We conducted a series of rotary-shear friction experiments on ground dolerite gouges, in which the amount of adsorbed moisture increases with grinding time (tgr), at room temperature and humidity, a normal stress of 2 MPa, and constant equivalent slip rates (V(eq)s) ranging from 20 mu m/s to 1.3 m/s. Their frictional strength changed with V-eq and tgr in three different ways depending on Veq and the gouge temperature (T). At Veq <= 1.3 cm/s, T did not exceed 80 degrees C, and the steady state friction coefficient (mu(ss)) ranged from 0.59 to 0.80. mu(ss) changes little with Veq, while mu(ss) at a given Veq systematically increases with tgr probably due to moisture-adsorbed strengthening of gouges. At Veq =4cm/s, T exceeded 100 degrees C, and dehydration of gouges resulted in roughly the same mu(ss) values (0.60-0.66) among gouges with different periods of tgr. At Veq >= 13 cm/s, T reached 160-500 degrees C, and mu(ss) dramatically decreases with Veq to 0.08-0.26 at Veq <= 1.3 m/s, while mu(ss) at a given Veq systematically decreases with tgr. At these fast Veqs, dehydration of gouges likely occurred too fast for water vapor to completely escape out fromthe gouge layer. Therefore, faster dehydration at faster Veq possibly resulted in a larger pore pressure increase and lower frictional strength. In addition, because gouges with longer periods of tgr contain larger amounts of adsorbed moisture, they became weaker due to larger increases in pore pressure and hence larger amounts of reduction in frictional strength.