Hydration characteristic and strengthening mechanism of municipal solid waste incineration fly ash cementitious composites modified by nano SiO2

As a by-product from the incineration of municipal solid waste (MSW), the incineration fly ash (MSWIFA) possesses potential hydration reaction activity and could be as an alternative supplementary cementitious material (SCM). In this study, 85 wt% cement and 15 wt% MSWIFA were blended to prepare the MSWIFA cementbased composites. Nano SiO2 (NS) was added at varying dosages to strengthen the performance of the composite materials. The fresh and mechanical properties of the nano SiO2-modified MSWIFA cement-based composite (NMCC) were examined to determine the optimal NS dosage. Moreover, the hydration behavior, phase composition, pore structure, and microstructure morphology of the NMCC were examined to dissect the hydration characteristic of NMCC and the strengthening mechanism of NS. The results showed that NS significantly accelerated the hydration process of the NMCC, reflected by a higher hydration degree, the formation of more C(A)-S-H gels formed, and superior mechanical properties. MSWIFA bestowed the NMCC with a high alkali content, which was beneficial to accelerating the hydration process. However, a low silicon content in MSWIFA failed to provide sufficient [SiO4]4- to react with Ca(OH)2 to produce C-(A)-S-H gels. This problem was effectively addressed by the addition of an optimal dosage of NS, which was rich in silicon element, so as to compensate for the decline in MSWIFA performance caused by insufficient Si elements in the composite. The NMCC sample, which consisted of 15 wt% MSWIFA and 1.0 wt% NS, exhibited the highest percentage (53 %) of harmless pores after 28 days. Additionally, its compressive strength reached 96.78 % (42.9 MPa) of the pure cement control indicating superior mechanical properties a compact microstructure. The utilization of NS as a viable material holds great potential in augmenting the application of MSWIFA as a SCM within the construction and building industry.