摘要
通过添加不同活性MgO,研究了其对碱-矿渣-偏高岭土基地聚物(ASM地聚物)干缩性能的影响,并利用X射线衍射仪(XRD)、扫描电镜(SEM)和压汞仪(MIP)等微观测试手段揭示了其内在影响机制.结果表明:随着MgO活性的增加,ASM地聚物的干缩率先减小后增大,当MgO活性适中时,ASM地聚物干缩率降低约15.0%;所生成的镁铝水滑石及水化硅酸镁(M‑S‑H)凝胶能够有效填充ASM地聚物孔隙,减小干燥收缩,增加结构致密程度.
关键词
与普通硅酸盐水泥(OPC)相比,碱-矿渣-偏高岭土基地聚物(ASM地聚物)具有更低的CO2排放量,且力学性能和耐久性能更
由于ASM地聚物中没有OPC中常用的如钙矾石类膨胀剂生长所需的Ca(OH)2环境,因此适用于OPC的膨胀剂对ASM地聚物的干缩性能没有明显抑制作
鉴于此,本文以菱镁矿为原料,将其高温煅烧得到不同活性MgO,并掺入ASM地聚物体系中,以研究活性MgO对ASM地聚物干燥收缩的作用机制;同时利用X射线衍射仪(XRD)、扫描电镜(SEM)和压汞仪(MIP)等微观测试手段,揭示MgO膨胀剂对ASM地聚物干缩特性的作用机制,以期为ASM地聚物在土木工程领域的应用提供理论依据.
偏高岭土(MK)产自山西大同,由煤系高岭土在600~700 ℃条件下煅烧24 h后得到,采用滴定络合法并依据文献[
MK和GGBFS的化学组成及特性见
Raw material | Chemical composition(by mass)/% | Diameter/µm | Specific surface area/( | ||||
---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | CaO | Na2O | MgO | |||
MK | 50.07 | 48.23 | 0.24 | 0.06 | 0.22 | <50 | 14 |
GGBFS | 35.00 | 15.38 | 38.19 | 0.34 | 7.06 | <10 | 15 |
为得到不同模数(Ma)的碱激发剂,先通过
(1) |
式中:M为水玻璃溶液的出厂模数,本文取为2.8.
首先采用差示扫描同步热分析仪(DSC‑TG)明确菱镁矿的合理煅烧温度范围;然后在此范围内制备不同活性MgO粉末;最后进行MgO活性分析,并利用XRD表征.目前,测定MgO活性的主要方法有物相分析法、比表面积法和柠檬酸法等.本文按照YB/T 4019—2020《轻烧氧化镁化学活性测定方法》,采用柠檬酸法测定MgO的活性:先称取(2.00±0.05) g MgO粉末,放入200 mL浓度为0.07 mol/L的柠檬酸溶液中;再滴入2滴酚酞指示剂;接着将盛有溶液的烧杯放在磁力搅拌器上,用秒表计时;当溶液呈红色时,即停止计时,采用该时间来表征MgO的活性.
依照JC/T 313—2009《膨胀水泥膨胀率试验方法》,使用比长仪测量ASM地聚物的干燥收缩率.首先将不同活性MgO、MK、GGBFS和碱激发剂混合并搅拌3 min,注入尺寸为40 mm×40 mm×160 mm的标准三联试模中;然后置于标准养护箱((20±2) ℃、相对湿度95%)中养护30 min;最后测量试件在不同养护龄期(n)时的长度(Ln,mm).每次试验分为3组,取其平均值作为ASM地聚物的干燥收缩率(L,%),其计算表达式为:
(2) |
式中:L0为ASM地聚物试件的初始长度,本文取为160 mm.
菱镁矿的热分析曲线见

