Following dehydration and dehydroxylation reactions, we obtained quite lower friction coefficient values under the undrained condition than under the fully drained condition. This difference between μUD and μDRapp can be explained by the generation of pore pressure in the narrow gouge zone under the undrained condition in which the dense alumina spacers confined the pore water generated from the clays. Therefore, comparing μUD and μDRapp at a certain displacement d and a certain high temperature T enabled us to estimate the pore pressure generation Pp caused by the clay's decomposition reaction. Figure 9 illustrates the relationship between the shear stress τ and (apparent) normal stress σn for both DR and UD conditions. Assuming that τDR is linearly proportional to σnDR with cohesion c, then Pp at a certain (d, T) can be calculated as
Assuming that c does not depend on d, T, or the undrained/fully drained conditions and is nearly equal to 0, then (2) becomes
Here we redefine the friction curve μRD as the reference for correcting the scattered curves of the experiments (see Figures 2a and 5a). The parameters μUD, μDRapp, and σnappUD are replaced by μ″UD, μ″appDR, and σ″nappUD, given as
Figures 2d and 5c give the values for μUDapp/μ′RT and μDR/μ′RT at a certain (d, T). Thus, (3) is rewritten as
Here Ppest is the estimated value of pore pressure by (5). We can then draw the alteration of pore pressure as a function of T, as shown in Figure 10a. The estimated pore pressure generation at T = 500°C, calculated by (5), corresponds to the quite high Ppest to Pc = 80 MPa and was 72 MPa for Na-montmorillonite gouge and 68 MPa for kaolinite gouge. Thus, in those narrow gouge zones under the undrained condition, the effective confining pressure (Pc − Ppest) was mostly canceled, leaving only about 10 MPa. On the basis of the TG-DTA tests, both the Na-montmorillonite and kaolinite gouges only released 6∼8 wt % of water. Further, Ppest for both gouge zones showed almost 70 MPa at T = 500°C. Therefore, only 6∼8 wt % of dehydrated/dehydroxylated water can potentially generate 70 MPa of pore pressure at a fault consisting of compacted and fine clay gouge.
Figure 10. (a) Estimation of the Pp in the gouges with increasing T to Tmax, based on calculations using (5), for Na-montmorillonite dehydration and kaolinite dehydroxylation during heating. The thick line represents the boundaries of the liquid (L), vapor (V), and supercritical (SC) states of water. The thin lines are isochoric lines of water, valued by these densities. (b) Phase diagrams from previous studies, excerpted from Koster van Groos and Guggenheim  for Na-montmorillonite (marked as SWy-1) and from Yeskis et al.  for kaolinite (marked as KGa-1) are also shown for comparison. Note that the vertical axis is not Pp but Ptotal = PAr + PH2O. The abbreviations K, MK, ML, and V represent kaolinite, metakaolin, metaliquid, and vapor, respectively.
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 While a linear, gentle increase of Ppest was shown for the Na-montmorillonite gouge with T increases from 80°C, an abrupt increase in Ppest could occur in the kaolinite gouge starting at 480°C. This difference in the increase of Ppest implies that dehydration of Na-montmorillonite interlayer water produces a more gradual reaction than does the dehydroxylation reaction of kaolinite. The dehydroxylation reaction of kaolinite, which is a chemically irreversible decomposition reaction, would proceed abruptly once the specimen attains enough heat energy. On the other hand, the interlayer water in clays such as smectite and vermiculite is bound weakly on the mineral surface. The water moves reversibly between the interlayers of the clay and pore space, according to the P-T condition [Koster van Groos and Guggenheim, 1984; Wu et al., 1997]. The increase in Ppest means that the pore water density in the pore space is increasing if the pore volume is unchanging. The Ppest path of Na-montmorillonite dehydration, however, illustrated in Figure 10a is not isochoric, and the pore water density decreases with increasing Ppest and T. The gradual increase in Ppest with increasing T for the Na-montmorillonite gouge could mean that the ratio of pore volume to the total volume of the gouge zone would increase, with interdependence on Ppest and T. On the other hand, the density Ppest path of the kaolinite gouge becomes larger as the reaction progresses, meaning the ratio of pore volume to total volume decreases.
 For reference, Figure 10b also presents phase diagrams of dehydration/dehydroxylation reactions of Na-montmorillonite [Koster van Groos and Guggenheim, 1984] and kaolinite [Yeskis et al., 1985] reported by previous studies. These previous studies investigated dehydration/dehydroxylation reactions in high-pressure DTA systems using argon gas as a pressure medium. For Na-montmorillonite (SWy-1), Koster van Groos and Guggenheim  proposed that total pressure Ptotal was nearly equal to PH2O under high pressure. Using kaolinite (KGa-1), Yeskis et al.  controlled the conditions of Ptotalas Ptotal ≈ PH2O and Ptotal ≈ PAr. Those curves in Figure 10b are equivalent to the Clapeyron curves of Na-motmorillonite dehydration and kaolinite dehydroxylation. At Ptotal = 0 MPa in Figure 10b, corresponding to the initial Pp condition of our experiments, both the curves of Na-montmorillonite dehydration and kaolinite dehydroxylation are positive; Ppest in the gouge zone can increase under the undrained condition. The difference in the dehydration/dehydroxylation temperatures for those clays could reflect geographic differences in the original location of the specimens. However, the shapes of the P-T paths resulting from our experiments are quite different from those of previous studies. This discrepancy was probably caused by two differences in the experimental procedure. First, the experimental determination of Clapeyron curves is usually carried out at Pp = Pc. In our experiment, Pp was not equal to Pc. Second, the gouge zone was sheared continuously with a certain normal stress σn and shear stress τ during dehydration/dehydroxylation. The Ppest during the reactions could be increasing in counterpoise between the stress condition (σn, τ) and the poroelastic properties of the gouge. It is therefore difficult to directly compare our results with the Clapeyron curves of the clays.