A study has been conducted in which a lightweight concrete was processed using ceramic microspheres, known as cenospheres, as a primary aggregate. The mechanical properties, including compressive strength, tensile strength, flexural strength and fracture toughness, were tested and cataloged. It was determined that the addition of high volumes of cenospheres significantly lowered the density of concrete but was also responsible for some strength loss. This strength loss was recovered by improving the interfacial strength between the cenospheres and the cement. The interfacial properties were quantified using interfacial fracture mechanics techniques. These techniques were also employed to find a suitable surface modifier with which to improve this interface. The admixture silica fume and the coupling agent Silane(TM) were found to be suitable candidates and both performed well in small-scale compression testing. Silica fume was eventually isolated as a prime candidate. The concrete produced with this admixture was tested and compared to a concrete with an equal volume fraction of cenospheres. The addition of silica fume improved the compressive strength of cenosphere concrete by 80%, tensile strength by 35%, flexural strength by 60% and fracture toughness by 41%.