First, micro-silica fume in the production of our country and the status of applications in concrete 

      Our applied research on micro-silica fume and recycling late start, most of the ferrosilicon producers recovery of micro-silica fume not pay enough attention, although some of the larger companies recovered silica fume, but did not find the value of which, And these recovery to the silicon dioxide content of most of the silica fume is not standard, is still extensive industrial by-products. In recent years, the importance of environmental protection because the state, gradually strengthen the management of polluting enterprises, especially the domestic demand for high-strength concrete increased, many ferroalloy producers with a dust collection equipment, and development, the introduction of encryption technology. Making our products has been the rapid development of micro-silica fume.
        China is mainly used in micro-silica fume concrete and refractories field. Micro-silica fume concrete has been used in the construction of major projects such as high-rise buildings, anti-erosion wear, leakage and other projects, as early as 1986, China Gezhouba gates have been used in mortar silicon repair and 2000-3000m3 reservoir seepage, the Three Gorges power plants, Ertan Hydropower Station, Zipingpu power plants, power plants Xiaolangdi, Fujian outlet power plants, power plants and other water conservancy and hydropower Longyangxia Qinghai and Shanghai Yangpu Bridge Engineering, Hong Kong Tsing Ma Bridge, Lianyungang, Jiangsu impermeable timber terminals and other projects abrasion-resistant parts, high strength and impermeability for projects such as the Shanghai Huangpu Jiang Yuejiang tunneling, micro-silica fume concrete is also used for anti-sulfate, perchlorate, and improve the durability of rust-proof steel works, such as the construction of China's coastal underwater oil platform, now with our high-speed rail and rapid development of nuclear power projects, micro-silica fume has also been vigorously promoted and used, such as the Beijing-Shanghai high-speed railway, Shanghai-Nanjing high-speed rail, Minqing nuclear power, such as Guangdong Taishan Nuclear Power .
  

Second, the properties of silica fume  

1, Overview    

Silica fume (scientific name "silica fume", Microsilica or Silica Fume), Department of the smelting ferrosilicon or silicon, through the flue of the steam oxidation of silicon, specially designed dust collector collected the amorphous powder -like silica (SiO2). Average particle size of silica fume in 0.15 ~ 0.20μm, surface area of 15000 ~ 20000m2/kg, with strong surface activity. The past 20 years, micro-silica fume in the domestic construction, chemical, metallurgy and other fields to obtain a wide range of applications. Which is mainly used for a large number of micro-silica fume cement or concrete admixture to improve the performance of cement or concrete, preparation of a high strength (C80 and above), wear resistance, erosion, corrosion, anti-penetration, frost, early strength The special concrete for dams, large reservoirs, hydropower, port terminals, railway bridges, highways, airport runways, tunnels and high-rise buildings and other projects.  

 2, the physical properties of silica fume 

(1) Color: silica fume is ferrosilicon or silicon metal production by submerged arc furnace in high purity quartz, coke and wood byproducts reduction, the main ingredient is SiO2, the general color of the light gray silica fume and between dark gray, SiO2 itself is colorless, the color depends on the carbon and iron oxide content, the higher the carbon content, the darker the color; another encrypted silica fume color darker than natural.
  (2) particle size: the average particle size of micro-silica fume is about 0.15 ~ 0.20μm, cement particle diameter of 1 / 100, it can be highly dispersed in silica fume concrete, fill in the cement particles and improve the density, also has high activity silica fume, faster and more complete hydration of cement produced by oxygen and hydrogen compounds react.
  (3) surface area :15000-20000m2 / kg
  (4) the proportion of density: There are two main supply silicon powder form, the former state densification of silica fume and silica fume.
a, the original state of silica fume, collected directly through the dust collector products, loose volume of about 150-300kg/m3, former state transportation bags commonly used silica fume, because the density is very small, less efficient long-distance transport, multi-use bags will break the artificial micro-silica fume directly poured concrete mixer, work environment, dust, low efficiency.
b, increasing the frequency micro-silica fume, silica fume to solve the original state should not be long-distance transport and the problem of low efficiency, developed to improve the density of loose silica fume micro silica densification technology. This technology enables the original state of micro-silica fume in the air flow under the effect of rolling particles gathered into small groups, which will loose silica fume volume increased to 350-700 kg/m3, greatly facilitate the use, increasing the frequency micro- silica particles of small particle group cohesion is weak, in the concrete mixer in the mixing process is very easy to spread, so the silica fume particles can aggregate into the mixer after feeding to ensure that increasing the frequency micro-silica particles spread and a good group dispersed.
The following table silica fume and cement, slag, fly ash and other physical properties of the comparison, the table shows, the proportion of silica fume concrete is about 2 / 3, but the density of cement is only 1 / 6.  

