Synthesis Of Calcium Silicate Hydrate And Its Composition, Structure And Morphology

The hydrated calcium silicate(C-S-H), the primary hydration product of Portland cement, accounts for most of physical, chemical, mechanical properties of cement-based composite materials. Research on the composition and structure of C-S-H have an important role in understanding, regulating and controlling the mechanical properties and the chemical stability of cement-based composite materials. This paper carried out a systematic study, which focused on the synthesis of C-S-H, its composition, structure and morphology, with major work and achieved important results as follows:Several series C-S-H samples were prepared by different methods-hydrothermal reaction of calcium oxide and nano-silica, solution reaction of alkali silicate and calcium salt, solution reaction of calcium oxide and nano-silica, hydration reaction of tricalcium silicate (C3S) and dicalcium silicate (C2S) at different temperature. Effect of synthesis methods, calcium silicon ratio (C/S), temperature, curing age, drying methods on the structure and morphology of C-S-H was investigated by the techniques of X-ray diffraction (XRD), Infrared spectrum (IR), Nuclear magnetic resonance (NMR), Scanning electron microscope (SEM), Transmission Electron Microscope (TEM).XRD Results indicated drying methods effected the structure of C-S-H. The drying method which C-S-H samples were dried and stored in a vacuum desiccator with silica gel hardly effect the structure of C-S-H, while the drying methods of vacuum drying at 60℃and D-drying method increased the degree of order of C-S-H to a certain extent; The basal spacing at C-S-H samples with low C/S (< 0.8) or high C/S (>1.7) was not seen in their XRD patterns. For the C-S-H samples synthesized by the above methods, its lattice parameter a increased, and b decreased, while the variation law of c depends on synthesis methods as C/S increases from 1.0 to 1.7.Polymerization degree, average chain length, the content of Q1,Q2 of the silica tetrahedron structure of C-S-H were studied by means of the techniques of IR,29Si DP MAS NMR. Results show that Si sites in silica tetrahedron structure of C-S-H with C/S of 0.8-2.0 are Q1 and Q2. The content of Q1 increased, and Q2 decreased, polymerization degree and average chain length decreased as C/S increased, while the variation law was just opposite to that as synthesis temperature increased. Morphology of C-S-H was studied through the techniques of SEM and TEM. Morphology of C-S-H depends on the composition, synthesis method and temperature, and curing age. There exists several types of C-S-H in SEM micrograph, such as fibrous, network shape, anomalous group bunch of pellet, curl thin laminated shape, stubby needle clavate as well as staggered tall and slender needle clavate; As can be seen in TEM images, the C-S-H with initial C/S of 1.5, which was synthesized by the solution reaction of calcium oxide and nano-silica at room temperature, exhibited fibrous pattern with a length of dozens or hundreds of nanometers. It looks like the fibrous C-S-H were formed from nearspherical C-S-H with a diameter of several nm or tens of nm. C-S-H with C/S of 1.0 exhibited fibrous pattern, however, some layered structure and crystal nucleai were also observed in TEM image, which did not appear in that of C-S-H with C/S of 1.5.