Sustainable concrete: the advantages of admixtures for mixes with supplementary cementitious materials (SMCs)

Mapei's contribution to the concrete industry's commitment to achieve carbon neutrality by 2050

Author

Giorgio Ferrari

HPSS Group R&D Manager – High Performance Sustainable Solutions

With admixtures of the MAPECUBE line, it is possible to reduce the dosage of clinker and increase the amount of Supplementary Cementitious Materials, or SCMs, in the concrete mixes to produce low carbon concrete (LCC), with performance equivalent to or better than traditional Portland cement-based concrete. The use of these admixtures therefore contributes to the reduction of carbon dioxide emissions associated with concrete production.

World concrete production has reached 14 billion m³/year with an impact, in terms of "greenhouse gas" emissions, of over 2.5 GTon/year of CO₂. This quantity is mainly attributable to its key ingredient, Portland cement, whose production has exceeded 4 billion tons and is expected to reach 5 billion tons by 2050⁽¹⁾.

For this reason, the main cement and concrete manufacturers are committed to implement, by 2050, decarbonization strategies to achieve "carbon neutrality", i.e. the ability to produce concrete in a sustainable way with a zero balance of CO₂ emissions.

Figure 1 shows the contribution of the various mitigation actions expected from 2020 to 2050. It can be observed that the formulation of the cement (orange section), relating to the replacement of the clinker with supplementary cementitious materials (SCMs) with a lower carbon impact, will contribute 9% (350 Mt CO₂) to the objective of offsetting the more than 2.5 Gt of CO₂ currently produced⁽¹⁾.

Figure 1. Actions for carbon neutrality of the cement/concrete industry. Source: Global Cement and Concrete Association, “Concrete Future – Roadmap to Net Zero”, 2021.

On the road to more sustainable concrete

SCMs include by-products from industrial processes, such as fly ash from coal-fired power plants, ground blast furnace slag from steelmaking and silica fume from silicon production and its alloys. Other SCMs are natural pozzolana and limestone powder. Since the 1990s, these materials have been used in the formulation of new binders, partially replacing clinker. Over the next decade this process will be further accelerated with the increased use of limestone and the introduction of calcined clays and concrete crushed fines (C&DW), which will compensate for the reduced availability of fly ash and of the slag. This will make it possible to reduce the clinker/binder ratio from the current value of 0.63 to the pre-established value of 0.58 by 2030, up to the ultimate value of 0.52 by 2050⁽¹⁾.

The use of SCMs for the formulation of new cements and as a "filler" in the concrete batching represents a key point for the production of the new low carbon concrete (LCC).

However, unlike clinker SCMs do not have autogenic hydraulic properties but form binding products only when activated. The activation takes place thanks to the hydrated lime produced by the cement through the so-called "pozzolanic reaction", from which hydration products are formed with binding properties and composition similar to those that originate from the hydration of the silicate phases of the clinker.

The formation kinetics are also different: while the hydrated calcium silicates (C-S-H) produced by hydration of the clinker form already after a few hours of mixing with water, those produced by the SCMs develop much more slowly and to a lesser extent. Furthermore, not all SCMs participate in the pozzolanic reaction. In fact, limestone powder (limestone) is not activated by hydrated lime and mainly acts as a "filler".

It should be added that the same hydration reactions of the clinker phases are not quantitative, but always leave aliquots of unreacted clinker. These fractions of non-hydrated clinker therefore do not participate in the development of mechanical strength⁽²⁾.

These observations limit the possibility of replacing high fractions of clinker in concrete without affecting its mechanical properties, compared to those of an equivalent concrete with a higher carbon footprint.

The new LCCAs admixtures

To overcome this drawback, a new class of admixtures has been developed with the function of compensating for the loss of mechanical properties that LCC concrete shows as the clinker-to-binder ratio decreases.

Compared to traditional admixtures, which have been developed to control the rheological properties of concrete in the fresh state (plasticisers and superplasticisers) and the setting and hardening times (retarders and accelerators), the functions of this new class of admixtures is aimed at controlling the reactivity of the binder system as a whole.

Since these admixtures have been specifically developed to match the sustainability needs and to improve the performance of LLC concrete, the name of Low Carbon Concrete Admixtures (LCCAs) has been proposed for them⁽³⁾.

The LCCAs of the MAPECUBE line

The MAPECUBE line was created to meet the present and future needs of sustainable concrete, increasingly oriented towards the use of binders with a low carbon footprint.

The LCCAs in the MAPECUBE line are the result of a multidisciplinary research project launched in the Mapei Research and Development laboratories, in collaboration with the Department of Geosciences of the University of Padua (Italy). This project is aimed at the analysis of the reactions of the hydration process of low carbon footprint binders through the study of hydration mechanisms and the complete characterization of hydration products, with the aim of maximizing their yield and directing them towards the optimal development of resistances mechanical, both in the short and long term. The conceptual scheme of the project, of which the new MAPECUBEs represent the central element, envisages three main areas of action, as shown in Figure 2.

