Study On Properties Of Nano-modified Negative Temperature Cement-based Materials

The quality of concrete in winter construction directly affects the safety and service life of the construction structure.How to prevent and reduce the frost damage of concrete and to ensure that the properties of concrete in the negative-temperature environment meet the design requirements are issues of general concern in engineering.The application of nanomaterials in construction engineering materials has gained wide attention and shown a bright future in construction engineering materials.Therefore,this thesis uses nanomaterials to improve the microstructure of cement-based materials under two frost-damaged environments（early-age freezing and negative-temperature curing）,thereby achieving the macro property enhancement,and exploring the working mechanism of nanomaterials.Furthermore,it also provides experimental and theoretical support for the application of nano-engineering in cold regions.The main contents are as follows:（1）Effect of nano-silica on the early-age hydration of Portland cement is studied.Influence of the specific surface areas and contents of nano-silica,and hydration temperatures on the early-age hydration is investigated,and it is found that the working mechanism of nano-silica with the different specific surface areas on the early-age hydration of Portland cement is the same,however,the modification effect of nano-silica with higher specific surface area is more effective.As the nano-silica content increases,the acceleration period and exothermic rate peak value increase,and the induction period decreases.The increase of hydration temperature can shorten the induction period and acceleration period,and increase the exothermic rate peak value.（2）Effect of nano-silica in the early-age hydration process of Portland cement is quantified by the hydration kinetic model.The Boundary Nucleation and Growth model is used to calculate the nucleation rate and growth rate of the hydrates corresponding to the second exothermic peak in the early-age hydration process.The regularity between the nucleation rate and growth rate and nano-silica content,and temperature sensitivity of nucleation rate and growth rate are investigated.And nucleation effect of nano-silica in the early-age hydration process is verified.The Krstulovic-Dabic model is used to simulate the hydration kinetics process of nano-silica modified Portland cement.It is found that the hydration reaction usually involves nucleation and crystal growth（NG）,interactions at phase boundaries（I）and diffusion（D）,however,excessively high hydration temperature and excessive nano-silica will directly transform the hydration mechanism into NG→D.The reaction exponent and reaction rate constant are calculated,and the regularity between the hydration kinetic parameters and nano-silica content is studied.（3）Study on the mechanism of nanomaterials on the microstructure of early-age freezing cement-based materials is carried out.Early-age frost damage can cause the pore volume less than 50 nm and larger than 100 nm to increase,but it can be repaired by positive-temperature curing.While the increase of pore volume between 50-100 nm cannot be repaired by positive-temperature curing.Early-age frost damage can severely inhibit cement hydration and reduce the polymerization of C-S-H gel silicate chains.The effect of three different properties of nanomaterials（zero-dimensional nano-titanium dioxide and nano-silica and one-dimensional carbon nanotubes）on the microstructure of early-age freezing cement-based materials is investigated.It is found that the nucleation effect and filling effect of CNT can repair the micro-cracks and defects caused by frost damage,thereby reducing the frost damage to the microstructure.The filling effect of nano-Ti O₂ can make the microstructure of the cement matrix be repaired well.However,nano-Ti O₂ has a limited effect on cement hydration.The working mechanism of the pozzolanic nano-Si O₂ on the pore structure is similar to that of nano-Ti O₂,but the modification effect is better than that of nano-Ti O₂.Moreover,nano-Si O₂ can effectively increase the degree of hydration and C-S-H gel content.Considering the modification effect and cost,it is believed that nano-silica has more advantages in the modification of early-age freezing cement-based materials.（4）Influence of negative-temperature curing environment on the pore structure of cement matrix and the working mechanism of nano-silica on the pore structure of cement matrix cured in negative-temperature curing environment is investigated.It is found that the negative-temperature curing environment can delay the development of pore structure,increase the pore volume less than 10 nm and larger than 50 nm,but has little effect on the pore volume between 10-50 nm.By giving sufficient curing time,the pore volume larger than 50 nm in the negative-temperature curing environment can be repaired to the positive-temperature curing level.The working mechanism of nano-silica on the pore structure cured in the positive and negative-temperature curing environment is the same,however,the modification effect is more obvious in the negative-temperature curing environment.Nano-silica mainly reduces the pore volume larger than 100 nm and less than 10 nm and increases the pore volume between 50-100 nm,which achieves the refinement of the pore structure.（5）Study on strength and permeability of negative-temperature cement-based materials modified by nano-silica is carried out.Nano-silica can effectively compensate for the strength loss caused by early-age frost damage and restore the strength to normal levels.The effect of nano-silica on the strength of negative-temperature curing cement-based materials with different water-cement ratios is different.In negative-temperature curing environment,the modification effect of nano-silica on the strength is mainly concentrated in the first 14 days.Nano-silica can effectively increase the tortuosity and reduce the threshold diameter to enhance the impermeability of cement matrix.