In this work, minimally processed sisal microfibres along with commercial eucalyptus pulp were used as reinforcement of composites based on magnesium oxide and silica cement (60/40 ratio by weight of MgO/SiO2) with high filler content. The fibre cement composites were submitted to a fast carbonation process, resulting in the formation of hydrated magnesium carbonate, which contributed to improved physical and mechanical properties. All composites presented a strain-hardening behaviour and high specific energy under flexural loading, in special the composites with 10% of fibres. We demonstrated that the sisal microfibres produced by a chemical/mechanical process can be used as an efficient reinforcing material in combination with commercial cellulose pulp for MgO-SiO2 composites.