摘要
为研究硫酸铝(AS)对喷射混凝土钙溶蚀过程的影响及作用机理,分析了含AS水泥砂浆试样在6 mol/L氯化铵溶液中的性能变化.结果表明:AS掺量对水泥砂浆试件的钙溶蚀过程有显著影响,掺2%AS有利于延缓砂浆试样在加速溶蚀过程中的性能退化;钙溶蚀过程导致水泥水化产物中的氢氧化钙(CH)溶解,水化硅酸钙(C‑S‑H)凝胶和钙矾石(AFt)脱钙,生成易溶的氯化钙,加速水泥石解体;加入AS会改变水泥水化产物的组成与结构,以及水泥石的微观结构,进而影响喷射混凝土的钙溶蚀过程.
中国隧道建设规模迅速增长,隧道工程所面临的环境越来越复杂,出现的安全事故也越来越
目前,混凝土钙溶蚀的研究对象以普通混凝土为
鉴于此,本文使用AS制备水泥砂浆试样,将其在6 mol/L 氯化铵溶液中进行加速溶蚀.通过抗压强度、质量变化率、吸水率和溶蚀深度对砂浆试样的劣化程度进行表征,同时制备净浆试样用于X射线衍射(XRD)、热重分析(TG)及扫描电子显微镜/能谱分析(SEM/EDS),以阐释喷射混凝土的钙溶蚀机理.
水泥为42.5级普通硅酸盐水泥,化学组成(质量分数,文中涉及组成、掺量等除特别注明外均为质量分数)见
CaO | SiO2 | Al2O3 | Fe2O3 | MgO | SO3 | K2O | Na2O | IL |
---|---|---|---|---|---|---|---|---|
61.65 | 21.03 | 5.84 | 3.27 | 2.00 | 2.41 | 0.39 | 0.26 | 2.43 |
将AS掺量取为水泥质量的2%、4%和6
Sample No. | Mortar sample | Paste sample | ||||||
---|---|---|---|---|---|---|---|---|
Cement | Sand | Water | AS | Cement | Water | AS | ||
Blank | 900 | 1 350 | 450 | 0 | 400 | 140 | 0 | |
AS‑2.0 | 900 | 1 350 | 450 | 18 | 400 | 140 | 8 | |
AS‑4.0 | 900 | 1 350 | 450 | 36 | 400 | 140 | 16 | |
AS‑6.0 | 900 | 1 350 | 450 | 54 | 400 | 140 | 24 |
Sample No. | Setting time/ min | |
---|---|---|
Initial | Final | |
Blank | 184.0 | 338.0 |
AS‑2.0 | 49.0 | 133.0 |
AS‑4.0 | 4.6 | 1.8 |
AS‑6.0 | 9.5 | 8.1 |
按照GB/T 35159—2017制备水泥砂浆试样,进行宏观性能试验.其中立方体试样尺寸为40 mm×40 mm×40 mm,每组6个,用于抗压强度、质量变化率及吸水率测试;圆柱体试样尺寸为ϕ40×70 mm,每组3个,用于溶蚀深度测试.各试样标准养护7 d后,浸泡在6 mol/L氯化铵溶液中进行加速溶
水泥净浆试样的凝结时间根据GB/T 1346—2011 《水泥标准稠度用水量、凝结时间、安定性检验方法》测定.水泥砂浆试样的抗压强度参照GB/T 17671—2021《水泥胶砂强度检验方法(ISO法)》进行.砂浆试样的质量使用电子秤(精度为0.1 g)称量,称量前用湿抹布擦去表面的浮渣和水分.砂浆试样的吸水率参照ASTM C642‑21 Standard Test Method for Density, Absorption, and Voids in Hardened Concrete进行.由于该标准中的干燥温度(105 ℃)过高,会导致水泥水化产物发生转变,故将干燥温度调整为40 ℃.砂浆试样的溶蚀深度采用酚酞法测定,溶蚀前将圆柱体试样的2个端面用环氧树脂封闭;测试时用切割机切除1个端面,然后将酚酞酒精溶液滴在断面上观察测试;测试完成后再次将断面用环氧树脂封闭,继续放入氯化铵溶液中溶蚀.
净浆试样粉碎研磨后用于X射线衍射(XRD)和热重分析(TG)测试.其中,XRD分析使用D/MAX‑3C Rikagu Advance型X射线衍射仪,TG分析使用Q600型同步热分析仪.取净浆试样中间部位进行扫描电子显微镜/能谱分析(SEM/EDS)观察和测试,采用线扫描和面扫描分析其从表面到内部的元素含量变化,具体取样方法和扫描方式示意图如