Promotion of clinker hydration - MAPECUBE contains new synthetic "hybrid" compounds, in which organic polymers and inorganic compounds interact to form "nanocomposites" which promote clinker hydration. Thanks to these compounds, larger quantities of hydrated phases are formed, characterized by a better microstructure and, therefore, more durable. The same compounds are capable of early triggering of the pozzolanic reaction, thus forming further quantities of hydration products in shorter times.
Alkaline activation - The presence of alkaline activators favors the dissolution of the SCMs, which release high concentrations of reactive species (silicates and aluminates) into the water of the pores which precipitate in the form of solid phases of calcium hydrated silicates and aluminates, contributing to the early development of mechanical strength.
Limestone activation – MAPECUBE admixtures are able to activate the calcareous filler, which is in itself poorly reactive, forming new carbonaluminate phases which contribute to develop mechanical strengths⁽⁴⁾
The combination of these ingredients with traditional superplasticizers, accelerating and retarding admixtures has made it possible to formulate different MAPECUBE with specific characteristics to optimize the yield of cements based on different SCMs.

Figure 2. Areas of action of LCCAs of the MAPECUBE line in relation to the various constituents of low carbon footprint binding systems.

Tests and results

The first test shows the development of mechanical strength of LCC concrete made with MAPECUBE 4 (1% in weight of the weight of cement) and CEMIV/AP 42.5R Pozzolanic cement, in which around 20% of clinker was replaced by a mix of Pozzolana and fly ash. A comparison between reference concrete produced under the same conditions with clinker-based cement only (CEMI 52.5R) indicates that MAPECUBE 4 increases strength after 28 days by 16%, in spite of it containing a lower class of cement.

In the second test, the effect of MAPECUBE 3 on a cement mix containing 55% clinker and 40% blast furnace slag was assessed. The results indicate that a dosage of MAPECUBE 3 corresponding to 3% by weight of cement considerably improves mechanical strength at all curing times, enabling concrete with a superior strength class to be produced.

The third test refers to a comparison between a reference concrete (with a cement dosage of 400 kg/m³) and a concrete with a lower cement dosage (350 kg/m³) with 1% of MAPECUBE 60 by weight of cement. The results confirm that, despite the different cement dosages, the two types of concrete have the same final strength. An evaluation of their carbon footprint, calculated using LCA (Life Cycle Assessment) methods, indicate that the concrete with MAPECUBE 60 has a carbon footprint of 333 kgCO₂eq/m³ compared with 375 kgCO₂eq/m³ for that of the reference concrete, with a reduction of 11.2%.

Lastly, in the fourth series of tests, a comparison was carried out between two types of concrete in which the composition of the binder was varied by mixing CEMI 52.5R cement and blast furnace slag (GGBFS) in different proportions. In the reference concrete the ratio between cement and slag was modified in favour of the cement (CEM/GGBFS = 60/40), whereas in the sample with 1% of MAPECUBE 60W by weight of binder, this ratio was inverted (CEM/GGBFS = 40/60). The results indicate that the concrete with MAPECUBE 60W, at all curing times, is characterised by higher compressive strength with respect to the reference concrete and confirm that the SCMs fraction in cement mixes may be increased without affecting mechanical properties. In this case, the carbon footprint passes from 240 kgCO₂eq/m³ for the reference concrete to 179 kgCO₂eq/m³for concrete with MAPECUBE 60W, with a reduction of 25%.

The table shows the results of concrete tests with MAPECUBE admixtures on different cements. *2 days ** 3 days

Less clinker, same performances

Mapei is now able to offer a complete range of Low Carbon Concrete Admixtures which are capable of carrying out a positive action on all the hydration reactions of the clinker and SCMs, increasing their yield and improving the microstructure of the hydrated phases.

Low Carbon Concrete Admixtures make possible the production of low carbon footprint concrete LCC (Low Carbon Concrete) having the same performance as Portland cement-based concrete with higher environmental impact.

Bibliography

[1] Global Cement and Concrete Association, “Concrete Future – Roadmap to Net Zero”, 2021, https://gccassociation.org/concretefuture/wp-content/uploads/2021/10/GCCA-Concrete-Future-Roadmap-Document -AW.pdf

[2] R.H. Mills, “Factors influencing cessation of hydration in water cured cement pastes”, 1966, Highway Research Board Special Report, 90, pp. 406-424

[3] G. Ferrari, “Where are admixtures going?”, Keynote at 13th International Conference on Superplasticizers and other chemical admixtures in concrete, Milan, July 13^th 2022, American Concrete Institute.

[4] G. Ferrari et al., “A new multifunctional additive for blended cements”, 2022, Construction and Building Materials, 354, 129086.