Cement manufacture contributes to 5 per cent of the western world’s global carbon dioxide emissions. This is due to the energy required to cook cement and carbon dioxide, which is released in the chemical reaction which binds cement’s basic ingredients of chalk, clay and calcium. Warren McLaren reports on some greener options.
Eco-Cement
Eco-Cement by TecEco in Tasmania attempts to improve cement’s carbon load by substituting the calcium component with reactive magnesia derived from magnesite. Common Portland cement is baked at about 1,450°C, whereas Eco-Cement’s magnesia only needs a kiln temperature half that — around 750°C. TecEco believes the lower temperature not only provides for significant reductions in carbon dioxide emissions, but it is a temperature which could be attained using renewable energy sources. Additionally, Eco-Cement is set by reabsorbing carbon dioxide out of the air. A sister product, Tec-Cement, can use a higher proportion of recycled aggregate materials, such as fly ash, than traditional cement. Although commercialised in 2007, Eco-Cement or Tec-Cement is not yet in widespread availability. However, testing by the Brick and Mortar Research Laboratory indicates that replacement of ordinary Portland cement with magnesium oxide can increase compressive strength by 150 per cent.
Ecoblend
Independent Cement’s Ecoblend range consists of ordinary Portland cement blended with a minimum of 30 per cent various supplementary cement materials such as ground slag and/or fly ash. Slag is the granulated waste from steel industry blast furnaces, whilst fly ash is a by-product of coal-burning power stations. Products that contain such supplementary cement materials may contain a small percentage of the human carcinogen hexavalent chromium. This chemical compound, also present in stainless steel, is not considered dangerous in a finished product, but inhalation of construction dust is best avoided. The benefit of a concrete like Ecoblend is twofold — waste materials are being recycled and the use of energy-intensive Portland cement can be significantly reduced, with corresponding drops in greenhouse gas emissions. Independent Cement suggest that 30 per cent slag blended cement has around 30 per cent less global warming impacts, heavy metals, carcinogens, water use and solid waste of the same weight of pure cement.
HySSIL
Born out of the labs of Australia’s CSIRO, HySSIL is a lightweight precast concrete building product. It is around half the weight of conventional aggregate-based concrete and up to five times more insulative than conventional concrete (its name reflects those properties — ‘high strength, structural, insulative, lightweight’). When used as a residential wall system, HySSIL is considered to have roughly 65 per cent less embodied energy and around 50 per cent less greenhouse gas emissions than conventional rendered brick veneer wall construction. HySSIL is best described as a lightweight cellular concrete or like an Aero chocolate bar, but with a very dense exterior skin. Currently the technology will produce wall panels up to 21 sqm. HySSIL is currently developing a new range of building products based on a novel form of foamed geopolymer with its research partner CSIRO. This geopolymer range of products currently under development delivers a lightweight product with significant greenhouse benefits (in contrast to cement-based concrete). HySSIL is also nearing the completion in the development of a new lightweight roof tile product with a major US roof tile company, which is expected to be in commercial production later this year.
E-Crete
E-Crete is a concrete product which uses fly ash and slag to displace the need for ordinary Portland cement (OPC). In this case, Melbourne company Zeobond employs what it calls geopolymer binder technology (an inorganic polymer formed at room temperature), which allows E-Crete to be made without any Portland cement, leading to a 80-90 per cent reduction in CO2 emissions. Decreased water use is another benefit of the E-Crete production process. Also, using 99 per cent industrial waste products does not impact on E-Crete’s performance. It is said to better both Portland cement’s fire and acid resistance whilst matching its compressive strengths. A similar technology was developed in the Ukraine in the late 60s, with resulting buildings now over 40 years old. E-Crete is produced commercially at a demonstration facility where daily output is the equivalent to the hourly yield of an OPC plant. Other production partnerships are being pursued to expand availability.
Hebel
Hebel is an autoclaved aerated concrete (AAC) product. Mostly produced as a precast block or panel, AAC is manufactured when silica (sand) is combined with cement, lime, water and aluminium powder. The powder reacts with the silica, creating hydrogen bubbles which allow the concrete to expand up to fives times its original volume. Once the hydrogen evaporates, a closed cell concrete is formed then steam cured in an autoclave. Being up to 80 per cent air, AAC products contain far less raw materials than standard concrete. Thus, some Hebel products have over 60 per cent less embodied energy than comparative concrete and brick veneer wall systems and 61 per cent less greenhouse gas emissions than comparative masonry products. All that air makes the products up to three-and-a-half times more thermally efficient than brick veneer or even double brick construction. The crystalline silica in AAC can be a respiratory issue as a construction dust, but is not considered a problem in the finished wall.
Hemcrete
The pithy centre of the hemp stalk, known as the shiv or hurd, is a material worth getting excited about. Once combined with lime, it forms a building material which hardens like concrete. Lime Technology, the British manufacturer of Tradical Hemcrete, has independent studies showing that for a 300 mm wall, Hemcrete locks up around 40 kg of carbon dioxide for every sqm of wall. It says that in comparison, a typical brick and block cavity wall creates around 100 kg of carbon dioxide per sqm per litre. Yet unlike the ingredients in other carbon absorbing concretes, hemp is a renewable crop. BPN has previously mentioned in these pages Hemcrete’s high insulative values, but we neglected to mention it also has a thermal inertia similar to thermal mass — it changes temperature, slowly reducing heating loads. This is a characteristic not lost on the English bewery Adnams, which used 90,000 hemp/lime blocks to construct its 4,400 sqm warehouse, the largest hemp building in the world.
Envirocrete
Like some of the other products mentioned here, Envirocrete earns its green merit badges by replacing the usual Portland cement with less energy consumptive alternatives. Anything from a modest 15 per cent cement replacement up to an impressive 60 per cent cement and 100 per cent aggregate substitution is possible, depending on end-use applications. The recycled aggregate comes from two sources. One is production waste that is returned to the plant “when concrete has been over-ordered, allowable time limits on-site have been exceeded or other issues such as pump breakdowns occur”. The other is demolition concrete, which Boral crushes up via its own patent pending process. In a similar spirit, Boral’s FireLight concrete blocks contain 75 per cent fly ash from power generation and 5 per cent recycled concrete. The company says it has reduced the energy needed to cure concrete blocks by 95 per cent.
