The strength and porosity relationship of very high strength cement based materials has been a famous research topic for many years. The author looks back on this relationship through the new class of macro-defect-free cement which is based on calcium aluminate cement and phenol resin. The flexural strength of this material is in the range of 120-200 MPa and has considerable stability to moisture and heat. The comparable composites made using alumina and aluminum hydroxide with phenol resin have given significantly lower strength, in spite of their low porous microstructure. Stress-deflection curves and scanning electron micrographs of the fracture surfaces of the composites revealed that the different fillers showed different behavior under load. The results demonstrated that the strong interparticle bonds created by the chemical interactions of the polymer and cement are significant in affecting the strength of calcium aluminate-phenol resin composite, whereas the alumina-phenol resin composite failed by debonding at the resin/filler weak interface.