The unit weights of the RHC samples, cured for 8 and 18 h, are indicated in Fig. 3. The unit weight of aerated concrete with RHA is reduced as RHA is introduced; however, the unit weight was stable or only slightly changed for the different autoclaving times. This result indicates that the unit weight reduction in RHC resulted from the relatively low specific gravity (2.13) and higher pore structure of RHA compared with quartz sand (2.59). The unit weight loss of the RHC samples compared with the control samples was in the range of 16–45% for the samples that were subject to 8 h and 18 h of autoclave curing.