摘要
为探究复掺偏高岭土和硅灰对混凝土耐盐酸的影响,在5%偏高岭土及5%、10%或15%硅灰混合置换胶凝材料水平下制备试样并进行盐酸浸泡试验,通过超声波、压汞法(MIP)、X射线衍射(XRD)、热重分析(TGA)及扫描电子显微镜(SEM)系列测试方法探讨腐蚀对混凝土损伤层厚度、孔隙结构、水化产物的影响,而后对比分析损伤层厚度与孔隙结构的演化关系. 结果表明:基于抗压强度、劈裂抗拉强度及抗折强度,M5S5耐酸性最优;损伤层厚度与超声波波速呈负相关,与孔隙率、平均孔径、总孔隙面积呈正相关;两者复掺可提高混凝土耐酸性,但硅灰掺量的递增使改善效益略有降低.
近年来,仍有不少关于混凝土构筑物遭受酸腐蚀的报道,如混凝土管道受酸性地下水、井水侵蚀;钢铁处理厂的酸洗废水池及农场围护结构持续处于酸性环境
研究表明当碱性混凝土受酸腐蚀时,内部碱度急剧降低,由于水化产物中氢氧化钙(CH)稳定性最差,成为最先被酸破坏的物质;随后依次是水化产物如水化铝酸钙(CAH)、钙矾石(AFt)、单硫型水化硫铝酸钙(AFm)、水化硅酸钙(CSH)及金属氧化物(如Al2O3、Fe2O3、MgO等)等被分解消耗;继而原本薄弱的界面过渡区进一步劣化,最终引发混凝土失效破
为此,将制备完成的试件置于设定好pH的盐酸溶液中进行全浸泡试验,相应周期后取出进行力学性能、超声波无损测试及微观检测,研究了腐蚀对混凝土损伤层厚度、孔隙结构和水化产物的影响;探讨了损伤层厚度与孔隙结构变化的关系;并分析了水化产物的演变特征及劣化机理,旨在揭示复掺偏高岭土及硅灰对混凝土耐酸性的影响,为改善其耐酸性提供一定的指导.
1 试验
1.1 材料
试验采用P·O 42.5硅酸盐水泥(C),偏高岭土(MK)和硅灰(SF)作为胶凝材料,主要化学成分详见
材料类别 | SiO2 | Al2O3 | Fe2O3 | CaO | SO3 | MgO | L.O.I. |
---|---|---|---|---|---|---|---|
C | 23.34 | 6.38 | 3.92 | 61.27 | 3.92 | — | 1.17 |
MK | 48.45 | 44.59 | 3.43 | 0.56 | 0.15 | 0.01 | 2.81 |
SF | 93.13 | 0.38 | 0.68 | 0.27 | — | 0.02 | 5.52 |
1.2 制备及试验设计
试验所用的4个配合比如
组别 | W/(kg· | C/(kg· | MK/(kg· | SF/(kg· | S/ (kg· | G/(kg· | SP/% |
---|---|---|---|---|---|---|---|
对照组 | 200 | 445 | 0 | 0 | 668 | 1 137 | 0.1 |
M5S5 | 200 | 404.5 | 22.3 | 22.3 | 668 | 1 137 | 0.1 |
M5S10 | 200 | 378.3 | 22.3 | 44.5 | 668 | 1 137 | 0.1 |
M5S15 | 200 | 356 | 22.3 | 66.8 | 668 | 1 137 | 0.1 |
1.3 测试方法
根据《超声法检测混凝土缺陷技术规程》(CECS 21: 2000
(1) |
(2) |
(3) |
(4) |
(5) |
式中:l0为超声波到达损伤层与未损伤层界面时的距离,mm;x为损伤层传播距离的水平投影;hf表示损伤层厚度,mm;Va和Vf分别为超声波在未损伤层和损伤层的传播速度(直线斜率),km/s;lf、la分别表示l0前后的距离;tf、ta分别表示l0前后lf、la对应的时间;a1和a2分别表示两条直线的截距. 联立方程可求解损伤层厚度hf.

