氧化镁膨胀剂对混凝土长期体积变化的影响

Effects of Reactivity and Dosage of Magnesium Oxide Expansive Agents on Long-Term Volume Variation of Concrete

观察了外掺不同活性、不同剂量氧化镁膨胀剂混凝土的限制膨胀率曲线,并结合混凝土强度发展和绝热温升特性,补偿收缩胶砂试件的限制及自由膨胀率,分析了不同活性氧化镁膨胀剂的补偿收缩特性。结果表明：适量掺加氧化镁膨胀剂对于补偿收缩混凝土的强度发展基本没有影响,但导致混凝土的温升值较大,温升开始时间延迟。在氧化镁膨胀剂的常用掺量为6%~8%时,导致混凝土的绝热温升增加值约为3~4℃。氧化镁膨胀剂的活性越低,用其制备的补偿收缩混凝土的早期膨胀率越小,但最终膨胀量越大,膨胀稳定期出现越晚,采用＂S＂型膨胀剂时直到140 d膨胀仍未停止,因而容易造成混凝土结构的后期安定性不良。实际应用时应尽量选用高活性、大掺量的氧化镁膨胀剂,掺量宜控制在6%~8%。

The restricted expansion rate of shrinkage-compensating concrete containing different amounts of magnesium oxide expansive agents（MEA） with different reactivities was investigated. The compressive strength and adiabatic temperature rise of concrete were measured. The restricted and free expansion rates of shrinkage-compensating mortar were also analyzed. The shrinkage-compensating characteristics of MEA with different reactivities were discussed. The results show that a suitable dosage of MEA does not affect the strength development of concrete but increases the temperature of concrete and delays its beginning time. The temperature increases by 3–4 ℃when the dosage of Mg O increases by 6%–8%. The lower the reactivity of MEA, the smaller the early age expansive rate of concrete, but the larger final expansive rate and the later the arrival of expansion stabilizing period will be. The expansion of concrete containing ＂S-type＂ Mg O dosen’t stop until 140 d, therefore, it is easy to result in a poor soundness of concrete structure in long-term. It could be better to prepare shrinkage-compensating concrete with higher dosages（i.e., 6%–8%） of MEA possessing a great reactivity.