In our projects in Malawi and Sierra Leone, we continuously face the same challenge: How can we construct durable and functional buildings in the most environmentally friendly way possible? The answer led us to a construction technique that combines traditional mud building methods with contemporary processing technology: Soil Stabilized Blocks (SSB).
By Annabella Ranft
Bonn, March 31, 2025 – Traditional fired brick production consumes enormous amounts of wood and contributes significantly to deforestation. Manufacturing cement blocks, on the other hand, is expensive due to high cement consumption, logistically difficult to transport to remote regions – and also costly and: Cement production burdens the climate and environment because of the huge amounts of energy and sand required. The construction industry is considered one of the largest emitters of climate-damaging carbon dioxide (CO2). According to the UN Environment Programme, about 40 percent of global greenhouse gases are caused by building construction.
When we began building the Namwawa Junior Primary School in Eleven Village, Malawi in late 2023, we wanted to take a different approach. For the production of SSBs, we use material that is abundantly available locally: local soil with a clay content of 15-30%. This is stabilized with a small addition of cement or lime. The blocks are not fired but pressed and dried – this method conserves resources and utilizes local materials.
Just get started? The path to the perfect block
Starting with SSBs in Eleven Village was like jumping into cold water. We had studied a lot of technical literature, observed the technique in other projects in Malawi, and spoken with local experts. However, our only practical experience came from an earlier project in Congo. So the first weeks were characterized by soil samples and experiments on mixing ratios and compressive strength.
Crucial for good SSBs is the right mixing ratio of soil and stabilizer, which varies depending on the composition of the soil. To test this, we applied a simple test well-known among clay experts: A glass is filled one-third with soil, topped with water, and shaken. After settling, the layers show the composition of the soil.
The manufacturing process itself follows a clear sequence: The sieved soil is mixed with about 5-7% cement or lime, water is added sparingly, just enough so that the mass can be formed into balls. This mixture is filled into a mechanical press and compacted by lever force or hydraulically. Sounds strenuous? Absolutely! With a motivated and routine team, you can make around 400 blocks per day, on very good days even more.
In this initial phase, tests were needed to learn more about stability and compressive strength. The first tests can be done after seven days. One of the most productive tests: You prick a block that has been soaking in water for 24 hours with a sharp object (like a nail or knife tip). Ideally, it doesn’t break. If the tip doesn’t penetrate more than 6mm, the block has good strength. Final tests can only be done when the block has fully cured after 24-28 days.
The fresh blocks must be covered and regularly watered for seven days. After that, they are restacked and reach their full strength after another three weeks. They are best covered with a tarp during this time. Alternatively, grass mats are sufficient, though the blocks must then be watered regularly.
Afterward, a final test for compressive strength and moisture resistance can be conducted. For SSB enthusiasts: A stabilized earth block should have a compressive strength of 25-35kg/cm². The compressive strength of a wet earth block that has been soaking in water for at least 24 hours should have a compressive strength of 15-23kg/cm². That’s approximately 60-65% of its dry strength.
The long drying time means that production cannot start too late, otherwise you run out of blocks during the building process. The entire team is all the more pleased when this piece-work is completed, all blocks are pressed and only need to be laid.
Masonry with SSBs follows similar rules to conventional masonry but requires, for example, adapted mortar and plaster due to the high soil content.
The aesthetic qualities of SSBs positively surprised us. In Eleven Village, the outer walls of the teacher accommodations remained unplastered as exposed masonry – a contrast that required careful work but led to an impressive result. The warm earth tones of the blocks blend harmoniously with the surroundings.
After the successful completion of the project in Eleven Village in Malawi, we began to use the technique in Sierra Leone as well: for the construction of the Junior Secondary School in Yarawadu (started August 2024) and the health station in Durukoro (started November 2024). In our current project, the construction of a Secondary School in Kalanga in South-West Malawi, the SSB technique is being further optimized.
Our experiences with SSBs have convinced us: This technique will be an integral part of future projects! Because this way, our buildings actively contribute to climate and environmental protection!
Current project reports from Malawi and Sierra Leone:
“Anything can be taken from you, but not your education”
We are celebrating a key handover in Sierra Leone: we are delighted to report on the opening of the new junior secondary school in Tambia Balia.
My life as a traveling carpenter – and as a green helmet
We wish you a Merry Christmas! Our long-time employee Bala from Sierra Leone tells us about his journey in this year's Christmas greeting. Just like us, he has the motivation to make a difference!
Third school built in Malawi
In the Neno district of Malawi, off the beaten track, lies the village of Eleven Village. Here, we Green Helmets have realized a new project together with the local community: the construction of the Namwawa Junior Primary School.