3/2023

Evaluating all possibilities to replace traditional materials for cement and concrete

Besides climate protection and biodiversity, the conservation of natural resources is one of the major ecological challenges of our time. Against this background, the path to a sustainable future must succeed in decoupling the use of resources from economic and population growth. This goal can only be achieved by reducing the use of natural resources, e.g. by expanding the circular economy and increasing resource efficiency.

Intelligent, material-saving, recyclable product designs and the reuse of products, but also waste and industrial by-products as valuable alternative resources are increasingly coming to the fore.

In particular, the use of alternative fuels, so-called secondary fuels, has been "state of the art" in the European cement industry for many years already. The replacement of fossil fuels such as coal, oil, and gas achieves in some plants 100 %. Even though the level of replacement is very high also here, some developments are visible through the use of more and more sustainable biogenic fuels or alternative fuels containing certain shares of biomass.

Focus on cement production

The level of replacement is somewhat different when it comes to the use of alternative raw materials. Secondary raw materials can replace both the raw materials for the clinker burning process as well as well as components for grinding cements. Although alternative raw materials such as granulated blast furnace slag and coal fly ash have been used for many years as substitutes for clinker in cement grinding, new approaches are being taken here as well. The reason is the decrease in available quantities of slag and fly ash due to the transformation of steel production and the move away from coal-fired power generation. On the one hand, these can be compensated by the use of recycled materials, such as crushed concrete sands, for example (Fig. 1). On the other hand, primary raw materials such as (calcined) clays and unburnt limestone and also volcanic ashes are being used to a greater extent than before to produce clinker-efficient cements.

In particular the use of fines from construction and demolition wastes has been investigated recently to improve the circular economy. It has been proven that this material in combination with limestone can replace a high amount of clinker in cement without a severe negative impact on its properties in the mortar or concrete. Most of the time recycled concrete fines - unless carbonated - do not show pozzolanic behaviour and can be considered as an inert filler. If the fines come from masonry waste, on the other hand, with some ground bricks or tiles included, a slight pozzolanic effect of the constituent can be observed.

Further research is currently being done with regard to the ideal particle size distribution to use the full potential of the recycled materials even better. At this point not only the traditional RRSB function is taken into consideration, but also multimodal particle size distributions which can be created by the separate ultra-fine grinding approach. This combines traditional cement grinding machinery with ultra-fine grinding devices like the dry stirred media mill. With such separate approaches for grinding and mixing cement, properties can be designed according to the need and the potential of the components.

Figure 1: Crushed concrete sand as an alternative raw material

Focus on clinker production

Recently, special attention has been paid to the possibilities of saving natural resources in the first process step of clinker production and reducing the CO2 emissions associated with calcination. Among other aspects, residual materials from certain high-temperature processes, such as slags from waste incineration or future slags from steel production which have not yet been investigated for their suitability for use in the cement industry, can play an increasing role here. In the meantime, larger research communities have been formed to investigate precisely this use more closely. It has been shown that a specific pre-treatment of waste incineration slags (Fig. 2), for example, leads to a depletion of valuable metals and heavy metals.

The objective is to produce a calcium-rich fraction which is suitable to be used  in the clinker burning process. In order to achieve these reductions, it is necessary to treat the slags with specific process steps. In the first steps, screens and magnets are used. The main advantage of this is the recuperation of different valuable metals e.g., copper. After this basic handling, selective comminution helps to further improve the properties of the materials. It has been shown that gentle comminution can result in products that have a significantly higher calcium content than the original materials on the one hand, and on the other hand much lower metal content at least for some components. This product would be therefore be ideal for the replacement of natural raw materials in clinker production. This can be partially explained by the different grindabilities of the components and therefore an accumulation of difficult to grind materials in very coarse fractions of the comminution process, and an enrichment of easy to grind materials in the finer fraction.

For such comminution, only energy-efficient grinding devices such as high-pressure grinding rolls or vertical mills are useful in combination with highly efficient working separators. In vertical mills, for example, 3 different “products” can be ground by using material passing the nozzle ring, rejects from the dynamic separator and the known product (separator fines).

In a future step, it is planned to use such produced alternative raw material in a plant trial in order to better assess the influence on the operating behaviour of the kiln and the properties of the clinker. If such use of waste incineration slags in the cement industry were possible, this would lead to a reduction in specific CO2 emissions and, on the other hand, it would no longer be necessary to deposit the slags.

Figure 2: Waste incineration slags

Focus on the future

In order to enable a climate-neutral future, it is necessary to explore all alternatives in the production of cements and concrete. This includes the substitution of raw materials which are needed for the production of clinker as well as the substitution of the main components of modern and high-performance cements and their use within concrete. Further investigation is in progress which alternative raw materials can be taken directly from the circular economy and which require further processing steps to enable their use within the production chain. Modern analytical methods play an important role as well as the latest processing and sorting techniques. Only in this way will it be possible to return existing residual materials to the value chain.