In order to reduce the carbon footprint, biochar can be used as CO2-negative concrete constituent. The paper shows results of experiments with cement paste and mortar mixtures with 5%, 15% and 30% of cement volume replaced by biochar. The density of the char is measured with a pycnometer I dried and wet conditions, workability is assessed using a spread flow test, and hydration characteristics are measured using isothermal heat flow calorimetry. In addition strength results are presented. The results show that char does not interfere with the cement hydration at early ages, but the water entrapped in the particles can have a slight retarding effect. In general workability and strength were negatively affected by the presence of biochar. However, small amounts of biochar can even have a positive effect on strength without negatively affecting the other properties of the material. The fact that at dosages above 5% the performance specifications of paste and mortar deteriorated has a detrimental effect on the use of biochar as carbon sink in structural concrete.