A few eye-opening facts about concrete: the concrete industry is the largest user of natural resources in the world, and Portland cement, a key ingredient in most conventional cement, accounts for roughly five percent of all carbon dioxide gas generated by human activities worldwide.
So when architects, engineers and contractors look at the environmental impact of their projects in light of Abu Dhabi’s mandated sustainability standards and with an eye to achieving higher numbers in the Estidama Pearl Rating System, concrete is one of the first places they are looking.
Worldwide each year, concrete production uses 1.6 billion tons of Portland cement, 10 million tons of sand and rock, and 1 billion tons of water. The quarrying of raw materials for concrete in open pits can result in soil erosion, pollutant runoff, habitat loss and ugly scars on the landscape. The total energy embodied in concrete varies, especially with the design strength. This is because higher-strength conventional concrete relies on a greater proportion of Portland cement in its mix, and the energy required to produce Portland cement is very high in comparison to concrete’s other ingredients. For average-strength concrete, the embodied energy ranges from about 0.5 – 0.7 Mj/kg.
The production of Portland cement is by far the largest user of energy in the concrete construction process, accounting for 85 percent of the total energy required. The manufacture of cement produces large amounts of air pollutants and dust. For every ton of cement clinker produced, almost a ton of carbon dioxide, a greenhouse gas, is released into the atmosphere.
For Masdar City, as for many regional projects, concrete was selected as a material of choice because it is relatively inexpensive, strong, versatile, resistant to fire, impact, water and weather, requires little maintenance, and can be sourced locally and recycled. The excellent thermal capacity or thermal mass of concrete means that it can absorb, store and later radiate heat, thereby stabilising the internal temperature of a building. This is important because in all buildings, heat is generated by people, electrical equipment, computers, lighting and solar gain, which means that buildings can overheat during the summer. Exposed concrete absorbs much of this heat, and can reduce daytime temperatures by up to 4°C or 5°C. Night-time ventilation is then used to cool the building, readying it for the coming day. Natural ventilation and good daylighting are features of many high thermal mass concrete buildings. Exposed concrete can reflect light far into a building's interior, helping reduce the need for artificial lighting.
However, there were challenges to using concrete in the city. We were faced with harsh conditions, chlorides, sulfates and hot weather – not to mention conventional concrete’s inherently high carbon footprint. Various steps were taken to increase the sustainability of the concrete used to build the city.
The good news is that the cement industry is committed to reducing greenhouse gas emissions per ton of product by 10 percent from 1990 levels by the year 2020. The amount of Portland cement used as an ingredient in concrete, and as a consequence, the energy required to produce the concrete, can be substantially reduced by the addition of certain industrial waste materials, which have cementing properties, to the concrete mix. Substituting such supplementary cementitious materials (SCM), including fly ash from power plants, silica fume, and blast furnace slag, for up to half the Portland cement in the concrete, can result in reductions in embodied energy of as great as one-third. Waste materials from other industries also can be used as cementing agents – examples include wood ash and rice-husk ash. Use of these materials, as well as mineral admixtures and pozzolans, lowers the embodied energy of the concrete by offsetting almost one ton of carbon emissions for every ton of Portland cement replaced. The use of green concrete cuts energy consumption, emissions and wastewater.
Concrete is a uniquely useful building material well suited to this region’s vast infrastructure and building needs, yet it traditionally has come with an enormous environmental cost. Green concrete is helping find a third way that not only retains or even enhances the many benefits of concrete, such as its durability, longevity, heat storage capability, and (in general) chemical inertness, but helps developers and the AEC community to build more sustainable developments.