Development of Sustainable Concrete

Limestone Calcined Clay Cement (LC3)

Contents

List of Figures

List of Tables

1 Introduction

2 Limestone Calcined Clay Cement (LC3)

2.1 Raw materials

2.2 Material processing

2.3 Cement reactions

3 Comparison of LC3 and OPC concretes

3.1 Mechanical Properties

3.1.1 Compressive strength

3.1.2 Elastic Modulus

3.1.3 Shrinkage

3.2 Microstructure

3.3 Durability

3.3.1 Chlorides

3.3.2 Carbonation

4 Conclusion – future applications

5 References

List of Figures

Figure 1: Forecast cement production (Scrivener, 2014)

Figure 2: Availability of common SCMs (Scrivener, et al., 2018)

Figure 3: Impact of kaolinite content on cement strength (blend: 50% clinker, 30% calcined clay, 15% limestone, 5% gypsum). Dashed lines indicate when LC3-50 cement has strength equivalent (with equal mix proportions) to CEM1 or OPC. (Scrivener, et al., 2018)

Figure 4: Small rotary calciner (Krishnan, et al., 2018)

Figure 5: Compressive strength development of cements with equivalent water/binder ratio, (Dhandapani, et al., 2018)

Figure 6: Elastic moduli (Dhandapani, et al., 2018)

Figure 7: Shrinkage strains of cements with equivalent water/binder ratio, (Dhandapani, et al., 2018)

Figure 8: Pore structure evolution of LC3 and OPC cement, (Dhandapani & Santhanam, 2017)

Figure 9: Surface resistivity of different concretes, (Dhandapani, et al., 2018)

Figure 10: World Soil Map – kaolinite-rich clays highlighted yellow and light-green, (Scrivener, 2014)

List of Tables

Table 1: Experimental data for carbonation depths, (Rathinarajan & Pillai, 2017)

Table 2: Energy and carbon footprint of OPC and LC3, (Gettu, et al., 2016)

Introduction

Climate change is becoming a larger concern in the 21st century and scrutiny falls on the construction industry, in particular, to assist in global CO₂ reduction. Concrete is especially fundamental to this issue, as the structural properties, ease of design and construction and widespread availability mean that concrete is a highly utilised material around the world. Today, approximately 2.8 billion tonnes of cement is produced per year accounting for 5-8% of all man-made CO₂ emissions (Krishnan, et al., 2018). As shown in Figure 1 below, the production of cement is expected to increase to 5.8 billion tonnes by 2050 (Krishnan, et al., 2018). Therefore, the sustainability of concrete production is a crucial opportunity for the construction industry to reduce its environmental impact.

Figure 1: Forecast cement production (Scrivener, 2014)

As 60% of the CO₂ emissions relating to cement production are produced during the decomposition of the ordinary Portland cement (OPC) raw materials (Scrivener, 2014) the usage of SCMS, substituting of a percentage of the OPC content with an alternative material, could have a significant effect on the environmental impact of concrete.

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Replacing traditional Portland cement clinker with SCMs is already being widely undertaken by the cement industry with excellent results. The two most commonly used SCMs, slag and fly ash, can substitute cement content by up to 70% and 30% respectively (Scrivener, et al., 2018). These materials are quite popular as they are waste products of other industrial processes – and therefore relatively cheap. However, the future usage of fly ash and slag presents its own challenges.

Slag, a by-product of steel production, is available worldwide, however, only has an availability of 5-10% of the amount of cement produced (Scrivener, et al., 2018) and as demand for steel is increasing at a slower rate than that of concrete (and more steel is being recycled) it is likely that this percentage will only decrease over time. There is a greater availability of fly ash, a by-product of coal combustion, around 30% of cement production (Scrivener, et al., 2018), however, the quality varies with geographic location and with the reduction in fossil fuel usage the availability of fly ash is also likely to decrease over time. Therefore, it stands to reason that in order to reduce CO₂ emissions of cement in future years alternative SCMs must be found.

One SCM which has shown promising progress is the subject of this report; limestone calcined clay cement (LC3).

1       Limestone Calcined Clay Cement (LC3)

2.1       Raw materials

LC3 is an SCM formed from two materials; calcined clay and limestone. Previously, both have been used separately, however, use of limestone alone has shown to have detrimental effects on the properties of concrete and use of calcined clay alone is economically unviable at higher substitution percentages (Scrivener, 2014). Tests have shown that a coupled substitution (30% calcined clay & 15% limestone) can be made without any impact on the mechanical performance of the concrete (Scrivener, 2014). The use of limestone offsets the cost of the calcination making the use of the SCM economically viable – a significant factor in industry adoption (Scrivener, 2014).

