Steinbeis experts at FiberCrete develop cement-free concrete for use in the base elements of maintenance holes
For more than a century, Portland cement has been used as the primary binding agent in concrete production – albeit at a high price, in both economic and ecological terms. As a result, there is increasing focus on developing alternative binder systems that will make significant contributions to reducing carbon emissions. Having also recognized this necessity, FiberCrete, a Steinbeis Innovation Center, is working with a variety of stakeholders as part of an alliance called recomine. Together, they have been developing alkali-activated slags based on roller slag with the aim of producing manhole bases with strong chemical resistance.
At more than 4 billion tons, the massive volume of Portland cement produced every year comes with a significant price tag. It takes around 100 kilowatt-hours of energy to produce one ton of Portland cement. In addition, around 600 kilograms of environmentally harmful carbon dioxide are released. The concrete industry causes roughly 9% of global carbon dioxide emissions. As a result, in recent years increasing use has been made of Portland composite cements. In addition to containing Portland cement clinker, these are made with secondary raw materials such as granulated blast furnace slag, natural or artificial pozzolanic materials, ground limestone, and bituminous coal fly ash. Due to the poor chemical resistance of concrete based on Portland cement, however, which has a low pH (under 7, indicating acidity), it is not used in all areas – such as wastewater systems or sewage treatment plants.
Alkali-activated slag: a climate-friendly alternative
The researchers are therefore turning their attention to alternative binder systems based on alkali-activated slags, the use of which opens up new fields of application and could also play an important part in reducing carbon emissions. Cement-free alternatives such as alkali-activated binders offer interesting potential to partially or fully substitute Portland cement in the production of concrete and mortar. Some of the alternatives deliver superior material properties, such as acid resistance. Compared to Portland cement, they also reduce the amount of greenhouse gases produced by around 70%. Alkaline activation often involves using two materials as binding agents: ground granulated blast-furnace slag, a byproduct of blast furnace processes, and bituminous coal fly ash resulting from the flue gas cleaning of coal-fired power plants.
The transition to alternative energy sources and the associated discontinuation of coal usage mean that at some point, bituminous coal fly ash will no longer be available, however, and it will also be rendered unusable by process optimizations. There will be a sharp decline in the availability of ground granulated blast-furnace slag in the future due to the reduced use of hydrogen in the steel industry and increasing levels of recycling. As a consequence, there will be limited access to sufficient quantities of composite materials and this will fuel significant price rises. Although other forms of slag contain reactive clinker minerals, unlike ground granulated blast-furnace forms of slag they have not yet been introduced as components in cements. As a result, the search is on for suitable alternatives.
Researching together and finding new alternatives
The FiberCrete Steinbeis Innovation Center is also looking into this area alongside other research institutions and partners in industry as part of an alliance called recomine. The focus of this partnership lies in the development of alkali-activated slags based on roller slag, which can be used to produce chemically resistant base elements for manholes. The alliance, funded through the WIR! program of the Federal Ministry of Education and Research, brings together companies, scientific bodies, educational institutions, public authorities, and non-governmental organizations from the extended region of the Ore Mountains. All share the same goal of finding new solutions for contaminated mining and metallurgy sites. On the one hand, these should alleviate the high reclamation costs associated with extracting dispersed materials, and on the other, they should ensure measures are broadly accepted by legislators and society in general.
The objective of this sub-project, which runs from January 1, 2023, to December 31, 2025, is to investigate slag obtained from the extraction of precious metals. Abbreviated to “Waelue” by the project team, this slag could be used as alkali-activated slag (AAS) to produce manhole bases without cement. The team is focusing on material composition – Waelue slag, alkali silicates, and alkali hydroxides – and the development and adaptation of material properties. The project should result in the development of a new product based on alkali-activated Waelue slag (AAWS) for use in the production of manhole bases.
To achieve this, the rheological properties of AAWS will need to be carefully adapted to the consistency of damp soil. AAWS has unique and innovative characteristics. For example, it has exceptional chemical resistance across a wide pH spectrum. It is also subject to low levels of shrinkage – in fact, compared to Portland cement-based systems, shrinkage is no longer an issue. Unlike conventional concrete manhole bases, components suffer almost zero shrinkage during setting, meaning that large manhole bases can be produced without any risk of critical shrinkage cracks forming within microstructures.
As part of the sub-project, currently the chemical, mineralogical, and physical properties of a type of Waelue slag are being investigated both before and after processing. Among other things, this has involved determining bulk density using a pycnometer according to Archimedes’ principle and conducting an eluate analysis. This can be used as a starting point for defining the requirements of the processed Waelue slag so that it can be used as a base material for alkaline activation in the production of manhole bases. To do this, experiments are underway using a scanning electron microscope to analyze inert and reactive phase components, which are also being assessed for their suitability to the intended application.
Research is currently being carried out on a cement-free, alkali-activated Waelue slag suited to material and application requirements. To this end, Waelue slag will undergo ultrafine grinding and sintering processes to reactivate strength-forming phase components. After processing, it must be ensured that the Waelue slag is sufficiently reactive to be alkalized for use in cement-free components. The material development process involves adapting the alkali-activated Waelue slag to the molding technique used for the manhole bases. This requires a moistened earth mixture, since the manhole bases are removed from molds immediately after shaping and as a result, they must be highly stable even before fully hardening.
To build on work being carried out on material development, a continual series of prototypes is being manufactured and tested in a precast plant run by the construction material specialist LIMEX-VENUSBERG, which is also a project partner. Initial testing has already been carried out as part of this project. This entailed first mixing the alkali-activated slag in a blending unit before transporting the mixture on a conveyor belt to an automated shaft ring system for production of the prototype. The prototypes were then removed from their molds – while still fresh, yet sufficiently stable before hardening – in order to create the precise shape in all areas. Initial results look extremely promising and point to plenty of potential to use the alkali-activated slag in the production of cement-free manhole bases.