Study On The Effect Of Modified Nanomaterials On The Properties Of High-performance Cement-based Materials

Cement-based materials have an unshakable position in the field of civil engineering.High-performance cementitious composites(HPCC)are more and more widely used in civil engineering due to their excellent workability,mechanical properties,and durability.In order to make HPCC have excellent performance,reducing porosity,optimizing pore structure,and improving matrix compactness are more effective methods.When the nanoparticles are incorporated into HPCC,the microstructure of the cement paste can be improved microscopically.The pore structure can be optimized to improve the mechanical properties and durability of HPCC.However,nanoparticles have a large specific surface area and a large number of hydroxyl groups on the surface,making them extremely easy to agglomerate in the cement matrix and reducing the effect of nanomaterials in cement-based materials.Therefore,effective dispersion methods(such as surface modification to generate steric hindrance between nanoparticles)should be used to achieve efficient dispersion of nanomaterials in cement-based materials and maximize the improvement of nanoparticles.This paper uses the silane coupling agent KH550 to modify the surface of nano-silica and nano-alumina.Thermogravimetric tests were carried out on the nanoparticles under different modification conditions,and the optimal modification process parameters were optimized according to the grafting rate.The optimum modification condition of nano-silica is that the content of the silane coupling agent is 7%,and the modification temperature is 60°C.The optimum modification condition of nano-alumina is that the content of the silane coupling agent is 4.5%,and the modification temperature is 80°C.Scanning electron microscopy and solution dispersibility tests were performed on the nanoparticles before and after modification.Compared with the unmodified nanoparticles,the agglomeration effect of the modified nanoparticles was weakened,and they could remain dispersed in the cement slurry filtrate for a longer time without agglomeration.The nanoparticles before and after modification were used as one of the raw materials of HPCC to replace part of the cement,and three series of HPCC materials were prepared,respectively: Single-doped modified nano-silica HPCC materials(called MNS series),singledoped modified nano-alumina HPCC materials(called MNA series),and double-doped modified nano-silica and nano-alumina HPCC materials(called MN series).The working performance,mechanical performance,and durability performance of these three series of HPCC were tested and analyzed,and various microscopic testing methods were used to systematically explore the influence of the modification and dosage of nanoparticles on the performance of HPCC.The main rules are as follows:Three series of HPCC were tested for fluidity,and it was found that compared with the reference group,the fluidity of HPCC added with 0.4% modified nano-silica was increased by0.6 cm.However,with the further increase of the content of modified nano-silica,the working performance of HPCC gradually decreased.When the content of modified nano-silica was 1.2%,the fluidity of HPCC decreased by 3.9 cm.For MNA series HPCC,the working performance of HPCC decreases with the increase of the content of modified nano-alumina.When the content of modified nano-alumina is 1.2%,the fluidity of HPCC decreases by 7.5 cm,which makes it difficult to form HPCC.For the MN series HPCC,the working performance decreased with the increase of the content of modified nanoparticles,and the fluidity decreased by 5.9 cm when the content of modified nanoparticles was 1.2%.The research results of static mechanical properties of three series of HPCC materials show that when the curing age is 3d,compared with the reference group,the compressive strength of HPCC mixed with 0.8% modified nano-silica and 0.8% nano-silica increased by 25% and 10%respectively;The results of the self-shrinkage performance test showed that compared with the reference group,the HPCC self-shrinkage values of 0.8% modified nano-alumina and 0.8%nano-alumina were reduced by 10.9% and 6.1% respectively;The chloride ion permeability resistance test results show that the chloride ion permeability coefficients of HPCC mixed with0.8% modified nano-alumina and 0.8% nano-alumina are 82% and 85.8% of the reference group respectively.From the test results of macroscopic properties,it can be seen that the content of modified nano-silica is more effective than that of modified nano-alumina on the macroscopic performance of HPCC.The results of the hydration heat test showed that the incorporation of nanoparticles accelerated the hydration rate of cement in HPCC and increased the cumulative hydration heat release of HPCC in the first 3 days.The cumulative heat release increases with the increase in the content of modified nanoparticles.The cumulative heat release of 1.2% modified nanosilica,modified nano-alumina,and multi-doped modified nanoparticle HPCC were 119.3%,111.7%,and 118.6% of the reference group,respectively.Moreover,the cumulative heat release of HPCC doped with modified nanoparticles is greater than that of HPCC doped with unmodified nanoparticles.XRD and TG test results showed that HPCC doped with modified nanoparticles had more amorphous phase content and lower calcium hydroxide content than HPCC doped with unmodified nanoparticles.SEM experimental results show that the surface morphology of HPCC doped with nanoparticles is more compact than that of the reference group,and the interface transition zone between cement matrix and sand is improved.The mercury intrusion test results show that,compared with unmodified nanoparticles,modified nanoparticles can more effectively refine the pore size of HPCC,convert some harmful pores into less harmful pores,and convert less harmful pores into harmless pores,further improving the compactness of the matrix.These several microscopic tests show that the addition of modified nanoparticles can provide more nucleation sites for the hydration of cement particles,accelerate the hydration rate of HPCC,and consume calcium hydroxide to convert into C-S-H gel.Compared with the unmodified nanoparticles,the dispersibility of the modified nanoparticles in the cement paste is higher,and the nucleation effect and the pozzolanic effect are better exerted,resulting in a more gel and denser cement matrix.