A composition for forming a high temperature, alkali resistant refractory fiber. The compositional formulation, by weight, is 45-76% silica, 12-32% alumina, 5-30% zirconia with a silica to alumina ratio in the range from 1.8 to 4.0. These fibers are chemically resistant in both acidic and alkaline environments even when heat treated to temperatures exceeding 500o (260oC) in such environments. The fibers are suitable for reinforcing calcium silicate cementitious products, which products are also claimed. Another compositional formulation, by weight, is 46-52% silica, 32-38% alumina, 13-18% zirconia with a silica to zirconia ratio in the range from 2.6 to 3.8. These fibers have a maximum service temperature in the range of 2550oF (1400oC) to 2650oF (1455oC) and exhibit better shrinkage resistance than similarly rated commercially available refractory fiber.