[en] Tests of alkali-silica reactivity were conducted on Devonian aggregates from the Belgian Arden massive. These are 'Bastogne sandstones' of the metamorphosed Mirwart layer consisting of the silica rich psammite rock family Petrography analysis reveals the presence of wavy extinction quartz and small size silica elements known to be potentially reactive to the alkalis in the base medium of concretes. Two accelerated tests were conducted according to the RILEM TC-106 and the French Standard NF P 18-590; they confirmed the strong reactive character of the sands,from these materials. The reaction is strongest in the presence of certain percentage of non-reactive sand. For this reason, the aggregates arc, described as being potentially reactive with pessimum content'. Their use in structures concretes must therefore be considered with the particular precaution of using cements that are low in free alkalis, and if necessary, with the addition of slag, fly ash or silica fume, all of which are known to reduce alkali silica reactivity. Verification of the dimensional stability of tat-get formulae must also be conducted on concrete test pieces.