The Science of ISSB

Posted on June 10th, 2017

ISSB are making attractive, sustainable and affordable buildings for families and communities – so how exactly do they work?


Spacious, stylish and sustainable – find out how we make the blocks that made this!


The Interlocking Stabilised Soil Block – or ISSB, for short – is the technology at the core of what HYT does. It’s a building material that Ugandans use to build robust, attractive and affordable buildings, whilst avoiding the deforestation and high carbon emissions of the conventional burnt brick.

What exactly IS an ISSB? And what makes it an excellent construction material?

The ‘interlocking block’ part of the name is fairly self-explanatory – it’s basically real-life LEGO! This allows for easy quality control on wall thickness, and reduces the amount of expensive mortar required to lay each course.


real-life LEGO

These giant LEGO blocks can even be curved for round structures like water tanks!


However, the ‘stabilised soil’ part of the name isn’t so obvious. What even is soil? And how is it stabilised? The soil referred to here – also known as murram – comes from about 50cm beneath the surface. It’s a mix of sand, silt and clay. It doesn’t contain any of the organic matter in topsoil, so using murram isn’t depriving farmers of their resources. Soil has been used for construction for millennia, and is still used in many parts of the world, from Devon to Delhi. However, there are a few problems with using unadulterated soil for building…


Mixing the Materials

Mixing the materials is a key skill taught to HYT trainees.


Firstly, it is a delicate material that requires skilful design and diligent maintenance. Although soil has good strength once compressed, it is vulnerable to the elements. Most threatening are water, which erodes the surface, and termites, which burrow through. Negating these threats requires competent planning and well-thought-out protection – procedures not to be rushed! Secondly, our tastes have changed over the years – just like most Brits, most Ugandans would rather not live in a mud hut if there’s other options. Burnt bricks and concrete blocks are aspired to because that’s what most people live in, right?


Stylish Ugandan Homes

The ISSB lets homeowners live in style without contributing to devastating deforestation.


ISSB retain many of the environmental advantages of building with just soil, but overcome these two problems – by making soil resilient and attractive. So how does ISSB do this? Well, this is where the ‘stabilised’ part comes in. When we stabilise a soil, this means that we change or add to its chemical composition in order to increase its strength and resilience. There have been many things added (i.e. stabilisers) to soil to stabilise it, such as lime, bitumen – even cheese! ISSB use cement as a stabiliser. The exact amount of cement added depends on the exact type of soil – judging this is one of HYT graduates’ skills after their 9 month training programme. Generally speaking, the amount of cement added to the soil is 5-10% by weight.


Material Stabilisation Diagram

Internal Microstructure of ISSB (cement amount has been exaggerated for clarity)!


The cement stabilises the soil by forming a network of strong crystals (calcium silicate hydrate) which surround the soil particles as the cement sets. The chemical reaction which causes the cement to set needs water – this is why HYT builders add water to the mixture of soil, sand, and cement before making the blocks. Over time, the cement sucks in the water, the setting reaction happens, and a few days later the cement has formed a network of strong crystals surrounding the soil particles. This structure makes the ISSB much stronger and more resilient than if it was just soil.


ISSB Corner Stack

Building in Uganda has turned a corner with the development of the durable yet economical ISSB.


Whilst cement itself has high carbon dioxide emissions from production, because it’s only used in small amounts in ISSB they give a much better environmental performance than burnt bricks. Using cement as a stabiliser avoids the need to fire bricks in a kiln, preventing deforestation for firewood. There’s still several improvements to be made to the technology, but in the meantime, Ugandans will continue using ISSB to build quality buildings for families and communities.


By Alastair Marsh

Postgraduate Researcher in the Department of Architecture and Civil Engineering

University of Bath

Twitter: @AlastairMarsh

Comments are closed.