Comparison of mechanical strengths and microstructure of Portland cement-calcium sulfoaluminate cement binders with cellulose ethers of different viscosities

Abstract

Portland cement-calcium sulfoaluminate cement (PC-CSA) binders possess special characteristics that are obviously different from that of a single component. This will affect the modification effect of cellulose ethers with different viscosities on cement. This study focuses on the mechanical properties, hydration process, type and content of hydration products, pore structure and interface structure of the binders with cellulose ethers, i.e. hydroxyethyl methyl cellulose (HEMC). The following results have been obtained that adding high content of CSA can improve the early mechanical strengths of the binders, while high content of PC is beneficial to the maintenance of the strengths at the middle and late stages. Compared with low viscosity HEMC, high viscosity HEMC reduces the mechanical strengths. Based on the hydration heat evolution results, high content of CSA significantly promotes the early hydration heat release of the binders. When the PC content is 85 %, adding high viscosity HEMC delays the hydration of the binders, but the hydration is promoted when the PC content is 50 %. Combined with the XRD and TG-DTG tests, the content of typical AFt and Al(OH)3 hydration product is relatively high in the binders with high content CSA. AFm has generated in the binders at mid-late age and adding HEMC improves its formation. Adding HEMC delays the formation of CH, while the HEMC viscosity has a limited effect. According to MIP results, the pore content in the binders tends to increase with the CSA content. The incorporation of HEMC significantly increases the pore content of all sizes, especially macro pores. And the greater the HEMC viscosity, the more obvious the above effect. The nanoindentation test results show that incorporating HEMC decreases the elastic modulus of the binders and weakens the interface structure between cement pastes and sands.