摘要
采用预浸石灰水碳化法对再生粗骨料进行强化处理,对强化前后再生粗骨料物理性能进行检测,研究了碳化再生粗骨料对混凝土力学性能的影响.同时结合微观测试手段,分析碳化对再生粗骨料的增强机理. 通过立方体抗压、劈裂抗拉及抗折强度试验,研究强化后混凝土各龄期强度变化. 结果表明:预浸石灰水碳化法能够显著改善再生粗骨料的物理性能,其吸水率降低15.2%~22.9%,压碎值降低15.2%~17.7%;碳化后粗骨料的界面过渡区更加密实,有利于再生粗骨料品质的提升;7、28 d的混凝土强度随粗骨料取代率的增大而降低;当粗骨料取代率为50%时,预浸石灰水碳化再生混凝土强度与普通混凝土相当.
当前,天然资源的开发与建筑固废大量排放存在矛盾关系,而再生混凝土(RAC)有利于缓和两者关系.与天然骨料(NA)相比,再生骨料随机性和差异性较大,强化再生骨料和拓宽RAC的应用已成为近年来的热
Zhan
综上,采用直接碳化法和复合碳化法均能有效改善RCA品质.然而,对于复合碳化法处理RCA的宏微观性能研究还有待进一步完善.鉴于此,本试验采用超饱和石灰水预浸泡复合碳化处理RCA,对RCA的物理性能、微观性能和再生粗骨料混凝土的基本力学性能进行探究.
试验采用P·O 42.5普通硅酸盐水泥、细度模数为3的河砂、聚羧酸减水剂、自来水、再生粗骨料(RCA)、饱和石灰水及CO2等原材料. 其中RCA为实验室废弃混凝土梁经破碎并筛选得到的5~10、10~20 mm骨料.另外,为了对比,试验选取相同粒径范围内的天然骨料(NA).NA和RCA的物理性能如
Physical property | RCA | NA | ||
---|---|---|---|---|
5-10 mm | 10-20 mm | 5-10 mm | 10-20 mm | |
Water absorption(by mass)/% | 4.8 | 4.6 | 0.6 | 0.6 |
Apparent density/ (kg· | 2 583 | 2 549 | 2 703 | 2 703 |
Crushing value(by mass)/% | 18.6 | 15.8 | 10.4 | 8.1 |
将RCA置于容器中,倒入石灰水并稳定搅拌;浸润24 h后将骨料沥出,移至(22±2) ℃、相对湿度为60%~70%的恒温恒湿箱中,确保骨料达到最佳含水率.研究表明,当RCA的含水率(质量分数,文中涉及的取代率、比值等除特别说明外均为质量分数或质量比)为60%~70%时有利于碳化反应的进

图1 碳化设备示意图
Fig.1 Schematic diagram of carbonization equipment
(1)转靶X射线衍射仪(XRD)
碳化后的RCA在烘箱(65 ℃)中干燥24 h.先通过锤击、研磨和筛分获得粒径小于80 μm的黏附砂浆样品,再用XRD进行扫描,扫描范围为5°~70°,速率为10(°)/min.
(2)扫描电子显微镜(SEM)
对小于10 mm的骨料样品进行打磨和修整,并在65 ℃的烘箱中干燥至恒重,然后通过SEM观察样品的微观形貌.
(1)碳化对RCA吸水率的影响
碳化前后RCA的吸水率如

图2 碳化前后RCA的吸水率
Fig.2 Water absorption of RCA before and after carbonation
(2)碳化对RCA压碎值的影响
碳化前后RCA的压碎值如

图3 碳化前后RCA的压碎值
Fig.3 Crushing value of RCA before and after carbonation
上述数据表明,经碳化处理后RCA的压碎值明显降低,说明碳化后RCA的品质得到有效改善, 这与Lu
(3)碳化对RCA表观密度的影响
碳化前后RCA的表观密度如

图4 碳化前后RCA的表观密度变化
Fig.4 Apparent density of RCA before and after carbonation
(1)微观组分

