The　g-C3N4/CoAl-LDH　nanoflowers　with　a　strong　photocatalytic　activity　under　UV-visible　light　irradiation　was　synthesized　and　applied　with　different　dosages　to　prepare　photocatalytic　cement　mortars　(PCM).　Besides　the　photocatalytic　assessment　by　NOx　degradation,　the　effect　of　g-C3N4/　CoAl-LDH　on　the　microstructural　and　mechanical　properties　of　cement　mortars　were　investigated　by　means　of　X-ray　diffraction　(XRD),　X-ray　computed　tomography　(X-CT),　mercury　intrusion　porosimetry　(MIP),　capillary　water　absorption　and　strength　tests.　The　results　show　that　in　addition　to　the　unique　air　self-purification　characteristics,　g-C3N4/CoAl-LDH　can　promote　cement　hydration　with　filling　effect　and　further　improve　the　microstructural　and　mechanical　properties　of　mortar.　Moreover,　the　ability　of　g-C3N4/CoAl-LDH　to　capture　CO32　　can　alleviate　the　influence　of　carbonation　on　photocatalytic　performance.　Therefore,　g-C3N4/CoAl-LDH　exhibits　lasting　photocatalytic　performance　in　cement-based　materials.　This　work　is　expected　to　contribute　to　the　effort　of　searching　for　new　multifunctional　nano-additives　for　improved　sustainability　of　building　materials.