The stress-strain behavior of the Na-montmorillonite clay samples
modified with 15% of four different CKDs and cured for 28 days
is presented in Fig. 10. The secant modulus calculated as a slope
of stress strain curve between the 0% strain and 1% strain was
used as a measure of the increase in the stiffness of CKD modi-
fied clay system. Generally, the addition of the CKD increased the
peak stress and reduced the peak strain considerably, i.e., induced
substantially brittle behavior. It appears that the chemical composition,
primarily the free lime content, has an important role in the
development of the stiffness. For example, the secant moduli at
1% strain for the Na-montmorillonite treated with 15% of CKDs
were 90, 130, 55, and 30 MPa, respectively, for CKD-1, CKD-2,
CKD-3, and CKD-4. The secant modulus of untreated clay was
only 10 MPa. The CKD-2 with the highest free lime content
showed the largest increase in the stiffness.