SA Affordable Housing March - April 2019 // Issue: 75 - Page 13

FEATURES Concrete’s green challenge Concrete has cemented its role in the affordable housing space but is criticised for not being ‘green’ enough. By Warren Robertson In 2016 alone, world cement production generated about 2.2 billion tonnes of CO 2 . M uch emphasis is put on the affordable housing sector to build homes in a sustainable way; everything from insulation to heating, bricks, plumbing and electricity are carefully planned to ensure that homes benefit residents and the environment. However, one aspect that is often taken for granted in the affordable sector is concrete, which has been around for millennia. The type of concrete that is most commonly used in our modern built environment was patented in the early 1800s by a bricklayer in England. Known as Portland cement this technique involves cooking limestone and clay in an oven and then grinding it to a powder to make ‘artificial stone’. This cement has helped build tens of thousands of buildings around the world from the Sydney Opera House in Australia to the Burj Khalifa in Dubai. However, despite its ubiquitous presence, concrete's environmental credentials are coming under increased scrutiny and the results are less than flattering. It turns out that the production of Portland cement is one of the single most environmentally destructive activities that humanity involves itself in. The first step in the process is the quarrying of raw materials mainly limestone and clay, which are then crushed. This process creates airborne pollution in the form of dust and these ground materials are mixed with others such as iron ore or ash and are fed into huge, cylindrical kilns and heated to about 1 450C (2 640F). Usually this heat is provided by burning fossil fuels, most commonly coal. This process of ‘calcination’ splits the material into calcium oxide and CO 2 and a new substance called clinker emerges as marble-sized grey balls. The clinker is cooled, ground and mixed with gypsum and limestone and this new cement is transported to ready-mix concrete companies. Each of these final steps requires more machinery and therefore more power. According to the Cement Sustainability Initiative (CSI) it takes 200kg of coal to make one metric ton of cement and about 300 to 400kg of cement is needed to produce one MARCH - APRIL 2019 11