A comparative study of the microstructure of both new and in-use aged blast-furnace slag cement coir reinforced composite was performed. Aged samples came from internal and external walls of a 12-year-old house, built in Sao Paulo. The panels of the house were produced using 1:1.5:0.504 (binder: sand: water, by mass) mortar reinforced with 2% of coir fibre by Volume. The binder was blast-furnace slag activated by 2% of lime and 10% of gypsum. Fibres were removed from the composite and Subsequently cleaned with acid solution (chloridric acid, 10%) in an ultrasonic bath. Both aged and new fibres were studied under low pressure BSE SEM with EDS analysis. The lignin content of the fibre was measured by the acetyl bromide method and qualitatively evaluated by Weisner reaction using an optical microscope. Cement transformations were Studied by X-ray diffraction and TG. Its pore water composition was also determined. Carbonation was measured by phenolphthalein. Composites were Studied under low vacuum SEM. Interfaces and deposition of inorganic species in the fibre lumen were also investigated. Methods are described. After 12 years, the cement was fully carbonated. Fibres removed from the old samples seem to be undamaged when examined under SEM. Qualitative lignin content determination by Wiesner reaction suggests that old samples have lower content of guaiacyl lignin units. Nevertheless. the total lignin content of old fibres when measured by using the acetyl bromide method, is comparable to that reported in literature. No significant difference was found in the lignin content of fibres removed from external and those removed from internal wails.