Drying shrinkage behavior of cement mortar under low vacuum conditions

Abstract

In this work, the time-dependent drying shrinkage behaviors of cement mortar under low vacuum and standard drying conditions were tested and compared, in which the specimens were prepared based on various mixtures. Also, a series of microscopic tests were conducted to investigate the hydration and microstructure of the prepared specimens, and thus, to explain the drying shrinkage behavior of different specimens. The results showed that compared with standard drying condition, the specimens under the low vacuum condition exhibited higher drying shrinkage and mass loss, but lower sensitivity of drying shrinkage to mass loss. Under both conditions, the incorporation of fly ash and silica fume or shrinkage-reducing agent could greatly decrease the drying shrinkage, with the better performance identified in the case of fly ash and shrinkage-reducing agent; while the addition of sulphoaluminate cement had no inhibiting effect on the drying shrinkage deformation. Although the specimen solely based on sulphoaluminate cement showed the lowest drying shrinkage under standard drying condition, it exhibited greater drying shrinkage than the Portland cement specimens under low vacuum conditions. The microscopic test results suggested that low vacuum treatment could decrease the hydration degree while the decomposition of ettringite in the sulphoaluminate cement could be observed under low vacuum conditions. Besides, the gel pore volume fractions decreased while the cumulative total pore volume increased after low vacuum treatment, which resulted in a loose and porous microstructure with high vulnerability to drying shrinkage.