Effects of fly ash on MgO-based shrinkage-compensating cement: Microstructure and properties.

• Proper application of MEA requires coordinating the relationship between expansion stress and the resistance response of the cement matrix. • Fly ash stabilizes the positive effect of MEA on increasing microstructure compactness. • The reduction in alkalinity results in a more uniform distribution of Mg(OH) 2 without concentrated expansion. The application of MgO expansive additive (MEA) in cement-based materials is one of the effective measures to compensate for shrinkage. However, related to the delayed excessive expansion, the incorporation of MEA has a potential risk of crack formation and decline of mechanical properties. To address the latter issue, an innovative approach using a large amount of fly ash with pozzolanic activity to coordinate the expansion stress of MEA and the resistance response of cement matrix is proposed in this paper. The effects of fly ash on MgO-based shrinkage-compensating cement are systematically investigated from the aspects of macroscopic properties and microstructural characteristics. The addition of 20% fly ash magnifies the effect of MEA compensation for early shrinkage and inhibits the later excessive volume expansion. In this way, the problem of mechanical properties decrease has been well resolved, and compared with 28 days, the compressive and flexural strengths with 20% fly ash at 90 days are increased by 23.3% and 18.4%, respectively. XRD and TG/DSC confirm fly ash slows down the early hydration process of MgO-based shrinkage-compensating cement and forms a stronger cement matrix through secondary reaction to offset the later reduced delayed expansion of MEA. NAD demonstrates fly ash stabilizes the positive effect of MEA on increasing microstructure compactness through expansion. SEM and EDS illustrate fly ash improves distribution of Mg(OH) 2 , avoiding the deterioration of microstructure caused by local excessive expansion. [ABSTRACT FROM AUTHOR]