Carbon-sequestering building materials from Finnish side streams (HIRAS) project develops low-carbon construction products that utilize significant Finnish side streams, such as surplus land masses from construction and mine tailings, as well as regionally significant process industry side streams, such as low-carbon steel industry slag, biochar, and captured carbon dioxide. In addition, the project will produce valuable information on the long-term durability of products based on side streams in Finnish climatic conditions, the lack of research on which has slowed down the broader utilization of side streams. The project utilizes diverse expertise in geochemistry, mineralogy, concrete technology, and geotechnical engineering, and it is implemented in cooperation with Aalto University, Geological Survey of Finland (GTK), Savonia University of Applied Sciences and the University of Oulu. The project investigates the utilization potential of new process industry side streams, such as low-carbon steel slags and biochar, in construction products such as concrete, which has hardly been studied in Finland. In addition, analytics and guidelines related to the study of the properties of such side streams will be developed to support the broader utilization of side streams. Aalto University has piloted a technology that binds carbon dioxide to artificial aggregates based on side streams during manufacturing, which improves their mechanical properties and the products also act as carbon sinks. As a result of the development work carried out in connection with the project, a new kind of solution will be created for the utilization of surplus clay from construction and captured carbon dioxide. In accordance with the principles of circular economy, the project develops composite materials such as concrete from regional side streams, which can be used to manufacture modular construction products. These will be used to build prototype structures in cities located in three different regions: Helsinki, Kuopio and Oulu. By monitoring these prototype structures, research data is produced on the long-term durability and environmental impacts of side-stream-based construction products in order to minimize possible risks related to utilization. In the long term, the project will advance the utilization of materials that are currently difficult to utilize, such as acid sulphate soils and mine tailings, and offer new, circular economy-based solutions to reduce the carbon footprint of construction products. The guidance and service offering for the productization of circular economy material flows will also be diversified.