Effect of steam curing conditions on mechanical properties and microstructure of electrolytic manganese residue cement mortar

The pozzolanic activity of electrolytic manganese residue (EMR) at different calcination temperatures (200℃ and 800℃), and the distribution of mechanical properties, hydration products, and pore characteristics of cement mortar with 0wt%, 10wt% and 20wt% EMR, subjected to 80℃ steam curing for 7 h and 7 d, as well as standard curing for 3 d and 28 d, were studied. The effects of EMR on the hydration products, pore size distribution and porosity of cement mortar were revealed by low field nuclear magnetic resonance technology, XRD quantitative characterization and SEM microstructure. The effects of different curing conditions on the hydration process of EMR cement mortar were also investigated. The results show that the pozzolanic activity of EMR calcined at 800℃ is significantly higher than that of EMR calcined at 200℃. When the substitution amount of EMR calcined at 800℃ is 10%, the initial setting time, final setting time and water requirement of normal consistency are 1.52, 1.06 and 1.05 times than that of the control group, respectively. The compressive strength of steam-cured 7 h and 7 d is higher than that of standard-cured 3 d and 28 d, respectively, while the flexural strength is only 17.3% higher than that of the control group at 7 d. The hydration products of pure cement mortar are mainly fibrous hydrate calcium silicate (C-S-H) and sheet-like calcium hydroxide (CH), while in EMR cement mortar, due to the high sulfate content, in addition to the above hydration products, the presence of gypsum phase and needle shaped ettringite (AFt) can be observed under steam curing or standard curing conditions. The CH, AFt and amorphous contents are 14.50%, 11.75% and 46.25% after 28 d of standard curing. These values are similar to the diffraction peaks after 7 d of steam curing. In addition, the porosity of steam-cured 7 h is 1.44% lower than that of standard curing 3 d, and the porosity of steam-cured 7 d is 0.06% lower than that of standard curing 28 d. In summary, steam curing can improve the pozzolanic activity of EMR calcined at 800℃, accelerate the hydration process, and refine the internal pore structure, thereby improving the mechanical properties of the mortar.