Clays are the product rock weathering and are an abundant material around the world, far outstripping the amount required for cement production, as shown in Figure 2. Limestone is a sedimentary rock which is also a readily available material, which is already utilised in the production of Portland cement clinker.

Figure 2: Availability of common SCMs (Scrivener, et al., 2018)

Figure 3, produced by Scrivener, et al., 2018, displays performance results of LC3 cements, substituting 50% clinker, formed from 46 different clays sourced from around the world. Three significant observations can be made about the LC3 raw material from this data:

1         Clays with a kaolinite content of ~40% produce cements which have comparable properties to OPC after approximately 7 days.

2         The kaolinite content required for LC3 production is lower than that required for other industries, such as paper or ceramic production.

3         Lack of significant variation in the test data demonstrates that secondary materials present in the clays (ex. quartz, minerals and iron oxides) do not have a detrimental impact on LC3 cement performance.

The final two observations mean that clays deemed as ‘low grade’ by alternative industries (often considered to be a waste product) can be used in LC3 production (Scrivener, 2014).

Additionally, the limestone required for LC3 cement is a lower quality than that required for clinker production, meaning existing limestone quarries can become more efficient utilising otherwise discarded material (Scrivener, 2014).

Figure 3: Impact of kaolinite content on cement strength (blend: 50% clinker, 30% calcined clay, 15% limestone, 5% gypsum). Dashed lines indicate when LC3-50 cement has strength equivalent (with equal mix proportions) to CEM1 or OPC. (Scrivener, et al., 2018)

2.2       Material processing

LC3 reactive materials are formed when clays containing kaolinite are calcined at a temperature of between 600-850°C to form metakaolin, an amorphous alumina silicate which is a highly reactive pozzolanic material (Scrivener, et al., 2018). The calcining temperature is much lower than that required for clinker production (1450°C) so this can have significant fuel (and therefore CO₂) savings.

Figure 4: Small rotary calciner (Krishnan, et al., 2018)

In theory calcining can take place in a multitude of standard, widely available, industrial machinery. Pilot studies conducted in India, summarised by Bishnoi & Soumen, 2018, found that existing rotary kilns used for clinker production are also suitable for LC3 production. Static kilns, however, were found to be not as suitable as uniform temperature throughout the clay was difficult to maintain.

The final processing stage of grinding of the clinker, calcined clay and limestone can be achieved in a conventional ball mill. However, as summarised by Bishnoi & Soumen, 2018, pilot studies found that as clinker is harder than the LC3 components grinding all components together will lead to coarse clinker particles. As particle size has a significant impact on cement performance, the current evidence, therefore, suggests that grinding each material separately and dry mixing together will lead to a better overall particle size distribution, however – this will require additional energy.

2.3       Cement reactions

Hydration of the alite and belite phases of Portland cement, detailed in Equations 1 and 2 below, produces calcium silicate hydrate gel (C-S-H) gel, the main strength giving compound in concrete, and calcium hydroxide.

Alite:  2(3CaO.SiO₂) + 6H₂O $\mathit{\to }$

3CaO.2SiO₂.3H₂O + 3Ca(OH)₂   [1]

Belite: 2(2CaO.SiO₂) + 4H₂O $\mathit{\to }$

3CaO.2SiO₂.3H₂O + Ca(OH)₂    [2]

Tricalcium aluminate (C₃A) within the Portland cement then reacts with gypsum to produce ettringite, detailed in Equation 3 below. Once the gypsum has been consumed the C₃A reacts with the ettringite to from monosulphates, shown in Equation 4.

C₃A + 3(C $\mathit{ }\widehat{\mathbf{S}}$

.2H) + 26H $\mathit{\to }$

C₃A.3C $\widehat{\mathbf{S}}$

.32H   [3]

2C₃A + 4H + C₃A.3C $\widehat{\mathbf{S}}$

.32H $\mathit{\to }$

C₃A.C $\mathit{ }\widehat{\mathbf{S}}$

.12H   [4]

Alumina from the metakaolin within the LC3 calcined clay reacts with the calcium hydroxide, Equations 1 and 2, to form calcium aluminate-silicate hydrate (C-A-S-H) gel and aluminate hydrates (Krishnan, et al., 2018). This alumina additionally reacts with carbonate ions from the limestone to form carboaluminates, as demonstrated in Equation 5 (Krishnan, et al., 2018).

