利用MgO膨胀剂的延迟微膨胀效应能显著提高大体积混凝土的力学性能，产生的预压应力还能有效提高大体积混凝土抵抗温度开裂的能力，但目前对其微观机理研究不多。采用宏观与微观相结合的方式，研究了不同活性指数、不同掺量的MgO混凝土的力学性能，利用SEM/EDS微观分析手段揭示了MgO对混凝土性能影响的作用机理。试验结果表明：MgO取代部分水泥掺入混凝土中，混凝土的力学性能降低；掺量为4%~5%时，活性指数为100s的MgO混凝土的膨胀率大于活性指数为50 s的MgO混凝土；MgO混凝土的膨胀特性与Ca，Si，Mg，Al等元素的分布情况有关，掺入的MgO将改变界面区Ca，Si，Mg元素的富集特性，Ca元素和Mg元素更易于在界面区富集，导致混凝土宏观力学性能的降低。

MgO expansion agent, with its delayed micro-expansion properties, could enhance the mechanical properties and deformation performance of mass concrete. The pre-compressive stress could effectively improve its thermal crack resistance, yet researches on its micro mechanism are inadequate. The mechanical properties of concrete with different MgO dosages and different activity indexes are researched by macro and micro approaches, and the mechanism of MgO influencing on the concrete performance is researched by using SEM/EDS. Results indicated that MgO replacing some amount of cement in the concrete would reduce the concrete's strength; when the MgO dosage was between 4% and 5%, concrete with activity index of 100s exhibited larger expansion than that with activity index of 50s. The expansion of MgO-mixed concrete was specifically dependent on the uniformity degree of the distribution of element Ca, Si, Mg and Al. By adding MgO in the concrete, enrichment of elements Ca, Si, Mg and Al in the vicinity of interfacial transition zone was altered whereas elements Ca and Mg were intended to accumulate and hence decreased the strength of concrete.