Roles of CLDHs and TEA on Portland cement mortar/paste: Strength, hydration and chloride binding

In this study, the synergistic effects of calcined Mg-Al layered double hydroxides (CLDHs) and triethanolamine (TEA) on the hydration properties and chloride binding capacity of cement paste were investigated. The roles of incorporation of CLDHs and TEA in the cement pastes were evaluated by bound chloride rate, physical properties, reaction kinetics and microstructure. Experimental results show that the chloride ions are efficiently trapped by CLDHs at early stage, and the addition of TEA provides further stability. The mechanical strengths are decreased and obtained the low values at the 3 wt% and 6 wt% substitution level of CLDHs, and the negative phenomenon is improved with the incorporation of TEA. From the isothermal calorimetry (ICC), X-ray diffraction (XRD) and thermogravimetric analysis (TGA) results, the induction period of C3S process is retarded, but hydration of C3A is accelerated. The enhancement of chloride binding ability is mainly attributed to the recrystallization generation of LDH-Cl and the formation of Friedel's salt (FS) with the decomposition of calcium hydroxide (CH), which is facilitated by TEA due to the rapid reaction of the aluminum phase. The analyses of scanning electron microscope and energy dispersive spectrometer (SEM-EDS) indicates that the enriched elemental chlorine originated from the immobilization effect of LDHs phase and FS, which is the significant support for the binding properties of chlorides in cementitious materials.