Henning Kruppa

Portland cement, the most widely used binder, generates large amounts of unavoidable process-related CO2 emissions that are hard to capture and store. Achieving carbon neutrality in construction therefore requires alternative binders. Alkali-activated binders (AAB) are a key option, but their manufacturability remains challenging. This study synthesised AAB using either basalt with ground granulated blast furnace slag or calcined clay with natural pozzolan. Extrudability was central, since suitable rheology is essential for continuous processes such as fine concrete extrusion and thus for advanced applications. A cradle-to-gate life cycle assessment (LCA) compared the environmental impacts of the AAB to a cement-based fine concrete, using a functional unit based on compressive strength and extrudability. The influence of slag addition and activator concentration on strength development was assessed, as were the effects of activator type and methylcellulose on setting behaviour. Setting was monitored by continuous ultrasonic pulse velocity measurements combined with ball penetration tests to evaluate suitability for extrusion. A low-risk, targeted optimisation method for AAB extrusion was developed. Results show that AAB substantially reduce environmental impacts, especially CO2-equivalents, mainly by avoiding clinker production. Electricity use, activator production, and transport remain hotspots, indicating further savings through renewable energy and supply-chain optimisation in this study.

Due to the demand for decarbonization of the construction sector, research on alkali-activated binders and material-minimized carbon-reinforced structures has gained momentum in recent years. Most of the research into alkali-activated binders is focused on developing market-ready alternatives, mainly using a trial-and-error approach. In this study, an attempt is made to identify and quantify the factors influencing compressive strength development. Due to their worldwide availability, investigations are being carried out into binders based on calcined clays and natural pozzolans. The goal is to develop a method to produce tailor-made AAB for continuous manufacturing methods to combine carbon reinforcement and alkali-activated materials. For this purpose, an experimental matrix with 20 variation parameters was set up, in which the activator solution and the precursor composition varied. The design of the experiments was used to minimize the number of experiments. It was shown that no single factor is responsible for the development of compressive strength but instead involves several interacting factors. It was possible to find empirical formulas for predicting the compressive strength after 2, 7, and 28 days.