| Terahertz(THz)refers to a section of electromagnetic waves with a frequency between0.1THz?10THz(0.03mm?3mm),which lies between microwave and infrared light in the electromagnetic spectrum.Compared with electromagnetic waves in other frequency bands,terahertz wave itself has excellent characteristics such as broadband,high penetration,coherence,and safety.Also because of this,terahertz spectroscopy technology is used in security inspection,national defense and military,chemical composition analysis and other fields.It has been widely used,especially in non-destructive testing in recent years.In the field of cement materials,internal pore structure and defects will directly affect the bearing capacity and durability of the structure.Therefore,it is very necessary to accurately measure the pore structure of building materials.However,most of the current detection of pore structure parameters such as porosity is destructive,and some even have a certain degree of harm.Therefore,it is also necessary to explore an efficient,non-destructive,and safe method for detecting the hole structure.Terahertz spectroscopy technology is fully equipped with such "characteristics."We know that using terahertz spectroscopy technology can only obtain the optical parameters of the material most directly,and there is no direct relationship with the pore structure parameters,but there must be a certain correlation between these two parameters.Therefore,this article will mainly explore the relationship between the optical parameters of the hardened cement paste and the pore structure parameters.At the same time,the changes in the refractive index and absorption coefficient of the sample in the frequency domain will be used to analyze the hydration of the cement.By comparing the terahertz spectrum,Infrared spectroscopy and Raman spectroscopy to fill in the "missing" information of the sample in the terahertz frequency band.The source of the research project in this article is an open project of the Key Laboratory of Civil Engineering Materials of Jiangsu Province.The project number is CM2018-08.This article mainly conducts research from the following aspects:1.The thickness and surface roughness of the hardened cement paste are two important factors that affect the terahertz signal.In order to obtain a stable signal,the experiment needs to unify these two parameters of the sample.Through preliminary experiments,it is found that the greater the thickness of the same sample,the weaker the intensity of the peak signal in the time domain,and when the thickness increases by 1 mm to 2 mm,the intensity of the peak signal will attenuate by about 9 to 10 times;Compared with the water-cement ratio,the thickness has a greater influence on the time-domain signal of the sample,and it is found that the best thickness in the experiment is.In addition,an industrial camera and an electron microscope were used to scan and observe the surface of the sample,and it was found that the surface of the sample treated with 1200 mesh sandpaper was very smooth,which could fully meet the requirements of the experiment.Therefore,in the whole experiment,the samples were treated with 1200 mesh sandpaper,and the thickness was controlled at about 3mm.2.The relationship between the optical parameters of the hardened cement paste and the pore structure parameters was explored.First calculate the equivalent refractive index based on the time-domain signal and reference signal of the sample,and obtain the pore structure parameters(porosity,most probable pore size,pore surface area,and total pore volume)of the sample through mercury intrusion experiments.The equivalent refractive index of the sample is numerically fitted with the pore structure parameters,and it is found that there is a strong negative linear correlation between the two parameters,that is,the refractive index keeps decreasing with the increase of the pore structure parameters,and in these few parameters Among the pore structure parameters,the negative linear correlation between refractive index and pore surface area is the strongest;at the same time,it is also found that the negative linear correlation between refractive index and porosity,most probable pore size,pore surface area and total pore volume is the largest at 28 d.And in particular,the relationship between hardened cement paste and porosity is studied,and the corresponding linear mathematical model is established according to the linear fitting line:It shows that this model can be used to predict the porosity of hardened cement paste in the future,and it may even be popularized and applied in other fields of building materials.Through nitrogen adsorption experiments,the BET multi-point method and Langmuir multi-point method were used to calculate the pore specific surface area of the sample.It was found that the specific surface area calculated by the two models increased with the increase of the curing age.The main reason is caused by the generation of hydrated calcium silicate(CSH).At the same time,the specific surface area value is numerically fitted with the equivalent refractive index of the sample,and it is found that the equivalent refractive index increases with the increase of the specific surface area.Comparing the equivalent refractive index of the sample with the linear correlation coefficients of the mercury intrusion experiment and nitrogen adsorption experiment,it is obvious that the correlation degree of the former is much greater than that of the latter,which shows that the linear mathematical model obtained by the mercury intrusion experiment predicts in turn The pore structure parameters of the sample(especially the porosity)are completely reliable and feasible.Finally,the Maxwell-Garnett and Bruggeman effective medium models and Wiener boundaries are used to verify the validity of the optical parameters.It is found that the equivalent dielectric constants are all within the upper and lower bounds of Wiener,indicating that the actual experimental results are completely valid.3.Research on optical parameters of hardened cement paste in terahertz frequency domain.On the basis of obtaining the time-domain signal,the Fourier transform can obtain the sample amplitude and phase parameters,and finally the refractive index and absorption coefficient of the sample can be calculated.It was found that only in the range of 0.2THz?0.77 THz,with the increase of frequency,the refractive index remained basically unchanged,while the absorption coefficient continued to increase;and the larger the water-cement ratio,the smaller the refractive index of the sample.At the same time,according to the absorption coefficient The size of the sample is used to analyze the degree of hydration of the sample.In order to fill in the "missing" spectral information of the sample in the terahertz frequency band,after obtaining the terahertz wave absorption spectrum of the sample,the infrared spectrum and the Raman spectrum of the sample were measured.Compare these three spectra,you can simply Qualitative analysis is performed on the changes of chemical substances in the sample. |
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