Mechanical properties and microstructure of nano-reinforced concrete containing etched fly ash cenosphere
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摘要: 粉煤灰漂珠是煤炭燃烧废弃物，用于替代水泥制备混凝土可有效降低环境负荷。本文提出针对粉煤灰漂珠的表面蚀刻工艺，并协同纳米SiO2以增强混凝土性能。通过扫描电子显微镜和X射线衍射仪对比了漂珠蚀刻前后的微观形貌和物相组成，利用热重试验表征蚀刻漂珠对浆体水化特征的影响，确定蚀刻方法的有效性。基于抗压、劈裂抗拉试验和扫描电子显微镜-能谱仪研究了蚀刻漂珠和纳米SiO2对混凝土力学和微观性能的影响规律与改性机制。结果表明，本文采用的蚀刻工艺加速了粉煤灰漂珠中Si和Al元素的释放，蚀刻的细微孔为水分迁移提供了有效路径，使漂珠具有内养护效应的同时提高了其反应活性，因此显著改善了浆体水化，提高了混凝土力学性能，但掺量增至40%时，粉煤灰漂珠中空结构的负面影响更为显著，混凝土强度下降。此外，蚀刻漂珠与纳米SiO2的协同增强效果显著，提高了界面Si/Ca和Al/Ca比率，改善了水化产物组成，优化了界面过渡区，有利于混凝土的致密稳定和强度发展。Abstract: Utilizing fly ash cenosphere, which is a coal-based waste, as a substitute for cement to prepare concrete can effectively mitigate its negative environmental effect. A surface etching process for fly ash cenosphere was proposed in this paper, and nano-silica was also adopted to coordinatively reinforce concrete performance. The microstructure morphology and phase composition of the cenosphere before and after surface etching were compared using scanning electron microscopy and X-ray diffraction and the influence of the surface etched cenosphere on the hydration properties was characterized by thermogravimetric testing to determine the effectiveness of the proposed etching procedure. The effects of surface etched cenosphere and nano silica were studied using compression and split-tensile tests and scanning electron microscopy-energy dispersive spectroscopy. Results show that the etching procedure proposed in this paper accelerates the release of Si and Al elements inside fly ash cenosphere. The etched micro-pores provide effective paths for water migration, enabling fly ash cenosphere to have internal curing effect and increased pozzolanic activity. Thus, cement hydration and mechanical properties of concrete are significantly improved. However, when the dosage of fly ash cenosphere increases to 40%, the negative impact of the hollow structure of fly ash cenosphere is more significant, resulting in decreased concrete strengths. Furthermore, the synergistic enhancing effect of surface etched cenosphere and nano silica is significant with increased Si/Ca and Al/Ca ratios at the interface, which improves the composition of hydration products and optimizes the interfacial transition zone and thus benefits the microstructure and strength development of concrete.
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图 8 不同SEFAC和NS掺量混凝土的抗压强度
Figure 8. Compressive strength of concrete containing different SEFAC and NS contents
图 9 不同SEFAC和NS掺量混凝土的劈裂抗拉强度
Figure 9. Split-tensile strength of concrete containing different SEFAC and NS contents
图 11 不同SEFAC和NS掺量混凝土的微观形貌
Figure 11. Microstructure of concrete containing different SEFAC and NS contents
图 13 20wt%FAC (a)、20wt%SEFAC (b)和 1wt%NS/20wt%SEFAC (c)的元素分布结果
Figure 13. Elemental distribution results of 20wt%FAC (a), 20wt%SEFAC (b) and 1wt%NS/20wt%SEFAC (c)
表 1 纳米SiO2的物理性能
Table 1. Physical properties of nano-silica
Purity/% Particle size/nm Specific surface area/(m2·g) Density/(g·cm−3) Bulk density/(g·cm−3) >99.99 20 >240 2.3 0.06 
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表 2 蚀刻粉煤灰漂珠(SEFAC)的吸水率和释水率
Table 2. Water absorption and water desorption of surface etched fly ash cenosphere (SEFAC)
Time/h Water absorption/% Water desorption/% RH=85.1% RH=75.5% RH=43.2% 1 63.2 12.4 20.8 31.8 6 66.5 40.5 50.6 63.9 12 67.4 55.3 73.8 82.5 24 68.7 66.4 83.5 92.4 48 70.2 76.2 92.1 98.9 Note: RH—Relative humidity
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表 3 混凝土配合比
Table 3. Mix proportions of concrete mixtures
Mixture ID Water/kg Cement/kg Gravel/kg Sand/kg FAC/kg SEFAC/kg NS/kg 20wt%FAC 147 310 1119 746 76 - - 20wt%SEFAC 147 310 1119 746 - 76 - 40wt%SEFAC 147 234 1119 746 - 152 - 1wt%NS/20wt%SEFAC 147 306.14 1119 746 - 76 3.86 2wt%NS/20wt%SEFAC 147 302.28 1119 746 - 76 7.72 1wt%NS/40wt%SEFAC 147 230.14 1119 746 - 152 3.86 2wt%NS/40wt%SEFAC 147 226.28 1119 746 - 152 7.72 Notes: NS—nano-silica; Example: 1wt%NS/20wt%SEFAC is that the concrete mixture was added with 20wt% SEFAC and 1wt% nano-silica.
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表 4 Ca(OH)2和结合水含量
Table 4. Ca(OH)2 and non-evaporable water contents
Mixture ID Ca(OH)2 content/% Non-evaporable water content/% 3 d 28 d 3 d 28 d 20wt%FAC 7.1 15.2 3.7 5.8 20wt%SEFAC 10.8 19.8 7.3 8.6
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