3D concrete printing (3DCP) offers new opportunities to create bioreceptive structures that support ecological colonisation. pH is a decisive factor because it controls the surface conditions where pioneer biofilms establish. There is no standard method to evaluate pH in relation to bioreceptivity. This study presents an initial assessment of pH development in 3D printed mortars and its link to early biofilm metabolic activity. A CEM I mortar, with and without an alkali-free accelerator, was cast into filament-like specimens. The specimens were exposed for six months to indoor, outdoor, and marine environments. Surface and bulk pH were measured to separate environmental interaction from internal carbonation. Biofilm productivity (photosynthesis and respiration) was evaluated through oxygen exchange measurements performed under controlled light regimes. Surface measurements reflected environmental effects more clearly than bulk values. Marine exposure caused the largest pH reduction and visible biofilm colonisation. Accelerated mixes consistently reached lower pH and higher biofilm productivity. These conditions indicate more effective bioreceptivity.