Effect of Ca(OH)₂ on shrinkage characteristics and microstructures of alkali-activated slag concrete

The study presents the shrinkage behaviour of Ca(OH)2-added alkali-activated slag (AAS) concrete. A series of tests were performed to determine the plastic shrinkage, autogenous shrinkage and drying shrinkage. In order to explain shrinkage mechanisms, the moisture loss was monitored; the pore structure was examined by nitrogen adsorption test; hydration process was assessed by isothermal calorimetry; hydration products and microstructure were characterised by XRD, SEM/EDS and TG/DTG. It was found that the use of Ca(OH)2 increased the plastic shrinkage of AAS concrete, but reduced autogenous and drying shrinkage. This is because the addition of Ca(OH)2 accelerated the hydration at very early stage, while non-noticeable impact could be identified after 120 h. The results from nitrogen adsorption test showed a decrease in mesopores by adding Ca(OH)2, which was responsible for the less drying and autogenous shrinkage. Meanwhile, the Ca/Si of C-S-H (I) increased from 0.89 to around 1.40 at 28 d when adding 5% and 10% Ca(OH)2. XRD patterns indicated C2ASH8 and C4AH13 were found in Ca(OH)2-added samples. These changes in hydration products also contribute less drying and autogenous shrinkage in Ca(OH)2-added samples.