Concrete is, without a doubt, one of the most widely used construction materials. After all, large cities lined with towering skyscrapers are known to be concrete jungles for a reason. The carefully curated mixture of cement, water, and aggregates form the structures of many buildings around us today. This isn't surprising, given concrete is a highly durable, economical, and increasingly sustainable construction material. Check out our article here for more info on sustainable materials and concrete. While concrete is becoming more sustainable, we aren't at a stage where concrete production is carbon neutral, but there is research into alternate cement types and efficient production processes. To understand how concrete ties into sustainability, we have to dive a bit deeper into the workings of cement, the core ingredient that forms concrete.
How cement is made
Cement is a fine powder that, when mixed with water, undergoes a hydration reaction to form a binding paste. It is then combined with a mixture of coarse and fine aggregates to create the concrete we use in construction.
Cement powder is generally made from lime, silica, alumina, and iron oxide . The raw minerals are mined and processed through a rotary kiln, as is in the case of the most commonly used Portland Cement. The rotary kiln dries the mixture of raw materials. It produces clinker by subjecting the raw materials to high enough temperatures that the chemical bonds break apart and reform. This can be achieved either through the Wet Process, whereby the minerals are supplied as a wet slurry, or the preferred Dry Process, where minerals are provided as a dry mixture and preheated before entering the kiln.
After the clinker is produced from the rotary kiln, it is cooled, grounded, and mixed with gypsum and other admixtures to form the final cement powder. The final product mainly consists of calcium silicates and aluminates. The proportion of clinker present in the cement to gypsum and other constituent minerals defines the final properties of the cement and the concrete it produces.
Types of cement
Cement used in construction is typically classified by its physical properties, which come from how it is manufactured and its constituent minerals. The standard designation of cement is based on several factors:
- Cement type
- Strength class
- Early strength
From AS 3972, cement is grouped into six major types across two categories:
- Type GP - General purpose Portland. Predominantly Portland clinker mixed with gypsum with up to 5% of a secondary additive as approved by AS 3972. They are cost-effective and good in most construction practices that don't require special properties such as high acid or water resistance.
- Type GB - General purpose blended. It is a blend of Portland cement with either fly ash or slag. Similarly to Type GP cement, blended cement applies to a wide range of large and small-scale construction. It brings the benefits of increased workability at a lower water demand and better resistance to sulfate attack.
- Type HE - High early strength. Type HE concrete is generally used for suspended slabs or precast structures where the building structure needs to be serviced quickly.
- Type LH - Low heat cement. Ideal for mass concrete pours with lower heat liberation. The cement is formed from cement, gypsum, and granulated blast furnace slag to generate minimal heat during hydration. LH cement is usually mixed at a high cement-to-water ratio to maximize concrete life expectancy.
- Type SR - Sulfate-resisting. Type SR cement reduces the chances of sulfate attacks by reducing the presence of sulfate salts in the mix. Tricalcium aluminate (C3A) is limited to a maximum of 5%, and the cement compound of 2C.3A + C4AF to 25%. SR cement is resistant to highly acidic environments and can lower the risk of corrosion in steel reinforcement.
- Type SL - Shrinkage limited. Type SL cement is a type of Portland cement with up to 7.5% of additive approved by AS 3972 to be applied in works requiring lower drying shrinkage. The performance of the cement ensures shrinkage stays well within the specified limit of 750 microstrains at 28 days from AS 3972.
Cement is typically used for its high compressive strength. However, it suffers from a far weaker tensile strength, so concrete structures typically accompany steel reinforcement. Steel is strong in tension and compression, which can compensate for the lack of tensile strength in concrete.
Properties of Ordinary Portland Cement Paste:
- Density: 2.01g/cm^3
- Compressive strength: 57.9MPa
- Tensile strength: 0.91MPa
- Specific heat capacity: 0.736kJ/kg K
Properties of Ordinary Portland Cement concrete:
- Density: 2240 - 2400kg/m^3
- Compressive strength: 20-40MPa
- Tensile strength: 2-5MPa
- Flexural strength: 3-5MPa
Chemical composition of Portland Cement
Chemical symbols used in the notation for cement compounds are abbreviated accordingly :
- Calcium Oxide(Lime): CaO -> C
- Silicon Dioxide(Silica): SiO2 -> S
- Aluminium Oxide(Alumina): Al2O3 -> A
- Ferric Oxide: Fe2O3 -> F
The four main compounds of Portland cement are:
Tricalcium Silicate (Alite)
Dicalcium Silicate (Belite)
Tricalcium Aluminate (Aluminate)
Tetra-calcium Aluminoferrite (Ferrite)
How do these chemical compounds contribute to cement properties?
Alites in Portland cement are mainly responsible for determining the initial strength of concrete in the first week of hardening. Belite determines the hardening of the concrete in the remainder of the 28 days of concrete hardening.
Concrete formation involves mixing cement with water, fine aggregates, and coarse aggregates. Cement serves as the adhesive to concrete, binding all the materials together. However, it lacks material strength, resulting in weak tensile strength of concrete. The aggregates are the components that determine the compressive strength of concrete instead. Coarse aggregates support the majority of compressive loading. Sand also contributes to compressive strength but mainly fills in the gaps of the crushed aggregates. Finally, water mixes with cement to help bind all the materials together and provides the workability of concrete during construction. Below are the associated mixing ratios for different concrete strength grades and their applications:
The talk on sustainability
Remember how in the beginning, we mentioned concrete is becoming more sustainable? Yes, there is plenty of discourse on the sustainability of concrete in construction, and a big concern is centered around cement manufacturing.
