CSA – the greener choice for concrete infrastructure
Communities across Australia and around the world are increasingly concerned about sustainability and the effects of global warming. Not only are they attempting to lower their individual carbon footprints, they are exertng pressure on governments of all levels to implement wider change through legislation. The result is being increasingly felt across all industries – particularly engineering and construction.
Engineers are looking towards new design and technology to meet both legislative and community expectations for carbon-neutral projects. However, sustainable building materials are increasingly popular as the leading method through which construction project sustainability targets can be met.
CSA cement has emerged as a simple and effective way to not only improve the “green credentials” of construction projects, but as a means to increase time efficiencies and, therefore, profitability.
What is sustainability?
Sustainability is widely accepted as being based on the simple principle that everything needed for survival and well-being depends, either directly or indirectly, on the natural environment. Sustainability efforts ensure resources needed to protect human health and the environment remain available.
According to Cement Concrete and Aggregates Australia, the nation’s peak body for the heavy construction materials industry, sustainable development can be broken into three parts: social environmental and economic. Social sustainability is the quality of life of individuals and their communities; environmental sustainability is the management and preservation of air, water, land and ecosystems; and economic is the level of prosperity of organisations and individuals.
The sustainability of construction material is affected by the amount of non-renewable materials entering into its manufacturing and its lifespan before it has to be replaced and manufactured again. These non-renewable materials include sand, rock, and limestone for cement manufacturing.
Durability is a key factor is the sustainability of construction materials. The longer a product lasts, and the less maintenance it requires, the more economical it is and the lower its impact on the environment.
The Cement Sustainability Initiative – a project of the World Business Council for a Sustainable Development – is an effort by 24 major cement-producers to look at the factors affecting concrete sustainability. It is still in its infancy and is attempting to deal with the many facts of concrete manufacturing. However, it cannot yet circumvent the fact that traditional cement technology and its carbon footprint has changed little since Joseph Aspdin obtained the patent in 1824.
A century-old technology
The traditional cement industry in the 21st century is facing the fact that the technology is significantly dated. Although the industry has made great strides in the efficiency of limestone burning, the material’s carbon footprint has not changed significantly, and is not likely to any time soon. The industry is locked into a technology that forces it to emit large amounts of carbon dioxide. (One ton of CO2 is released into the atmosphere for each ton of cement manufactured.)
Given the global industry produced nearly four billion tons in 2012, traditional cement production accounted for about five percent of total greenhouse gas emissions worldwide.
Sustainability efforts have generally been limited to reducing the cement content in concrete or increasing the amount of filler, such as limestone, slag, or fly ash. While fly ash, blast furnace slag, and other pozzolans provide cementitious properties, such fixes can only reduce the carbon footprint by a fraction. They do not address the core issue of a 200-year-old chemistry, making it difficult to consider traditional cement concrete as a sustainable building material.
A greener solution
CSA cement is a “green” alternative that uses less into natural resources. Manufacturing this rapid-setting cement requires burning mixtures of limestone, bauxite, and gypsum at lower temperatures than its traditional cousin. (1482°C versus 1232°C.)
The lower burning temperature of CSA cement reduces the amount of energy and carbon dioxide emissions associated with traditional cement production.
CSA cement also requires less limestone, which is the primary source of carbon dioxide released during the chemical sintering process. CSA clinker is also easier to grind, reducing the energy needed during the milling process. A particularly interesting type of CSA cement is the CSA-belite (C2S) cement that provides some of the advantages of traditional cement while lowering its carbon footprint.
CSA cement can play a significant role in improving the sustainability of construction materials by simply reducing the quantity of non-renewable resources used during manufacturing. The use of resources is just one part of the equation – durability or lifecycle, is the second.
CSA cements have been available for several decades and are extensively used in runway or highway repair as well as underground construction and pipework projects.
Sustainability should be easy for engineers to quantify when selecting a building material. One method for assessing the sustainability of a construction material could be to divide its lifecycle by the amount of non-renewable resources required in its manufacturing process.
Using this method, the Sustainability Index can be defined as: Sustainability Index (S) = Lifecycle Resources.
In this equation, “lifecycle” refers to the durability of concrete in years. It is linked to fatigue life and other material properties such as shrinkage, cracking, and porosity.
“Resources” refers to the quantity of non-renewable resources used in concrete manufacturing (including carbon footprint). Finally, the Sustainability Index can have the unit of [m3*years/ ton-CO2].
In this equation, sustainability is tied not just to resources used, but also to lifecycle. If the lifecycle of one cubic meter of concrete were infinite, it would be sustainable. However, if all Earth’s resources were required to manufacture that same amount of concrete, it would not be sustainable. The Sustainability Index helps to quantify the sustainability of producing and using one cubic meter of concrete in simple, measurable terms.
Further, LCA and EPD calculations, as defined by concrete PCRs, can be used as the numerator and the denominator, respectively, in the Sustainability Index equation.
The life cycle of concrete
The Sustainability Index can help rate the sustainability of various materials and mix designs and helps decision-makers choose materials consistent with stated sustainability goals. It brings lifecycle into the equation, which is vital, as greater lifecycle decreases the burden on resources.
