Building out of poverty
The future of urban development requires planning and appropriate materials. Alastair Marsh argues that geopolymers could be a part of the solution.
One of the greatest overarching challenges facing humanity is population growth. Quito, Ecuador, hosted the United Nations Conference on Housing and Sustainable Urban Development (known as Habitat III) in October 2016. In a nutshell, the challenge this gathering set itself was to produce a roadmap for sustainable urban development for the next few decades.
Since much urban development is built on bricks and mortar, materials will have a significant role in making this vision a reality. However, the role of materials research in housing for the urban poor in the least economically developed countries (LEDCs) needs to be examined. What are the current challenges in the urban habitat? How is materials research working to address these? What challenges lie ahead?
There are three facets to the current situation of population growth and housing – scale of supply, quality of life and environmental impact.
The United Nations Department of Economic and Social Affairs estimates that by 2050 there would be 9.7 billion people on this planet, up from 7.4 billion in 2015. This growth is predicted to be most intense in the world’s cities, especially within Africa and Asia. Building enough homes for all these extra people will be no mean feat.
Many of the most rapidly growing countries are also among the world’s poorest. Slums are prevalent in many cities worldwide, causing problems such as overcrowding, unstable structures, low thermal comfort and poor flood resistance. The combined effect of inadequate dwellings often conspires to trap residents in situations of very poor quality of life.
Added to all this is climate change and other environmental problems. Traditional materials in many regions, such as reed poles in coastal Tanzania, are under stress or already depleted. Standard modern materials such as fired clay bricks and concrete have an unacceptably high carbon impact given the scale of building needed, especially when production efficiencies are generally lower in LEDCs.
So the current conundrum is something of a three-sided storm – how can we collectively provide adequate shelter for a decent quality of life, at an acceptable environmental impact and the scale of supply needed?
There is little detailed data on current building practices by the urban poor in LEDCs. Given the geographical diversity, the sheer number of buildings involved and the fact that data is often lacking (in government surveys and censuses, slums often officially do not exist), it is very difficult to give a representative overview.
Case studies reveal several common features. Materials production and housing construction is dominated by the informal sector. This ‘underground economy’ is extremely effective at producing dwellings cheaply, but often at an inadequate quality. Houses themselves lie on a spectrum between using traditional, natural materials – such as wattle and daub, reed poles – standard modern materials – such as concrete blockwork, fired clay bricks – and some materials originally intended for other uses – such as metal and timber, as well as plastic sheeting for wall and roof materials.
Over the past 50 years, there have been several innovations in sustainable, appropriate construction materials from commercial companies, NGOs and academic researchers. Examples include sandbag construction, composite polymer panels and compressed stabilised earth blocks. However, the limited evidence suggests that, despite the invention of cheaper, stronger, more sustainable alternatives to the status quo, their adoption has so far been negligible. The reasons for this are not clear.
Soil-based materials have often been hailed as practical alternatives to conventional construction materials, partly because they tend to be cheap, with a low environmental impact. However, because of the water-adsorbent properties of clay, they typically require stabilisation to prevent physical degradation in the presence of water. Many substances can be used in stabilisation mixtures (including certain cheeses, surprisingly), but the most common in modern use are cement and lime. Although effective, the embodied carbon and cost of these substances reduce the overall attractiveness of stabilised soil materials. An additional problem is that, for many in LEDCs, soil is associated with traditional dwellings and therefore anything involving soil is seen as backwards or undesirable.
Hybrid materials that could make soil materials an improved proposition are geopolymer-stabilised soil materials (GSSMs). Also known as inorganic polymers, geopolymers are synthetic, solid, amorphous aluminosilicates produced through a reaction with an alkaline solution. Synthesis is via a condensation reaction of the precursors at an elevated temperature of typically 100°C or less.
In GSSMs, the clay particles are used as the aluminosilicate reaction precursors, which are transformed into a geopolymer gel by the addition of an alkaline solution and curing at an elevated temperature. The production process involves adding an aggregate to the soil (typically sand), preparing an alkaline solution, mixing the solution with the soil to create a slurry, compacting the soil in a press to make a block and then curing it.
The proposed advantage of stabilising soil with geopolymerisation over lime or concrete is lower embodied energy and carbon. The temperature required to cure a geopolymer-stabilised earth block (around 100°C) is far lower than that required to calcine cement or lime, or fire a clay brick (typically 700°C). There is possibly scope to use waste materials to produce the alkaline solution activator, further reducing the environmental and economic costs.
Social scientists working in the field have long called for more research into sustainable, affordable construction materials that the urban poor in LEDCs can build with. GSSM is a relatively undeveloped and young research field, with geopolymers only discovered around 40 years ago. But, in recent years, research groups from various countries including Togo, Jordan, Senegal and Malaysia have carried out experimental studies into the fundamental behaviour of these materials, as well as characterising materials made with real soils.
The key variables in the production process can be divided into composition and processing. Composition factors include clay mineral composition of the soil, alkali activating substance, mass ratio of water to clay mineral and mass ratio of activator to clay mineral. Processing factors include temperature, time and atmosphere of curing. There is now a reasonable understanding of different compositional and processing conditions. Behavioural trends and useful ranges have been established for kaolinite (the most common clay mineral). Results from real soils have been impressive, demonstrating dry strength values that can meet typical standards for stabilised earth construction.
Ultimately, for such a material to be of practical use, a ‘recipe’ needs to exist for determining how much activating substance and what curing conditions are required for a given soil. Soils vary in composition between regions, especially the types and proportions of clay minerals present. Most studies to date have focused on kaolinite. However, real soils often contain a mix of clay minerals, with tropical soils often containing rarer clays such as halloysite. To understand how effective GSSMs could be in different regions, we will need to understand how the reaction varies for different combinations of clay minerals. In addition, the geopolymerisation reaction itself is still not fully understood, and so the role in the reaction of minor elements in soil, such as iron, are still unknown.
On the civil engineering scale, there are several practical constraints that must be borne in mind. Firstly, the moisture content of such a mix must tread a fine balance. Too much water, and there is the risk of shrinkage cracking. Too little water, and the wet mix will not be workable.
On the social side, there are issues around perception and aspiration. There are aspirations by some in LEDCs to live in concrete-based houses regardless of the benefits of alternative materials. In urban areas of LEDCs, the majority of materials production and construction is by the informal sector. In this underground economy, health and safety procedures are often non-existent. It is the responsibility of materials designers to design out risks as far as possible. In particular, the need for alkaline substances for the reaction requires care to be taken.
Our collective ability to provide adequate housing for the growing global population requires sustainable, practical and affordable materials that do not currently exist. This issue is most acute for poor families in urban areas of LEDCs. GSSMs are one category of material under development that could fulfil this brief. Results from research studies so far are positive, although there are many questions still to be answered about how such materials would work on both a chemical and civil engineering level. Alastair is a PhD student in the Centre for Decarbonisation of the Built Environment at the University of Bath, UK. Prior to this he worked as a Graduate Sustainability and Physics Consultant at BuroHappold Engineering. He holds a degree in Materials Science from the University of Oxford.
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