A Bio-based Unfired Clay Brick Made Without Portland Cement
Developing Options for All Economies
Research & Development
Background
Since late 2018, Wheeler Kearns Architects has been formulating, producing, and testing ChitoClay, a material for making a biobased unfired clay brick that eliminates the portland cement content of modern concrete, rammed earth, and stabilized earth block construction. The ten percent portland cement content of all three prevailing materials carries a substantial carbon footprint since the manufacturing of portland cement accounts for seven percent of global CO2 emissions. We aim to replace the portland cement binder with biopolymers to improve the natural binding capabilities of clay.
Already, we have achieved an unfired biobrick with the compressive strength of a concrete block, tested two-and-a-half years after casting, including a full year exposed in Chicago’s extreme weather. With continued iteration, we believe we can improve its performance.
We are refining ChitoClay formulations on two parallel paths - one for developed economies and one for developing economies - each designed to diminish global reliance on portland cement use. Each economy offers different challenges. Developed economies can leverage access to energy, advanced materials, sophisticated equipment, and transportation to avail more complex processing options to obtain higher-performance materials. In contrast, developing economies like those in Africa and South America rely heavily on mud-brick construction and don’t have equivalent access to energy, equipment, and transportation. Consequently, we are developing a bagged additive that users can manually combine with local materials under ambient temperature and pressure. We began pursuing this path first because of its potential to forestall the adoption of portland cement where it is not yet common. It also stands to raise the standard of living for the greatest number of people.
The Biopolymer Backbone
Chitin and Chitosan
In pursuit of a circular economy, ChitoClay valorizes the shells of crustaceans as a source of chitin, the second most abundant natural polymer in the world after cellulose. Chitin forms the hard exoskeletons of insects and the cell walls of fungi, too. Since chitin is so resistant to decomposing in the natural world, processors must first “upcycle” it into chitosan, a derivative that combines more easily with other materials.
To date, chitosan and other additives have improved the axial compressive strength of unfired ChitoClay biobrick by a factor of six over mud bricks and by a factor of four over compressed earth blocks stabilized while dramatically improving long-term water stability. In rural economies, users can dissolve chitosan and other ChitoClay additives in water and manually mix it with local ball-milled clays and aggregates to produce a material that will set in less than 6 hours and harden in a day. After 14 days, it will have the compressive strength of a concrete block and over half of the compressive strength of ordinary concrete. It will be able to be repeatedly submerged in water for 12 hours and withstand freeze-thaw cycling.
Most importantly, the ChitoClay additives users will handle are nontoxic and solely made from abundant natural biopolymers. So, ChitoClay will deliver outsize advantages for a modest cost and a minuscule carbon footprint. ChitoClay recombines the chitosan biopolymer, used in tissue engineering, drug delivery, and agriculture, with ordinary clay and aggregates. We’ve enhanced its performance by crosslinking it with other biopolymers in novel ways. Although its constituents are known, ChitoClay’s recombinations result in a spectacular advance for a humble building material shared by the developed and developing world.
Although chitin produces hard, water-resistant shells in nature, it must be processed into chitosan before you can combine it with other materials. Like its cousin cellulose, chitin is a polymer derived from glucose. Processors first deacetylate chitin’s monomer acetylglucosamine in an alkaline solution. Here it becomes glucosamine, which exists in long chains of the derived biopolymer chitosan.
The resulting chitosan is a polysaccharide which is a carbohydrate made from bonded sugar molecules. Notably, chitosan is a polycationic organic polymer, offering multiple protonable amino groups that bind with anionic molecules like inorganic clay minerals and other biopolymers. This rare characteristic is its unique advantage. See Figure 5.
In nature, the negative charges of clay minerals confer the ability to support plant growth by binding to cationic nutrients in the soil. No other soil constituent can do this. Of the twelve soil-borne nutrients required by plants, seven are cations. That is why clay soils are the world’s most fertile.
Chitosan protonates in acidic solutions, which allows its cationic groups to bond with anionic groups of other molecules. It is effective at low concentrations, especially when highly deacetylated. Consequently, we formulate ChitoClay with 90% deacetylated chitosan made from shrimp shells. Films we've cast from chitosan solutions approximate the hardness of the parent chitin.
Stay Tuned
We are now in our second round of testing and will publish our findings shortly.