| Silicon dioxide nanoparticles are widely used as reinforcing fillers in cementitious materials and silicone rubber materials.Nano-silica in cementitious materials has effects such as volcanic ash effect as well as crystalline nucleation effect,which is more than 10 times higher than silica fume;therefore,even at trace amounts,nano-silica can significantly improve the workability,compressive strength and durability of cementitious materials.This is because nano-silica forms hydrated calcium silicate by reacting with calcium hydroxide during cement hydration to form a dense paste structure,which in turn improves the mechanical properties of cementitious materials.Due to the presence of abundant Si-OH on the surface of nano-silica,it is easy to form hydrogen bonds and thus adsorb each other,while the small size of nano-particles has high surface energy,so nano-silica tends to exist in the form of agglomerates.Therefore,how to achieve uniform dispersion of nano-silica in cementitious materials to improve the rheological behavior and mechanical properties,and how to effectively and visually characterize the dispersion state of nano-silica in cementitious materials still need further systematic research.In addition,nano-silica is widely used in silicone rubber to improve the mechanical strength and thermal stability of silicone rubber.Silicone rubber needs to be reinforced to be useful,and nanoparticle reinforcement is the most important way to achieve rubber reinforcement.However,the prerequisite for these improvements is good dispersion of the silica filler in the silicone rubber matrix,and the more evenly silica filler is dispersed,the better the reinforcing effect of the nano-silica is.In the production of silicone rubber,timely and accurate observation of the dispersion state of silica is important for controlling the process parameters.The surface Si-OH of silica can provide reaction sites for surface grafting fluorescence as well as improving dispersion,and fluorescent nano-silica is optically transparent in silicone rubber,which is beneficial for its application in the tracing of nanoparticles and observation of dispersion status.In this work,we synthesize fluorescent silica nanoparticles and use their fluorescence tracing ability and the imaging effect of laser scanning confocal fluorescence microscopy（LSCM）to clearly reflect the exact location,i.e.,the dispersion state,of silica nanoparticles in cement matrix and silicone rubber matrix.Numerical characterization of the dispersion state is further achieved by calculating the inter-particle distance through Python programming,which provides the conditions to reveal the influence of the dispersion state of silica in the matrix material on the mechanical properties.The specific study of this paper is as follows:1.Preparation of fluorescent nano-silica and its confocal imaging in cementNano-silica（NS）was used as the raw material,and NS was first surface modified by hydrolytic condensation of alkoxy groups with 3-aminopropyltriethoxysilane（APTES）,and then fluorescent silica（Si O2-FITC）was synthesized by the reaction of the amino group at the other end of APTES with the thiocyano group of fluorescein isothiocyanate（FITC）.The results of dynamic light scattering and fluorescence spectroscopy were used to optimize the experimental procedure and obtain the best experimental raw material ratio.The particle size analysis by dynamic light scattering showed that the average size of the modified NS was reduced from 555.45 nm to 283.17 nm,indicating the improvement of the NS dispersion state.The successful bonding of NS with APTES and FITC was demonstrated using infrared spectroscopy,X-ray photoelectron spectroscopy and fluorescence spectroscopy.the optimal excitation wavelength of Si O2-FITC was 495 nm and the optimal emission wavelength was 520nm.the Si O2-FITC still had a strong fluorescence intensity in an alkaline environment comparable to that of cement.The concentration-dependent test results show that the fluorescence intensity of Si O2-FITC is essentially positively correlated with the concentration,which is a prerequisite for imaging in silicate cements.The fluorescence region of Si O2-FITC in the cement matrix can be observed by LSCM.2.Dispersibility and numerical characterization of fluorescent nano-silica in cement matrixSilicate cement slurries and blocks were prepared using different doping of Si O2-FITC（FS）and NS,FS was characterized for dispersibility and numerical properties.The FS-doped cement slurry was observed by LSCM and the theoretical values of fluorescence area for each doping amount were obtained by visualization rate calculation equation.Nine-point sampling of the above-mentioned samples with different FS doping was performed,and the fluorescence area ratio of each area obtained by Image J.When the average fluorescence area ratio of the nine areas agreed with the theoretical value,it showed that the method of fluorescence imaging in cement matrix using FS was feasible,which proved that the fluorescent filler could be visualized and observed.The visualization rate results show that it is applicable at the conventional admixture（10%and below）of nano-silica in the silicate cement paste.Comparing the values of the fluorescence area ratio in the LSCM imaging photographs of nine regions,the dispersion state of FS in cement can be roughly estimated.The dispersion state of FS in cement slurries with low FS dosing（0.02 wt%-0.2 wt%）was investigated.A five-point sampling method was used and the results were programmed using the computer Python language for statistical particle spacing to determine a numerical characterization of FS dispersion,using the magnitude of the dispersion coefficient to indicate whether the particles were uniformly dispersed.In addition,the effects of NS and FS on the setting time,hydration process and compressive strength of silicate cements at different dosing levels were investigated.the addition of NS shortens the setting time and the addition of FS has a retarding effect;NS accelerates the hydration process of ordinary silicate cement（OPC）and FS does the opposite;both NS and FS increase the early compressive strength of silicate cements,and since FS is better dispersed,the The improvement of compressive strength is more obvious due to better dispersion of FS.Meanwhile,NS and FS have the same trend on the compressive strength of cement with the increase of dosing,and FS can provide guidance for the influence of NS dispersion in cement on compressive strength.3.Characterization and three-dimensional imaging of fluorescent nano-silica dispersion in silicone rubberThe fluorescent amide small molecules were first synthesized by chloride method using N-（β-aminoethyl-γ-aminopropyl）methyl dimethoxysilane（AEAPMDS）and citric acid（CA）as raw materials,and then grafted onto the NS surface by hydrolytic condensation of alkoxy groups to obtain fluorescently labeled silica（CA-A-Si O2）.CA-A-Si O2 exhibits super bright blue fluorescence in the solid state.CA-A-Si O2 exhibits super bright blue fluorescence in the solid state.This blue CA-A-Si O2 was incorporated into a transparent polymer matrix silicone rubber,and the fluorescence imaging obtained by LSCM successfully observed the two-dimensional planar dispersion of the fluorescent silica filler in the silicone rubber matrix and performed labeling and particle statistics.Finally,the 2D photos were reconstructed in 3D by avizo software to visualize the internal structure,providing a new way to evaluate the dispersion of silica filler in polymer matrix. |