图1 测试过程图解
Fig.1 Diagram of the test process

图2 声时与声距的关系曲线图
Fig.2 Relationship curve between the sound time and distance
力学性能试验均参照《混凝土物理力学性能试验方法标准》(GB/T 50081—2019
(6) |
式中:mB、mC和mD分别表示样品在70~350 ℃、350~470 ℃、600~730 ℃范围内的质量损失率,%;m(H2O)表示化学结合水的质量分数(含量),%;m[Ca(OH)2]表示CH的质量分数(含量),%;M为H2O、CO2、 Ca(OH)2的相对分子质量,分别为18、44、74.
2 结果与讨论
2.1 力学性能
不同掺量试件抗压强度、劈裂抗拉强度及抗折强度的变化趋势如

(a) 不同掺量试件抗压强度的变化

(b) 不同掺量试件劈裂抗拉强度的变化

(c) 不同掺量试件抗折强度的变化
图3 不同掺量试件的抗压强度、劈裂抗拉强度和抗折强度的变化
Fig.3 Variation of compressive strength, splitting tensile strength and flexural strength of specimens with different dosages
力学性能 | 平方和 | 均方 | f值 | p值 | 结果 |
---|---|---|---|---|---|
抗压强度 | 1 380.06 | 460.02 | 7.17 | 0.00 | 显著 |
劈裂抗拉强度 | 3.69 | 1.23 | 19.69 | 0.00 | 显著 |
抗折强度 | 1.47 | 0.49 | 1.81 | 0.17 | 不显著 |
2.2 损伤层厚度
盐酸腐蚀混凝土是一个由外到内的过程,而超声波可无损评估混凝土的劣化程度. 各龄期试件的损伤层厚度如

图4 损伤层厚度增长趋势
Fig.4 Increasing trend of the thickness of damage layer

图5 超声波传播路径示意图
Fig.5 Schematic diagram of ultrasonic propagation path
(a) (b) (c)
2.3 孔隙结构
样品24 d和48 d孔隙率及平均孔径如
Ca(OH)2+2 | (7) |
CSH+2 | (8) |

图6 样品孔隙率及平均孔径
Fig.6 Porosity and average pore diameter of the samples

图7 总侵入体积及总孔隙面积
Fig.7 Total intrusion volume and total pore area
其余水化产物如CAH、AFt及金属氧化物继而被盐酸分解消耗,如式(9)~
CAH+2 | (9) |
C3A·3CaSO4·32H2O+12 | (10) |
Fe2O3+6 | (11) |
Al2O3+6 | (12) |
MgO+2 | (13) |
CaO+2 | (14) |
此时,C
对照组在酸性环境暴露较长时间后,孔隙不断连通贯穿形成微裂缝,超声波的多次绕射导致波速减小更为迅速,因而损伤层增长幅度最大. 该缺陷的存在除了使得扩散通量增加之外,还可通过对流来输送溶解的物质,从而提高输送速率,加速溶解溶出. 换言之,裂缝是水泥材料在脱钙过程中体积收缩并转化为大量腐蚀产物的结
CaCO3+2 | (15) |
此时超声波难以在
如

图8 样品累积孔径分布
Fig.8 Cumulative pore size distribution of the samples

图9 样品差异孔径分布
Fig.9 Differential pore size distribution of the samples

图10 样品孔隙体积分布
Fig.10 Pore volume distribution of the samples
还应注意到M5S5、M5S15的大毛细孔(>100 nm)占比在48 d后仅有30.50%、47.18%,低于对照组的65.73%,其存在对混凝土的耐久性和渗透性产生不利影响. 此外,Delagrave等论证了氯离子的存在会加剧钙流失,通过CH置换为易溶的CaCl2,而后由钙的流失最终导致孔隙率升高,水泥浆体脱钙硬
2.4 热重分析

(a) 24 d

(b) 48 d
图11 样品腐蚀24 d及48 d后的TGA曲线
Fig.11 TGA curves of samples after 24 and 48 days of corrosion

