摘要
利用交流阻抗技术对早龄期碳化养护下不同水灰比、砂灰比和试件尺寸的水泥基材料进行研究,建立优化的等效电路模型并获得相应电学参数,对比分析早龄期碳化养护下水泥基材料的质量增加率和交流阻抗谱变化.结果表明:试件质量增加率随水灰比增大而增大,随砂灰比降低而增大,净浆组的质量增加率均值为11.29%,为砂浆组的2~3倍;试件尺寸越大,质量增加率越大;在碳化养护条件下水泥基材料交流阻抗谱高频区圆弧直径呈增大趋势,低频区直线斜率呈减小趋势;建立了试件质量增加率与阻抗谱参数之间的关系模型,采用电荷传递电阻Rct2预测碳化养护下水泥基材料的CO2吸收量,早龄期碳化养护下水泥基材料的交流阻抗谱研究有助于建立相应的无损监测技术.
随着工业的快速发展,烟气的排放导致大气中CO2含量迅速增长,引起温室效应等一系列环境问题.建筑领域中被广泛应用的水泥基材料可以吸收CO2,发生碳化反应,具有庞大的固碳潜力.近年来,早龄期碳化养护水泥基材料的固碳研究成为热点,其在封存CO2的同时还可提高水泥基材料的密实度和强
准确地表征水泥基材料在碳化过程中微观结构的演变,对于深入研究混凝土碳化行为和揭示碳化机理具有重要意
水泥基材料微观结构可为碳化反应提供场所,而反应物与生成物的摩尔体积存在差异,材料微观结构也会因为碳化而改变.为了更好地研究水泥基材料的微观结构变化,可以将其视为由固相、液相和固液间界面三部分组成的体系.在外加电场作用下,水泥基材料微观孔隙和介质形成导电通路,碳化则会引起孔隙结构和介质中可溶性盐浓度的变化,导致电流响应变化,交流阻抗谱发生规律性改
本研究采用交流阻抗技术表征水泥基材料的早龄期碳化养护进程,探究碳化养护对水泥基材料质量增加率与细微观结构的影响规律.针对不同水灰比、砂灰比和试件尺寸,研究水泥基材料在碳化养护过程中交流阻抗谱的变化规律,建立等效电路模型对阻抗谱曲线进行拟合,得到关键参数,进一步探究等效电路元件参数与水泥基材料碳化程度和细微观结构之间的关系.
水泥(C)采用P·O 42.5普通硅酸盐水泥,其化学组
CaO | SiO2 | MgO | SO3 | Al2O3 | K2O | Na2O | Fe2O3 | IL |
---|---|---|---|---|---|---|---|---|
63.80 | 20.78 | 1.72 | 3.82 | 3.57 | 0.75 | 0.26 | 3.99 | 1.31 |
Group | Size/mm | Amount/g | mW/mC | ||
---|---|---|---|---|---|
Cement | Sand | Water | |||
A4(S/M/L) | 10/15/20 | 100 | 300 | 40 | 0.4 |
A5(S/M/L) | 10/15/20 | 100 | 300 | 50 | 0.5 |
A6(S/M/L) | 10/15/20 | 100 | 300 | 60 | 0.6 |
B4(S/M/L) | 10/15/20 | 100 | 200 | 40 | 0.4 |
B5(S/M/L) | 10/15/20 | 100 | 200 | 50 | 0.5 |
B6(S/M/L) | 10/15/20 | 100 | 200 | 60 | 0.6 |
C3(S/M/L) | 10/15/20 | 100 | 0 | 30 | 0.3 |
C4(S/M/L) | 10/15/20 | 100 | 0 | 40 | 0.4 |
C5(S/M/L) | 10/15/20 | 100 | 0 | 50 | 0.5 |
将脱模后带有铜导线的试件放置于碳化养护箱中养护.碳化养护箱参数为:相对湿度(70±5)%,CO2质量分数(20±2)%,温度(20±2) ℃和压强0.1 MPa.需要特别说明的是,试样在放入养护箱之前,用底层垫有饱水海绵的塑料盒盛载,以保证水泥水化需要,盒盖留有若干小孔,以维持盒内CO2质量分数在20%左右.整个养护和测试过程见

图1 试件养护和测试过程
Fig.1 Curing and test process of specimens
在养护龄期为1、3、7、14、28、56 d时采用烘至绝干增重法对试件碳化养护过程中的CO2吸收量进行测

