Abstract The performance of both E-glass and AR-glass fiber reinforced composites with four low alkali/low lime cementitious matrices made of glass, calcium aluminate phosphate, magnesia phosphate and ettringite cements, was studied. After testing, SEM observations were carried out to resolve the mechanisms involved in the aging process, in particular those of microstructural origin (i.e. the densening of the matrix at the interfacial zone), and those of chemical origin (i.e. those which caused defects to form on the fiber surface). Keywords: Glass, cement, fiber-reinforced composites, aging, durability, SEM.