Ultra-high performance concrete has high compressive strength, high resistance to deformation, high density and other characteristics, mainly because of a variety of mineral admixtures added in the formulation process, the more obvious effect is silica fume. The role of silica fume in ultra-high performance concrete is not only to increase the strength, there are other more obvious role.
The role of silica fume in ultra-high performance concrete
Reduce the heat of hydration
When the amount of silica fume is increased, the hydration rate of cementitious material becomes smaller, the peak of hydration exotherm decreases, and the amount of exotherm decreases, which means that the incorporation of silica fume delays the acceleration period of cementitious material and reduces the total amount of exotherm.
Lower calcium hydroxide content
The calcium hydroxide content of the hardened ultra-high performance concrete is inversely proportional to the amount of silica fume. Silica fume reacts with calcium hydroxide and forms secondary hydrated calcium silicate. Silica fume is more reactive and reacts more easily with calcium hydroxide, thus reducing the calcium hydroxide content. When only cement and silica fume are used as cementitious materials and hot curing is used to prepare ultra-high performance concrete, the calcium hydroxide content after hardening is less than 5%, thus reducing the carbonation rate.
Reducing water absorption porosity
The water absorption porosity of ultra-high performance concrete is inversely proportional to the amount of silica fume. The particle size of silica fume is much smaller than that of cement and has volcanic ash activity. The incorporation of silica fume can fill the pores in the hardened ultra-high performance concrete on the one hand, and the C-S-H gel produced by its volcanic ash effect also optimizes the pore size distribution of ultra-high performance concrete and reduces the water absorption porosity. When only cement and silica fume are used as cementitious materials and the ultra-high performance concrete is prepared by hot curing, the capillary pore content of the ultra-high performance concrete is zero.
The strength of ultra-high performance concrete is proportional to the amount of silica fume admixture. From the test results of water absorption porosity, it can be seen that the incorporation of silica fume reduces the porosity of UHPC, therefore, the incorporation of silica fume can increase the strength of UHPC. However, when the amount of silica fume is too large, the viscosity of fresh UHPC increases and the fluidity decreases, which will increase the porosity of UHPC and reduce the strength of UHPC.
Dosing of silica fume in ultra-high performance concrete
Usually, when adding silica fume, we need to add water-reducing agent, otherwise we need to increase the water consumption (water-cement ratio) if we want to maintain the flow of concrete, which will lead to the strength of concrete is difficult to ensure, but adding water-reducing agent can ensure that the water-cement ratio remains unchanged, to enhance the strength of concrete.
The amount of silica fume
Silica fume in the concrete is too little, not much improvement in concrete performance, but too much, the concrete is too sticky, not easy to construct, and dry shrinkage deformation, poor frost resistance. Therefore, when mixed with silica fume, should be used according to the silica fume, cement type and the nature of the aggregate, choose a few doping ratio for concrete specimen analysis, generally speaking, the doping should be less than 10%, many projects on the different requirements of concrete, silica fume doping more than 3% to 5% or so.
The amount of water reducing agent
Usually with the use of silica fume water reducing agent are high-efficiency water reducing agent, its dose is generally within 1% of the amount of gel material, sometimes in order to reduce the water-cement ratio of ultra-high performance concrete, water reducing agent dose can reach 2% to 3%.
It can be seen that the role of silica fume in ultra-high performance concrete is very powerful, but in most cases, in order to meet the needs of the project, will also be mixed with some other mineral admixtures, which requires a targeted test to determine before the preparation.
As one of the largest suppliers of silica fume in China, Henan Superior Abrasives has provided high quality silica fume for many projects using ultra-high performance concrete, if you need, you can leave a message on our website or call our customer service for detailed consultation.