Biochar in Concrete - Beginner's Guide

Biochar is a carbon-rich material created by heating organic waste—like wood, crop residues, or even manure—in a low-oxygen environment. This process is called pyrolysis. Because there isn’t enough oxygen, the material doesn’t burn completely. Instead, it transforms into a highly stable form of carbon.

Now, why is biochar special? Because of its unique structure and chemical properties, it can be used in agriculture, water filtration, and even concrete. 

Today, we’re focusing on its role in concrete and how it affects the hydration process of cement.

The Microstructure of Biochar

Now, imagine we zoom into biochar under a powerful microscope. What would we see? 

1. A highly porous structure—biochar is full of tiny pores and channels. These pores come from the original cellular structure of the plant material it was made from.
2. A large surface area – Because of its porous nature, biochar has a huge surface area compared to its size. A tiny amount of biochar can have hundreds of square meters of surface area!
3. Functional groups on its surface – Biochar contains hydroxyl (-OH) and carboxyl (-COOH) groups. These chemical groups make biochar reactive with water and cement particles.

Now, let’s compare this with the microstructure of concrete to understand how they interact.

The Microstructure of Concrete

Concrete is a composite material made of cement, water, sand, and aggregates. The key component that binds everything together is cement paste, which undergoes a hydration reaction when mixed with water.

When cement hydrates, it forms:

• Calcium Silicate Hydrate (C-S-H) – This is the most important product. It’s what gives concrete its strength and durability. It forms tiny gel-like structures that fill the space between sand and aggregates, hardening over time.
• Calcium Hydroxide (CH) – This is a byproduct of hydration. It’s not very strong, but it helps maintain the alkalinity of concrete.
• Ettringite & Other Hydration Phases – These help with early-stage setting and volume stability of concrete.

Read More On: Structure of Hydrated Cement Paste

However, traditional concrete has some problems at the microstructural level:It has capillary pores, which allow water and chemicals to seep in, leading to cracks. It releases a lot of CO₂ during cement production, making it environmentally harmful.

Now, let’s explore what happens when biochar is added to concrete.

How Biochar Interacts with Concrete?

When biochar is mixed into concrete, it changes the hydration process in several ways:

1. Water Absorption & Internal Curing

Because of its porous structure, biochar absorbs extra water from the concrete mix.
But instead of just holding it, biochar slowly releases water back over time.
This keeps the hydration process going for a longer period, reducing cracks and improving long-term strength.

Think of it like a tiny reservoir inside the concrete, helping the material cure better from the inside.

2. Strength & Durability Enhancement

The large surface area of biochar provides extra nucleation sites for C-S-H formation. This means more binding material is created, making the concrete stronger.
The pores in biochar can trap harmful ions like chloride and sulfate, reducing corrosion in reinforced concrete.

3. Carbon Sequestration & CO₂ Absorption

Biochar traps carbon permanently, preventing it from returning to the atmosphere as CO₂.
Some studies show that biochar-enhanced concrete can absorb CO₂ from the air during curing, making it even more environmentally friendly.

4. Workability & Setting Time

Because biochar absorbs water, adding too much can make fresh concrete stiff and less workable.
To fix this, the water content in the mix may need to be adjusted.
In small amounts, biochar doesn’t delay setting time significantly, but at higher doses, it can slow down the process.

Comparing Traditional Concrete vs. Biochar Concrete

Feature	Traditional Concrete	Biochar Concrete
Microstructure	Dense but with capillary pores	Porous but with water-retaining pores
Strength	Relies on C-S-H formation	Enhanced due to biochar’s nucleation effect
Durability	Can suffer from shrinkage cracks	Improved due to internal curing
CO₂ Emissions	High due to cement use	Lower, as biochar captures carbon
Water Absorption	Minimal	High, but helps with curing

Final Thoughts

So, what’s really happening when biochar is added to concrete?

1. At the microstructural level, it absorbs water, stores carbon, and provides extra binding sites for cement hydration.
2. At the performance level, it improves strength, durability, and resistance to cracking.
3. At the environmental level, it reduces CO₂ emissions and makes concrete more sustainable.

The key is getting the right balance. Too little biochar won’t have much effect. Too much can reduce workability and require mix adjustments. But when used correctly, biochar has huge potential to make the construction industry greener and more durable.

Read More On: Sustainable Construction