As　a　metal-free　and　visible-light-responsive　photocatalyst,　polymeric　carbon　nitride　(PCN)　is　an　ideal　alternative　to　traditional　titanium　dioxide　employed　in　photocatalytic　cementitious　materials　(PCM)　for　environmental　remediation.　However,　the　interaction　between　the　composition　and　microstructure　of　cementitious　materials　and　PCN　is　far　from　clear.　Herein,　the　hydration　behavior,　phase　composition,　microstructure,　compressive　strength　and　nitrogen　oxides　removal　of　the　PCN-modified　paste　with　different　dosage　of　PCN　at　varying　curing　ages　were　investigated　and　the　key　factors　governing　the　photocatalytic　activity　of　the　PCM　were　studied.　The　results　show　that　the　porosity　of　the　PCN-modified　pastes　presented　an　increasing　trend　compared　to　the　reference,　but　the　micro-filler　effect　of　PCN　contributed　to　the　varying　reductions　in　the　volume　of　harmful　pores,　which　are　mainly　responsible　for　the　highest　compressive　strength　of　0.5%　PCN-added　pastes　at　3　days　and　28　days.　Although　the　kinds　of　hydration　products　were　not　affected　with　the　addition　of　PCN,　the　carbonates　could　be　regulated　to　the　well-crystallized　and　flower　flaky　calcites　in　the　presence　of　PCN　at　28　days.　The　well　crystallized　calcite　would　be　regarded　as　potential　acceptors　of　photo-induced　holes　to　promote　the　separation　of　electron-hole　pairs　on　the　PCN.　The　promoting　effect　of　electron　transfer　could　outperform　the　negative　effect　of　the　decreased　porosity　caused　by　ongoing　hydration　and　carbonization,　resulting　in　an　enhanced　photo　catalysis　of　PCM　with　the　increase　of　curing　ages.　The　findings　from　this　study　would　provide　a　novel　understanding　on　the　improvement　of　photocatalytic　efficiency　for　PCM　against　the　shielding　effects　of　hydrates　and　carbonates.