In this study, incinerated sewage sludge ash (ISSA) is mixed with cement in a fixed ratio of 4:1 for use as
a stabilizer to improve the strength of soft, cohesive, subgrade soil. Five different ratios (in wt%: 0%, 2%,
4%, 8%, and 16%) of ISSA/cement admixture are mixed with cohesive soil to make soil samples. In order
to understand the influences of admixtures on the soil properties, tests of the pH value, Atterberg limits,
compaction, California bearing ratio (CBR), unconfined compressive strength, and triaxial compression
were performed on those samples. The study shows that the unconfined compressive strength of specimens
with the ISSA/cement addition was improved to approximately 3–7 times better than that of the
untreated soil; furthermore, the swelling behavior was also effectively reduced as much as 10–60% for
those samples. In some samples, the ISSA/cement additive improved the CBR values by up to 30 times
that of untreated soil. This suggests that ISSA/cement has many potential applications in the field of
geotechnical engineering.