. Thus, carbonation is essential for creating strong, stable structures.

Calcium Hydroxide

Calcium hydroxide (\( \mathrm{Ca(OH)}_2 \)), is a vital chemical compound in the construction industry. Known commonly as slaked lime, it plays a crucial role in the setting and strength of mortar, plaster, and concrete.

• It forms when quicklime (\( \mathrm{CaO} \)) is added to water.
• This compound's chemical properties make it indispensable in various applications, including water treatment, as it neutralizes acidity.

In the context of mortar setting, calcium hydroxide interacts with carbon dioxide through the carbonation process to form calcium carbonate, cementing the structure together.

Chemical Reaction

Chemical reactions involve the transformation of substances through the reorganization of atoms to form new compounds. In the setting of mortar, the reaction between slaked lime and carbon dioxide is an excellent example.
Through this chemical reaction, two substances, slaked lime (\( \mathrm{Ca(OH)}_2 \)) and carbon dioxide (\( \mathrm{CO}_2 \)), combine.

• The end products are calcium carbonate (\( \mathrm{CaCO}_3 \)) and water (\( \mathrm{H}_2\mathrm{O} \)).
• This transformation is what contributes to the hardening of the mortar.

Chemical reactions like these are fundamental in nature, as they account for the formation and change of many materials in our world.

Carbon Dioxide

Carbon dioxide (\( \mathrm{CO}_2 \)), a colorless and odorless gas, plays a significant role in the setting process of mortar. Present naturally in the air, it interacts with slaked lime during the carbonation process to form calcium carbonate.

• This gas is absorbed from the atmosphere by the mortar surface during setting.
• Despite being surrounding us, its interaction is slow and gradual, taking place over time.

Without carbon dioxide, the hardening of slaked lime in mortar through carbonation would not occur, emphasizing the importance of this gas in construction and many other natural processes.