The effects of the developed microstructure on the mechanical

artificially cemented clays

To further evaluate the micro-structural results for the cemented slurried clays, hydraulic conductivity and undrained shear strength were measured for the clays and cement contents described previously. Permeabilities, obtained by performing oedometer tests on undisturbed, reconstituted, and cemented samples of Nanticoke and Ottawa clays, are plotted in Fig. 3-16. For Nanticoke clay (Fig. 3-16a), the permeability of the specimen with 2% cement is close to that of the uncemented samples. This is in agreement with the observed resemblance between the pore size distributions of reconstituted Nanticoke and the 2% cement treated specimen (Fig. 3-11) and indicates that the dispersed structure observed in both samples governs their permeability behaviour. In contrast, as cement content increases to 4.2 and 8.7%, there is a considerable reduction in the permeability of the cemented Nanticoke clay. As observed in Fig. 3-11, artificial cementation of more than 2% creates a flocculated unimodal pore structure in Nanticoke clay and appreciably reduces the amount of inter-connected pores (with a diameter higher than 4000 nm) that exist between the soil aggregates. The cementitious material produced by cement hydration most likely blocks some of these water-channels, hence reducing the

permeability of the clay. A similar pattern exists in Fig. 3-16b for the Ottawa clay specimens. The results are in agreement with the suggestion by Garcia-Bengochea et al. (1979) that permeability is predominantly related to magnitude and frequency of macropores rather than micropores. These observations are also in accord with those of Locat et al. (1996), who reported an order of magnitude reduction in the permeability of a clay specimen treated with 10% lime.

It was shown earlier in the paper that unlike cement content, moisture content does not significantly affect the pore size distribution pattern of cemented EPK kaolin. Similarly, as Fig. 3-16 shows, the cement content has a more important effect than the moisture content on the permeability of Ottawa clay. Despite having different total pore volumes, the two specimens with a similar 6.4% cement content, but with different moisture contents, have almost the same permeability. Hence, the flocculated structure produced in these samples due to cementation should play a vital role in their pore size distribution and permeability behaviour. Furthermore, the important effect of mineralogy and activity on the behaviour of cement treated clay is again illustrated in Fig. 3-16; the addition of cement reduces permeability in Ottawa clay more than it does for Nanticoke clay. On average, the permeability of reconstituted Ottawa clay is 65 times that of the same clay treated with 6.4% cement. However, this ratio reduces to 30 when reconstituted Nanticoke is compared to 8.7% cement treated sample of Nanticoke clay.

The laboratory shear vane was employed to measure the undrained shear strength and sensitivity of the cement treated clays (Fig. 3-17). Focusing on the behaviour of the Nanticoke specimens, we can find correspondance between MIP and shear vane test results. Fig. 3-11 showed that at 98% moisture content, Nanticoke samples with less than

2% cement have a bimodal pore size distribution, similar to that of reconstituted material. Moreover, Fig. 3-14 suggests that almost no effective bonding is developed in samples with 1% cement content, since when they are air dried, they have shrunk to the same total pore volume as that of the reconstituted material. These findings are in agreement with the observation in Fig. 3-17 that theNanticoke specimens with 1% cement content have not gained any measurable strength. In addition, as illustrated before by SEM and MIP results, samples with higher cement contents have a more flocculated structure and a larger dominant pore diameter, explaining their brittle behaviour and higher sensitivity.

Fig. 3-17 also shows the important connection between the clay mineralogy and activity, and the effectiveness of artificial cementation. At a certain cement content and after 28 days of curing, Ottawa clay samples gain much higher strength, cu, 28 days, and sensitivity, st, 28 days, than do samples of Nanticoke clay or EPK kaolin. This is in agreement with the highly flocculated structure observed by the SEM and MIP analysis in cemented Ottawa clay. Compared to those of Nanticoke clay or EPK kaolin, Ottawa clay particles appear to have more chemical interactions with the added cement. Some researchers have mentioned the presence of amorphous iron, silica, and alumina oxides as a reason for the high sensitivity of Champlain/Leda clays (Bentley and Smalley, 1978; Yong et al., 1979; Quigley, 1980;Locat et al., 1984; Locat et al., 1985). The presence of such minerals could explain the higher strength of the cemented Ottawa clay. Besides silica and alumina dissolved from clay surfaces, the amorphous sesquioxides previously present in Ottawa clay react more readily with the produced lime and create more secondary cementitious bonds within the material (Wissa et al., 1965). Townsend (1985) also suggested that the presence of amorphous silica produces higher pozzolanic strengths

in cemented clays. Choquette et al. (1987) postulated that a more significant change in volume distribution due to cementation is also accompanied by a higher strength gain of lime treated clays. We can see that for Ottawa clay, a unimodal pore distribution has formed with a much wider peak than that of the other two clays. In addition, SEM results showed that more significant structural changes occur to cemented Ottawa clay, when compared to cemented Nanticoke or EPK material. The higher strength gain of cemented Ottawa clay, therefore, can be attributed to the more significant structural modifications that occur for this soil after the addition of cement.

Pore size analysis of the cemented clays showed that after a certain cement content (for example 2% for Nanticoke clay), when a flocculated structure is formed in the material, further addition of cement does not significantly change the pore size distribution pattern (Fig. 3-11 and Fig. 3-13). However, the permeability and strength continue to change with more cement being added to the soil. Although it does not change the pore size distribution pattern, the addition of more cement partitions and eliminates the remaining macropores by forming cementitious bridges, further reducing the permeability of the material. Moreover, further addition of cement to the flocculated soil creates more and stronger cementitious bonds in inter-particle contacts, as well as inter-aggregate pore spaces, increasing the strength of the crystal lattice formed within the material. On the other hand, more cementation appears not to change the size of the micropores significantly. These intra-aggregate pore sizes should depend on particle size distribution, activity, and clay mineralogy rather than cementation.