Synergistic effects of fly ash and MgO expansive additive on cement paste: Microstructure and performance

To expand the use proportion of fly ash (FA) and MgO expansive additive (MEA) in engineering, this paper explores the feasibility of a new cementitious material, which conforms to the dual-carbon background. The synergistic effects of FA and MEA on the expansion behavior and mechanical properties of cement paste are confirmed by designing experiments with the dosage and curing temperature as variables. Moreover, the formation mechanism of synergism is analyzed through isothermal calorimetry, XRD, TG, 29 Si NMR, MIP, SEM and EDS in combination. The experimental results show under 40 °C water curing, the addition of 40 % FA and 10 % MEA not only amplifies the early expansion, but also controls well the later expansion in a stable state. Besides, the 90-day compressive and flexural strengths increase by up to 20.8 % and 15.6 %, respectively. FA reduces the effect of MEA on the hydration heat of cement significantly, making the hydration rate curves of the two systems about Krstulovic-Dabic model highly consistent. The MEA-enhanced alkaline environment promotes the secondary hydration of cement by improving the hydration degree of FA. The reduction of cumulative pores volume and refinement of pore size are promoted obviously, resulting in only the gel pores and transition pores in the matrix. The existence of FA avoids the centralized distribution of Mg-containing hydration products, while there is more fibrous C-S-H generated on the surface of FA. In addition, the hydration model of the new cementitious material has been established.