摘要
以Ca含量和细度不同的粉煤灰制备碱激发粉煤灰(AAFA)胶凝材料,并在水化早期对其进行微波加热养护,研究其抗压强度和微观结构的发展规律,探讨微波加热养护对强度发展的影响机理.结果表明:增加粉煤灰的Ca含量及适当减小粉煤灰的细度均可显著提高微波加热养护后AAFA试件的抗压强度,但会加剧其28 d龄期后的强度倒缩;水化硅(铝)酸钙(C‑(A)‑S‑H)凝胶中的C
碱激发粉煤灰(AAFA)胶凝材料具有优异的抗化学侵
选用不同电厂提供的粒径分布相近的2种粉煤灰FA(D50=12.19 μm)和FB(D50=12.87 μm),其化学组
Type | SiO2 | Al2O3 | CaO | Fe2O3 | K2O | TiO2 | SO3 | MgO | Na2O |
---|---|---|---|---|---|---|---|---|---|
FA | 44.83 | 32.05 | 4.98 | 3.66 | 1.28 | 1.16 | 0.99 | 0.76 | 0.60 |
FB | 42.24 | 29.32 | 10.22 | 4.33 | 1.13 | 0.95 | 1.02 | 1.22 | 1.81 |

图1 粉煤灰的矿物组成
Fig.1 Mineral compositions of fly ashes

图2 粉煤灰的粒径分布
Fig.2 Particle size distributions of fly ashes
采用质量分数大于96%的颗粒状NaOH作为碱激发剂.拌和水为去离子水.
AAFA胶凝材料的配合比如
Specimen | FA | MFA | UFA | FB | NaOH | Water |
---|---|---|---|---|---|---|
AFA | 100 | 15 | 21 | |||
AMFA | 100 | 15 | 21 | |||
AUFA | 100 | 15 | 21 | |||
AFB | 100 | 15 | 21 |
将AAFA新拌浆体注入25 mm×25 mm×25 mm的聚醚醚酮(PEEK)模具中成型,然后将试件分为2组,分别进行微波加热养护(M)与蒸汽加热养护(S).第1组试件放入Moilelab微波材料学工作站中,采用

图3 AAFA试件热养护用多级微波加热制度
Fig.3 Multistage microwave‑heated mode adopted for curing of AAFA specimens
采用YAW‑300型全自动压力试验机以1.5 kN/s的加荷速度对微波加热养护结束(M‑over)与蒸汽加热养护结束(S‑over)的AAFA试件及继续在标准养护条件下养护至7、28、90 d的试件(如M‑90 d、S‑90 d)进行抗压强度测试.
从养护至预定龄期的AAFA试件的中心部分取块状样品后用酒精终止水化备用.一部分块状样品研磨至颗粒细度小于80 μm的粉体样品,分别采用D/max‑2200型X射线衍射仪(XRD)和Nicolet IS5型傅里叶变换红外光谱仪(FTIR)分析其矿物组成和结构;XRD测试采用Cu靶Kα射线,管电压40 kV,管电流40 mA,扫描速率10(°)/min,扫描范围5~80°;FTIR测试采用400~4 000 c

图4 粉煤灰性质对微波加热养护AAFA试件强度发展的影响
Fig.4 Effect of fly ash properties on strength development of microwave‑heated AAFA specimens
由
由
微波加热养护AAFA试件在标准养护90 d后的抗压强度倒缩现象非常显著,这在实际应用中会带来较大的安全隐患.

图5 AAFA试件经加热养护后与继续标准养护90 d后的抗压强度
Fig.5 Compressives strengths of AAFA specimens after heatedcuring and continued standard curing for 90 d
综上所述,微波加热养护的AAFA试件随养护时间延长普遍出现强度倒缩现象,然而相比于蒸汽加热养护,AAFA试件的后期抗压强度仍具由优势.粉煤灰的Ca含量和颗粒细度显著影响AAFA试件的强度发展,Ca含量的增加与细度的适当减小均可显著提高AAFA试件微波加热养护结束时的抗压强度,但会使继续标准养护时强度出现倒缩的时间提前、增大强度倒缩值;采用超细粉煤灰会大幅降低AAFA试件的抗压强度,即利用超细粉煤灰制备的AAFA不适合采用微波加热养护.

图6 AAFA试件经微波加热养护后与继续标准养护90 d后的XRD图谱
Fig.6 XRD patterns of AAFA specimens after microwave‑heated curing and continued standard curing for 90 d

图7 AAFA试件经微波加热养护后与继续标准养护90 d后的FTIR图谱
Fig.7 FTIR spectra of AAFA specimens after microwave‑heated curing and continued standard curing for 90 d

图8 AAFA试件经微波加热养护后与继续标准养护90 d后FTIR图谱中Si—O—T键的峰位
Fig.8 Peak positions of Si—O—T bond in FTIR spectra of AAFA specimens after microwave‑heated curing and continued standard curing for 90 d

图9 AAFA试件经微波加热养护后与继续标准养护90 d后的SEM图片
Fig.9 SEM images of AAFA specimens after microwave‑heated curing and continued standard curing for 90 d
Specimen | Curing condition | Porosity(by volume)/% | Increase ratio of porosity/% | Median pore diameter/nm | Increase ratio of median pore diameter/% |
---|---|---|---|---|---|
AFA | M‑over | 24.01 | 0 | 426.25 | 0 |
M‑90 d | 28.64 | 19 | 465.16 | 9 | |
AMFA | M‑over | 25.36 | 0 | 369.26 | 0 |
M‑90 d | 26.31 | 4 | 414.39 | 12 | |
AUFA | M‑over | 41.78 | 0 | 221.54 | 0 |
M‑90 d | 38.43 | -8 | 178.10 | -20 | |
AFB | M‑over | 27.11 | 0 | 16.78 | 0 |
M‑90 d | 29.50 | 9 | 32.03 | 91 |

图10 AAFA试件经微波加热养护后与继续标准养护90 d后的孔体积分布
Fig.10 Pore volume distributions of AAFA specimens after microwave‑heated curing and continued standard curing for 90 d
(1)粉煤灰中Ca含量的增加可显著提高AAFA试件经微波加热养护后的抗压强度并使其达到峰值,水化硅(铝)酸钙(C‑(A)‑S‑H)凝胶中的C
(2)适当减小粉煤灰的细度,可显著提高经微波加热养护后AAFA试件的抗压强度并使其达到峰值,水化硅铝酸钠(N‑A‑S‑H)凝胶分解与凝胶产物体积缩小导致AAFA试件结构致密性降低,使其在继续标准养护90 d后的抗压强度有所降低,但仍可与高Ca含量AAFA试件的抗压强度相当,即适当减小粉煤灰的细度,可弥补AAFA中Ca含量不足导致的强度发展缓慢.
(3)粉煤灰过细时,AAFA试件的抗压强度大幅降低,但继续标准养护90 d后,由于AAFA聚合反应的持续进行,强度不会产生倒缩.
(4)相比于传统蒸汽加热养护,微波加热养护可显著提高AAFA试件的后期抗压强度.
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