Preparation And Characterization Of C-S-H Doped With Polyurethane And Its Effect On Mechanical Properties Of Cement Paste

Calcium silicate hydrate（C-S-H）is the most important hydration product of Portland cement,and the modification of its micro-nano structure has practical significance,which can directly affect the structure and performance of cement-based materials.In this paper,pure calcium silicate hydrate and waterborne polyurethane（PUA）-doped calcium silicate hydrate materials were prepared by hydrothermal synthesis,and the effects of calcium-silicon ratio,synthesis temperature,reaction time,and polymer content on the micro-nano structure interaction of PUA and C-S-H system were studied,and the effect of calcium silicate hydrate with different synthesis parameters on the mechanical properties of cement paste was studied.The relevant research conclusions are as follows:The doping of PUA has a certain influence on the interlayer spacing and crystallinity of calcium silicate hydrate.After doping with PUA,the interlayer spacing and crystallinity of calcium silicate hydrate increased,and the doping of excess PUA would distort the structure of calcium silicate hydrate,resulting in the disappearance of the d_（002）peak.With the increase of calcium to silicon ratio,the interlayer spacing of calcium silicate hydrate decreases slightly.With the prolongation of the reaction time,the interlayer spacing of calcium silicate hydrate gradually expanded.The doping of PUA changed the microscopic morphology of calcium silicate hydrate.The morphology of calcium silicate hydrate doped with PUA（5 g/l）changed from flocculation to a dense network;with the increase of PUA content,the morphology gradually changed to a layered structure.And the increase in the calcium-silicon ratio,synthesis temperature,and reaction time can promote the transformation of C-S-H gel into a Tobermorite phase with higher crystallinity,resulting in a more compact structure of the product.In addition,doping with PUA also changed the degree of polymerization of calcium silicate hydrate.After doping with PUA（5 g/l）,the degree of polymerization of calcium silicate hydrate increased,and the degree of polymerization decreased gradually with the increase of PUA content.With the increase in the calcium-silicon ratio,the degree of polymerization of calcium silicate hydrate decreases.However,with the increase in synthesis temperature and reaction time,the degree of polymerization of calcium silicate hydrate increases.The results of TG-DTG analysis indicated that PUAs formed at different positions on calcium silicate hydrate,with a part of PUA inserted into the interlayer region of C-S-H,while the other part was adsorbed on the"surface of the C-S-H layer"or"between the stacked C-S-H layers".The increase in the calcium-silicon ratio,synthesis temperature,and reaction time are all beneficial to promoting the interaction between PUA and C-S-H,thereby increasing the specific surface area of the product.In addition,the interaction between PUA and C-S-H is limited,and the doping of excess PUA can block the pore structure of calcium silicate hydrate,which in turn leads to a decrease in the specific surface area and number of mesopores of the product.The addition of calcium silicate hydrate with different synthesis parameters improves the early strength of the cement paste without impairing the long-term strength of the paste.Compared with the compressive strength,the addition of calcium silicate hydrate is more conducive to enhancing the early flexural strength of the paste,and its enhancement effect on the strength of the paste mainly depends on the specific surface area of the calcium silicate hydrate,the larger the specific surface area,the better the enhancement effect.