Portland cement matrices have been reinforced by 5 wt% of "CemFIL 1" alkali-resistant glass fiber and isothermally wet cured for up to 1 yr at 20-degrees and 55-degrees-C. Degradation of mechanical properties occurs as a consequence of chemical attack on the fibers. Various stages of the degradative process are followed by electron microscopy and analysis. The original composite microstructure is described in terms of a solid fiber reinforcing system. However, the fibers gradually become hollow, leaving a concentric shell of cement-fiber reaction product which still gives a useful measure of reinforcement. These changes, together with fiber shortening arising from local impingement of Ca(OH)2 crystals, give rise to what is termed a hollow cylinder reinforcement system. The hollow shell structures consist of gel and semicrystalline material; glass is absent or nearly so. The role of zirconia in stabilizing the hollow cylinder structures is described. It is considered that the microstructure developed at approximately 20-degrees-C, while not as effective a reinforcement as solid fibers, still provides useful tensile reinforcement relative to plain paste matrices.