图12 样品腐蚀24 d及48 d后的CH和化学结合水含量
Fig.12 The content of CH and chemically bound water of samples at the corrosion age of 24 d and 48 d
2.5 腐蚀产物

(a) 24 d

(b) 48 d
图13 样品腐蚀24 d及48 d后的XRD图谱
Fig.13 XRD patterns of samples after 24 and 48 days of corrosion
值得注意的是,所有样品均无法检测到CH和AFt,一方面,基质中的CH和AFt大部分被盐酸消耗殆尽;另一方面,腐蚀致使两者的晶体结构发生改变,衍射后难以识别. 当pH值在4.0~6.5之间时,少量C
2.6 微观结构分析
如

(a) 对照组

(b) M5S5
图14 样品腐蚀24 d后的SEM图
Fig.14 SEM image of samples after 24 days of corrosion

(a) 对照组

(b) M5S5
图15 样品腐蚀48 d后的SEM图
Fig.15 SEM image of samples after 48 days of corrosion
3 结 论
1)损伤层厚度与超声波速度呈负相关,与孔隙率、平均孔径以及总孔隙面积呈正相关.
2)方差分析中除抗折强度外,其他p值均小于0.05显著性水平,表明SF掺量对抗压强度和劈裂抗拉强度影响显著,对抗折强度影响较小.
3)M5S5组别耐酸性最优,而耐酸性随SF掺量的增加而略有降低,归因于富余的SF消耗更多的CH,从而削弱混凝土中和盐酸的能力.
4)复掺的MK及SF通过填充效应及二次水化作用优化混凝土微结构,使混凝土耐酸性提高,劣化速率得以延缓.
参考文献
CHANG H B,CHOI Y C.Accelerated performance evaluation of repair mortars for concrete sewer pipes subjected to sulfuric acid attack[J].Journal of Materials Research and Technology, 2020,9(6):13635-13645. [百度学术]
EKOLU S,DIOP S,AZENE F,et al.Disintegration of concrete construction induced by acid mine drainage attack[J].Journal of the South African Institution of Civil Engineering,2016,58(1):34-42. [百度学术]
NOCHAIYA T,SURIWONG T,JULPHUNTHONG P.Acidic corrosion-abrasion resistance of concrete containing fly ash and silica fume for use as concrete floors in pig farm[J].Case Studies in Construction Materials,2022,16:e01010. [百度学术]
宋志刚,李贤胜,谢世华,等.硫酸侵蚀混凝土的腐蚀产物附面层及其影响[J].建筑材料学报,2019,22(3):348-355. [百度学术]
SONG Z G,LI X S,XIE S H,et al.Corroded product surface layer of concrete corroded by sulfuric acid and its effect on corrosion rate[J].Journal of Building Materials,2019,22(3):348-355.(in Chinese) [百度学术]
郑山锁,关永莹,黄莺歌,等.酸雨环境下约束混凝土本构关系试验[J].建筑材料学报,2016,19(2):237-241. [百度学术]
ZHENG S S,GUAN Y Y,HUANG Y G,et al.Experiment on the constitutive relation of confined concrete under acid rain environment erosion[J].Journal of Building Materials,2016, 19(2):237-241.(in Chinese) [百度学术]
张英姿,范颖芳,刘江林,等.模拟酸雨环境下C40混凝土抗压性能试验研究[J].建筑材料学报,2010,13(1):105-110. [百度学术]
ZHANG Y Z,FAN Y F,LIU J L,et al.Experimental study on compressive performance of concrete C40 in simulated acid environment[J].Journal of Building Materials,2010,13(1):105-110.(in Chinese) [百度学术]
BEDDOE R E,DORNER H W.Modelling acid attack on concrete:part I.the essential mechanisms[J].Cement and Concrete Research,2005,35(12):2333-2339. [百度学术]
PAVLÍK V.Corrosion of hardened cement paste by acetic and nitric acids part I: calculation of corrosion depth[J].Cement and Concrete Research,1994,24(3):551-562. [百度学术]
PAVLÍK V,UNČÍK S.The rate of corrosion of hardened cement pastes and mortars with additive of silica fume in acids[J].Cement and Concrete Research,1997,27(11):1731-1745. [百度学术]
王凯,马保国,张泓源.矿物掺合料对混凝土抗酸雨侵蚀特性的影响[J].建筑材料学报,2013,16(3):416-421. [百度学术]
WANG K,MA B G,ZHANG H Y.Effect of mineral admixtures on deterioration of concrete under acid rain attack[J].Journal of Building Materials,2013,16(3):416-421.(in Chinese) [百度学术]
