Sustainable binders for concrete

(Philip Van den Heede, Elke Gruyaert, )

One way to minimize the environmental impact, is the use of industrial by-products as cement replacing material. As a consequence, the CO2 emissions (production of 1 tonne cement = production of 1 tonne CO2) and energy demands associated with the Portland clinker production can be reduced. In high-volume fly ash (HVFA) concrete,  at least 50 % of the cement is replaced by fly ash, a by-product of coal fired electrical power plants. At the Magnel laboratory, its concrete mix design is being optimized for use in environments with exposure to carbonation, chlorides and freeze-thaw. The environmental benefits of these HVFA compositions in each environment are quantified using the life cycle assessment (LCA) software SimaPro (Philip Van den Heede).

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SEM image of fly-ash particles

Also blast-furnace slag, a by-product of the steel industry, can partly replace cement. However, for mixes containing blast-furnace slag, the microstructure development, the mechanical characteristics and the durability behaviour are different from that of ordinary Portland cement concrete. These aspects were investigated for mixes in which up to 85% of the cement is replaced by blast-furnace slag. (Elke Gruyaert)

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Image analysis of BSE image of cement paste containing blast-furnace slag

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Testing apparatus for accelerated degradation tests – Accelerated carbonation test – Frost-salt scaling

The use of secondary copper slag in ultra-high performance concrete

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In Belgium, secondary copper slag is produced by a recycling plant. Since this material needs a large area for landfilling, of which the availability is insufficient, and to avoid problems related to the leaching of heavy metals and other harmful elements, it would be interesting to upgrade these ‘waste’ products in high-value applications. Moreover, the amount of natural resources is declining due to a large consumption in the cement and concrete production. A possible breakthrough can thus be sought in exploiting by-products within cement and concrete production.


a) Planetary ball mill          b) Intensive vacuum mixing         c) Mercury intrusion porosimetry

d) SEM image of copper slag (2000x)          e) BSE image of RPC containing copper slag

Copper slag obtained from recycling plants is ground in a planetary ball mill to achieve a product with a higher specific surface area (SSA) and assess the effect of fineness on the reactivity of the copper slag. The reaction and pozzolanic activity of copper slag were assessed using isothermal calorimetry and Frattini tests. The latter was compared with the Chapelle test and strength activity index.
The physical properties of copper slag can enhance concrete strength, and improve the cohesion in the concrete matrix. Additionally, the combination of vacuum mixing and heat curing can still increase the compressive strength of UHPC. The pore size of UHPC is investigated by mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM).

Replacement of cement by supplementary cementitious materials (SCMs)

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The combination of ordinary Portland cement with supplementary cementitious materials (SCMs) gives origin to blended cements, which currently represent a big portion of the cement market. A main advantage of these types of cement is that they imply an effective reduction of the environmental impact of the concrete industry without affecting competitiveness. The knowledge of hygroscopic and transport properties of concrete with SCMs contributes mainly in three aspects. First, regarding performance-based design for durability of reinforced concrete structures built with this kind of concrete. The quantification of concrete transport properties allows predicting the interaction of the structure with the surrounding environment, and inferring variations in the moisture content and the ingress of aggressive substances. Second, related to dimensional stability of concrete and its conductive properties. Drying causes concrete shrinkage, and, therefore, the development of residual stresses caused by external or internal restrictions. Moreover, under load, creep is produced as a function of moisture content. Finally, a better understanding of chemical modifications in the pore liquid offers knowledge to determine the formation of hydration products in the presence of SCMs, and to evaluate the interaction between SCMs and clinker.

Continuous measurements of water uptake in cementitious materials
Continuous measurements of water uptake in cementitious materials

 

Processed Incineration Ash-Based Sustainable Binders

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ASH-CEM is a Belgian multi-partner project co-financed by SIM and it investigates the use of fine bottom ash in cement-based products and looks also into carbonation binder types.


