Study On Mechanical Properties And Carbonation Of Calcium Silicate Hydrate At Micro-and Nanoscale Scale

Cement-based materials are the most widely used man-made materials in the world,and their performance is related to the implementation of national major projects and the promotion of national strategies.The destruction of cement-based materials is a development process from micro to macro.It is generally believed that the basic performance of concrete is affected by the performance of micro-and nano-scale materials.Understanding the performance characteristics of materials on a microscopic scale has an important guiding role in improving their macroscopic properties.However,due to the complex and disordered arrangement of various cement hydration products,it is very difficult to prepare cement materials on the micro scale.Calcium silicate hydrate（C-S-H）,as the main product of hydrated Portland cement pastes,plays a decisive role in the mechanical properties and durability of cement-based materials.At present,the research on the C-S-H micro-scale performance is mainly carried out by means of simulation.In this paper,micro-and nano-scale C-S-H materials were prepared,on which the micro-mechanical properties and carbonization of C-S-H were studied.The preparation of materials is the premise and foundation of the research.In this paper,micro-scale C-S-H powders with multiple calcium to silicon ratios（Ca/Si）were successfully prepared by chemical co-precipitation.The composition of C-S-H is the same for different Ca/Si ratios,but the component content of the material is different.With the increase of Ca/Si ratio,the content of C-S-H（Ⅱ）increases gradually.C-S-H is a layered structure,and the layer thickness increases with the increase of Ca/Si ratio.The increase of Ca/Si ratio makes the chain length of[Si O₄]tetrahedron in C-S-H decrease,the amount of bridging oxygen decreases,the silicate chain gradually depolymerizes,and the density of microstructure gradually increases.In this paper,C-S-H nano films were prepared by radio frequency magnetron sputtering.The C-S-H thin films prepared by this method have high uniformity and flatness.The C-S-H nano films has a good molecular structure correspondence with C-S-H in cement hydration products.The C-S-H films and the powders synthesized by chemical method maintain a high degree of uniformity in element composition,and the composition of thin films maintain a high degree of uniformity in different depths;the density and crystal structure of C-S-H thin films are consistent with the existing research results.In this paper,the nano-material preparation method is introduced into the preparation of cement-based materials for the first time,which makes up for the lack of the preparation method of the material on the nano scale.It provides a good material basis for the performance research of C-S-H on the nanometer scale.C-S-H powder tablets were used for nano-indentation detection,the porosity has a great influence on test results.The lower the porosity of the tablet,the higher the mechanical properties of the sample,and the smaller the dispersion of the mechanical test results.The results show that the limitations of powder and cement paste as research objects of micromechanical properties.The surface flatness of C-S-H thin films prepared by the magnetron sputtering method is very high,which can effectively ensure the accuracy of the nano-indentation test results.It is shown that the indentation modulus and hardness of C-S-H decrease with the increase of Ca/Si ratio,but the decreasing rates are gradually reduced.The indentation modulus and hardness of C-S-H are 78.13 GPa and 4.96 GPa respectively when the Ca/Si ratio is 0.8,and gradually decreased to 52.03 GPa and 1.84 GPa when the Ca/Si ratio is 2.3,and eventually stabilized.The indentation modulus and hardness of C-S-H thin films with different Ca/Si ratios are consistent with the calculated simulation results.Unlike C-S-H,which is directly selected from cement hydration products and synthesized by chemical method,C-S-H nano film is not affected by other hydration products and voids,and can truly reflect the mechanical properties of C-S-H,and its advantages in micro-mechanical experimental research are reflected.The application of C-S-H thin film in the study of mechanical properties provides experimental technical support for improving the mechanical properties of materials from the micro-and nanoscale.It is helpful to realize the collaborative development of computational simulation and experimental research.Carbonization is an important factor that affects the durability of cement-based materials.Previous studies generally believed that carbonization is a chemical reaction between alkaline substances（calcium hydroxide）in cement hydration products and carbon dioxide.As the main hydration product of cement,C-S-H has not been paid enough attention to its carbonation.The study of C-S-H carbonization is of great significance to improve the durability research system of cement-based materials and further enhance their durability.In this paper,the carbonization study of C-S-H found that the carbonation reaction was very rapid.Three types of calcium carbonate crystals of vaterite,aragonite and calcite were formed in short-term contact with carbon dioxide,and accompanied by the formation of amorphous silica（Si O₂）.With the prolongation of the carbonization time,the unsteady vaterite and metastable aragonite disappeared in succession,and finally transformed into stable calcite crystals.The final carbonized product material system contains only irregular gel and calcite crystals.Carbonization leads to a reduction in the chain length of the[Si O₄]tetrahedron in the C-S-H molecule,the amount of bridging oxygen is reduced,and the silicate chain is gradually depolymerize.The carbonization reaction causes the C-S-H structure to gradually disintegrate,and the adsorbed water between the layers is released;during the dissociation of the silicate chain,part of the bound water is lost.The Ca/Si ratio has a great influence on the carbonization reaction rate of C-S-H.The time needed to complete carbonization decreases with the increase of Ca/Si ratio,but the rate of decrease gradually decreases with increasing Ca/Si ratio.An analysis of the thickness of the C-S-H thin films before and after carbonization found that carbonization causes the volume shrinkage of C-S-H,and the volume shrinkage rate increases as the Ca/Si ratio increases.Shrinkage cracking is one of the main reasons leading to the deterioration of the durability of cement-based materials.This conclusion further improves the theoretical system of the durability research of cement-based materials.The temperature and humidity in the environmental conditions have a great influence on the carbonation speed of C-S-H.For the C-S-H samples with the same Ca/Si ratio,the time needed to complete carbonization decreases significantly with the increase of temperature and humidity;when the concentration of carbon dioxide exceeds 40%,the continuous increase of concentration has little effect on the carbonation speed.C-S-H carbonization in natural environment is a long process,and the impact on the durability of concrete should be paid enough attention.Temperature,humidity and carbon dioxide concentration are the main control parameters in the process of carbonation curing of cement-based materials.The conclusion of this study has guiding significance for accelerating carbonation curing of cement-based materials.The effect of carbonization on the mechanical properties of C-S-H found that the calcite formed by carbonization filled the microstructure voids of C-S-H,which reduced the porosity of the entire material and improved the mechanical properties of C-S-H samples.Under the influence of carbonation products and the depolymerization of silicate chains,the indentation modulus and hardness of all C-S-H thin films after carbonization are relatively average.The indentation modulus is in the range of 64.53～70.01 GPa,and the hardness is in the range of 2.93±0.08 GPa.Carbonation can improve the mechanical properties of C-S-H（Ca/Si ratio is 1.7）in cement hydration products,which provides a theoretical basis for early high-concentration carbon dioxide curing of cement-based materials.