[5]: (Krishnan, et al., 2018)

$\begin{array}{c}\begin{array}{c}\mathit{Alumina\; from\; the}\\ \mathit{calcined\; clay}\end{array}\\ \overbrace{A{S}_2}\end{array}\begin{array}{c}\mathit{ }\\ +\end{array}\begin{array}{c}\begin{array}{c}\mathit{Carbonates\; from}\\ \mathit{the\; limestone}\end{array}\\ \overbrace{\mathit{Cc}}\end{array}\begin{array}{c}\mathit{ }\\ +\end{array}\begin{array}{c}\begin{array}{c}\mathit{Calcium\; Hydroxide }\\ \mathit{from\; reactions }12\end{array}\\ \overbrace{\mathit{CH}}\end{array}\begin{array}{c}\mathit{ }\\ +\end{array}\begin{array}{c}\begin{array}{c}\mathit{ Water}\end{array}\\ \overbrace{H}\end{array}\mathit{ }\begin{array}{c}\to \end{array}\mathit{ }\begin{array}{c}\begin{array}{c}\mathit{calcium\; aluminate}\\ \mathit{silicate\; hydrate\; gel }\end{array}\\ \overbrace{C–A–S–H}\end{array}\begin{array}{c}\mathit{ }+\mathit{ }\end{array}\begin{array}{c}\begin{array}{c}\mathit{Aluminate\; Hydrates}\\ \mathit{Carboaluminates}\end{array}\\ \overbrace{C_4\mathit{Ac}{H}_{11}+C_{4}A{c}_{0.5}{H}_{12}}\end{array}$

These carboaluminates stabilise the ettringite produced by the Portland cement reactions, preventing it from reacting with C₃A to form monosulphates. Ettringite fills more space compared to monosulphates leading to the improved mechanical performance of the concrete (Krishnan, et al., 2018).

As calcined clays are pozzolanic materials they react very similarly to traditional Portland cement. Therefore, the process by which LC3 cement works does not significantly deviate from that of traditional OPC with the same products being formed, just in different quantities.

2       Comparison of LC3 and OPC concretes

The following section aims to compare the structural performance of LC3 concrete against that of traditional OPC concrete.

3.1       Mechanical Properties

Dhandapani, et al., 2018 conducted mechanical testing on LC3 concrete (50% clinker, 31% calcined clay, 15% limestone, 4% gypsum) and traditional OPC clinker. Three mixes were assessed, two with equivalent strength 30MPa and 50MPa and one with equivalent water/binder ratio.

3.1.1      Compressive strength

The compressive strength test results are displayed in Figure 5 below. The graph shows that for an equivalent water/binder ratio the LC3 concrete demonstrates increased strength potential compared to traditional OPC concrete at all ages. This is likely to be a result of the more efficient LC3 reactions  – where ettringite is stabilised reducing the formation of monosulphates.

Figure 5: Compressive strength development of cements with equivalent water/binder ratio, (Dhandapani, et al., 2018)

3.1.2      Elastic Modulus

The elastic moduli for both OPC and LC3, results displayed in Figure 6, were found to be similar for all test mixes. Which suggests that LC3 concrete can expect to have a similar mechanical performance to OPC for structural applications.

Figure 6: Elastic moduli (Dhandapani, et al., 2018)

3.1.3      Shrinkage

The experimental testing demonstrates that water/binder ratio has a dominant effect on the shrinkage of the concretes. This is displayed in Figure 7 as for equivalent water/binder ratios both concretes displayed broadly equivalent shrinkage strains.

Figure 7: Shrinkage strains of cements with equivalent water/binder ratio, (Dhandapani, et al., 2018)

3.2       Microstructure

Dhandapani & Santhanam, 2017 conducted research into the microstructure of LC3 concrete (55% clinker, 30% calcined clay, 15% limestone) and traditional OPC clinker.

Their testing found that LC3 demonstrated early refinement of pore structure, shown in Figure 8. It is thought that the early pore refinement is due to the composition of the C-(A)-S-H gel where LC3 has lower Ca/Si composition compared to OPC (Gettu, et al., 2018). The early pore refinement leads to improved properties of the concrete at an early age, in contrast to fly ash concretes which react slowly and require extended curing.

Figure 8: Pore structure evolution of LC3 and OPC cement, (Dhandapani & Santhanam, 2017)

3.3       Durability

3.3.1      Chlorides

Surface resistivity experimental results performed by Dhandapani, et al., 2018 are displayed in Figure 9. These results demonstrate that LC3 concretes achieve a higher surface resistivity for all test mixes, which results in an increased resistance to chloride ingress (Gettu, et al., 2018). It is likely that this occurs due to the refined pore structure described above and the reactive aluminates present in the clay-binding with the chloride ions slowing the build-up.