Rapid urbanization has resulted in the generation of increasing amounts of household waste. The management of this waste forms an important challenge. In developed countries different types of waste are already re-used, but still a huge volume goes back to landfills. Besides recycling of paper, plastics and (non-)ferrous metals and composting of organic waste, large amounts of waste are incinerated in furnaces which results in three types of ashes: bottom ash, fly ash and air pollution control (APC) residue ash. Most of these residues are presently landfilled, which can have various drawbacks in terms of excessive land usage, undesired leaching of components, cost of management of an engineered landfill, etc.


Almost 70% by volume of the incineration residues is composed of bottom ash. The composition of bottom ash is similar to slag and fine ash of coal burning which are already applied worldwide in concrete as pozzolanic and latent hydraulic materials. Thus the use of incineration bottom ash as a replacement of Portland cement is of considerable interest.


In the project, initially, bottom ashes from an incineration plant combined with a state-of-the-art ash treatment facility are collected and characterized. Then the optimized bottom ash is used both as a raw material for ordinary Portland cement production and as a pozzolanic additive in combination with Portland cement. The presence of elemental Al in the ash is the main challenge in using it as a pozzolana.  Beneficiation processes at the laboratory scale are studied for efficiency and sustainability and in order to improve and enhance the quality for the specified application. The mechanical properties and durability of concrete made with these cements are assessed and the optimum blend is selected based on this study. Ground bottom ash is also added as a corrective material in raw meal for cement clinker production. Raw meal composition is optimized for maximum addition of bottom ash without deteriorating the quality of clinker produced.  Furthermore, the project looks into the cradle to cradle approach of using the demolition wastes from concrete made with ashes as raw materials for the next generation concrete production, both as recycled aggregate and as raw material for cement production.

Figure 1. Mortar bars for strength testing (Expansion due to elemental Al content is also visible)   Figure 2. Set up for elemental Al quantification

 

Iron-rich slag as substitution for Portland clinker

(Vincent Hallet, joint PhD KULeuven – UGent)

There is a large production of non-ferrous slags in Belgium, currently mostly used in low-value applications. Using these slags as substitution for Portland clinker in blended cements presents an opportunity for both valorization of the slags as well as environmental impact reductions of cement. However, these slags typically have a lower dissolution rate than ferrous slags such as blast furnace slag as a consequence of the high iron content, leading to lower early strengths.
Using various additions, the properties of the blended cements with iron-rich slags can be improved. Most notably, the addition of alkali activators leads to increased slag dissolution and thus to higher (early) strengths. The resulting binders can be seen as hybrid cements due to their hybrid character between Portland clinker binder and inorganic polymers.
Fundamental understanding of the effect of such additions is still scarce and both blended and hybrid cements with iron-rich precursors require further study. In this project, the focus lies on the effect of various additions (e.g. alkali activators) and material parameters, such as the slag fineness, on the microstructure and the link with the macroscopic properties. This is complemented with a study on the viable applications for these materials, with the goal of linking the various mix-designs to the most suitable application.

 

Modified copper residue as cement and aggregate replacement

(Pithchai Pandian Sivakumar, joint PhD, UGent and KU Leuven

A possible solution to minimize the CO2 footprint caused by cement industry and to enhance the transition towards circular economy is to use residues as supplementary cementitious materials (SCM) in “concrete” applications. Modified Copper (Cu) residues can be a viable source as an alternative SCM due to the presence of “zero” heavy metals, earning a term “clean residues”. By incorporation of modified Cu residues in concrete mixtures at high cement and/or aggregate replacement levels, we strives for (i) maximum residue valorization and avoidance of waste disposal, and (ii) more durable and sustainable reinforced concrete structures, with a low carbon footprint, high service life and good recyclability. However, usage of modified Cu residues in concrete is just in the onset stage and further understanding of reaction mechanism, residues contribution towards the mechanical properties and durability aspects are needed. In collaboration with industrial and institution partners these aspects will be investigated in order to turn modified Cu residues into a secondary raw material for building applications.   
 
For more information: http://www.sim-flanders.be/project/dusc-icon