图2 碳化养护中试件质量增加率
Fig.2 Mass increasing ratio of specimens in carbonation curing
(1) |
式中:M为试件质量增加率;t为碳化时间;a、b和c均为系数.
从
Size | A4 | A5 | A6 | B4 | B5 | B6 | C3 | C4 | C5 |
---|---|---|---|---|---|---|---|---|---|
S | 2.459 | 3.197 | 2.536 | 4.217 | 3.886 | 4.162 | 7.945 | 9.023 | 10.957 |
M | 3.462 | 4.752 | 4.598 | 4.643 | 5.387 | 7.232 | 9.881 | 13.126 | 15.269 |
L | 3.591 | 5.176 | 5.479 | 5.247 | 5.680 | 6.029 | 9.461 | 13.422 | 12.532 |
对比分析
为了更好地利用交流阻抗谱表征水泥基材料在早龄期碳化养护过程中的一系列物理化学变化,需要结合测试方法,选取适合的等效电路,从而拟合得到阻抗谱参数,获取细微观结构和可溶性盐含量等信息.本文采用等效电路模型Rs(CRct1(QRct2)

图3 等效电路模型
Fig.3 Equivalent circuit model

图4 A组试件的阻抗谱
Fig.4 AC impedance spectrum of specimens in group A
对比分析养护龄期28 d时不同水灰比的砂浆试件阻抗谱曲线(见

图5 B组试件的阻抗谱
Fig.5 AC impedance spectrum of specimens in group B
由
C组试件的阻抗谱如

图6 C组试件的阻抗谱
Fig.6 AC impedance spectrum of specimens in group C
根据等效电路分析,水泥基材料的阻抗谱参数Rct2与水泥相孔隙结构的连通度和可溶性盐含量关系最密切.早龄期碳化养护会造成水泥基材料中可溶性盐转变为难溶物,沉积在微孔隙中,从而改变孔隙结构和可溶性盐含量,因此建立Rct2与碳化影响因素之间的关系,有助于进一步认识碳化对水泥基材料微观结构的影响.利用所选等效电路对试件碳化养护28 d的阻抗谱曲线进行拟合,得到参数Rct2,建立其与早龄期碳化养护试件水灰比、砂灰比和试件尺寸之间的关系,如
Size | A4 | A5 | A6 | B4 | B5 | B6 | C3 | C4 | C5 |
---|---|---|---|---|---|---|---|---|---|
S | 2 469 | 7 340 | 6 604 | 8 360 | 10 050 | 6 809 | 3 160 | 3 197 | 3 606 |
M | 2 827 | 8 240 | 8 556 | 12 550 | 11 820 | 8 710 | 3 882 | 3 352 | 3 826 |
L | 3 341 | 8 269 | 8 732 | 15 297 | 13 008 | 8 462 | 3 667 | 3 454 | 3 906 |
由
随砂含量降低,各组试件电荷传递电阻Rct2呈现先增大后减小的规律.对于C组净浆试件,由于水泥含量丰富,可溶性盐含量较高,故电阻较低;对于砂灰比2∶1的B组试件,其电阻较高,这可能是因为砂粒和水泥颗粒的级配良好,固化后试件微孔隙减少,加上含量略高的水泥发生碳化反应进一步填充微孔隙,导致B组试件电阻最大;A组试件中砂含量进一步提升,使得水泥不能良好地填充砂粒之间的孔隙,导致连通孔隙丰富,电阻因而下降.
随着试件尺寸的增加,A、B组砂浆试件电荷传递电阻Rct2呈现增大趋势,而C组净浆试件表现出相反规律.一般而言,试件尺寸越大,意味着有更多的路径可以传递电荷,但是传递路径也随尺寸增大而变长.二者平衡后呈现出更大的电荷传递电阻Rct2,这说明对于砂浆试件,传递路径的曲折延长具有更显著的影响,这也间接说明早龄期碳化养护有助于形成更加致密的微观结构.
根据Fick第二定律,碳化渗透速率与成正比例关
(2) |
式中:和y为拟合参数.
假定合适y值,以作为未知数,按照正比例函数进行拟合,以所得相关系数

图7 与碳化质量增加率之间关系规律
Fig.7 Relationship between and carbonation mass increase ratio
综上可知,可以建立试件质量增加率与阻抗谱参数之间的关系模型,采用交流阻抗方法测得的电荷传递电阻来预测碳化养护水泥基材料的固碳量(可由质量增加率转换得到),从而实现对混凝土碳化养护的检验与预测.由
(1)早龄期碳化养护的水泥基材料质量增加率随水灰比增大而增大;随砂灰比降低而增大,净浆试件的质量增加率均值达到11.29%,为砂浆试件质量增加率的2~3倍;试件尺寸越大,质量增加率越大.
(2)水泥基材料在碳化养护过程中电化学阻抗谱呈规律性变化,高频区圆弧直径呈增大趋势,对应电荷传递电阻Rct2增大;低频区直线斜率呈减小趋势,对应更致密的微观孔隙结构.
(3)水泥基材料随砂灰比降低,电荷传递电阻Rct2呈现先增大后减小的趋势.这说明不同砂灰比下试件形成不同致密程度的微结构,即使高含量水泥可提供更多带电粒子,其对水泥基材料碳化养护的影响也不如内部结构的影响显著.
(4)早龄期碳化养护下水泥基材料的交流阻抗谱研究有助于建立混凝土碳化的无损监测技术.
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