[en] Grouting of water-conducting fractures with low-alkali cement is foreseen for the potential future repository for spent nuclear fuel in Finland (ONKALO site). Results from a previous modeling study showed a quick (2 days) sealing of porosity at the grout-rock interface due to the precipitation of C-S-H (also C-A-S-H) and ettringite. The spatial extent of this alteration and sealing of the rock was in the order of a few tens of micrometers. Following on these results, experiments were designed to observe this possible effect of the low-pH grout on the rock (migmatitic gneiss). A series of centimetric rock cubes were placed in contact with the low-pH grout. After different times (4 - 19 days), electron microprobe analyses (EMP) were carried out to evaluate any variation in the original rock composition. The results show that contact between the low-pH cementitious grout and gneiss from ONKALO did not induce any alteration in the rock, at least at the scale of the EMP observations (micrometers to tens of micrometers). The low porosity of the bulk rock (1%) compared to the porosity used in the previous calculations (5%; zone of increased porosity near fractures), together with the large reactive surface areas used in the calculations (all surfaces assumed to be in contact with water), are probably the cause of the apparent discrepancy between model results and observations (lack of alteration). (orig.)