Calcium silicate hydrate (C-S-H), as the main hydration product of Portland cement, is limited by the ability of traditional characterization methods when establishing theoretical models. However, the information, which is used to establish the theoretical model like chemical composition and structural parameters, is based on the results of cement particle groups. Since the composition and structure of C-S-H are susceptible to the change of hydration condition, the average results need to be further optimized to accurately explain C-S-H. Optical tweezers technology have been used in single particle research of many fields, because it can capture small particles nondestructively. Introducing optical tweezers into the research of cement-based materials can manipulate the individual cement particle and observe its hydration, which is beneficial to achieving the precise control of cement hydration conditions and reducing the interaction effect between different particles. By using optical tweezers, morphological characteristics, composition and structure of C-S-H at the micro-nano scale can be directly explored under specific hydration conditions. This is of significance for establishing a theoretical model of C-S-H which is closer to the actual system. Optical tweezers have gone through three main development stages according to the difference of capture accuracy as follow: (i) Far-field optical tweezers are more suitable for capturing micron-sized particles. The capture objects mainly include microbial cells and gradually expand the field of organic and inorganic materials. (ii) Near-field optical tweezers are suitable for capturing micron and sub-micron particles. They break through the classical optical diffraction limit through the evanescent field of near field optics. (iii) Nano-aperture optical tweezers are based on the self-induced back-action (SIBA) effect, which increases the capture accuracy of optical tweezers to nanoscale. Nano-aperture optical tweezers can reduce the laser power, decrease the thermal damage and enrich the capture objects. At present, the capture medium of the three optical tweezers includes liquid, gas and vacuum, while the capture objects extend to organic particles, inorganic particles and metal particles, making it applied to the particles in various fields in theory. The parameters of optical tweezers to capture particles include many factors such as laser wavelength, beam power, capture medium and particle properties. For the particle, capture is mainly related to its refractive index. The larger the refractive index is, the more helpful it is to produce "pull force"(gradient force). This paper counts the properties of some particles that have been captured, and the refractive index ranges from 0.30 to 2.86. For cement particles, the refractive index is about 1.7. The capture medium of air will not affect cement hydration, and the laser power for capture is at milliwatt level which is not enough to damage the cement particles. Thus it is realizable to achieve the capture of cement particles stably. This paper reviews the research about optical tweezers. Their application objects, capture medium and capture accuracy are focused, and the application of optical tweezers in cement-based materials is explored. © 2022, Materials Review Magazine. All right reserved.
