Interlocking stabilised soil blocks — soil, a small cement fraction, compressed in a press, cured, then dry-stacked — have moved from NGO demonstration projects to genuine commercial use in Uganda over the past decade. The interest is well founded, and the enthusiasm sometimes outruns the evidence.
The case for
- Less cement. A typical stabilised block uses 5–10% cement by weight, against fired brick's fuel load or concrete block's binder content.
- Local soil. On sites with suitable lateritic soil, the material is already there. Transport emissions and cost fall sharply.
- Less mortar. The interlock means blocks dry-stack with mortar only at intervals, which cuts cement again and speeds laying.
- Local employment. Pressing happens on site with a manual or hydraulic press and a trained local team.
- Thermal performance. Denser than hollow block, with useful mass.
The case for caution
The soil has to be right. This is the constraint that decides everything. You need a soil with an appropriate clay-to-sand ratio and low organic content. Test it properly — grading, Atterberg limits, and trial blocks tested for wet compressive strength — before committing. A site with the wrong soil means importing soil, and the environmental argument weakens considerably.
Quality control is everything. Block strength varies enormously with compaction pressure, moisture at pressing, and curing discipline. A well-run site produces excellent blocks. A poorly supervised one produces a wall you cannot certify.
Wet strength, not dry. Specify and test wet compressive strength. Dry figures flatter the material and are not the condition that matters.
Structural limits. ISSB is a load-bearing masonry material suited to one and two-storey construction. It is not a substitute for a frame on a taller building.
Approvals. Expect to submit test certificates. Some authorities and most lenders are still unfamiliar with the material and will ask for evidence a concrete block wall would never be asked for.
Where we specify it
Single and two-storey residential, rural and peri-urban institutional buildings, boundary and landscape walls — on sites where soil testing supports it and where the build can be properly supervised. It is a good material used in the right place, and it is not a universal answer. Very few materials are.
