The reduction in the use of primary raw materials through alternative raw materials or industrial by-products (e.g. blast furnace slag, fly ash) as well as recycling materials from the dismantling of buildings have played an important role in the cement and concrete industry for many years. Thinking and acting in terms of a circular economy already takes place on three levels:
- Cement with recycled building materials
- Concrete with recycled aggregates
- Reuse of concrete components
Cement with recycled building materials
Based on experience from research as well as practical experience in countries like Switzerland and the Netherlands, a new cement standard EN 197-6 “Cement with recycled building materials” has been drafted which is currently in enquiry. This standard will allow the use of recycled concrete fines (RCF) as a main constituent of cements. Acc. to prEN 197 6:2022, recycled concrete fines are specially selected and prepared mineral materials coming from plants producing recycled concrete aggregates and/or sands. These cements have the potential to be used in several applications.
Figure 1: Freeze-thaw resistance of concrete in the cube test acc. to CEN TS 12390-9
Figure 1 shows as an example that concrete using cements with 30 % recycled clay bricks, mixed recycled masonry rubble or recycled concrete fines can have a sufficient resistance to freezing and thawing for the exposure classes XF1 and XF3.
Concrete with recycled aggregates
The European concrete standard EN 206:2013 gives recommendations for the use of coarse recycled aggregates with d ≥ 4 mm. The standard gives limits for the replacement of natural normal-weight coarse aggregates by coarse recycled aggregates in relation to exposure classes. The recommendation is valid for coarse recycled aggregates conforming to EN 12620 and the categories stated therein. In indoor and normal outdoor concrete (XC1-XC4, XF1) a replacement of 30 % of natural normal-weight coarse aggregates by coarse recycled aggregates Type A is possible. Type A is a material with more than 95 % concrete, mortar and unbound aggregates. This is the basis for the national annexes to the non-harmonised standard.
Figure 2: Recycled concrete as a source of raw materials for new buildings
In research it has been shown that also higher amounts of the coarse natural aggregates can be replaced by recycled aggregates; partially up to 100 %. In these cases the water demand of the aggregates requires careful attention and whether the design principles according to the Eurocode are still valid has to be checked.
Reuse of concrete components
Direct reuse of components is even more resource-efficient than recycling. Especially in the area of serial buildings, there is great potential for reuse. When planning reversible component connections, entire components can be dismantled and reused. The additional costs of systematic dismantling can basically be amortised, so that corresponding projects are economically viable and, if necessary, future income can even be generated from the sale of the elements. Corresponding digital platforms are emerging. Elements or components made of concrete have bound CO2 during their use. In further use, new energy expenditures arise only through dismantling, transport (possible cleaning) and installation. The distance between the component “store” and the new building should be taken into account. For the comprehensive reuse of components, solutions for dismantling (standardisation of the technologies to be used) and interim storage must be further developed.
Figure 3: Building with prefabricated concrete elements as a component of the circular economy © Deutsche Bundesstiftung Umwelt (DBU)