摘要
评价了再生砖细骨料(RFCB)的几何形状和表面形态,研究了RFCB颗粒级配和初始饱水度对混凝土流变行为与力学性能的影响.结果表明:相比天然河砂,RFCB几何形状不规则且形状参数较低,表面形态呈凹陷凸起状且粗糙度较大;RFCB复杂的物理特性导致新拌混凝土的屈服应力与塑性黏度增大,劈裂抗拉强度有所提高且对抗压强度无较大不利影响;适当降低RFCB的最小粒径和初始饱水度,虽会使混凝土的流变性能变差,但有助于在混凝土中形成以再生砖细骨料为软核、界面为硬壳的结构,从而提高了混凝土的力学性能.
随着城镇化的快速发展,建造新建筑、拆除旧建筑等导致天然资源过度消耗,建筑固废大量产生,从而造成不可逆的环境污染.据统计,拆除旧建筑所产生的砖混废弃物在建筑固废中的占比巨大,综合利用率较低,若采用简单堆放或填埋处置方式,则会占用大量土
目前,砖混废弃物经破碎、粉磨和筛分等处理后,得到的再生骨料和再生微粉可部分取代砂石骨料或掺合料来制备砂浆和混凝土,符合发展循环经济的要
鉴于此,本文评价了不同种类、不同粒径再生砖细骨料的几何形状和表面形态,对比分析了其与天然河砂之间的物理特性差异;同时,探究了再生砖细骨料颗粒级配和初始饱水度对混凝土流变行为、抗压强度和劈裂抗拉强度的影响,揭示其对混凝土宏观性能和微观形貌的影响机理,以期为实现废砖再生利用提供科学依据.
水泥选用P·O 42.5普通硅酸盐水泥,各项性能指标符合GB 175—2007《通用硅酸盐水泥》要求;减水剂采用聚羧酸高效减水剂,减水率(质量分数,文中涉及的减水率、水灰比等除特别注明外均为质量分数或质量比)25%,固含量为35%;天然粗骨料为5~20 mm连续级配的碎石;天然细骨料为普通河砂(NAF);再生砖细骨料由拆除砌体结构中的废旧黏土砖和砂浆经二次机械破碎后筛分而得,控制其最小粒径,将再生砖细骨料分为RFCB‑A(0~5.00 mm)和RFCB‑B(0.15~5.00 mm)2种级配.天然河砂和再生砖细骨料的基本物理性能和颗粒级配曲线如
Material | Apparent density /(kg· | Bulk density /(kg· | Fineness modulus | Saturated surface dry absorption (by mass)/% |
---|---|---|---|---|
NAF | 2 500 | 1 465 | 2.65 | 1.5 |
RFCB‑A | 2 300 | 1 220 | 2.79 | 9.0 |
RFCB‑B | 2 411 | 1 326 | 2.88 | 11.4 |

图1 天然河砂和再生砖细骨料的颗粒级配曲线
Fig.1 Grading curves of NAF and RFCB
本试验配合比设计目标强度等级为C40,主要考虑再生砖细骨料的颗粒级配和初始饱水度.再生砖细骨料混凝土的水灰比(mW/mC)为0.45,其配合比如
Specimen No. | Water | Cement | Coarse aggregate | River sand | RFCB | Initial water saturation |
---|---|---|---|---|---|---|
R0 | 171 | 380 | 1 077 | 780 | 0 | 0 |
R50AF | 171 | 380 | 1 077 | 390 | 356 | 32 |
R50BF | 171 | 380 | 1 077 | 390 | 375 | 42 |
R50AP | 171 | 380 | 1 077 | 390 | 356 | 25 |
R50BP | 171 | 380 | 1 077 | 390 | 375 | 36 |
进行几何形状测试时,先将骨料按[0.075,0.15)mm、[0.15,0.3)mm、[0.3,0.6)mm、[0.6,1.18)mm、[1.18,2.36)mm和[2.36,4.75)mm这6类粒径区间进行分类;再取等质量的骨料均匀分散在测试面板上,采用Occhio Scan 700型粒度粒型分析仪进行图像扫描.基于骨料图像数据,借助Callisto 3D图形分析软件进行计算,获得骨料的长宽比、球度值和棱角度等参数.
进行表面形态测试时,将骨料切割、打磨至平整块状样品,暴露骨料固有的表面形态,采用原子力显微镜(AFM)技术和NanoScape Analysis分析软件,获得再生砖细骨料和天然河砂的表面形态及其特征参数(轮廓算数平均偏差Ra、高度均方根Rq和轮廓最大高度Rz).
根据GB/T 50081—2019《混凝土物理力学性能试验方法标准》,制备尺寸为100 mm×100 mm×100 mm的立方体试件,测试7、28、60、120、180 d立方体抗压强度,以及28 d劈裂抗拉强度.试验值取3个试件的平均值.
骨料的几何形状和表面形态是其物理特性研究的关键,不仅影响颗粒之间的相互运动,而且影响骨架结构的堆积程
基于图像处理技术,不同粒径区间天然河砂和再生砖细骨料的二维几何形状如

图2 天然河砂和再生砖细骨料的几何形状
Fig.2 Geometrical shapes of NAF and RFCB
为进一步对比分析再生砖细骨料与天然河砂几何形状的差异,借助图形分析软件,量化表征不同粒径细骨料的几何形状参数(长宽比、球度值和棱角度),结果如

图3 天然河砂和再生砖细骨料的几何形状参数
Fig.3 Geometrical shape parameters of NAF and RFCB
采用AFM技术观测天然河砂和再生砖细骨料的表面形态,其三维形貌照片如

图4 天然河砂和再生砖细骨料的三维形貌照片
Fig.4 3D morphology images of NAF and RFCB
Material | Ra/nm | Rq/nm | Rz/nm |
---|---|---|---|
NAF | 6.736 | 8.813 | 59.210 |
RFCB | 27.967 | 32.615 | 453.230 |

图5 新拌混凝土的流变性能
Fig.5 Rheological properties of fresh concretes
由

图6 不同养护龄期下再生砖细骨料混凝土的抗压强度
Fig.6 Compressive strength of recycled concrete containing RFCB at different curing ages
由

图7 再生砖细骨料混凝土养护28 d龄期时的劈裂抗拉强度
Fig.7 Splitting tensile strength of recycled concrete containing RFCB curing for 28 d
由

图8 再生砖细骨料混凝土养护180 d龄期时的SEM照片
Fig.8 SEM images of recycled concretes containing RFCB curing for 180 d
由
(1)相比天然河砂,由于高温烧制工艺和二次机械破碎的影响,再生砖细骨料的几何形状参数较低,表面粗糙度较大,其特征参数(轮廓算数平均偏差Ra、高度均方根Rq和轮廓最大高度Rz)分别为天然河砂的4~8倍.
(2)再生砖细骨料的掺入增大了新拌混凝土的屈服应力与塑性黏度,提高了硬化混凝土的劈裂抗拉强度且对抗压强度无较大不利影响.此外,再生砖细骨料最小粒径越小、初始饱水度越低,新拌混凝土的流变性能就越大,硬化混凝土的劈裂抗拉强度最大提高14%.
(3)适当降低再生砖细骨料的最小粒径和初始饱水度,有助于在混凝土中形成以再生砖细骨料为软核、界面为硬壳的结构,该结构可抵消多孔骨料的内在缺陷,有利于增强混凝土的力学性能.
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