Saw dust can be rendered chemically inert by boiling it in water containing:
🔬 Understanding the Problem with Sawdust in Concrete
Using sawdust as a lightweight aggregate in concrete is an attractive idea for sustainability and reducing weight. However, raw sawdust poses a major chemical problem. Wood contains sugars, starches, and other organic compounds (hemicelluloses) that are soluble in water.
The Issue: Cement Retardation. These organic compounds act as powerful retarders for cement hydration. They interfere with the chemical reactions that cause cement to set and harden. If untreated sawdust is used, the concrete may fail to set properly, or it will have extremely low strength.
To use sawdust safely, it must be "rendered chemically inert," meaning these harmful organic compounds must be neutralized or removed.
⚖️ Detailed Analysis of the Treatment Options
The question asks which chemical is used in a boiling water treatment to make sawdust inert.
(a) Ferrous sulphate (FeSO₄)
Why it's correct: Boiling the sawdust in a solution of ferrous sulphate is a well-known and effective method for neutralizing the harmful compounds. The ferrous sulphate reacts with the sugars and other organic matter, rendering them insoluble and preventing them from leaching out and interfering with the cement's hydration process. This chemical treatment effectively makes the sawdust inert in the context of a concrete mix.
(b) Potassium chloride (KCl)
Why it's incorrect: Potassium chloride is a neutral salt. While it could potentially accelerate cement hydration slightly, it does not have the chemical properties to react with and neutralize the organic sugars in the wood. It would not solve the primary problem of retardation.
(c) Ammonia (NH₃)
Why it's incorrect: Ammonia is a basic compound. While it might have some effect on the wood chemistry, it is not the standard or effective treatment for making sawdust inert for use in concrete. Ferrous sulphate is the specifically recommended chemical for this purpose.
💡 Study Tips for Concrete Admixtures and Aggregates
- Wood Sugars are Cement's Enemy: Remember that the natural sugars in wood are a major problem for cement setting.
- Ferrous Sulphate "Fixes" the Sawdust: Create a direct mental link between the problem (sawdust) and the solution (ferrous sulphate treatment).
- Inert means Non-Reactive: Understand that the goal of the treatment is to stop the sawdust from reacting with the cement paste.