Strengthening mechanism of lightweight cellular concrete filled with fly ash.

• Lightweight cellular concrete (LCC) filled with fly ash (LCCFFA) is developed. • Strengthening mechanism of LCCFFA is revealed from microstructure point of view. • 25% content of fly ash is recommended with the Si/Ca ratio as a judging criterion. Fly ash has been used to fill the lightweight cellular concrete (LCC) as the primary admixture to promote comprehensive engineering applications. However, the strengthening mechanism of LCC resulted from fly ash is not well understood. In this paper, the strengthening mechanism of LCC filled with fly ash (LCCFFA) was investigated with respect to the microstructures including porosity and skeleton characteristics. The variation rules of average pore size (PS), pore roundness value (PRV), and fractal dimension of pore distribution (FDPD) with different fly ash contents were explained. The relationship between microscopic parameters of PS, PRV, and FDPD and the overall compressive strength, along with the wet bulk density was established. Furthermore, the difference of skeleton structure evolution mechanism between LCC and LCCFFA was investigated with the results showing that the difference comes from the effects of ball and particle filling of fly ash when before 28 days. While after 28 days, the pozzolanic reaction of fly ash and hydration reaction of cement mutually promote the densification of skeleton structure. Finally, the SEM energy spectrum diagram was analyzed to further reveal the strengthening mechanism. From the perspective of characteristic pore parameters and skeleton hydration products, the recommended content of fly ash is approximately 25%. The ratio between Si and Ca (Si/Ca) can determine the degree of reaction of fly ash, and can therefore be used as a judging criterion to determine whether the content of fly ash is appropriate. [ABSTRACT FROM AUTHOR]