•Traces of CH in the C4A3$-C2S system were characterized using XRD, TG, FTIR, and pH.•The RQPA method was used to determine the compositions of the C4A3$-C2S hydration.•The hydration process and evolution of the C4A3$-C2S system were investigated. A C4A3$-C2S (ye'elimite-belite) clinker system with different C2S and C4A3$ contents is synthesized from analytical reagents and a C4A3$-C2S hydration system is established with a water-cement ratio of 0.4. The traces of calcium hydroxide or portlandite (CH) in the C4A3$-C2S hydration system were characterized using X-ray diffraction (XRD), thermo-gravimetric (TG) analysis, Fourier transform infrared (FTIR) spectroscopy and pH tests. The Rietveld quantitative phase analysis (RQPA) method was used to determine the compositions of the C4A3$-C2S hydration products. The hydration process and microstructure evolution of the system were also investigated. The hydration of the C4A3$-C2S involves two key process: one the consumption of AH3 phase and another the generation and stabilization of CH phase. The occurrence of the CH traces in C4A3$-C2S hydration system can be described in terms of the changes in the C2S content and the curing age. When the C2S content is less than 55 wt%, the newly generated AH3 preferentially reacts with the C2S to generate C2ASH8. When the content of C2S is between 55 wt% and 85 wt%, the C2S begins direct hydration but the generated CH is immediately consumed by the C2ASH8. When the C2S content is higher than 85 wt%, the CH generated by the hydration of C2S becomes stable in the C4A3$-C2S hydration system.