Effect of Sodium Stearate on the Microstructure and Hydration Process of Alkali-Activated Material

In response to the high water sorptivity of alkali-activated slag–red mud–steel slag cementitious material (AASRSS), water and aggressive ions are easily transferred to the interior of cementitious materials, which poses a significant threat to their durability. In order to limit the transfer of moisture and aggressive ions to the interior of AASRSS, sodium stearate (NaSt) was utilized in this paper to reduce its water sorptivity. The aim of this study was to elucidate the role of NaSt in the AASRSS system. The changes in the compressive strength and water sorptivity of AASRSS mortar were tested by mixing different amounts of NaSt into the mortar. The changes in the hydration process and microstructure of AASRSS after mixing with NaSt were analyzed by using the test methods of the isothermal calorimetry, resistivity, SEM and MIP. The results showed that NaSt plays an important role in the pore structure characteristics of AASRSS and that the use of NaSt significantly reduces its water sorptivity. The improvement in the water sorptivity was caused by two main mechanisms, namely the optimization of the pore structure (reduction in defects of the microstructure and alteration of pore size distribution) and the introduction of a hydrophobic film on the pore surface. The addition of NaSt did not change the type of AASRSS hydration product, but it inhibited the hydration reaction, leading to a reduction in the hydration product. The combination of the increased porosity and reduced hydration products mainly accounts for the decrease in the compressive strength of AASRSS due to NaSt.