摘要
通过试验结合分子动力学,模拟研究了Al掺杂影响水化硅铝酸钙(C‑A‑S‑H)的机理.结果表明:Al掺杂对于C‑A‑S‑H凝胶中硅铝氧链链长的影响可等效为同摩尔量Si的影响,但会使C‑A‑S‑H凝胶层间距增加的幅度更大;提高养护温度促进了C‑A‑S‑H凝胶中硅氧四面体的聚合,当养护温度达到80 ℃时,C‑A‑S‑H凝胶的层间距减小,相邻主层上硅铝氧链之间形成了交联结构,部分转变为交联C‑A‑S‑H凝胶,其力学性能大幅度增加;Al掺杂桥接了断裂的硅氧链且形成了硅铝氧长链,提升了C‑A‑S‑H凝胶沿y轴方向的力学性能,对其沿x轴和z轴方向力学性能的影响较小.
随着粉煤灰、矿渣等含活性Al的工业废渣在混凝土中的广泛应用,材料体系中活性Al的含量明显增加,更易形成水化硅铝酸钙(C‑A‑S‑H)凝胶.因此,对于C‑A‑S‑H凝胶微结构和性能的研究变得愈发重要.诸多学者研究了Al掺杂对C‑A‑S‑H凝胶结构的影
C‑A‑S‑H凝胶具有多尺度的复杂结构,其力学性能的表征方法也多种多
本文拟通过合成C‑A‑S‑H凝胶,研究Al掺杂C‑A‑S‑H凝胶结构的演变规律,随后采用分子动力学模拟方法来建立C‑A‑S‑H凝胶的分子动力学模型,并研究Al掺杂对其力学性能的影响.
采用活性反应法合成C‑A‑S‑H凝胶.CaO由国药集团分析纯CaCO3制备.SiO2为美国Sigma Aldrich生产的气相纳米SiO2.
C3A矿物由CaO与Al2O3高温烧成.首先,将CaO与Al2O3按摩尔比3∶1混合压片;然后,将压片在高温炉内经3 h由室温升至1 400 ℃(升温速率约为7.6 ℃/min)并保温3 h;随后,将样品取出并在空气中急冷.将冷却样品粉磨后重新按上述制度煅烧,反复煅烧3次后即制得C3A粉末样品,化学滴定法测试表明其中f‑CaO含量(质量分数,文中涉及的含量、水固比等除特别说明外均为质量分数或质量比)为0.73%.
C‑A‑S‑H凝胶合成步骤为:根据目标产物的化学组成确定原材料(CaO、SiO2和C3A)比例,按照水固比50∶1将原材料与去离子水(电导率小于0.01 μS/cm)混合好后置入锥形瓶中,在指定温度的密闭条件下通过磁力搅拌器搅拌,使得原材料充分反应,反应龄期达30 d时,将混合液过滤并在40 ℃下真空干燥,防止碳化.将干燥得到的样品磨为粒径小于75 μm的粉末,保存在真空干燥皿中用于测试.
C‑A‑S‑H凝胶分子模型的建立:首先,以1.1 nm的Tobermorite模
相分析采用日本理学的RU‑200B/D/MAX‑RB型XRD,在100 mA、40 kV条件下测试,采用Cu(Kɑ)射线,步进扫描,步长为0.02°,角度范围为5°~40°.
核磁分析采用Bruker AVANCE III 400 MHz型固体核磁共振波谱仪(MAS NMR),磁场强度为9.4 T
分子动力学模拟基于ReaxFF力
采用模拟单轴拉伸来进行力学性能测试,应变率为0.08 p
为研究Al掺杂量对C‑A‑S‑H凝胶结构的影响,在20 ℃条件下合成了n(Al)/n(Si)=0、0.05、0.10、0.15、0.20的C‑A‑S‑H凝胶.

图1 不同n(Al)/n(Si)值C‑A‑S‑H凝胶的XRD图谱和层间距
Fig.1 XRD patterns and basal spacing of C‑A‑S‑H gel with different n(Al)/n(Si) values
通过XRD图谱中(002)晶面的衍射
(1)随着n(Ca)/n(Si)的增加,C‑S‑H凝胶的层间距趋于减小.这是由于高n(Ca)/n(Si) C‑S‑H凝胶的硅氧链存在桥位置缺陷,三元重复硅氧链结构在z轴方向的尺寸减小,导致层间距减
(2)当n(Ca)/n(Al+Si)相同时,C‑A‑S‑H凝胶的层间距随着n(Al)/n(Si)的增加而增加.一般来说,n(Ca)/n(Si)降低会导致硅氧链上桥硅氧四面体的数量增加,C‑S‑H凝胶的层间距增加.Al掺杂进入硅氧链的桥位置缺陷,也导致C‑A‑S‑H凝胶的层间距增加.
(3)n(Al)/n(Si)增加导致C‑A‑S‑H凝胶的层间距增加,说明等量Al掺入C‑S‑H结构中,对层间距的影响比Si更大.这是因为与硅氧链桥位置上的Si相比,Al[4](四配位Al原子)进入硅氧链桥位置会引入额外的负电荷,导致主层间斥力增强和层间平衡电荷阳离子数量增

