Quicklime is a key component of Autoclaved Aerated Concrete (AAC), with typical quicklime contents of order 15 wt.% in Europe. It is the mostly used source of calcium oxide that eventually ends up as tobermorite in the fi nal product. In this paper, we describe the research performed at Lhoist in order to assess the infl uence of quicklime on AAC. Several types of quicklimes were used, differing by their origin and reactivity. They were incorporated together with other fi xed ingredients (cement, quartz,...) in several AAC recipes with fi nal densities ranging from 350 to 550 kg/m3 , corresponding to the European P2 – 0.35, P2 – 0.4 and P4 – 0.55 classes (EN 771-4). The study consisted fi rst in following the green cake expansion of the different recipes. Then, the green cakes were autoclaved for 10 hours at 11 bars of steam pressure. The obtained AAC blocks were then tested for compressive strength and density. The study showed that quicklime reactivity had low effect on green cake penetration value. However, quicklime reactivity had a major effect on green cake expansion. Too reactive a quicklime generated a poor pore structure as detected by a fallback effect and a below-specifi cation compressive strength of the low density AAC. Then, quicklime slaking curve was not suffi cient to anticipate the green cake expansion for a given recipe. Quicklimes with similar reactivity but different origin could had very different behaviour in the AAC. Finally, the behaviour of the too-reactive quicklimes would probably have been easily corrected by decreasing the water temperature.