studied types and amount of hydration products in cement paste in waste concretes and effect of the cement paste in waste concrete for compressive strength of autoclaved waste concrete samples simulated by hydrated cement with X-ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Infrared Spectroscopy (IR). The results showed, compressive strength of autoclaved waste concrete products increased with cement paste in waste concrete increasing, and the effect reduced obviously when amount of cement paste exceed 12.5% in waste concrete. Cement paste calculated amount exceeds 12.5% according to present national specification for mix proportion design of ordinary concrete. And it shows that effect of cement paste in waste concrete is weakly for compressive strength of autoclaved waste concrete sample. And compressive strength of autoclaved waste concrete samples is unrelated to mix and age of waste concretes. Hydrated products such as hydrated calcium silicate (C-S-H), α-C2SH and hydrated calcium aluminate sulfate (AFm or AFt) in cement paste forming during normal temperature curing play a role of crystal seed in autoclaved waste concrete samples.