Investigation of the mechanism of slow hydration in low w/c ratio RPC matrix under long-term autoclaved curing

•Long-term hydration of RPC matrix cured under high temperature autoclaved condition was studied.•Amorphous quartz powders were utilized to wipe off the quartz peaks in the XRD results.•Via XRD, no crystalline hydrates can be detected in the RPC matrix autoclaved cured for 480 h.•Adverse environments caused the slow hydration of RPC matrix during long-term autoclaved curing.•Crystallization of the micro hydrates in RPC matrix could be restricted by its crystalline stress. Reactive Powder Concrete (RPC) is a cementitious composite which is famous for its excellent mechanical properties and durability. Long-term evolution of hydrated products in RPC is important for the studies about durability of RPC. In this paper, under high temperature hydrothermal condition, long-term hydration process and evolution of hydrated products of the RPC matrix have been studied. During the 0–72 h of autoclaved curing, hydration of the RPC matrix was relatively fast. However, after 72 h of curing, hydration of the silica fume (SF) and fused quartz powders (FP) in RPC matrix was very slow. After 480 h of autoclaved curing, few crystalline hydration phases in the RPC matrix were observed. Major phases in the RPC matrix cured for 480 h were calcium silicate hydrate (C-S-H gel), unhydrated cement clinker and SF/FP. According to the different hydration behaviors in the RPC matrix and entrained pores, the lack of liquid environment and micro-pore waters were put forward to explain the slow hydration in the matrix. In addition, C-S-H in the RPC matrix tended to form tobermorite and jennite under autoclaved condition. However, the crystallization of C-S-H was accompanied with expansion and crystalline stress. Hence the formation of crystalline products was restricted in the RPC matrix. The limited hydration process and slow crystallization of the hydrates of RPC matrix under long-term hydrothermal condition provide the possibility for the RPC materials to be applied in some special environments.