Investigation of microstructure of C-S-H and micro-mechanics of cement pastes under NH4NO3 dissolution by 29Si MAS NMR and microhardness

Abstract The hydraulic concrete structures commonly suffer from severe dissolution. Dissolution has a great influence on the mechanical properties and durability of concrete, which would be detrimental to the normal operation and even cause structure failure. In this study, the effects of an accelerated dissolution method, namely NH4NO3 dissolution, on the microstructure and micro-mechanics of hardened cement pastes have been investigated via X ray fluorescence (XRF), Fourier transform infrared spectrometer (FTIR), X-ray diffraction (XRD), 29Si magic-angle spinning nuclear magnetic resonance (29Si MAS NMR) and microhardness techniques. Besides, an attempt was made to reveal the relationship between the C-S-H microstructure and micro-mechanics of cement paste during the NH4NO3 dissolution process. The results demonstrated that the decalcification process of the cement pastes under NH4NO3 erosion can be classified into two stages. The first stage was the dissolution of Ca(OH)2 in cement pastes. The Ca(OH)2 was almost completely removed, and C-S-H was partly decalcified in this stage. In the second stage, as the C/S of cement pastes decreased to a constant value about 1.83, C-S-H was further decalcified. It was also observed that the continuous dissolution would significantly weaken the microhardness of cement pastes. The long-term hydrated cement paste decalcified for 500 min exhibited nearly no microhardness. In addition, the results indicated that the microhardness of the cement pastes was related to the paste C/S, the proportion of Q1 and the polymerization of C-S-H. Finally, it was concluded that the Ca(OH)2 dissolution and degradation of C-S-H structure are the main causes for the degradation of cement pastes under NH4NO3 dissolution.