Interaction between composition and microstructure of cement paste and polymeric carbon nitride

•Composition and microstructure of PCN-added paste were studied at different ages.•A flower flaky and well-crystallized calcite was observed with the addition of PCN.•NOx removal of PCN-added paste was improved with increase of curing ages.•A novel understanding on the enhanced photocatalysis of PCM was proposed. 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 photocatalysis 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.