Samples of Pinczow limestone, and Nietulisko and Zerkowice sandstone were impregnated by OH strengthener (Wacker), and then subjected to various tests to assess how ageing and hardening of an alkoxysiloxane product affects the properties of the treated stone. Results of tests showed that these processes are very slow and do not influence the stone properties, including its mechanical strength. Graph. Data tables. 3 bibliographic references.