The effect of adding finely ground construction and demolition (C&D) waste fines, containing a significant amount of clay-brick, to mixtures of Portland cement and quartz sand autoclaved at 180 degreesC was investigated by DSC. The incorporation of up to 41.25 mass% C&D waste fines enhanced the formation of aluminium-substituted tobermorite indicating that the mullite in the clay-brick fines is reactive under autoclaving conditions and supplied the aluminium required for the formation of this critical phase. C&D waste fine additions in excess of 41.25 mass% promoted the formation of aluminium-substituted C-S-H, over aluminium-substituted tobermorite.