Silica fume is microsilica, in the concrete as a mineral mixer, the right amount of admixture can effectively improve the performance of the concrete, and silica fume can produce such a great effect, and its own activity is inseparable, higher activity determines a better filling and reaction, that silica fume activity index is how much? And how to determine its activity?
The standard activity index of silica fume
Silica fume has different specifications due to its silica content, but no matter what specifications of silica fume, its activity index should be in line with national standards, that is, in the 7d rapid determination method, the activity index to ≥ 105%, and this larger activity index is mainly related to the silica in silica fume.
Silica in nature is divided into crystalline and amorphous, silica in silica ash is a kind of amorphous silica, its silica-oxygen structure is very chaotic, which also leads to it is very unstable, and the reason why it can be agglomerated into silica ash, and it is captured and then cooled. The soot produced by ferrosilicon smelting has a large amount of free silica inside, which, when left behind by the dust collector filter, is subjected to rapid cooling without time for crystallization, thus forming a silica ash with great activity.
The activity measurement method of silica fume
The activity index of silica fume is mainly determined by the 7d rapid determination method, and the specific activity index is obtained by measuring the compressive strength at 7d age of the specimens mixed with and without silica fume, respectively, as follows:
1.The choice of raw materials for the specimen
The raw materials of the specimen are mainly standard reference cement, standard sand, high efficiency water reducing agent and tap water, which requires water reducing agent with water reduction rate greater than 18%, and tap water can be replaced by distilled water.
2.Specimen making
First, the various raw materials and appliances will be placed in the laboratory for more than 24 hours, so that the temperature is the same as the laboratory. Specimen production needs to mix cement and silica fume first, then add water and admixture to the mixing pot, then add the mixed silica fume cement, then stir according to the standard, first low speed stirring 30s, in the second 30s at the beginning of the uniform will be added to the standard sand, after stirring to high speed to maintain 30s and then stop 90s, the first 15s of this 90s will be stirring pot blade and pot wall of the rubber sand hanging The first 15s of these 90s will be hung into the pot, and then stirred at high speed for 60s after 90s. After molding, 1d before demoulding, should be placed at a temperature of 20±2℃, humidity of 95% or more in the environment maintenance, after demoulding should be placed in a closed steam box, at a temperature of 65±2℃ steam for 6d.
3.Calculation of silica fume activity
Silica fume activity calculation needs to be determined before the 7d compressive strength of the specimens mixed with silica fume and not mixed with silica fume, required after cooling to a constant temperature in the compressive strength test, obtained after the calculation of silica fume activity according to the following formula:
A= R1/R0×100%
The formula A represents the activity index of silica fume (%), R1 represents the 7d age compressive strength of silica fume mixed with colloidal sand (MPa), R2 represents the 7d age compressive strength of colloidal sand without silica fume (MPa). It is required that the activity index of silica fume is calculated accurately to 1%.
