Effects of Using Silica Fume and PP Fibers on Foamed Concrete

Preparation

Materials

• Cement: ASTM C150 Type I portland cement, 28-day compressive strength 72.5MPa;
• Silica fume: G92D silica fume (min. SiO₂ content 92%);
• Fly ash: ASTM C618 Class F fly ash;
• Polypropylene (PP) fibers: length 15mm, diameter 100μm, density 0.9g/cm³, modulus of elasticity 8GPa, elongation at break 8.1%, tensile strength 800MPa;
• Foaming agent: protein based foaming agent, the mixing ratio of the forming agent to water is 1:40.

Preparation of foamed concrete

The preparation process is as follows: First mix cement, fly ash, and silica fume (if added) for 1.5 minutes. Then add water (not including the water for foam preparation) and admixtures, stir for 2 minutes. Add PP fibers and stir for 4 minutes. (If fibers are not added, skip this step.) While the slurry is being stirred, mix the foaming agent and water in proportion with a high-speed mixer to create foam. Add the foam to the slurry and stir for 2 minutes. Finally, pour the formed concrete into the mold, flatten the surface with a wooden board. Leave the test specimens to stand in the laboratory for 24 hours, then demold them and send them to a standard curing room for curing to specified ages.

Testing

100mm x 100mm x 100mm test specimens are used for tests of apparent density, compressive strength, and splitting tensile strength. 100mm x 100mm x 515mm specimens are used for the drying shrinkage test.

Results

Compressive strength

The influence of ages

1. The compressive strength of foamed concrete specimens with different mix ratios increases with age.
2. For foamed concrete without silica fume, the 7-day compressive strength is 70%-75% of the 28-day compressive strength. Whereas for foamed concrete with silica fume, the compressive strength at 7 days is 85%-90% of that at 28 days. It indicates that silica fume has a good contribution to the development of the early age compressive strength of foamed concrete.
3. For those without silica fume, the compressive strength at 28 days is 80%-85% of that at 90 days. For those with silica fume, it is 85%-90%. This shows that silica fume improves the long-term compressive strength of foamed concrete.

The influence of the foam content and apparent density

1. The compressive strength of foamed concrete decreases and increases almost linearly, respectively, with the increase of foam content, and apparent density.
2. The increased rate of the compressive strength of the foamed concrete with silica fume is greater than that of without silica fume.

Effects of silica fume

Silica fume can significantly improve the compressive strength by up to 25%: The foam content influences the increased rate of compressive strength by silica fume. When the foam content is 20%, the compressive strength is increased by 21.8%; and when the foam content is 40%, the compressive strength is increased by 25.0%.

Effects of polypropylene fibers

Polypropylene fibers also can improve the compressive strength of foamed concrete. When the foam content is 20%-50%, PP fibers increase the compressive strength of concrete without silica fume by 16%-45%, and that of with silica fume by 5.6%-27.5%. Since silica fume concrete already has a relatively high compressive strength, the effect of the PP fibers is less significant. It can also be seen that for low-strength foamed concrete, adding PP fibers is an effective reinforcement method.

During the preparation process, the fluidity of all the slurries mixed with polypropylene fibers is greatly reduced. In order to ensure good workability, it is necessary to increase the content of superplasticizers. And a suitable dosage of polypropylene fibers is 0.8%, which not only ensures the fluidity of the slurry, but also can give full play to the reinforcement effect.

Splitting tensile strength

The splitting tensile strength of foamed concrete decreases with the increase of the foam content, and increases with the increase of the compressive strength. The test results show that polypropylene fibers and silica fume significantly improve the splitting tensile strength of the foamed concrete, and the higher the foam content, the greater the improvement. When the foam content is 20%-50%, PP fibers increase the splitting tensile strength by 32%-50% without silica fume, and 28%-45% with silica fume.

Drying shrinkage

Normal foamed concrete has a drying shrinkage strain of (1300-1700)x10^-6 at 90 days. Whereas the value of PP fiber reinforced foamed concrete is (700-1100)x10^-6. It can be said that polypropylene fibers significantly reduce drying shrinkage.