Study on the Characteristics of Fly Ash Slurry - Examination of Controlled Long-term Strength Material -

Civil Engineering Research Section

Fly ash slurry is a slurry material, a mixture of fly ash, water, cement and sand, mainly used as backfill for building foundations and backfill of structures. This material is excellent in fluidity, self-hardening ability and self-compacting ability and its strength can be determined according to the amount of cement used. However, the composition of materials, which provides the initial strength required in the construction, will increase long-term strength, resulting in difficult re-drilling by backhoe. To solve this problem, the conventional fly ash slurry components are improved to expedite the start of the initial strength after constructing the slurry. The resulting material, with a controlled increase in long-term strength, will be outlined.

The conventional slurry containing ordinary Portland cement () shows continued strength increase at long-term age. On the other hand, slurry containing cement-based solidification material () and slurry containing cement-based solidification material and gypsum () demonstrate higher initial strength than () and their strength at long-term age is controlled. Consequently, when the slurry () and () are re-excavated after a long period, the strength can be controlled so as to be mechanically excavated by backhoe (under 1,400kN/m2).

Fig-1 Relationship between age and strength

We focused on Al2O3 substance that accounts for about 20% of fly ash. By reacting Al2O3 with components eluted from cement-based solidification material and gypsum, a large amount of characteristic hydration products (3CaO･Al2O3･3CaSO4･32H2O: Ettringite) is produced at the initial phase to control the progress of long-term pozzolanic reaction. Particularly, in slurry including desulfogypsum at age of 14 days, capillary crystal of Ettringite is produced in large volume. In fact, Ettringite contributes to the increase in strength and controls the elution of heavy metal.

Photo-1 SEM observation (age: 14 days）