图2 C‑A‑S‑H凝胶
Fig.2
C‑A‑S‑H凝胶的Si原子所处的化学环境可以用
(1) |
(2) |
基于试验数据建立了C‑A‑S‑H凝胶的分子模型,保证层间距与硅氧链聚合度与试验数据一致,结果如

图3 C‑A‑S‑H凝胶的分子结构
Fig.3 Molecular structure of C‑A‑S‑H gel
不同n(Al)/n(Si)值C‑A‑S‑H 凝胶沿x、y 和z 轴方向的杨氏模量和抗拉强度如

图4 不同n(Al)/n(Si)值的C‑A‑S‑H凝胶沿x、y和z轴方向的杨氏模量和抗拉强度
Fig.4 Young's modulus and tensile strength of C‑A‑S‑H gel with different n(Al)/n(Si) values along x‑, y‑ and z‑axis
(1)随着n(Al)/n(Si)从0提高至0.20,C‑A‑S‑H凝胶沿y轴方向的杨氏模量从71.42 GPa提高到了94.31 GPa,抗拉强度从7.71 GPa提高到12.66 GPa,力学性能提升明显.这是由于Al掺杂促进硅氧链短链转变为硅铝氧链长链,以共价键增强了材料原本沿y轴方向的离子键连接,提高了C‑A‑S‑H凝胶的力学性能.
(2)与y轴相比,Al掺杂对C‑A‑S‑H凝胶沿x轴方向的力学性能影响较小,这是由于x轴方向主要为Si—O—Ca离子键结合,形成的硅铝氧链长链结构并不能增加材料沿x轴方向承受荷载的能力.
(3)Al掺杂对C‑A‑S‑H凝胶沿z轴方向力学性能的影响也较小.这是由于Al掺杂一方面加强了主层间电荷的吸附,增强主层间作用,另一方面Al掺杂导致C‑A‑S‑H凝胶的层间距扩大,层间水含量增加,层间水屏蔽主层间的作用增强,弱化了主层间的结合.
为研究合成温度对C‑A‑S‑H凝胶微纳米结构的影响,分别在20、50、80 ℃下合成了n(Al)/n(Si)为0和0.10的C‑S‑H和C‑A‑S‑H凝胶并进行了XRD和NMR测试,结果如

图5 不同合成温度的C‑S‑H和C‑A‑S‑H凝胶的XRD图谱和层间距
Fig.5 XRD patterns and basal spacing of C‑S‑H and C‑A‑S‑H gel with different synthesized temperatures
由
由

图6 不同合成温度下C‑S‑H和C‑A‑S‑H凝胶
Fig.6
以试验数据为基础,建立了50、80 ℃的C‑A‑S‑H凝胶模型.

图7 不同养护温度的C‑A‑S‑H凝胶沿x、y和z轴方向的杨氏模量和抗拉强度
Fig.7 Young's modulus and tensile strength along x,y and z axis for C‑A‑S‑H gel at different synthesis temperatures
(1)Al掺杂进入C‑A‑S‑H凝胶结构中时,Al[4]可以占据硅氧链桥位置处的缺陷,桥接断裂的硅氧链,其对于硅铝氧链分子链长的影响可以等效为同摩尔量Si的影响.但是Al掺杂增加了主层的电负性,导致层间距增加.
(2)养护温度达到80 ℃时,C‑A‑S‑H凝胶的结晶程度增加,层间距减小,相邻主层上的硅铝氧链之间形成了交联结构,部分转变为交联C‑A‑S‑H凝胶.
(3)Al掺杂有助于提升C‑A‑S‑H凝胶沿y轴方向的力学性能,对沿x轴方向力学性能的影响较小.无交联结构形成时,Al掺杂对C‑A‑S‑H凝胶沿z轴方向的增强效果一般.高温促进Al引导形成交联后,C‑A‑S‑H凝胶的层间结合增强,力学性能大幅度增加.
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