Study On The Effect And Mechanism Of Nano-SiO₂on The Properties Of Cement-based Materials Incorporating Waterproofing Agent

Durability of cement-based materials is a worldwide issue. Waste of resources, economic loss and social problems due to poor durability of cement-based materials has been attracting attention. There are many reasons for insufficient durability, and almost all are water-related. Water is not only the carrier of the aggressive media that migrate into cement-based materials, but also a necessary condition for the material deterioration. Almost all chemical and physical processes that affect the durability of concrete structures involved in water and its movement in the pores and cracks in the concrete. Therefore, how to effectively improve waterproofing properties and infiltration resistance of cement-based materials is of great significance to improve the durability of cement-based materials and structures.Results of previous studies showed that waterproofing agent can improve the waterproofing properties and infiltration resistance of concrete, but it can also adversely affect the early hydration process and mechanical properties of cement-based materials. Considering that nanomaterials can improve many aspects of performance and even create new properties of cement-based materials, waterproofing agent incorporating nanomaterials can help to improve overall performance of cement-based materials.This paper studies the effect and mechanism of nano-SiO2on the properties of cement-based materials incorporating waterproofing agent, which is specifically divided into four parts:Firstly, the effects of nano-SiO2on fluidity and mechanical properties of cement paste incorporating waterproofing agent KIM was studied. Secondly, the influence and impact mechanism of nano-SiO2on microstructure of cement paste incorporating KIM was investigated through a variety of means of microscopic tests and analysis. Then, combing hydration heat test with Krstulovic-Dabic hydration kinetics model, the influence and mechanism of nano-SiO2and KIM on cement hydration process was studied. Finally, take into account the fact that cement-based materials are rarely saturated in practical applications, experimental study on the influence of KIM and nano-SiO2on the capillary suction process of cement mortar was conducted, and the relationship between capillary absorption coefficient and pore structure of cement-based materials was preliminarily explored.The data presented in this study showed that:1. KIM waterproofing agents will significantly increase water demand, thereby affecting the cement paste fluidity. Meanwhile, hydrophobic film could be formed within cement-based materials after incorporating KIM, thus inhibiting the cement hydration process at early age, and influencing the refinement of pore structure of cement-based materials.2. Nano-SiO2can improve the mechanical properties and microstructure of cement paste, effectively compensating for the adverse effects of waterproofing agent. After the incorporation of nano-SiO2, the compressive strength of the cement paste increased rapidly at early age, while the increasing rate remained stable at between15%-20%in the later period, compared with the cement paste incorporating KIM only. In a proper range, increasing the dosage of nano-SiO2can improve the compressive strength of cement paste. Microstructure analysis showed that incorporation of nano-SiO2could contribute to microstructure uniformity and density of cement paste with less pores. Meanwhile, the second hydration reaction of nano-SiO2significantly reduced the number of Ca(OH)2crystals and refined the crystal size, thus pore structure was optimized.3. KIM can significantly prolong the nucleation and crystal growth (NG) process and improve hydration degree in the later process dominated by interactions at phase boundaries (I). Meanwhile, KIM can delay hydration process into the diffusion (D) process. While nano-SiO2can significantly shorten the time of NG process, making hydration enter I process earlier. In addition, nano-SiO2can significantly shorten the time of I process, and reduce hydration degree in I process, so that hydration reaction become diffusion-dominated earlier.4. Given that cement-based materials are rarely saturated in actual engineering practice and water absorption is mainly driven by capillary suction effect, the effects of KIM and nano-SiO2on capillary water absorption was studied. KIM showed inconspicuous influence on capillary water absorption coefficient, while nano-SiO2can significantly reduce capillary water absorption coefficient by optimize the distribution of pore structure and reduce the content of harmful pores.5. The relationship between capillary water absorption coefficients and pore structure characteristics was analyzed and showed to be complicated. The impact of mixed material on physical and chemical properties and microscopic morphology of cement-based materials needs to be taken into consideration. However, with the doping of KIM, the smaller the critical pore size, the smaller the capillary water absorption coefficient. And capillary water absorption coefficient is smaller in cement-based materials with lower content of harmful pore.