The sintering flue gas desulphurization ash (DA) has emerged as the most significant solid waste, second only to granulated blast furnace slag and steel slag (SS), generated by steelmaking plants. However, its high CaSO3 content has rendered it challenging to recycle effectively. This study delves into the hydration characteristics of a low-carbon cementitious material composed of DA and SS, across different sulphur contents (3.14%, 6.28%, 9.42%, 12.56%) and water to binder (W/B) ratios (0.30, 0.35, 0.40, 0.45). The hydration products of the DASS composite cementitious material encompass C–S–H, C–A–S–H, Ca(OH)2, and K2Ca5(SO4)6·H2O. The setting time and fluidity exhibit a diminishing trend with increasing sulphur content. Notably, the hydration exothermic peak for pastes with 3.14% sulphur content emerges at 3 h, signifying a 2-h advancement compared to pure steel slag paste. The compressive strength of cured DASS paste gradually increases with sulphur content ranging from 3.14% to 9.42% over the initial 28 d, but experiences a decline with further sulphur content increases. A reduction in the W/B ratio significantly reduces setting time and achieves a two-fold increase in 28 d compressive strength. In DASS paste featuring a W/B ratio of 0.35 and a sulphur content of 9.42%, a substantial quantity of newly generated Ca(OH)2 is consumed through the pozzolanic reaction with steel slag after 60 d, leading to the production of more C–S–H to fill the interstitial voids between raw material particles. In conclusion, the recommended formulation for the DASS cementitious material should incorporate approximately 6%–9% sulphur supplied by DA, while greater mechanical properties can be attained by minimizing the W/B ratio. This innovative low-carbon cementitious material mitigates the impact of the massive amount of cement demand and promise an alternative to industrial waste management.