The effect of unreacted cement content on the processing, structure, and properties of macro-defect-free (MDF) composites fabricated from calcium aluminate cement (CAC), alpha-alumina (Al2O3), and polyvinyl alcohol-acetate (PVAA) has been investigated. Three systems were formed with initial CAC : Al2O3 ratios of 50:50, 35:65, and 25:75 by volume in their respective formulations. The amount of unreacted cement was reduced from 68.1 vol% which is present in standard (100% CAC) MDF cement, to 14.9 vol% for composites with an initial CAC : Al2O3 ratio of 25:75, while the hydration product content was reduced from 18.1 vol% to 11.4 vol% for these respective systems. A hard core/soft shell continuum percolation model was used to determine that alumina substitution did not significantly affect the percolative nature of the interphase and bulk polymer regions. However, experiments showed that the reduction in unreacted cement content through Al2O3 substitution affected both the processing and microstructural features of these composites. The moisture absorption kinetics and flexural strength of composites exposed to 100% relative humidity were also evaluated, and it was found that their moisture sensitivity improved with decreasing unreacted cement content. A hypothesis is presented to explain the role of unreacted cement on the moisture sensitivity of these materials.