Figure 9: Surface resistivity of different concretes, (Dhandapani, et al., 2018)

Further testing has suggested that the chloride threshold required for corrosion to occur is lower in LC3 concrete than OPC concrete (Gettu, et al., 2018).

3.3.2      Carbonation

Carbonation testing has been conducted by Rathinarajan & Pillai, 2017 has demonstrated that LC3 concretes have a lower carbonation resistance when compared to OPC concrete, as shown in Table 1. This creates potential issues for the usage of LC3 cement and is currently undergoing further research.

Table 1: Experimental data for carbonation depths, (Rathinarajan & Pillai, 2017)

3       Conclusion – future applications

The decline of conventional SCMs such as slag and fly ash over the coming decades will mean that the concrete industry will be looking for alternative SCMs. In my opinion LC3 cement, in particular, is well placed to become a mainstream construction material as: 

• Its raw materials are abundant worldwide, far exceeding the amount required for cement production.
• Low-grade raw materials are acceptable for use, often greatly improving the efficiency of existing material sources.
• LC3, as a pozzolanic, reacts similarly to OPC, so can be easily understood and trusted by industry.
• The production process is similar to OPC, can utilise existing industrial equipment with the potential to be manufactured by a low skilled workforce (Bishnoi & Soumen, 2018).
• Economical to produce, this factor is particularly important for mainstream adoption.
• LC3 has mechanical properties comparable to OPC so design processes would not need to be amended.
• Good durability potential, however, currently this aspect of LC3 requires further research.
• Blended cement will reduce the CO₂ emissions associated with OPC.

Additionally, there is an excellent opportunity for developing countries such and India and China to adopt LC3 as a mainstream construction material as they lie within regions where clay has a high kaolinite content, shown in Figure 10, – which is ideal for LC3 cement reactions. As these countries are expected to significantly increase their concrete production over the next 30 years, demonstrated in Figure 1, mainstream LC3 adoption has the opportunity to make a significant impact on the reduction of CO₂ emissions. Introduction of LC3 as a mainstream material may already be underway as several pilot studies conducted in India, described by Bishnoi & Soumen, 2018, have already shown promising results.

Figure 10: World Soil Map – kaolinite-rich clays highlighted yellow and light-green, (Scrivener, 2014)

As LC3 can substitute Portland clinker by up to 50% the use of this SCM would lead to low carbon concrete construction. Fuel requirements will reduce as clay is calcined at a lower temperature than used to produce Portland clinker. Additionally, the LC3 reactions do not produce chemical CO₂ like Portland cement leading to significant CO₂ savings. Table 2 below displays a life cycle assessment conducted by Gettu, et al., 2016 which shows a reduced carbon footprint of LC3 for all scenarios when compared to equivalent OPC.

Table 2: Energy and carbon footprint of OPC and LC3, (Gettu, et al., 2016)

References

• Bishnoi, S. & Soumen, M., 2018. Limestone Calcined Clay Cement: The Experience in India This Far. In: F. Martirena, K. Scrivener & A. Favier, eds. Calcined Clays for Sustainable Concrete. Dordrecht: Springer, pp. 64-68.
• Dhandapani, Y. et al., 2018. Mechanical properties and durability performance of concretes with Limestone Calcined Clay Cement (LC3). Cement and Concrete Research, Volume 107, pp. 136-151.
• Dhandapani, Y. & Santhanam, M., 2017. Assessment of pore structure evolution in the limestone calcined clay cementitious system and its implications for performance. Cement and Concrete Composites, Volume 84, pp. 36-47.
• Gettu, R. et al., 2016. Sustainability Assessment of Cements and Concretes in the Indian Context: Influence of Supplementary Cementitious Materials. Las Vegas, Sustainable Construction Materials and Technologies.
• Gettu, R. et al., 2018. Recent Research on Limestone Calcined Clay Cement (LC3) at IIT Madras. Lausanne, Ecole Polytechnique Federale de Lausanne.
• Krishnan, S. et al., 2018. Industrial production of limestone calcined clay cement: experience and insights. Green Materials , 0(00), pp. 1-13.
• Rathinarajan, S. & Pillai, R. G., 2017. Carbonation rate and service life of reinforced concrete systems with mineral admixtures and special cements. Mumbai, CORCON.
• Scrivener, K. L., 2014. Options for the future of cement. The Indian Concrete Journal , 88(7), pp. 11-21.
• Scrivener, K., Martirena, F., Bishnoi, S. & Maity, S., 2018. Calcined clay limestone cements (LC3). Cement and Concrete Research, Volume 114, pp. 49-56.