图1 线扫描和面扫描的取样和扫描方式示意图
Fig.1 Sampling and scanning method diagram of line scan and area scan
水泥砂浆试样的抗压强度如

图2 水泥砂浆试样的抗压强度
Fig.2 Compressive strength of cement mortar samples

图3 水泥砂浆试样的质量变化率
Fig.3 Mass change rate of cement mortar samples
水泥基材料具有多孔性,当与水接触时,外界水会渗入到试样表面的孔隙中.孔隙数量越多、孔隙尺寸越大,外界水就越容易渗透,因此可用吸水率来表征试样的孔隙状

图4 水泥砂浆试样的吸水率
Fig.4 Water absorption of cement mortar samples
因为水泥基材料的pH值可达到14左右,能使酚酞酒精溶液变成红色;而溶蚀后水泥水化产物中的CH被溶解,导致材料的pH值降

图5 水泥砂浆试样的溶蚀深度
Fig.5 Erosion depth of cement mortar samples
为验证AS对水泥砂浆溶蚀过程的影响,采用XRD对水泥水化产物的组成进行分析.水泥净浆试样溶蚀前水泥水化产物的XRD图谱如

图6 水泥净浆试样溶蚀前水泥水化产物的XRD图谱
Fig.6 XRD patterns of cement hydration products of cement paste samples before leaching
采用热重-微商热重(TG‑DTG)曲线对水泥水化产物中各组分含量进行分析,结果如

图7 水泥净浆试样溶蚀前水泥水化产物的TG‑DTG曲线
Fig.7 TG‑DTG curves of cement hydration products of cement paste samples before leaching
Sample No. | CH | Bound water |
---|---|---|
Blank | 24.85 | 12.22 |
AS‑2.0 | 24.22 | 13.31 |
AS‑4.0 | 23.97 | 14.50 |
AS‑6.0 | 22.29 | 14.03 |
结合
使用SEM观察水泥水化产物的微观结构,并使用EDS对溶蚀7 d后强度最高的试样AS‑2.0和空白组试样截面进行线扫描和面扫描,以分析钙溶蚀过程引起试样内部元素分布的变化.水泥净浆试样溶蚀7 d后的元素线扫描结果如

图8 水泥净浆试样溶蚀7 d后的元素线扫描
Fig.8 Element line scan results of cement paste samples after leaching for 7 d
由

图9 水泥净浆试样溶蚀前水泥水化产物的微观结构
Fig.9 Microstructure of cement hydration products of cement paste samples before leaching
水泥净浆试样溶蚀28 d后的元素面扫描结果如

图10 水泥净浆试样溶蚀28 d后的元素面扫描结果
Fig.10 Element area scan results of cement paste samples after leaching for 28 d
结合XRD、TG和SEM/EDS测试结果,以NH4Cl溶蚀为例,分析普通混凝土的溶蚀机理:当混凝土与外界环境接触时,水泥水化产物中的CH被溶解,同时,C‑S‑H和AFt也会经历脱钙反应,并分别伴随着脱硅和脱铝过程,最终导致水泥石被分解;从混凝土中溶出的C
对于喷射混凝土,AS主要通过以下两个方面影响其钙溶蚀过程:
(1)AS会改变水泥水化产物的组成和结构.掺加AS导致水泥水化产物中的CH含量降低,而CH的溶解速率决定了混凝土的溶蚀速率.AS还会降低C‑S‑H的钙硅比,使得更多的Ca元素以主层钙的形式存在,不易被脱
(2)掺加适量AS会增加喷射混凝土的孔隙率,CaCl2等侵蚀产物积聚在混凝土原有的孔隙中,降低了喷射混凝土的劣化速率.但是如果AS掺量过大,会造成水泥凝结硬化时间过短,导致硬化混凝土中存在较多裂缝,为外界侵蚀物质提供了通道,从而使喷射混凝土的劣化速度更快.
(1)硫酸铝(AS)掺量对喷射混凝土的溶蚀过程有显著影响.掺加2% AS能够减缓水泥在加速溶蚀过程中的性能退化.
(2)在溶蚀过程中,水泥水化产物中的氢氧化钙(CH)被溶解,水化硅酸钙(C‑S‑H)凝胶和钙矾石(AFt)发生脱钙反应,从水泥中溶出的C
(3)AS会改变水泥水化产物的组成和结构,以及喷射混凝土的微观结构.适量掺加AS可以改善喷射混凝土的钙溶蚀性能.
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