Phase pure hydroxyapatite (HA), 0.8 wt.% silicon-substituted hydroxyapatite (0.8SiHA) and 1.5 wt.% silicon substituted hydroxyapatite (1.5SiHA) were isostatically pressed into discs and sintered between 900-1300degreesC for 2 hours in order to assess the effect of silicon/carbonate content on the density, microstructure, hardness and Young's modulus of HA with sintering temperature. C-H-N chemical analysis of the as-prepared SiHA samples showed increased carbonate absorption with silicon content compared to HA samples, but carbonate levels were not detected when the samples were heat-treated. At low sintering temperatures (900-1000degreesC) the densification of SiHA samples was inhibited, with this effect being more significant for the high level of silicon substitution (1.5SiHA) ceramics which was reflected in the hardness and Young's modulus values of the ceramics. However, for high sintering temperatures (1200 and 1300degreesC), the sintered densities, hardness and Young's modulus of HA and SiHA were comparable. Furthermore, examination of the microstructures by scanning electron microscopy showed that silicon substitution inhibited grain growth at high sintering temperatures.