(5) particle morphology and mineralogical structure: silica fume in the formation process, because the process of phase change by the surface tension, forming a non-crystalline amorphous phase spherical particles, and the surface is smooth, while others are more spherical aggregates of particles stick together. It is a large surface area, active high ash material. Material mixed with silica fume, small spherical body can play the role of lubrication. 

3, the chemical composition of silica fume  

The chemical composition of silica fume table below to TOPKEN ® 90 Series micro-silica fume chemical indicators, for example:  

Third, the silica fume concrete application in concrete

1, the working principle
With fine particles of silica fume (average particle size is 0.1um, only a few percent of the cement particles), surface area (15000m2/kg) features. After adding silica fume concrete, the role of
(1) physical effects. Due to fine particles, which fill the pores between the cement particles, the slurry is more compact, improving cement, aggregate microstructure, thereby increasing the concrete macro-physical - mechanical properties (strength, impermeability, durability , elasticity modulus, etc.).
 (2) chemical reaction. The main components of silica fume silica (SiO2) excitation in alkaline conditions, the precipitation of cement hydration with Ca (OH2) from the reaction of a hydraulic nature of the silicate gel - calcium silicate hydrate condensate glue (also known as CSH gel), these gel plug in the capillary, the capillary hole smaller, but not continuous, thus greatly improving the compaction of concrete. And calcium silicate hydrate gel strength is much larger than Ca (OH) 2 gel. Therefore, the effects of volcanic ash and silica fume particle effects with two advantages, fresh concrete can improve the secretion of water and cohesiveness, to comprehensively improve the concrete quality. 

2, the silica fume concrete to improve performance and enhance the performance of 

(1) increase the early strength and ultimate strength of concrete
       When silica fume cement replacement rate of 30% or less, the vapor temperature of 80 ℃, the day compressive strength of mortar is not mixed with silicon powder 2 times, (100MPa); without the use of autoclave curing, it is almost up to 3 times (150MPa); standard conservation, mortar compressive strength has improved markedly. When the micro-silica composite with high alkaline agent used, concrete water-cement ratio (W / C + Si) fell to 0.13-0.18, silica fume cement particles are filled with dense, concrete compressive strength is not silica fume 3-5 times.
 
       Concrete mixed with silica fume increased the silicon content from the reaction in electron microscopy, gap-doped silicon cement concrete in crystal growth. In addition, micro-silica particles that are small, evenly fill the concrete pores. Determination of foreign cement injection, whatever the curing conditions of concrete mixed with silica fume cement pore volume were significantly reduced.
 (3) to improve the performance of concrete segregation and bleeding
        After concreting, often resulting separated water from the concrete phenomenon, namely the formation of water film on the surface, also known as floating pulp, so that the uneven distribution of the upper concrete, affecting the quality of construction. Foreign studies have shown that incorporation of silica fume replacement of Si / (Si + C) the more difficult the more concrete segregation and bleeding. When the replacement rate of 15%, the concrete slump, even up to 15-20cm, almost without segregation and bleeding; when replacing rate of 20-30 percent, the concrete directly into the tap water should not produce segregation. As the silica fume concrete segregation and bleeding on performance improvement, so mixed with silica fume concrete can be used as seaports, tunnels and other underwater projects.
 (4) increase the permeability of concrete, chemical resistance and resistivity
         As the incorporation of silica fume increased the density of concrete, cement significantly reduces the gap, so to improve the micro-silica fume concrete impermeability. Overseas studies suggest that, when the silica fume concrete was replaced by 10-20%, significantly improved the impermeability of concrete, chemical resistance, and corrosion resistance of steel also improved. This is because of the increased density and SiO2 content increased, effectively blocking the acid ion intrusion and corrosion. In addition, due to the high resistivity silicon powder (up to 1.2 × 1011Ω • cm, 90 ~ 150 ℃), so the resistivity of concrete can increase 1.9 to 16 times.
 

