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

Keeping up with Kadungulu: Construction Complete!

Posted on May 22nd, 2017

April 25th was a well-marked day not only in the calendars of HYT staff, but also in the minds of Kadungulu’s residents, its schoolchildren, and its burgeoning team of new masons. Though Kadungulu Secondary School’s new dormitory and water tank were completed the month before, the community wanted time to prepare a fitting opening ceremony to celebrate the efforts of the HYT trainees and the generosity of the project’s funder, Andrew Billington.


Leaving Billington Vocational Training Centre

Michael, director of Billington Vocational Training Centre, and HYT Assistant Trainer Alamanzani prepare to follow the truckload of students and band members to the ceremony.


The ceremony was indeed unforgettable, and featured a procession around the town of Kadungulu. The marching band of Billington Vocational Training Centre in Serere, from which the trainees are drawn, merrily accompanied the festivities. A crowd of staff, pupils, donors, builders, government officials and townspeople marched together to the anthems of Teso and Uganda in what was described as “the biggest event ever held in the town”! Such an accolade is not unrealistic, considering the parade collected over 3,000 people at its height.


Processing past the dormitory

Any townspeople who had not previously visited the highly popular new structure were able to see it at the parade’s end.


Having wound through the town accumulating revellers enthused by the new construction, the column finally made its way back through the school gates. From here, everyone was able to see the building, nestled in a stand of trees preserved thanks to HYT’s pioneering technology. According to research from Oteng’i et al. (2007), a building the size of Kadungulu’s dormitory would require the felling of approximately 9 mature trees if built using traditional fired brick methods. [1] That would destroy the shade currently cooling the building, and lead to further disruption of the local water cycle (already erratic, at best). For a country that loses 1% of its biodiversity per year, the conservation of such trees is of vital importance.[2]


Kadungulu Dormitory

The dormitory, with its iconic red roof, has been described as the “very best building in the vicinity”!


In order to combat Uganda’s alarming deforestation rates, HYT instructs trainees in the use of Interlocking Stabilised Soil Blocks (ISSB). This makes Kadungulu’s new masons a highly specialised team able to build with both traditional and modern, environmentally-friendly techniques. Not only does the ISSB cure in the sun, removing the need for firewood. It also exists in a number of different shapes, one of the most important being the curved block. These are used in the construction of water tanks, key structures that fired bricks would never be able to accomplish. The trainees would explain all these benefits to the local community after the official opening, encouraging the technology’s wider application.


Tour of the Tank with Chris Billington

Among the guests of honour was Chris Billington, son of Andrew and Penney and a new ISSB enthusiast!


Following recognition of the hard work and contributions of all, the ribbon was cut and the building officially opened. Guests filed into the large, airy space whose temperature is regulated by the ISSB’s excellent thermal insulation. The trainees then gave a tour of the building’s unique aspects, before commencing their graduation.


Traditional Dance

Pupils from all over the school, including the arts and army sectors, joined in a celebratory traditional dance.


Accompanied by traditional music from the school’s musicians, each trainee danced up to the presenters, a team made up of local officials, sponsors Andrew and Penney Billington, and HYT Operations Manager Philip Yiga. They were given certificates marking the completion of their training and confirming their new status as ISSB masons. Their experience will continue to grow as they work on more projects, either through HYT or for independent employers. By the looks of the dormitory, they are well on their way to becoming experts in the field!


Graduation with the Billingtons

The trainees were pleased to receive their certificates from presenters including Andrew and Penney Billington.


So there we have it, another school furnished with safe accommodation and water facilities, another set of youths imbued with employable skills, and another oasis of greenery preserved by HYT’s ISSB training programme. We may work ‘One Village at a Time’, but the collaboration of forward-looking benefactors like the Billingtons is invaluable in maximising the powerful effects of the technology. Thanks to them, we are pleased to welcome to the HYT family 11 youths, newly empowered to join the quest for stability, sustainability and success!



Congratulations to the graduates on successfully completing both the structures and the training programme!


You can continue to keep up with Kadungulu and other HYT projects via our Facebook, Twitter and Instagram pages, as well as our main website.


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HYT is proud to be shortlisted for an Ashden International Award 2017.


[1] OTENG’I, S. B. B. & NEYOLE, E. M. (2007) Brick Making Activities and their Environmental Impacts in Busia, Siaya, Bondo and Butere-Mumias Districts of the Lake Victoria Basin of Kenya. International journal for Disaster Management and Risk Reduction. Vo. 1 No. 1, pp. 24-28.

[2] DR. ROBERT NABANYUMYA (2017), NatureUganda, 25th Annual General Meeting Chairman’s Report, p.2.