摘要
为研究加速碳化对白色硅酸盐水泥石强度、碳化程度和微观结构的影响,采用热重分析(TG)定量表征水泥石中的Ca(OH)2(CH)及CaCO3含量,通过扫描电镜-能谱分析(SEM‑EDS)和压汞仪(MIP)测试水泥石的微观结构.结果表明:与同龄期的自然养护试样相比,碳化养护3、14、28 d时水泥石抗压强度分别提高10.7%、7.3%和5.8%,抗折强度分别提高17.9%、16.1%和14.3%;碳化14 d时的试样继续干湿循环养护7 d仍可明显提高CH含量,碳化28 d后的碳化程度趋于稳定;碳化试样的孔隙率略高且平均孔径更低,其50~200 nm的较大孔数量明显减少,而小于20 nm的微孔数量相对更多;常压下加速碳化反应直接发生在水泥水化产物CH晶体的表面,在CH晶体棱角处的碳化程度最高,碳化产物与之共生.
碳化养护可提高水泥石的表面硬
水泥(C)为白色硅酸盐P‧W 52.5水泥,产自阿尔博波特兰(安庆)有限公司;减水剂(WJ)为聚羧酸系减水剂,产自武汉华轩高新技术有限公司;碳化养护用CO2,纯度≥99.5%(质量分数,文中的纯度、水灰比等均为质量分数或质量比),产自上海智光化工有限公司;拌和水(W)为自来水.水泥的化学组成见
由
(1)试样制备与养护
先按照水灰比0.4制备水泥净浆,然后振动成型90×30 mm的碳化圆饼试样和40 mm×40 mm×160 mm的抗压/抗折强度试样.上述试样先在自然条件((20±5) ℃、相对湿度(50±5)%)下养护1 d,脱模后再分别进行碳化养护(CO2 质量分数(20±2)%、(20±5) ℃、相对湿度(50±5)%、压力0.5 MPa)、自然养护((20±5) ℃、相对湿度(50±5)%)和干湿循环养护(DYC)至不同龄期.
干湿循环采用自制装置——在放于水槽中的玻璃罩内放置1个转轮,罩内2个上角各设置1个红外电暖器,保持罩内温度为35~38 ℃;试样固定在轮周上,鼓风机沿切向吹风使其转动,对试样进行风干;由定时器控制的微型水泵从槽中定时抽水并淋湿试样,设置每天12个干湿循环,每个循环进行120 min(淋湿15 min+干燥105 min).
(2)测试方法
碳化圆饼试样养护至规定龄期后,先在30 ℃烘箱中烘干,再置于台钻上使用16钻头对其半部采用九点法取样,取样深度2 mm,用于热重分析,取样示意图见

图1 TG‑DTG取样示意图
Fig.1 TG‑DTG sampling diagram
场发射电子扫描电镜(SEM)采用美国FEI公司产Quanta 200;热重分析采用美国TA公司产STA 650型联合热分析仪,保护气氛为N2,升温速率为20 ℃/min;日本产压汞测孔仪型号为Autopore Ⅳ 9500.上述试样均用切割机在圆饼试样中截取距离表面2 mm深的块体中得到.

图2 水泥石的抗压强度和抗折强度
Fig.2 Compressive and flexural strength of hardened cement
由

图3 各试样的TG‑DTG曲线
Fig.3 TG‑DTG curves of each sample

图4 各试样的CaCO3及CH含量
Fig.4 Contents of CaCO3 and CH of each sample
对比
对比

图5 各试样的孔径分布结果
Fig.5 Pore diameter distribution test results of each sample
对比
选取上述试样中养护龄期为14、28 d的孔隙率数据与CaCO3含量数据进行对比,见

图6 孔隙率与CaCO3含量的数据对比
Fig.6 Comparison of porosity and CaCO3 content

图7 各试样的SEM照片
Fig.7 SEM images of each sample
Khoshnazr
(1)碳化养护对各龄期的白色硅酸盐水泥石强度,尤其早期强度具有显著增强作用.碳化养护3、14、28 d时水泥石的抗压强度分别提高10.7%、7.3%和5.8%,抗折强度分别提高17.9%、16.1%和14.3%.
(2)与自然养护试样不同,加速碳化14 d的试样继续干湿循环养护7 d仍可明显提高其中的CH含量,并且碳化28 d后的碳化程度趋于稳定.
(3)与同龄期的自然养护试样相比,碳化养护14 d和28 d试样的孔隙率略低,平均孔径均降低约10%,且其50~200 nm的较大孔数量大幅减少,而小于20 nm的微孔数量相对更多.
(4)SEM‑EDS显示,加速碳化反应直接发生在水泥水化产物CH晶体的表面,在CH晶体棱角处的碳化程度最高,碳化产物与之共生.
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