Generally, 5-10% of the cementing material. The doping method of silica fume is divided into internal doping and external doping:
（1）Internal mixing: Under the premise of the same amount of water added, 1 part of silica fume can replace 3-5 parts of cement (weight) and keep the concrete compressive strength unchanged and improve other properties of concrete.
（2）External admixture: The amount of cement remains unchanged, and the addition of silica fume can significantly improve the strength and other properties of concrete. When concrete is mixed with silica fume, there is a certain slump loss. This point needs to be paid attention to during the mix ratio test.
Silica fume must be used in conjunction with water reducing agent. It is recommended to mix fly ash and ground slag to improve its workability.
When using silica fume to prepare concrete, the weight ratio to cementitious material is generally:
High performance concrete: 5-10%;
Hydraulic concrete: 5-10%;
Shotcrete: 5-10%;
Pump aid: 2-3%;
Wear-resistant industrial floor: 6-8%;
Polymer mortar, thermal insulation mortar: 10-15%;
Unshaped refractory castable: 6-8%.
Please select a reasonable and economical dosage based on actual needs through experiments before use.
2. Adding method
The construction mix ratio of silica fume concrete and castable shall be made by the laboratory. In the mixing of silica fume concrete, silica fume should be added to the mixer immediately after the aggregate is added. There are two ways :
(1) Put in aggregate, then put in silica fume, cement and dry mix, then add water and other additives.
(2) Put in coarse aggregate + 75% water + silica fume + 50% fine aggregate, stir for 15-30 seconds, then put in cement + concrete admixture + 50% fine aggregate + 25% water, and stir until uniform. The mixing time is 20-25% or 50-60 seconds longer than ordinary concrete. Never add silica fume to the mixed concrete.
3. Construction method
There is no major difference between the construction methods of silica fume concrete and ordinary concrete, but good organization and vibrating compaction are necessary in construction. The early strength of silica fume concrete will advance the final setting time, so care should be taken when plastering the surface; at the same time, adding silica fume will increase the viscosity of the concrete and greatly reduce bleeding, making it slightly difficult to plaster the surface.
