The Effect on Mechanical Properties in Biochar Replaced Cement & Aggregate in Concrete Before and After Fire Exposure

Sammanfattning: Concrete is good as a construction material regarding fire and its properties regarding strength. However, the concrete expands and cracks due to fire resulting in structural damage. According to the literature the concrete usually loses its strength during and after fire exposure due to themechanical and physical changes.Concrete consists of cement, water, aggregates of different sizes, and usually some sort of plasticiser depending on its final use. Globally the concrete production releases about 4.5 billionmetric tons of carbon dioxide, which is about 8% of all the emissions of carbon dioxide in the world (Naturskyddsföreningen, 2022).The main components in cement are limestone and marlstone which are melted and turned to clinker. The clinker is mixed with sand and gypsum to make cement. Due to the high releases of carbon dioxide from the production of cement and the destruction of the environment mining of the raw materials, Sweden had a concrete-crisis in the summer of 2021, this was due to the government denied the main cement company Cementa AB to continue to mine limestone on the island of Gotland. Due to this decision new sustainable components to replace the components to produce concrete is critical. An alternative material called biochar may be suitable as a component in concrete. Biochar is a renewable product from pyrolysis of biomass. The favorable properties of biochar such as low density, high specific area and low thermal conductivity has the potential to lower the carbon footprint of concrete. This thesis evaluates the properties and performance for different biochar ratios mixed within concrete before and after exposure to fire in a furnace that followed the standard ISO 834 curve up to 650 ℃. In particular, experiments were conducted to observe how the mechanical properties (e.g., tensile and compressive strengths) are affected by exposure to fire that caused a temperature rise of up to 650 ℃. By analyzing the results from the experiments, it is seen that the workability of the concrete decreases with higher ratios of biochar due to the biochar’s water absorption properties. The compressive and tensile strength tests before fire exposure, for both aggregate and cement replaced samples, resulted in the average strength decreasing with higher ratios of biochar. Interestingly, iiithe results after fire exposure represented higher compressive strengths for both cement and aggregate replaced samples for all ratios of biochar. However, the tensile strength after fire exposure generally decreased with higher ratios of biochar. Differential scanning calorimetry and infrared spectroscopy were performed to gain an insight into the reason for the increase in compressive strength after fire exposure. Most probably, when the silica, present in the cement, was exposed to 650 °C under fire, it softened and fused the other components, which led to stronger compressive strengths.