According to the activity of fine filler used in dry mix mortar, it can be divided into two kinds- active filler and inert filler.
The active filler is made from natural mineral material or industrial waste residue, which can be directly used or pre-milled. It is a kind of fine powder material directly mixed into the mixture when the dry mortar is mixed. It does not have hydration activity or only very weak hydration activity, but it can be hydrated in alkaline condition or in the presence of sulfate and produce strength. It mainly includes coal ash, granulated blast furnace slag, silica ash, etc.
Coal ash has potential chemical activity, the particle is very fine, and contains a large number of vitreous micro beads. The active agents in coal ash can reach over than 70% and hydrate under the effect of Ca(OH)2 and sulfate salts, which results in high strength. The micro beads in coal ash are spherical which reduce the internal friction resistance of mortar and improve the workability of mortar. Besides its particle size is mostly less than 0.045mm, smaller than cement, which can be filled in the capillaries and pores of cement gels to make cement gels more compact.
Granulated blast furnace slag powder mainly contains SiO2, CaO and Al2O3, total amounting to over 90%. The proportion of different oxides results in different types of activities. It is hard and difficult to grind with cement clinker, which means poor water retention, but if grinded separately or with auxiliaries, the active contents can be hydrated soon.
Silica fume, also known as silica powder or silica fume, is very fine particles collected from the fume discharged in the production of ferro-silicon alloy or silicon steel. It is noncrystalline and smooth spheroidal particle with high specific surface area. So it demands lots of water during application, which is commonly applied together with water reducing agent to ensure the workability. It can increase the mortar strength through the Ca(OH)2 results from silicate hydration, and improve the pore structure. It is a kind of expensive material and addition volume normally doesn’t exceeds 10%.
As for inert filler, it is inactive material or material that does not produce strength, mainly including limestone powder, quartz powder, calcium carbonate, talcum powder, mica powder, etc. At present, the most commonly used type is limestone powder, which can be divided into heavy calcium carbonate and light calcium carbonate due to different production methods.
Heavy calcium carbonate is mainly used as filler material of putty in dry mortar products, which has good anti-abrasion and certain covering power. It is also one of the important fillers in building coating. Adding heavy calcium carbonate into some dry mortar can change the density and workability of mortar. Light calcium carbonate granules are fine, insoluble in water, slightly alkaline, not suitable for sharing with non-alkali pigments, which are used as filler in building waterproof coating.