The construction and real estate sectors have a history of being focused on reducing energy consumption and emissions in the operation and maintenance stage of buildings – which continues to be important considering their top spot in the worldwide ranking of energy hogs (see dia­gram on next double-page spread). Yet looking at the energy input during the construction stage and the required materials is becoming increasingly important as well. For achieving a truly carbon-neutral footprint of buildings, it will even be necessary to look at their entire lifecycle – from raw material extraction to demolition and disposal of the construction materials used.

Share of building construction in worldwide energy consumption and CO₂ emissions

This energy input for the production, transportation, storage and disposal of the materials used is called “gray energy.” The proportion of gray energy installed in a building may amount to a third of the total amount of energy and emissions during the operation and maintenance stage, depending on the materials and efficiency standard selected. According to the International Energy Agency (IEA), the construction sector on average accounts for five percent of the worldwide energy requirement and ten percent of the global CO₂ emissions. Disposal has to become a focus of attention too. In Germany, for instance, the construction sector causes 53 percent of the total waste generated nation-wide. Some of it is special waste that can be sorted and recycled only with a major effort (including energetic input).

These facts provide an impression of the construction sector’s impact on resource consumption, CO₂ emissions and disposal. They show the urgent need of thinking about alternative construction materials with a better carbon footprint. Such innovative solutions are already on the market. The rediscovery of timber as a construction material is a case in point.

Use of renewable raw materials

Timber as a construction material has many advantages. It’s renewable, stores CO₂ during the growth phase, and releases oxygen. Particularly important is the fact that wood as well as bamboo are regional construction materials in many countries. Today, timber is already in use for large-scale projects including high-rise buildings and can replace concrete or steel. In 2019 alone, three skyscrapers were completed worldwide with a timber content of at least 75 percent: the Dushan Shuisi Building in China (99.9 meters / 328 feet tall), the Norwegian Mjøsa Tower (85.4 meters / 280 feet) and the HoHo in Vienna (84 meters / 276 feet). Others, including a 73-meter (240-foot) high-rise in Amsterdam, are either under construction or in planning. Even such large-scale architectural projects use cross-laminated timber (CLT). Due to the biaxial load transfer of CLT, timber buildings in which it’s used can even be erected in earthquake-prone areas.

Generally speaking, timber, besides its carbon friendliness, has other benefits: Weighing less than steel and concrete, it’s easier to deliver in prefabricated elements. As a poor thermal conductor by nature, it has good insulation properties as well (the same, by the way, applies in terms of acoustics). Plus, the demolition of timber buildings has a lesser environmental impact than that of conventional ones.

Renewable raw materials such as wood fibers, sheep wool, flax, hemp and straw are also suitable for insulating buildings. Many of these materials were used for construction purposes centuries ago and are now experiencing a revival. But just like in the case of timber the question inevitably arises if such raw materials can grow back fast enough to cover the construction industry’s enormous material demand. Or at least to such an extent that its climatic impact can be improved significantly by their utilization. At the moment, the worldwide timber market is depleted and prices have risen to new record highs. Even so, the current potential of worldwide wood harvesting can cover the lion’s share of a timber-based transformation of construction, provided it’s based on the current space requirement, according to a study by the Potsdam Institute for Climate Impact Research.

Climate-positive construction materials

The façade of Audi’s branch on Trudering in Munich consisting of hexagonal plastic elements is more than a sleek-looking shell – it’s a CO₂ storage system, because its manufacturer, a company called Made of Air, extracted more CO₂ from the air than it generated in the material’s production process. The basis of such carbon-negative materials is the thermochemical process of pyrolysis during which organic waste such as sawdust, grass, leaves and tree cuttings, as well as industrial waste, are split up under application of oxygen-free heat (400–700 °C / 750–1,300 °F). The process produces biocarbons that – for instance as a petroleum substitute – serve as the basis for a wide variety of materials also in the construction sector. Carbonauten, another company pioneering carbon-negative technology, is planning to use biocarbon – which offers non-combustibility and very light weight as two further advantages – as a base material for concrete, asphalt, brickwork, wall structures, frame profiles, floor, wall and ceiling panels, and even for thermosolar roofs and façades. Every metric ton of biocarbon stores 3.3 metric tons (3.6 short tons) of a CO₂ equivalent, according to the “carbonauts.” Plus, the company says that this is not the only sustainability advantage of biocarbon. Both suppliers emphasize that their manufacturing process produces more energy than it consumes. This surplus can be converted into district heat or green electricity.

Cement-free concrete

Cement production is the “bad carbon dioxide apple” of the construction sector. It alone is supposed to be responsible for seven to eight percent of worldwide CO₂ emissions. During the production of one metric ton (1.1 short tons) of cement around 600 kilograms (1,320 lbs) of CO₂ are released into the atmosphere. But there are some promising innovations in this area as well.

They include “CarbiCrete,” another carbon-negative and cement-free construction material from the same-named Canadian startup company that uses ground steel slag to replace cement as a binding agent. The fresh concrete injected with it is cast into molds for hollow concrete masonry units (hollow CMUs) and cured in a carbon dioxide atmosphere that binds more CO₂ than the amount released during the entire production process. Another advantage is that the setting time of the CMUs is reduced from 28 days to 24 hours. The technology is still being tested. However, a rollout to the field might be hampered by the price of the special furnace for the CMUs that’s supposed to amount to the equivalent of around two million euros, according to “Deutsche Bauzeitung.”

Scientists from the Institute of Industrial Science at the University of Tokyo have developed a method for producing cement-free concrete using alcohol as a binding agent. To obtain a viable end product, the researchers dedicated several years to systematically investigating and determining the right mixing ratio of sand, alcohol, a catalyst and a dehydration agent as well as the suitable heating temperatures and reaction times. Another advantage of this technology is its suitability for the use of desert sand, which is too round for conventional methods.

Dr. Gnanli Landrou, a professor in the department of Sustainable Construction at ETH Zurich, has developed a chemical mixture called Oxacrete. It liquefies mud that can subsequently replace cement as a binding agent for concrete. Since mud is excavated at most construction sites anyway, this technology is supposed to improve not only the carbon footprint but save up to 20 percent of construction costs, according to Landrou – to the delight of builders, contractors and the environment alike.

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Made of air
carbonauten