Experimental study on mechanical properties of silica fume modified steel fibre reinforced geopolymer recycled aggregate concrete
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摘要: 钢纤维增强地聚物再生混凝土(Steel fiber reinforced geopolymer recycled concrete, SFRGRAC)具有碳排放量低、节约天然矿物资源以及延性韧性好等优点,具有广泛应用前景。为改善SFRGRAC力学性能,本文以硅灰为增强材料,通过立方体抗压、劈裂抗拉、抗折和弹性模量试验,研究硅灰取代率、钢纤维体积掺量和再生骨料取代率等因素对SFRGRAC力学性能的影响规律,并基于SEM扫描电镜和低场核磁共振测试结果揭示硅灰的改性机制。结果表明:掺入硅灰可延长SFRGRAC的凝结时间,当硅灰取代率为15%时,初凝和终凝时间分别提高了29.68%和22.98%;由于硅灰与碱激发溶液快速发生发应,加快了水化反应的速度,SFRGRAC3d抗压强度和劈裂抗拉强度可达到28d强度的85%以上;随着钢纤维体积掺量从0%增至1.5%,其抗压强度可提高17.44%,随着再生骨料取代率从0%增至50%,其强度降低了9.79%。掺入10%硅灰,总孔隙率降低了37.38%,能显著提高其抗压、劈裂抗拉和抗折强度,但当硅灰掺量为15%时,因过量硅灰降低了基体的碱度,导致地聚物水化反应不完全,使其力学性能表现出下降趋势。研究成果为再生混凝土相关规范的修订和完善提供参考依据。Abstract: Steel fiber reinforced geopolymer recycled concrete (SFRGRAC) presents several benefits, including reduced carbon emissions, conservation of natural minerals, and enhanced ductility and toughness, rendering it a promising material for broad applications. This study aims to augment the mechanical characteristics of SFRGRAC by incorporating silica fume as a reinforcing agent. It examines the influence of silica fume content, steel fiber volume fraction, and recycled aggregate substitution ratio on mechanical performance of SFRGRAC through cube compression, split tensile strength, flexural strength, and modulus of elasticity test. Additionally, the study elucidates the modification mechanism of silica fume via SEM and low field NMR test. The findings indicate that silica fume addition extends the setting time of SFRGRAC, with 15% substitution rate of silica fume leading to a 29.68% increase in initial setting time and a 22.98% increase in final setting time. The accelerated hydration reaction, prompted by the prompt pozzolanic reaction between silica fume and the alkali activator, results in the SFRGRAC achieving over 85% of its 28d compressive and tensile strengths within 3d. The compressive strength improves by 17.44% as steel fiber volume fraction increases from 0% to 1.5%, whereas a 50% substitution of recycled aggregate causes a 9.79% reduction in strength than 0% substitution rate. A 10% inclusion of silica fume diminishes total porosity by 37.38%, substantially enhancing compressive, tensile, and flexural strengths. When the silica fume substitution rate reaches 15%, the excessive silica fume reduces the alkalinity of matrix, resulting in an incomplete hydration reaction of the geopolymer. Consequently, this causes a decline in its mechanical properties. These findings offer valuable insights for the revision and enhancement of specifications related to recycled concrete.