图5 碳化前后RCA的微观组分变化
Fig.5 Change in micro‑component of RCA before and after carbonation
(1) |
式中:、和分别为样品在150、550、800 ℃时的质量分数.
(2) |
式中:为样品在500~800 ℃下的脱碳质量分数;和分别为CO2和CaCO3的相对分子质量.
Type of RCA | /% | m/% |
---|---|---|
C‑RCA | 26.37 | 47.10 |
LC‑RCA | 28.96 | 50.75 |
(2) 微观形貌
碳化前后RCA与所粘结砂浆间ITZ及水化产物的微观形貌如

图6 碳化前后RCA与所黏结砂浆ITZ及水化产物的微观形貌
Fig.6 Change of ITZ and hydration products of RCA before and after carbonation
再生混凝土立方体抗压强度随RCA取代率、碳化处理方式和养护龄期的变化如

图7 再生混凝土立方体抗压强度随RCA取代率、碳化处理方式和养护龄期的变化
Fig.7 Change of cube compressive strength of recycled conerete with RAC substitution rate, carbonation treatment method and curing age
再生混凝土劈裂抗拉强度随RCA取代率、碳化处理方式和养护龄期的变化如

图8 再生混凝土劈裂抗拉强度随RAC取代率、碳化处理方式和养护龄期的变化
Fig.8 Change of splitting tensile strength of recycled concrete with RCA substitution rate, carbonation treatment method and curing age
再生混凝土抗折强度随RCA取代率、碳化处理方式和养护龄期的变化如

图9 再生混凝土抗折强度随RCA取代率、碳化处理方式和养护龄期的变化
Fig.9 Change of flexural strength of recycled concrete with RCA substitution rate, carbonation treatment method and curing age
与NA相比,RCA具有较高的吸水率和压碎值,主要原因是RCA在破碎过程中产生大量的微裂纹;且RCA表面附着的旧砂浆疏松多孔,导致骨料的品质较
Ca(OH)2+CO2CaCO3+H2O | (3) |
C‑S‑H+CO2CaCO3+SiO2 · nH2O | (4) |
由此可见,碳化能够有效提升RAC的力学性能.RAC的CH层间联结较弱是发生受力破坏的主要根源,经碳化反应后,CH转化为热稳定性较好的无机碳酸盐,起到了良好的填充效应,有效提升了RAC的强度. 研究发现,C2S和C3S是硅酸盐水泥中的主要矿物组分,但C3S具有较高的水化活性,其早期与CO2反应生成C‑S‑H和CaCO3;随着反应的进行,C‑S‑H与CO2发生脱钙反应,生成无定型硅胶填充在孔隙中,从而提高了混凝土早期力学强

图10 预浸石灰水碳化增强机理示意图
Fig.10 Schematic diagram of mechanism of pre‑soaking in lime water and carbonation
(1)碳化能够有效改善RCA的品质,且预浸石灰水碳化对RCA的物理性能提升效果更加显著.与未碳化RCA相比,LC‑RCA的吸水率降低15.2%~22.9%,压碎值降低15.2%~17.7%.
(2)RCA的粒径越小、砂浆含量越高、比表面积越大,其碳化反应效率越高,碳化后骨料品质提升效果越明显.碳化处理可提高RCA的CaCO3衍射峰强度,反应产物CaCO3和无定形硅胶有效改善了骨料与旧砂浆间ITZ的密实性,对旧砂浆中的孔隙和微裂纹具有充填作用.
(3)预浸石灰水碳化比直接碳化对RAC抗压强度的提升幅度更大,LC‑RAC的抗压强度比C‑RAC提高了3.9%~7.6%. 随着RCA取代率的增加,RAC的抗压强度呈下降趋势. 预浸石灰水碳化改善了较高取代率下RCA对混凝土抗压强度的不利影响.
(4)碳化有利于强化旧砂浆的强度,增强基质间的黏结作用,形成更加密实的混凝土结构. 预浸石灰水碳化处理后,RAC的劈裂抗拉强度和抗折强度均有明显提升.
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