 3, the silica fume concrete construction methods  

(1) silica fume concrete mixing. Mixing with the mixer, the material according to formula accurate weighing, feeding order for water stones → sand → → → cement mixer mixing with silica fume → 1.5min, then add a latex additive mixing 1.5min the machine again. Concrete is a symbol of marinated: uniform mixing of various materials, liquidity is good, not isolation, slump control in less than 3cm, you may not use more than 4cm.
(2) the transport of silica fume concrete. The level of silica fume concrete marinated transport and vertical transport time shall not exceed 10min.
(3) warehousing silica fume concrete pouring. Pouring process should be finished pouring 1 shall not stop halfway. Due to mechanical failure and other reasons have cold joints, should stop pouring, so the first poured concrete with a certain strength (usually after 24h, etc.), according to the construction joint treatment (treatment with the same base surface treatment), and then pouring the remaining part of the concrete. Pouring should be from left or right direction to a warehousing, the open after the first plug-in vibrators, after the pulp with a plate vibrator to mention, vibrated a little longer time than ordinary concrete, vibrating compaction can not be missed Zhen. Plastering open vibrated close shot hit the workers should seize paste and wipe the surface, usually due wiping 3 times 4 times, so that the surface smooth, non-empty, pockmarked face and non-penetrating cracks, not flatness control in 5mm or less.
(4) conservation. New silica fume concrete poured after wiping received by the vibrator stop 1h ~ 3h (determined according to site specific temperature), immediately stamped with plastic film to prevent evaporation of rain and surface cracks, 24h is prohibited stepped on, or put their things. After 36h ~ 48h after removing the plastic film, sealed straw bags, concrete curing procedures in a timely manner by hydraulic sprinkler maintenance.
 

4, mixed with silica fume concrete should pay attention to the problem: 

(1) mix design
 a, Compatibility: silica fume and superplasticizer should be, fly ash, slag, fiber and other good compatibility.
 b, the slump: interstitial because of its effect, the higher viscosity of concrete, the slump of concrete mixed with silica fume than conventional concrete design requirements of high 2 ~ 3cm;
 c, incorporation: the manufacture of high strength and sprayed concrete, cement, silica fume incorporation of the amount of 6 to 12%; for underwater engineering concrete, the dosage is slightly higher; while the micro-silica fume as an agent used to help pump When mixed with minus 2 percent. Through with the ratio test to determine the final mix.
 (2) feeding and mixing
 a, usually concrete, increasing the frequency of industrial use micro-silica fume, so choose artificial feeding or automatic feeding.
 b, as a unique ultra-fine silicon powder characteristics, micro-silica fume required immediately after feeding in the aggregate investment in micro-silica powder may be added has been properly mixing the concrete.
 c, mixed with silica fume concrete mixing time than ordinary concrete to extend the 30 ~ 40s, in order to obtain good uniformity.
 d, incorporation of silica fume did not specifically require the transport of concrete, the same as with ordinary concrete. Note that transit time is no more than 10min.
 (3) The casting and curing
 a, micro-silica fume concrete vibrator required density, no leakage vibration, less vibration, vibration is not too. General the vibration time is 10 ~ 30s;
 b, requires a continuous supply of concrete to prevent cold Jiecha, causing problems. Jiecha time shall not exceed 100min;
 c, silica fume concrete curing process to prevent premature evaporation of moisture is very important. Therefore, the casting is completed, immediately coating and laying sacks, water conservation.
(2) increase the density of concrete