•Novel composite-cement technology with anti-microbial co-functionality.•Cetylpyridinium chloride modified montmorillonite (CPC-Mont).•Continuous anti-biofilm activity by gradual CPC release.•No reduction in mechanical strength and bonding performance. The aim of this study was to evaluate the mechanical properties, bonding performance and anti-microbial activity of a novel composite cement containing cetylpyridinium chloride (CPC) modified montmorillonite (‘CPC-Mont’), and using these parameters to determine the optimal particle size and concentration of CPC-Mont the composite cement can be loaded with. CPC-Mont particles with a median diameter of 30 and 7 µm were prepared and added to a composite cement at a concentration of 2, 3, 4, 5 and 7.5 wt%. Mechanical properties and bonding performance of the experimental composite cements were evaluated by 3-point bending and micro-tensile bond-strength testing. The amount of CPC released from the cement disks was quantified using a UV–vis recording spectrophotometer. The anti-biofilm activity was studied using scanning electron microscopy (SEM). Adding 30-μm CPC-Mont decreased the mechanical properties and bonding performance of the composite cement, while no reduction was observed for the 7-μm CPC-Mont loaded cement formulation. Although CPC release substantially decreased during the 7-day period assessed, 5- and 7.5-wt% CPC-Mont loaded composite cement inhibited biofilm formation for 30 days. Loading composite cement with CPC-Mont with a median diameter of 7 µm at concentrations of 5–7.5 wt% was effective in achieving continuous anti-biofilm activity, while maintaining mechanical strength and bonding performance.