#47 Mineral Admixtures | Agricultural Ashes | Part 2 Rice Husk Ash | Admixtures & Special Concretes

Summary

This video explores the structure and reactivity of agricultural ashes, particularly rice husk ash (RHA) and sugarcane bagasse ash. It delves into how these ashes can be processed for cement and construction purposes, highlighting their high surface areas, pozzolanic properties, and various chemical compositions. The script discusses key factors like particle size, temperature control during burning, and the role of additives like lime in enhancing reactivity. Additionally, it covers the use of agro-ashes in brick production, emphasizing the potential for creating durable, eco-friendly building materials without the need for high-temperature processing. The research also investigates how agricultural residues can provide sustainable alternatives to traditional materials like fly ash.

Takeaways

• 😀 Bagasse ash has different particle shapes (irregular, prismatic, spherical), indicating varying degrees of crystallinity depending on the burning temperature.
• 😀 Rice husk ash (RHA) has a very high surface area and is highly pozzolanic due to its high purity amorphous silica content.
• 😀 The particle size of RHA can be as small as 150 nanometers, but typical RHA particles are larger, with a surface area of about 77.4 m²/g.
• 😀 RHA can contain a significant amount of alkalis, like potassium oxide, similar to sugarcane bagasse ash.
• 😀 The potential for using RHA is large, as it has one of the highest ash percentages (around 25%) compared to other agricultural residues like wheat straw and bagasse.
• 😀 Proper heat treatment, such as controlled burning, is crucial to optimize the reactivity of rice husk ash for use in concrete.
• 😀 RHA particles have a cellular structure with honeycombed pores around 10 microns in size, contributing to its high surface area.
• 😀 Increased RHA content in concrete can lead to higher viscosity, reduced workability, and potentially higher segregation resistance.
• 😀 RHA has a beneficial impact on concrete compressive strength when used as a cement replacement, but too high a replacement (beyond 20%) can reduce its effectiveness.
• 😀 Durability of concrete with RHA is generally better than plain OPC, as RHA improves the microstructure and reduces permeability, enhancing chloride resistance.

Q & A

What types of particles were observed in bagasse ash, and what does their shape indicate?

Bagasse ash was found to have irregular, prismatic, or spherical particles, indicating varying degrees of crystallinity due to differences in burning temperature. These particle shapes reflect the crystallinity formed during the burning process.

Why is rice husk ash considered highly pozzolanic?

Rice husk ash (RHA) is highly pozzolanic due to its high surface area, which allows for a large amount of surface available for pozzolanic reaction. Its high purity amorphous silica content further enhances its reactivity, similar to silica fume.

How does the particle size of rice husk ash compare to cement particles, and why is this important?

Rice husk ash typically has a particle size similar to that of cement particles, but its surface area is much larger. This increased surface area facilitates a higher pozzolanic reactivity, crucial for its role as a cement replacement.

What is the significance of the low loss on ignition (LOI) in processed rice husk ash?

A low LOI (about 3%) in processed rice husk ash indicates that most of the organic carbon has been removed during processing. This improves the quality and reactivity of the ash, making it more suitable for use in construction materials.

What is the typical ash content of rice husk, and how does it compare to other agricultural residues?

Rice husk contains about 25% ash, which is higher than other agricultural residues such as wheat straw (10%) and sugarcane bagasse (15%). This high ash content increases the potential use of rice husk ash as a material in construction.

How does the burning temperature affect the reactivity of rice husk ash?

At higher burning temperatures, especially around 700°C, the decomposition temperature of potassium oxide can be reached, potentially trapping unburned carbon in the melt. This reduces the reactivity of the ash, so precise control of burning temperature is crucial to obtaining high reactivity.

What is the impact of rice husk ash on the workability of concrete?

Rice husk ash increases the water demand and viscosity of concrete due to its high surface area, which can reduce workability. However, it also increases segregation resistance, meaning water is less likely to separate from the mix.

How does the use of rice husk ash affect the compressive strength of concrete?

Rice husk ash enhances the compressive strength of concrete due to its pozzolanic reactivity. However, beyond a certain percentage of replacement (about 20%), the benefit diminishes because the lime required for further pozzolanic reactions becomes insufficient.

What role does rice husk ash play in improving the durability of concrete?

Rice husk ash improves the durability of concrete by reducing permeability, resulting in better resistance to chloride penetration. This is primarily due to the denser microstructure formed by the pozzolanic reaction, which reduces pore connectivity.

How can rice husk ash be used in brick production, and what benefits does it offer?

Rice husk ash can be mixed with clay and hydrated lime to form bricks, either through traditional or geopolymeric processes. The pozzolanic and geopolymeric reactions lead to hardened bricks with sufficient strength, reducing the need for high-temperature sintering and energy consumption.