High-volume ground granulated blast furnace slag (GGBS) is often used in cementitious materials to produce environmentally friendly building materials . However, high-volume GGBS (HVS) can lead to numerous problems including low strength and loose structure of concrete at the early age. To overcome these drawbacks, nanosilica (NS) is used to compensate the early strength insufficiency of high-volume granulated blast furnace slag (HVS) cementitious material by improving the pore structure. Currently, it is still a challenge to perform accurate testing of the pore structure. Although mercury intrusion porosimetry is widely used to test the pore structure of cementitious materials, there is still some controversy. In this study, the 1 H NMR tests are used to measure the pore structure of NS and HVS blended materials and compared with standard MIP tests to comprehensive evaluate the effect of NS on the pore structure of HVS cementitious materials. Results indicate that NS can significantly improve the early strength of HVS cementitious materials and the greater the volume of GGBS, the greater the degree of enhancement exhibited by NS. The compressive strength increases of 23.5% and 30.2% were presented by 3-day S6N2 and S8N2 samples while flexural strength increases of 25.3% and 68.7% was resulted. NS significantly reduces the amount of harmful pores and increases the amount of harmless pores (smaller than 100 nm) in the HVS cementitious materials, it was also found that NS had no significant effect on the reduction of the total pore volume. Results showed that the strength of samples with high porosity was not necessarily low, the pore distribution and pore composition have a greater effect on the strength than the total porosity. The pore volume (>3 nm) obtained from the MIP test was larger than the pore volume measured by 1 H NMR for early age samples. The difference in the porosity and pore size distribution obtained from the MIP and 1 H NMR test was large at early age. The effect of NS on the pore structure of HVS system was investigated in more detail by 1 H NMR and MIP techniques to provide a further theoretical basis for the application of NS in HVS cementitious materials.
Jing Dai,Chenchen Gong,Yanfei Wang,Liuqing Huo,Lingchao Lu
Hydrating-hardening properties of high sulfate resistance Portland cement (HSRPC) mixed with 35 wt% mineral powder (MP) or fly ash (FA) and the action mechanism of nano-silica (NS) under the condition of low-calcium hydrating phase were studied in this paper. Rheology, fluidity, setting time and compressive strength were investigated to explore the effect of NS on the basic physical properties of HSRPC doped with MP or FA. Mercury intrusion porosimetry (MIP), ultrasonic crack analysis, hydration heat and thermogravimetry (TG) were investigated to analyze the hardened structure and the hydration mechanism. The conclusions were as follows: For HSRPC with MP, the 0.1–0.5 wt% NS increased the yield stress, decreased the fluidity and prolonged the setting time. However, when NS content increased from 0.1 wt% to 0.5 wt%, the cumulative porosity increased by 9.8% and the 60 h (60 h) cumulative hydration heat decreased by 14.6% because NS with high specific surface area inhibited the hydration of HSRPC, but its particle filling increased the porosity of harmless pores to improve the 3day (3d) compressive strength slightly. After 28d of hydration, the compressive strength increased by 2.8%, ultrasonic amplitude increased by 19.3%, and calcium hydroxide (CH) decreased by 6.6%, indicating that NS fully reacted with CH generated by HSRPC hydration. For HSRPC with FA, the 0.1–0.3 wt% NS increased the yield stress, setting time and 3d compressive strength and decreased the cumulative porosity. The 60 h cumulative hydration heat increased from 155.9 J/g to 198.5 J/g. The ultrasonic amplitude of 28d hardened paste increased from 10.91 V to 17.01 V and the CH content decreased from 15.29% to 14.1%. It indicated that the 0.1–0.3 wt% NS with high activity reacted with CH and further promote cement hydration to generate a large number of hydration products to improve the densification and performance, especially at the early age.
Concrete, in particular application scenarios, such as crack repair, disaster rescue and relief, and 3D printing, needs to be cured quickly. In this work, a novel synthesis strategy, UV-initiated frontal polymerization (FP), for fast preparation of high-strength polymer concrete at room temperature is proposed, and the whole fabrication progress within several minutes, in which coarse aggregates and a small amount of short carbon fibers (CF) are used to enhance mechanical properties and heat conduction and reduce the amounts of chemical adhesives. SEM, DSC, MIP, Ultra depth of field microscope, and Rheometer are employed to characterize the properties of chemical adhesives, polymer concrete, and polymerization process. In addition, the finite element method is used to investigate the internal temperature field status of the FP process. The results indicate that the compressive and flexural strengths of the polymer concrete with 1.0 wt%-1 mm-CF-50 vol.%-quartz sand exceeded 70 and 20 MPa, respectively, in which the average self-propagation velocity reached 58 mm min −1 . The investigation provides a promising approach for the rapid construction of structural facilities, emergency repair after disasters, 3D printing concrete, etc.
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