Research has been carried out with the aim of solving the problem of veyetable fibre mineralization in Portland cement mortars. It consists of two parts. The first part deals with pore sealing, mechanical strength and alkaline reduction; the second with fibre impregnation including a short description of the compounds used and the ageing of mortars, with or without admixtures, which are reinforced with sisal fibres impregnated or unimpregnated under different conditions. Products used in this solution are mainly compounds derived from timber, insoluble in water and unextractable by water or water vapour. The results of the research look promising.