Microstructural analysis of recycled brick aggregate concrete modified by silane

There are essential differences between recycled brick aggregate (RBA) and recycled concrete aggregate (RCA). The weak area of RCA is the old mortar attached to the aggregate surface, while the weak area of RBA is its own larger porosity. Silane has a small molecular structure, which can penetrate into the concrete with water as the medium and react with the active functional groups in the concrete to form a network structure of silane polymer. In this paper, 8% silane solution was used to modify recycled brick aggregate concrete (RBAC) in two schemes. The first scheme was that RBA was soaked in 8% silane solution, and the second scheme was that 8% silane solution was added in the mixing process of RBAC instead of water. The results showed that compared with the unmodified RBA, after silane modification of RBA its apparent density increased by 5.48%, water absorption decreased by 61.12%, crushing index decreased by 11.09%. The average compressive strength of RBAC with RBA modified by silane was 11.72% higher than that of unmodified RBAC, and the average compressive strength of RBAC mixed with silane was 9.12% lower than that of unmodified RBAC. The pores and cracks on the surface of RBA were significantly reduced and the interface transition zone (ITZ) of RBA was compacted after silane modification of RBA. RBA contained Si, Fe, Al, Ca elements and a small amount of Na, K, Mg elements brought by mineral powder; the content of Ca, O, Si elements in ITZ and cement mortar were rich, which indicated that there were a large amount of calcium silicate hydrate (C-S-H) in ITZ and cement mortar; ITZ and cement mortar contained Al elements, which indicated that there were ettringite and monosulfate; the content of Ca element in RBA and ITZ increased after silane modification of RBA, which indicated that silane hydrolyzed and reacted with free calcium ions to form calcium silicate.