Stronger Hempcrete Retains Its Carbon-Negative Potential

A building material can have an appealing environmental story and still struggle to win over builders. Walls must withstand handling, dry predictably, manage moisture, and perform consistently. For hempcrete, turning those practical requirements into reliable products may matter as much as demonstrating that hemp can store carbon.

A new study from Stellenbosch University offers an encouraging step in that direction.¹ Researchers found that replacing a small portion of hempcrete’s binder with potassium sulphate increased its compressive strength while reducing drying shrinkage. Both the modified material and the original formulation retained estimated negative net greenhouse gas emissions within the study’s assessment.

The finding points toward a useful opportunity for industrial hemp: improving a familiar formulation rather than asking construction companies to adopt an entirely unfamiliar building system. However, the results also show why stronger hempcrete and commercially proven hempcrete are different milestones.

What Hempcrete Does in a Building

Hempcrete combines the woody inner portion of hemp stalks, called shives or hurds, with water and a mineral binder. The plant material creates a lightweight composite, while the binder holds it together. MyCannabis’s hempcrete construction guide explains its established role as insulating infill around a separate structural frame.

That distinction is essential. Despite its name, hempcrete generally does not perform the same job as reinforced concrete. A timber or other approved frame carries the building’s structural loads. Hempcrete fills the spaces around it and contributes to the building envelope.

Strength still matters in that role. A nonstructural material must survive production, installation, and service without excessive damage. Better mechanical performance could help manufacturers produce more dependable blocks or panels. It could also improve resistance to handling damage, although this study did not test those commercial outcomes.

How a Small Binder Change Improved Hempcrete Strength

The researchers compared two mixtures. The reference formulation used a binder containing lime and metakaolin, a processed clay material that participates in binder reactions. The modified formulation replaced 3% of that binder’s mass with potassium sulphate.

This was a targeted adjustment. It was not a 3% replacement of the entire hempcrete mixture, and the experiment did not establish 3% as the optimal dosage. The researchers selected that amount using earlier work on sulphate activation in related binder systems.

At 28 days, the ambient-cured modified specimens reached a compressive strength of 0.681 megapascals, compared with 0.413 megapascals for the reference. That represents an improvement of approximately 65%. Strength also improved at seven and 14 days, while the material retained a similar lightweight density.

The researchers suggest that the sulphate encouraged the formation of ettringite, a mineral phase that can contribute to the binder’s internal structure. However, they recommend additional analysis of the hardened material to confirm that mechanism. The measured performance improvement is clearer than the proposed chemical explanation.

Why Lower Shrinkage Could Matter to Builders

Drying shrinkage occurs when a material changes dimensions as it loses moisture. In a wall assembly, excessive movement can complicate connections, finishes, and dimensional tolerances. The modified hempcrete’s lower shrinkage therefore deserves attention alongside its headline strength gain.

For a manufacturer, a promising implication is greater predictability. A block or panel that changes dimensions less during drying could be easier to produce to a consistent specification. For an installer, predictable components could simplify fitting and finishing. These are potential benefits that require product-scale testing, rather than outcomes demonstrated by the laboratory specimens.

This reflects a broader direction in hemp materials research. Recent work covered by MyCannabis on improving hemp insulation durability also investigates how mineral ingredients can improve practical performance. Different formulations serve different purposes, but both approaches move beyond simply showing that plant-based materials are possible.

Moisture Performance Remains a Balancing Act

The water absorption findings prevent an overly simple success story. Adding potassium sulphate reduced the initial rate of water absorption. However, the 24-hour absorption coefficient remained essentially unchanged, and the modified mixture showed a higher coefficient after 144 hours.

In other words, slower initial wetting did not translate into uniformly lower absorption during prolonged exposure. A builder should not interpret the additive as waterproofing. Rain protection, appropriate finishes, drainage, and drying conditions remain questions for the complete wall system.

The distinction between water vapor and liquid water is also useful. A porous material’s ability to exchange moisture with surrounding air does not mean it can tolerate persistent saturation without consequences. Performance must be assessed under the conditions a building will actually encounter.

The authors identify freeze-thaw resistance, accelerated aging, and biological deterioration over multiple years as priorities for further research. Those tests would help establish whether the improvements persist across different climates and service conditions.

What Carbon-Negative Hempcrete Actually Means

Hempcrete’s carbon balance draws on two processes. Hemp absorbs carbon dioxide during growth, and part of that carbon remains in the plant material incorporated into the composite. Lime-containing binders can also take up carbon dioxide through carbonation.

The study estimated net balances of -0.491 kilograms of carbon dioxide equivalent for the reference specimen and -0.470 kilograms for the modified specimen. Each specimen measured 290 by 140 by 90 millimeters. These are specimen-level estimates, not figures for a house.

The authors describe their assessment as cradle-to-gate, but their methodology also credits binder carbonation associated with the use stage. Construction activities and end-of-life outcomes were excluded. Readers should therefore understand the result as a negative balance within defined accounting boundaries, rather than proof that every finished hempcrete building remains carbon-negative throughout its lifetime.

The modified mix was also slightly less negative than the reference. Its advantage was stronger material with broadly preserved environmental credentials, rather than improved carbon removal.

Carbon Storage Needs Comparable Evidence

For commercial adoption, manufacturers will need environmental information that buyers can compare alongside technical specifications. EPD International’s review of environmental product declarations reports growing use of verified environmental data, with construction products dominating its 2025 activity.

A declaration does not automatically establish that one product is better than another. Comparisons still require compatible boundaries and equivalent functions. For hempcrete, that means considering the wall assembly and its useful service life, rather than comparing a kilogram of hemp composite with a kilogram of a material that performs a different job.

From Hemp Research to Repeatable Building Products

Industrial hemp’s opportunity extends beyond one additive. Recent research into hemp protein bioplastics illustrates how different parts of the crop can become engineered materials. For construction, the corresponding challenge is connecting farm output with predictable processing and manufacturing.

There are signs of that infrastructure developing. An ANDRITZ report on scaling hemp insulation production (AZ2.DE ) describes a manufacturing line commissioned for Ekolution in January 2025. Hemp fiber insulation is a different product from hempcrete, but the example demonstrates the industrial processing capabilities available to the broader hemp building sector.

For the formulation studied here, the next useful steps are straightforward:

• Test additional potassium sulphate dosages and confirm the strengthening mechanism.
• Measure long-term durability under realistic moisture and climate exposure.
• Evaluate full-size products, production costs, and installation performance.

A Practical Step Forward for Hemp Construction

The strongest takeaway is that hempcrete’s performance can improve through relatively modest formulation changes. This experiment delivered substantial strength gains and lower shrinkage without sacrificing its lightweight character or reversing its estimated negative carbon balance.

That gives the hemp industry a credible development path. The goal is to produce building materials that are environmentally attractive and dependable enough for routine specification. With further durability testing and manufacturing validation, improvements like this could help hemp move from an interesting construction option toward a more practical one.

References:

¹ Steyn, K., De Villiers, W., & Babafemi, A. J. (2026). Performance of hempcrete for carbon-negative construction: Density, compressive strength, durability and carbon sequestration. Case Studies in Construction Materials. Advance online publication. https://doi.org/10.1016/j.cscm.2026.e06597