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表 1 胶凝材料主要化学成分
Table 1. Chemical composition of cementitious materials
Binder Compositions/% Specific surface
area/(cm2·g−1)Density/
(kg·m−3)CaO SiO2 Al2O3 Fe2O3 MgO SO3 K2O Na2O MnO TiO2 Slag 36.82 26.75 19.66 0.32 11.1 2.65 0.29 0.84 0.37 0.94 428 2.9 Silica fume 0.12 96.65 0.31 0.07 0.11 1.21 0.22 0.67 0.17 0.83 200000 2.1 表 2 SFRGRAC设计配合比
Table 2. Designed mix proportions of SFRGRAC
No. Specimen Proportions/(kg·m−3) Slag Sand Natural
aggregate (NA)Recycled
aggregate (RA)Silica fume Steel fiber Water Sodium
silicateSodium
hydroxidePlastisizer 1 R50F00S00 417 724 724 543 0 0 61.08 161.75 3.59 3.3 2 R50F00S05 396.15 724 724 543 20.85 0 66.77 168.68 12.08 3.3 3 R50F00S010 375.3 724 724 543 41.70 0 72.47 159.80 11.45 3.3 4 R50F00S15 354.45 724 724 543 62.55 0 78.17 150.93 10.81 3.3 5 R50F10S00 417 724 724 543 0 78.5 61.08 161.75 3.59 3.3 6 R50F05S10 375.3 724 724 543 41.7 39.25 72.47 159.8 11.45 3.3 7 R50F10S05 396.15 724 724 543 20.85 78.5 66.77 168.68 12.08 3.3 8 R50F10S10 375 724 724 543 41.7 78.5 72.47 159.8 11.45 3.3 9 R50F10S15 354.45 724 724 271.5 62.55 78.5 78.17 150.93 10.81 3.3 10 R25F10S10 375.3 724 724 271.5 41.7 78.5 72.47 159.8 11.45 3.3 11 R00F10S10 375.3 724 818 0 41.7 78.5 72.47 159.8 11.45 3.3 12 R50F15S10 375.3 724 626 543 41.7 117.75 72.47 159.8 11.45 3.3 Notes:R—Addition of recycled aggregate; F—Volume fraction of steel fiber; S—Addition of silica fume. R50F10S10represent the 50% recycled aggregate content, 1.0% steel fiber volume fraction and 10% silica fume content. 表 3 SFRGRAC强度和弹性模量测试结果
Table 3. Test results of machine strength and elastic modulus of SFRGRAC
Specimen Compressive strength/MPa Splitting tensile strength/MPa Flexural strength/MPa Modulus of elasticity/GPa 3d 7d 28d 3d 7d 28d 28d 28d R50F00S00 38.07 43.05 47.51 3.39 3.71 4.16 3.79 13.02 R50F00S05 39.15 48.53 51.24 3.72 4.10 4.41 4.18 13.35 R50F00S10 41.36 48.60 53.96 3.91 4.27 4.32 4.26 14.75 R50F00S15 28.95 36.39 47.80 2.66 3.10 3.19 3.64 11.29 R50F10S00 40.30 48.44 54.05 3.98 4.42 4.91 4.30 13.85 R50F05S10 47.78 49.31 55.68 4.57 5.26 5.80 4.48 18.48 R50F10S05 49.15 53.75 57.25 5.54 5.91 6.26 4.83 16.43 R50F10S10 49.85 54.3 58.76 5.61 6.15 6.61 5.18 22.51 R50F10S15 40.75 46.93 55.91 5.00 5.51 6.16 4.06 14.22 R25F10S10 52.15 56.62 60.21 5.64 6.56 6.88 5.32 24.37 R00F10S10 55.91 61.27 65.14 6.16 7.01 7.24 5.68 27.52 R50F15S10 53.49 58.95 65.39 5.75 6.46 6.68 5.34 24.35 表 4 混凝土基质中孔隙率的总体积分数
Table 4. Total volume fraction of porosity in the concrete matrix
Specimen Curing age Small pores (<4μm) Medium pores (4~6μm) Large pores (≥10μm) Total R50F10S00 3d 3.15 0.29 0.61 4.05 28d 2.31 0.08 0.94 3.32 R50F10S05 3d 3.06 0.12 0.72 3.91 28d 2.11 0.06 0.61 2.78 R50F10S10 3d 3.13 0.061 0.86 4.06 28d 2.07 0.08 0.37 2.53 R50F10S15 3d 4.01 0.38 0.80 5.19 28d 2.71 0.12 0.88 3.71 -
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