In fiber reinforced cement, the interface between fiber and cement paste is a weak area. When micro silica fume is added to fiber cement, pozzolanic effect of low calcium silicon is more amorphous and dense than C-S-H gel, it makes the unhydrated cement particles and C-S-H interface become denser and more uniform, between pores and big hole it cannot be coherent and interconnected than the cement paste, thus the transition zone width of cement – fiber becomes small, become micro transition zone, improve the structure of the fiber – cement stone interface.
Advantages of Silica fume in fiber reinforced cement:
Adding silica fume significantly improve the flexural strength and impact strength of fiber reinforced cement, and with the increase of silica fume content, fiber reinforced concrete flexural strength and impact strength constantly improve and can effectively reduce the fiber reinforced cement water absorption rate, improve the volume mass of fiber reinforced cement, and have certain effects on the dry shrinkage.
1. particle filling
The average diameter of silica fume is about 0.1 um, which is 100 times smaller than that of cement particles. The particles can fill the gap between cement particles or fiber – cement paste, and increase the density of fiber reinforced cement.
2. volcanic ash effect
Micro silica has highly amorphous nature, the main component SiO2 and cement can hydrate and produce CH crystal, which can occur volcano ash, generating C-S-H (I) gel with high calcium silicon ratio, and continue react to produce C-S-H (II) gel with low calcium silicon ratio, more advantageous to the strength, at the same time, due to two reaction between SiO2 and CH in microsilica, make CH on the surface of cement particles has been consumed, the fresh surface exposed to hydrate, making the hydration of cement particles more fully, to speed up cement hydration rate. Finally, a very dense hardened cement paste with a large number of C-S-H gels and little CH crystal content has been formed, which strengthens the strength of the hardened cement paste itself.
3. backwater effect
Micro silica has huge specific surface area, fiber reinforced cement mixing micro silica fume, can prevent the water accumulation on the surface of the fiber, reduce the interfacial water cement ratio, improve the compactness of fiber reinforced cement.
In fiber cement, the CH crystal grows larger and has strong orientation parallel to the surface of the fiber, and these CH crystals are easy to crack. Micro silica fume can make CH crystal orientation weakened, reduced size, improve the compactness between fiber and cement and adhesive strength, so that the hydration products C-S-H gel in fiber reinforced cement increased, CH crystal decreased, crystal refinement, compactness is further enhanced, the ultimate performance is flexural and impact strength improved, volume mass increased, reduce the water absorption.
The best dosage of silica fume in fiber reinforced cement should be controlled in 4%-6%.
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