Treatments with silica fume slurry were applied to improve the durability of alkali resistant (AR) glass fiber reinforced cement composites (GFRC). Two treatments were studied: fiber treatment obtained by immersion of the reinforcing strand in the slurry, prior to their incorporation in the composite, and matrix modification of the matrix by replacing with 10% silica fume. Fiber treatment alone was found to be extremely effective, leading to 50% or more toughness retention after 5 to 9 months of accelerated aging. Additional modification of the matrix led to better performance, with the toughness retention exceeding 50%. Matrix modification alone did not yield any significant improvement. The effects of the silica fume treatments were discussed in terms of the microstructural and chemical aging mechanisms.