Influence of sample preparation and the addition of BFS on water vapour sorption isotherms

Blast-furnace slag (BFS), a by-product of the steel production, can be added to a concrete mix as cement replacing material. However, the presence of BFS has a significant influence on the microstructure of the matrix. To study the pore structure of pastes in the micro- and mesopore range, dynamic water vapour sorption experiments were performed. This method allows to determine the water vapour adsorption/desorption isotherms by continuously monitoring the weight change of a sample placed in an environment with controlled temperature (20°C) and relative humidity. To obtain complete adsorption/desorption cycles, the relative humidity was increased/decreased in steps by mixing dry and humid nitrogen gas. The tests were performed on pastes with water-to-binder ratios of 0.5 and BFS-to-binder ratios of 0 (CP0), 0.5 (CP50) and 0.85 (CP85). First of all, the effect of the predrying method on the sorption isotherms was investigated. The test results indicate that the water removed by vacuum drying can probably re-enter at high relative humidity. At low relative humidities, specific interactions possibly occur by formation of hydrogen bonds. This topic will be investigated in more detail in the near future. Furthermore, the mass water content at 100% RH after a step-by-step desorption/adsorption cycle never reaches the mass water content of saturated test specimens. A step-by-step adsorption to 100% RH cannot fill all of the large meso and macro pores. Moreover, a decrease of the RH to 0% induces no irreversible changes to the pore structure. Secondly, the sorption isotherms of CP0 were compared with those of CP50 and CP85. Although differences are noticed between BFS and OPC pastes, additional tests are necessary to draw conclusions.

ISBN:9789727521326

Publication year:2011

Accessibility:Closed