<p>Portland cement is mixed with calcium sulfoaluminate cement (CSA) to obtain composite cement. Adding cellulose ethers can endow this cement with more excellent properties. In this study, different proportions of cellulose ethers modified&#xa0;composite cement are used to explore the effect of water to cement ratio on the mechanical strengths and microstructure. The results reflect that the mechanical strengths show an obvious decrease with the water to cement ratio increasing. When the CSA content increases, the mechanical strengths of composite cement mortars increase significantly at early age. The microstructure results show that after increasing the water to cement ratio, the hydration heat also obviously increases. The 24&#xa0;h cumulative hydration heat increases with the CSA content. Increasing water to cement ratio can obviously improve the formation of AFt, C–S–H and AH₃ in the binders with 70 and 85% CSA. The above hydration products also tend to increase with the CSA content. When the water to cement ratio increases, the cumulative pore volume and total porosity obviously increase. Meanwhile, when the water to cement ratio is 0.6, adding CSA results in the increase in the total porosity.</p> Graphical abstract <p></p>