| Calcium silicate hydrate (C-S-H) is the main cement hydration product; it has complex structure and has an great effect on the properties of cement and concrete. The dehydration phases of C-S-H have abilities of rehydration. It is of important value and theoretical significance to study the structure evolution and rehydration characteristic so as to realize the resource utilization of cement paste in waste concrete.The series of C-S-H samples with different C/S were synthetic in this paper. The structural disintegration and reconstruction of C-S-H series, C3S paste and C3S paste add with mineral and admixtures during heating, and its hydration characteristic of dehydration were investigated by the modern techniques of x-ray diffraction (XRD), infrared spectrum (IR), Raman spectrum, nuclear magnetic resonance (NMR), scanning electron microscope (SEM).The change of the C-S-H phase and C3S paste in the thermal decomposition process were studied by XRD.The results indicated that the C-S-H sample with lower C/S has a high stability, when calcinate at400℃it have the basal spacing, that the structure of the C-S-H is not changed greatly. The basal spacing at C-S-H sample was not seen in the XRD patter, however, only a small amount of C-S-H decomposition at650℃. After calcinating at900℃, only calcium silicate formed. The C-S-H sample with higher C/S have the basal spacing at200℃, they were basic decomposition at400℃, the β-C2S formed as the temperature raised to650℃. After heating at900℃, the P-C2S translate into a-C2S. In the hydration paste of pure C3S and C3S blend with admixture, the β-C2S formed at about600℃.The evolution of Polymerization state of [SiO4]4" in C-S-H phase and C3S paste during the heating process were investigated by the techniques of FT-IR﹑Raman and29Si MAS NMR. These result indicated that the content of Q1and Q2in the C-S-H sample which with lower C/S changes less and little change of the polymerization state of [SiO4]4'.After heated at650℃,the Q0is detected, but its content is very low, it indicates that only a small amount of C-S-H decomposition. In the heated sample only Q3exist, this result is consistent with XRD which indicated only calcium silicate and quatz formed at900℃. When heat at650℃, the Q2content in the paste of C3S mixed with silica fume decreased. A lot of Q0, Q1and little Q2in the sample, which indicated that the lager number of C-S-H generated in the paste decomposed at650℃.Morphology of C-S-H dehydration phase was studied by the techniques of SEM. Experimental investigation showed that network morphology of C-S-H (Ca/Si=1.0) suffering the calcination of400℃changed a little, while it changed into frame shapes after calcination of650℃and appears with both well-ordered stub and disk shapes after calcination of900℃.Frame shapes of C-S-H (Ca/Si=1.5) after the calcination of400℃and500℃respectively decreased with the reduction of order degree. Spherical shapes appeared after calcination of650℃, and the network morphology changed into well-ordered spherical and stub shapes after calcination of900℃.The structure evolution of the rehydration phase of C-S-H and C3S paste after heating at different temperatures were investigated by the techniques of XRD﹑FT-IR﹑29Si MAS NMR and hydration heat. Results showed that C-S-H generated after rehydration in the hydration system of C-S-H(Ca/Si=1.5) and C3S suffering both the calcination of400℃and650℃. C-S-H mainly including Q1and Q2created again during the rehydration process.The dehydration phase of C-S-H with lower and higher C/S all have exothermic peak when heat at400℃and650℃.However after heating at900℃,only C-S-H with high Ca/Si ratio has the lower hydration exothermic peak and the rest of the hydration heat curve is almost a straight line during hydration. Rehydration process of C3S dehydration phase formed by calcine C3S paste at600℃,650℃and700℃, presents low exothermic peak. |
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