PANDEY A,KUMAR B.Investigation on the effects of acidic environment and accelerated carbonation on concrete admixed with rice straw ash and microsilica[J]. Journal of Building Engineering,2020,29:101125. [百度学术]
CHAND G,HAPPY S K,RAM S.Assessment of the properties of sustainable concrete produced from quaternary blend of Portland cement,glass powder,metakaolin and silica fume[J].Cleaner Engineering and Technology,2021,4:100179. [百度学术]
KUZIELOVÁ E,ŽEMLIČKA M,BARTONIČKOVÁ E,et al.The correlation between porosity and mechanical properties of multicomponent systems consisting of Portland cement-slag-silica fume-metakaolin[J].Construction and Building Materials,2017,135: 306-314. [百度学术]
HE S,QIN Y,YU P,et al.Synergistic effect of metakaolin and silica fume on hydrochloric acid resistance of concrete[J].Construction and Building Materials,2022,359:129498. [百度学术]
超声法检测混凝土缺陷技术规程:CECS 21:2000[S].北京:中国城市出版社,2000:19-20. [百度学术]
Technical specification for inspection of concrete defects by ultrasonic method:CECS 21:2000[S].Beijing:China City Press,2000: 19-20.(in Chinese) [百度学术]
混凝土物理力学性能试验方法标准:GB/T 50081—2019[S].北京:中国建筑工业出版社,2019:12-28. [百度学术]
Standard for test methods of concrete physical and mechanical properties:GB/T 50081―2019[S].Beijing: China Architecture & Building Press,2019:12-28.(in Chinese) [百度学术]
VEJMELKOVÁ E,PAVLÍKOVÁ M,KEPPERT M,et al.High performance concrete with Czech metakaolin:experimental analysis of strength,toughness and durability characteristics[J].Construction and Building Materials,2010,24(8):1404-1411. [百度学术]
ZIVICA V,BAJZA A.Acidic attack of cement based materials—a review.part 1. principle of acidic attack[J]. Construction and Building Materials, 2001, 15(8): 331-340. [百度学术]
陈正,陈犇,郑皆连,等.青藏高原低气压环境下钢管混凝土的核心混凝土密实性评估方法研究[J].土木工程学报,2021, 54(8):1-13. [百度学术]
CHEN Z,CHEN B,ZHENG J L,et al.Methodology on evaluating the compactness of core concrete in CFST serving under low atmospheric pressure over the Qinghai-Tibet Plateau[J].China Civil Engineering Journal,2021,54(8):1-13.(in Chinese) [百度学术]
NEŽERKA V,BÍLÝ P,HRBEK V,et al.Impact of silica fume,fly ash,and metakaolin on the thickness and strength of the ITZ in concrete[J].Cement and Concrete Composites,2019,103:252-262. [百度学术]
DELAGRAVE A,PIGEON M,MARCHAND J,et al.Influence of chloride ions and pH level on the durability of high performance cement pastes (part Ⅱ)[J].Cement and Concrete Research,1996,26(5):749-760. [百度学术]
CHU S H,KWAN A K H.Co-addition of metakaolin and silica fume in mortar:effects and advantages[J].Construction and Building Materials,2019,197:716-724. [百度学术]
PAVLÍK V.Corrosion of hardened cement paste by acetic and nitric acids part II:formation and chemical composition of the corrosion products layer[J].Cement and Concrete Research,1994,24(8):1495-1508. [百度学术]
GUTBERLET T,HILBIG H,BEDDOE R E.Acid attack on hydrated cement—effect of mineral acids on the degradation process[J].Cement and Concrete Research,2015,74:35-43. [百度学术]