图1 菱镁矿的热分析曲线
Fig.1 Thermal analysis curves of magnesite

图2 不同煅烧条件下MgO的活性
Fig.2 Activity of MgO under different calcination conditions

图3 不同煅烧条件下菱镁矿及活性MgO的XRD图谱
Fig.3 XRD patterns of magnesite and activated MgO under different calcination conditions
不同碱激发剂模数条件下ASM地聚物与OPC的干缩率随养护龄期的变化如

图4 不同碱激发剂模数条件下ASM地聚物和OPC的干缩率随养护龄期的变化
Fig.4 Drying shrinkage change of ASM geopolymers and OPC with curing age under different moduli of alkali activators
Collins

图5 ASM地聚物和OPC的孔径分布曲线
Fig.5 Pore size distribution curves of ASM geopolymers and OPC
当碱激发剂模数为1.4时,ASM地聚物的干缩率最大,可达0.4%(见
将不同活性MgO以相同比例掺入ASM地聚物中,测定其干缩率,结果见

图6 不同活性MgO对ASM地聚物干缩率的影响
Fig.6 Effect of different activated MgOs on drying shrinkage of ASM geopolymers
综上所述,活性MgO对ASM地聚物干燥收缩性能的抑制效果如下:当养护龄期小于7 d时,MgO活性越高,ASM地聚物干缩率越小;当养护龄期大于7 d时,MgO活性越高,ASM地聚物干缩率先减小后增大.

图7 掺入不同活性MgO的ASM地聚物的XRD图谱
Fig.7 XRD patterns of ASM geopolymers with different activated MgOs

图8 掺入不同活性MgO的ASM地聚物的孔径分布和累积孔隙率
Fig.8 Pore size distribution and cumulative porosity of ASM geopolymers added activated MgOs
Type of pore | d/µm | Cumulative porosity(by volume)/% | |||||||
---|---|---|---|---|---|---|---|---|---|
Blank | 600 ℃,1 h | 700 ℃,1 h | 800 ℃,1 h | 900 ℃,1 h | 600 ℃,2 h | 600℃,3 h | 600 ℃,4 h | ||
Mesopore | <1.25 | 2.5 | 1.2 | 1.2 | 2.2 | 1.8 | 1.1 | 1.1 | 1.2 |
1.25-25.00 | 27.1 | 20.6 | 22.3 | 26.0 | 29.5 | 19.3 | 19.4 | 24.1 | |
Macropore | >25.00 | 70.4 | 78.2 | 76.5 | 71.8 | 68.7 | 79.6 | 79.5 | 74.6 |
由

图9 掺入不同活性MgO的ASM地聚物的SEM照片
Fig.9 SEM images of ASM geopolymers added different activated MgOs
Element | Point A | Point B | Point C | Point D |
---|---|---|---|---|
O | 49.15 | 44.83 | 46.06 | 49.36 |
Na | 3.83 | 3.36 | 6.69 | |
Al | 16.39 | 25.73 | 18.43 | 12.68 |
Si | 24.30 | 29.44 | 27.26 | 22.77 |
Ca | 6.33 | 3.26 | 4.69 | |
Mg | 1.63 | 3.81 |
(1)在有效煅烧温度范围内,菱镁矿的煅烧温度越低、煅烧时间越长,所获得的MgO活性越高.
(2)当MgO活性过低时,在碱-矿渣-偏高岭土基地聚物(ASM地聚物)中生成的镁铝水滑石和水化硅酸镁(M‑S‑H)凝胶较少,以至于无法填充原有孔隙结构,从而导致ASM地聚物的干缩率较大;当MgO活性过高时,所生成的镁铝水滑石和M‑S‑H凝胶较多,M‑S‑H凝胶能够形成大量微孔结构,ASM地聚物的干缩率亦会增大.
(3)当菱镁矿的煅烧温度为600 ℃且煅烧时间为2 h时,MgO活性适中,所生成的镁铝水滑石和M‑S‑H凝胶能够有效填充原始微孔,从而使得ASM地聚物结构更加致密,其干缩率较不含活性MgO的空白试件降低约15.0%.
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