Injectable and gas-permeable composite cements, polydimethylsiloxane/hydroxyapatite (PDMS/HAp), were synthesized using bi-component polysiloxane precursors (RTV-A and RTV-B), which react by hydrosilylation, and different mass fractions of hydroxyapatite (HAp) having particle size of about 100 nm. The effect of HAp mass fraction (0-60%) on the thermodynamic and kinetic parameters of polysiloxanes cross-linking, i.e., PDMS/HAp composite preparation, was investigated by means of differential scanning calorimetry (DSC). It was determined that small mass fractions of HAp (up to about 10%) cause a decrease in temperature (T-max), and activation energy (E-a) of the investigated cross-linking process, while the total heat of the process (H-tot) increases. For the HAp mass fraction greater than 20%, an opposite effect was observed. The temperature and the time interval of cross-linking increase rapidly for smaller, but slowly, almost insignificantly, for larger HAp mass fractions. The determined order (n) of the reaction shows a trend similar to H-tot, but due to small variations found, this does not merit discussion.