In this paper, preliminary results about both the chemical and physical behaviours of hemp fibres in calcium rich medium are presented. The influence of hemp fibres introduction in Portland cement pastes on the setting time has been investigated. It seems that pectin contained in the fibres can form complex molecules with calcium ions and could be responsible for the observed delay. The role of fibres surface treatment has been also studied. Chemical attack of the fibres surface by an alkaline and calcium rich solution degrades hemicelluloses contained in the fibres and seems to roughen the surface. This surface modification seems to play a major role in the strengthening of the cement/fibres interface. These results show that hemp fibres introduced in cement pastes exhibit a typical composite behaviour compared to cement sample, which lead to an improvement of the mechanical properties.