In concrete, internal bleeding is blocked by aggregate particles and accumulates under the aggregate to form a porous interface. More Ca(OH)2 is formed in the aggregate interface filter area than in other areas. Ca(OH)2 crystals grow larger and have strong orientation parallel to the aggregate surface.
Large Ca(OH)2 crystals parallel to the aggregate surface are easier to crack and are weaker than hydrated calcium silicate gel (C-S-H). The interface filtration zone between the cement slurry and the aggregate becomes a weak zone within the concrete due to its porous nature and many directionally arranged large Ca(OH)2 crystals. Due to the inclusion of a certain amount of microsilica fume in HPC, its strength is significantly improved compared with ordinary concrete (without microsilica fume). Some scholars have calculated that if 15% of microsilica fume is used to replace cement, the ratio of the number of cement particles to the number of microsilica fume particles is 1:2000000, that is, two million microsilica fume to one cement particle, so microsilica fume Has a great impact on HPC strength.
In HPC, microsilica fume, which is smaller than 100 times the diameter of cement particles, fills the pores of cement slurry and fills the gaps of cement particles. The effect is like cement particles filling the gaps of aggregates and fine aggregate filling the gaps of coarse aggregates. The gaps between aggregates are the same, increasing the density of HPC from a microscopic scale and improving the strength of HPC. This is the "filling effect" of microsilica powder. In HPC, the microsilica powder filled in the cement slurry significantly reduces the number of pores in the cement slurry and improves its homogeneity, while the total void ratio remains basically unchanged.
The microsilica fume in the transition zone between cement slurry and aggregate reduces the bleeding of HPC, prevents moisture from gathering under the aggregate, and makes the aggregate interface transition zone similar to the microstructure of cement slurry, thus improving the interface filtration zone. The density and effectively reduce the thickness of the interface transition zone. Tiny microsilica particles become the "seeds" of Ca(OH)2, making the Ca(OH)2 crystals smaller and more randomly oriented.
Therefore, the incorporation of silica powder improves the bonding strength between cement paste and aggregate in HPC, eliminates the "weak connection" problem of different composite components in concrete, and makes HPC have the characteristics of composite materials. Bone particles play a reinforcing role in HPC and are not just inert fillers. Microsilica fume does not have a great impact on the strength improvement of cement paste (without aggregate), but it can make the strength of concrete with the same water-cement ratio significantly higher than the strength of its matrix (cement paste).