A 100-year-old pavement would have five times the Sustainability Index of a 20-year pavement, all other parameters being equal. At equal lifecycle, a mix design with half the carbon footprint would have twice the sustainability. This simple concept ties economic decisions to materials properties.
The concept also highlights the sustainability of concrete is, above all, a materials property. For example, lower shrinkage and lower porosity increase lifecycle. Doubling the lifecycle while halving the carbon footprint, quadruples the Sustainability Index. Using this approach, the sustainability of concrete can be defined as the lifecycle of the material per unit of non-renewable resources.
Using this definition of sustainability for concrete enables decision-makers to choose between materials of different lifecycles and made with different resources.
Concrete, water, porosity and structure
Concrete is a highly porous material, with the amount of water used in the mix controlling its porosity. A highly porous concrete will have lower strength and shorter lifecycle. Also, concrete shrinks as it ages, resulting in cracks. Any factor that reduces concrete shrinkage decreases its tendency to crack. This, in turn, increases its lifecycle and therefore sustainability.
Traditional cement-based concrete shrinks because more water is needed to make a workable mixture than is required for chemical hydration. Most of the shrinkage of traditional cement concrete, and its lifecycle, is directly connected to the amount of water needed to make the mixture. Adding more water than necessary for hydration is one reason why the goal of 100-year pavement is still elusive.
In contrast, CSA concrete permanently retains the mixing water in its crystalline structure, making it less prone to shrinkage or cracking. Moreover, CSA concrete is less porous, stronger, and has lower shrinkage than traditional cement concrete, which increases its lifecycle. CSA concrete’s lifecycle has been tested to be as long as 80-years, as opposed to 40-years for traditional cement. Civil engineers, using the Sustainability Index, can include lifecycle as part of their evaluation of the sustainability of a mix design.
A long lifecycle for concrete is a significant advantage not only sustainability, but economic costs. Not only is replacing cracked or worn concrete a cost in terms of purchase and labour, there are significant economic costs associated with lost productivity and disruption to business activity.
CSA cement can play an important role in improving the sustainability of construction materials from the perspective of raw materials use, energy demand, carbon footprint, and pavement longevity.
A combination of low calcium content and low burning temperatures allows CSA cement to yield concrete with a lower carbon footprint. When used as a shrinkage-compensating additive to traditional cement, CSA can also improve sustainability through a decrease or total elimination of shrinkage, resulting in an increase in longevity.
The improved sustainability of CSA-based concrete is due to a combination of factors including:
- Lower greenhouse gas emissions
- Decreased nitrogen oxide emissions
- Use of recycled raw feedstock materials
- Longer lifecycle through higher strength, lower shrinkage, and lower porosity.
Rapid setting concrete and sustainability
There are several additional benefits that should be taken into account when considering CSA cement for a project. While traditional cement can set in about three hours, it can take several days for it to reach structural strength. CSA cement achieves structural strength in three hours. As a result, CSA concrete has been used in time-sensitive pavement rehabilitation projects around the world.
The concrete slab repair/replacements at Melbourne International airport in Victoria and Sydney International Airport in New South Wales are just two examples of the benefits of CSA cement-based concrete. The Melbourne and Sydney Taxiways and Thresholds were shut down during night-time closures (Sydney Airport Curfew hours 11pm to 5am, Melbourne from 11pm), old concrete slabs removed, and fresh CSA concrete poured. 2-3 hours after placement of CSA Concrete test results greater than 3.0MPa Flexural Strength were achieved (exceeding specification requirements), the areas were trafficked by aircraft – resumption to normal operation in hours, not days as is the case with traditional binders.
There are substantial sustainability benefits to the rapid-setting characteristics of CSA concrete. For example, shorter closing times mean less fuel burned in traffic and lower economic impact on the travelling public. Additionally, it can be used in post-tensioned concrete buildings, or precast concrete work, where high, early strength means sooner post-tensioning and faster construction times.
Cost is a significant factor in the selection of sustainable building materials. CSA cement is a premium product and can be more expensive than traditional cement, but its durability and lifespan make it more cost-effective because of its longer lifespan.
When it comes to publicly-funded infrastructure, the overall benefits of a longer-lasting pavement outweigh the short-term impact on a budget. A less-expensive material that will need to be replaced in a few years simply shifts the burden to future generations.
The Sustainability Index can be an important tool in helping engineers and specifiers make decisions based on cost and lifecycle considerations and environmental impact. A common misconception is that the chemical composition of all cements is the same. This is not true and understanding the differences between traditional and CSA cements in terms of properties, cost, and sustainability, as well as use of the Sustainability Index, is crucial to making an informed decision on the best material to use in individual projects.
The construction industry should look to CSA cement with a fresh eye because of its impact on sustainable development and the improvement of infrastructure.
While there is always reluctance to specify a material that deviates from long-established standards, it is vital that the industry educates itself on the advantages of specifying CSA cement if it is to reach the “greener” sustainable standards demanded by the community and increasingly required by government.