During the production of Portland cement, the kiln is heated up to 1450 degrees Celsius, which requires large amounts of energy. At this stage, calcination occurs in the mixture of minerals. The calcium carbonate within the limestones is converted to calcium oxide, producing carbon dioxide as a byproduct and releasing it. The empirical expression for this reaction is as shown:
5CaCO3 + 2SiO2 → (3CaO,SiO2)(2CaO,SiO2) + 5CO2
Calcium Carbonate + Kaolinite -> Alite*Dicalcium Silicate + Carbon Dioxide
❗Producing 1 tonne of Portland clinker will also directly yield 0.55 tonnes of carbon dioxide gas as a product .
Let's consider the amount of fuel it needs to generate the energy to raise the kiln to 1450 degrees Celsius. We are looking at about another 0.40 tonnes of extra carbon dioxide produced. This would mean an associated 0.95 tonnes of carbon dioxide per tonne of Portland clinker produced.
This process alone accounts for half of the carbon emissions in cement production and 4% of the globe's total carbon emissions. The other half of carbon emissions in cement production comes from creating energy to fuel the process .
It's a given that sustainability is a big issue within the construction industry. So, what's the solution?
We can tackle it in two ways: reduce carbon emissions by addressing the carbon in the materials or reduce the carbon from production.
Let's go with the latter first. There is a rising carbon minimization strategy in the form of Carbon Capture and Storage (CCS). The idea is to capture all the carbon dioxide released in the cement production process and transport them to an underground storage system . The process involves combusting the fuel in oxygen to allow for the greatest concentration of CO2 in its emissions. The emissions are then captured and compressed into a liquid before finally getting transported through pipelines into the storage system.
There are some limitations to this method. CCS does not entirely rid cement production of carbon dioxide; CCS has yet to be tested on an industrial scale. About 72-90% of CO2 is produced, with the remainder coming from energy costs.
If Portland cement isn't the way to go, what will it be?
Hope is far from lost. While Portland cement is still the most popular cement option in construction, we currently have areas where we can continue toward a carbon-neutral future. One answer is geopolymer cement. Geopolymer is made from alkali aluminosilicate in waste materials being put through polycondensation . Such materials include fly ash from coal burning, ground-granulated blast-furnace slag, and clay, all abundant in Australia.
Geopolymer concrete produces up to 80% fewer carbon emissions than Portland cement as it does not rely on calcium carbonate . It also primarily uses the waste materials mentioned above instead of raw minerals.
Geopolymer cement also has performance benefits in the production of concrete and construction. Initial and final setting times increase as the cement is produced with a high alkali concentration. Its initial compressive strengths are lower than Ordinary Portland Cement. However, strength increases through concrete aging. It has been shown that the final compressive strength can match that of Portland Cement and, at times, even higher strengths .
What could the future of cement hold?
Research around geopolymer cement and sustainable concrete has been conducted in the construction industry for a while. We are still transitioning into a future that is less reliant on Portland cement. It's not an easy path, for sure. Still, there are plans and studies on integrating more environmentally friendly cement into the industry from an engineering and business perspective.
These strategies lead Australia to a carbon-neutral future for cement production and beyond.
A report by Beyond Zero Emissions in 2017 proposes a 5-step process by which Australia can transition our cement production to a carbon-neutral future with the possibility to venture into carbon-negativity .
- Strategy 1 - Use of geopolymer cement: Geopolymer cement does not require a kiln, and the cost of a plant costs 10% of a Portland cement plant. These clays are produced from fly ash and ground-granulated blast-furnace slag or clay, all currently made in Australia. This stage will replace half of the current cement market.
- Strategy 2 - Use of high-blend cement: Blending Portland cement with other materials will reduce its cement intensity and assist with a transitioning phase in decarbonization. Other materials, such as fly ash, slag, and clay, make up 70% of the final cement. This stage will replace the remaining half of the current cement market.
- Strategy 3 - Carbon mineralization: Unlike Carbon Capture and Storage (CCS), carbon waste from Portland cement production is captured and chemically sealed within a rock. This prevents the risk of leakage or post-capture monitoring. The carbonated mineral can produce commercial materials such as magnesium carbonate and silica. This seeks to reduce Portland cement emissions to zero.
- Strategy 4 - Cement use minimization: Reduce cement usage by efficiently designing concrete structures. This can be done using high-strength concrete or incorporating timber into design structures. An outcome of a 15% reduction in cement usage is expected.
- Strategy 5 - Use of carbon-negative cement: This strategy aims to bring cement production to a carbon-negative era. Magnesium-based cement has the potential to absorb carbon over time to become a carbon sink. Research is still being performed in this area, and commercialization will be the desired outcome.
Geopolymer cement has been known for a while. Still, despite its commonly accepted environmental and strength benefits, it faces challenges in its adoption into the construction industry. With Alkali Activated Materials (AAM) generally, the first is the high costs associated with the sodium silicate and sodium hydroxide used to activate the materials . These activators are costly and corrosive, requiring special handling instead of large-scale transportation for Portland Cement.
Other than the properties of the activators, geopolymer cement has short setting times and high curing temperatures . Temperatures of 60 to 80 degrees Celsius are required for geopolymer concrete to achieve the same strength as Portland cement. Thus, geopolymer cement is limited in its practicality in large applications outside precast concrete production, where production conditions are more easily controlled.
We probably won't be seeing a carbon-neutral cement industry very soon. Still, based on recent works in geopolymer cement, it has cemented itself as a vital alternative to Portland cement in producing concrete structures.
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