This paper systematically studied the modification of cement-based materials by nano-SiO2 particles with an average diameter of about 20 nm. In order to obtain the effect of nano-SiO2 particles on the mechanical properties, hydration, and pore structure of cement-based materials, adding 1%, 3%, and 5% content of nano-SiO2 in cement paste, respectively. The results showed that the reaction of nano-SiO2 particles with Ca(OH)2 (crystal powder) started within 1 h, and formed C–S–H gel. The reaction speed was faster after aging for three days. The mechanical properties of cement-based materials were improved with the addition of 3% nano-SiO2, and the early strength enhancement of test pieces was obvious. Three-day compressive strength increased 33.2%, and 28-day compressive strength increased 18.5%. The exothermic peak of hydration heat of cement increased significantly after the addition of nano-SiO2. Appearance time of the exothermic peak was advanced and the total heat release increased. Thermogravimetric-differential scanning calorimetry (TG-DSC) analysis showed that nano-SiO2 promoted the formation of C–S–H gel. The results of mercury intrusion porosimetry (MIP) showed that the total porosity of cement paste with 3% nano-SiO2 was reduced by 5.51% and 5.4% at three days and 28 days, respectively, compared with the pure cement paste. At the same time, the pore structure of cement paste was optimized, and much-detrimental pores and detrimental pores decreased, while less harmful pores and innocuous pores increased.
In this study, the effects of graphene oxide (GO) on the microstructure of cement mortars were studied using scanning electron microscopy (SEM), thermogravimetric (TG), and X-ray diffraction (XRD) techniques. Cement mortar samples with different proportions of GO (0.02, 0.04, 0.06, and 0.08 wt % based on the weight of cement) were prepared. The test results showed that GO affected the crystallization of cement hydration products, C–S–H (calcium silicate hydrate is the main hydrate product) and CH (calcium hydroxide). The morphology of hydration products changed with the increase of GO content. Furthermore, the results of XRD analyses showed that the diffraction peak intensity and the crystal grain size of CH (001), (100), (101), and (102) for GO samples increased considerably compared with the control sample. Based on the results, it can be understood that GO can modify the crystal surface of CH, leading to the formation of larger crystals.
In this study, hydrocalumite-like Ca2Al-NO3− layered double hydroxides (Ca-Al LDHs) with different microstructures were synthesized. The crystalline properties, structure composition, morphology and particle size distribution of the Ca-Al LDH (CAL) samples were illustrated. To obtain the chloride uptake performances of CAL, the influences of contact time, initial concentration of Cl−, pH of reaction solution and coexistence anions on the chloride uptake were examined systematically. Compared to the CAL samples obtained at a higher aging temperature, CAL synthesized at 60 °C demonstrated the minimum average particle size (6.148 μm) and the best Cl− adsorption capacity (211.324 mg/g). Based on the test results, the main adsorption mechanism of chloride ion on CAL was recognized as an interlayer anion exchanging reaction other than the dissolution-precipitate mode. With the increase in the pH value of reaction solution from 7 to 13, it was found that the amount of chloride ion adsorbed by CAL increased slightly, and the solution could remain at relatively high pH value even after the adsorption. The presence of CO32− and SO42− reduced the adsorption capacity of CAL dramatically as compared with OH− due to the destruction of layered structure and the formation of precipitates (CaCO3 or CaSO4). The interference sequence of the investigated anions on the chloride uptake of CAL was SO42−, CO32− and OH−, and the order of interlayer anionic affinity was Cl− > OH− > NO3−. The results illustrated that the synthesized CAL could be used as a promising chloride ion adsorbent for the corrosion inhibition of reinforcement embedded cement-based materials.
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