B. Summary of the problems that arise in the process of using micro-silica powder.
Throughout the development of micro-silica powder applications, we have accumulated a wealth of experience on how to best use micro-silica powder. The following are some summaries:
1. Micro-silica powder is not suitable for all occasions. Those who develop the technical requirements for concrete must carefully analyze the specific application and confirm that the use of micro-silica powder will give the desired results.
2. Micro-silica powder must be used in concrete of the right composition (mix ratio), which should be low water-cement ratio (w/cm) and concrete with high water-reducing agents. Early studies found that the use of micro-silica powder in the concrete mix ratio of 0.6 or higher water-cement ratio is not reasonable.
3. The use of micro-silica powder in combination with other auxiliary cementitious materials is growing. At first, these materials were seen as competing materials, but now it is increasingly widely recognized: The best concrete is often the result of the combined use of these materials.
4. The amount of micro-silica fume per unit volume of concrete is decreasing, especially for concrete exposed to chloride salts. The initial bridge deck concrete contained 12 % to 15 % micro-silica fume (by weight of cement), and then in the following years the amount of micro-silica fume was reduced to about 8%. At present, if used in combination with fly ash or finely ground slag, the amount of micro-silica fume is reduced to about 5 %.
5. There is no empirical method for designing and predicting the performance of micro-silica fume concrete mixes, but existing guidelines and examples of mixes can be consulted. Laboratory and field tests must be conducted with the actual materials used in the project to determine the final mix ratio.
6. Cracking used to be a problem in micro-silica fume concrete flat structures, especially overlays.
7. Wet maintenance for at least 7 days can basically solve or mitigate the cracking problem. However, some of the cracks that occur on some longer span bridges cannot yet be explained or prevented by construction procedures. Such cracks also occur in concrete that does not contain micro-silica fume, and therefore may need to be considered more in terms of structure rather than material. Research in this area is ongoing.
8. Micro-silica fume concrete must be properly maintained. Without curing, the benefits that micro-silica fume brings to concrete cannot be realized. If the requirements for proper maintenance are not incorporated into the technical standards for concrete and the maintenance requirements are not implemented in the project, it is best not to use micro-silica concrete.
9. Concrete using micro-silica powder must pay attention to every aspect of concrete production and construction. The mixing process, the adequate dispersion of micro-silica powder is the key to the performance of concrete. Poor mixing and poor dispersion of micro-silica powder may not get the expected concrete properties.
10. For determining the ‘hard index’ properties of concrete, the use of compressive strength may be an appropriate method. However, performance must be determined with care and a full understanding of the circumstances and conditions. Concrete with the same strength, tested for other performance parameters, will not necessarily perform the same.
11. Many flat concrete structures are over-plastered. Experience with micro-silica concrete plastering has shown that performing minimal plastering results in more durable concrete while preventing plastic shrinkage cracks.
12. Micro-silica fume concrete has the desired resistance to chloride ion penetration in a chloride salt environment. To date, a number of tests conducted on actual structures have proven that micro-silica fume concrete performs well. One of the reports shows that the rate of chloride penetration is decreasing substantially with time.
The type of micro-silica powder product changes. Today, formulated products and slurry micro-silica powders have disappeared from the market. Almost all commercial concrete and precast concrete companies use densified microsilica powders.
13. In terms of workability, the performance benefits of micro-silica fume concrete in the fresh state (plastic state) are becoming increasingly important. These advantages include: improved adhesion and hardening properties of shotcrete, reduced plastering work for flat structure construction, improved stability of self-compacting concrete and improved early concrete performance, etc.
C. How much is the market price of micro-silica powder? Why is there a big difference?
The current market price of micro-silica powder depends on what manufacturers to choose, if it is a domestic manufacturer, the general supply price is about two thousand RMB a ton, if you choose a well-known brand production of micro-silica powder, due to its more stable performance, better quality reasons, so the selling price will often channel three thousand RMB a meal or so price, and choose which type of manufacturers produced, depending on their own requirements for the material high or low.
The reason for the large price difference.
As for the reason why the price difference is so large, mainly because some industries for the product content, fluidity and PH value of the high requirements are relatively high, so this high-end requirements are more difficult to meet, the market is also relatively few such sources, so the price will generally be higher, which is also due to the demand for more, less supply of reasons caused by.
G: The physical and chemical properties of micro-silica powder application range.
The physical and chemical properties of micro-silica powder:
1. Micro-silica powder: the appearance of off-white powder ﹑ refractoriness > 1600 ℃. Bulk weight: 200-250 kg/m3.
2. Micro-silica powder fineness: micro-silica powder fineness of less than 1um accounted for more than 80%, the average particle size in 0.1 ~ 0.3um, specific surface area: 20 ~ 28m2/g. Its fineness and specific surface area of about 80 to 100 times the cement, 50 to 70 times the fly ash.
3. Particle morphology and mineral phase structure: Micro-silica fume in the formation process, due to the process of phase change by the role of surface tension, the formation of non-crystalline phase amorphous spherical particles, and the surface is relatively smooth, and some are multiple spherical particles stuck together in agglomerates. It is a volcanic ash material with large specific surface area and high activity. The material mixed with micro-silica fume, the tiny spherical body can play the role of lubrication.
Function:
Micro silica fume can fill the pores between cement particles, and at the same time generate gel with hydration products and react with alkaline material magnesium oxide to generate gel. In cement-based concrete, mortar and refractory casting materials, mixing the right amount of silica fume can play the following roles:
1、Significantly improve the compressive, flexural, seepage resistance, corrosion resistance, impact resistance and wear resistance.
2、It has the function of water retention, preventing segregation, water secretion and significantly reducing the pumping resistance of concrete.
3、Significantly prolong the service life of concrete. Especially in harsh environments such as chlorine salt pollution erosion, sulfate erosion and high humidity, it can double or even multiply the durability of concrete.
4、Significantly reduce the landing ash of sprayed concrete and casting material, and increase the thickness of single spray layer.
5、It is a necessary component of high-strength concrete, and there are already engineering applications of C150 concrete.
6、It has about 5 times the effectiveness of cement, and can be used in ordinary concrete and low cement castables to reduce costs and improve durability.
7、Effectively prevent the occurrence of alkali aggregate reaction in concrete.
8、Improve the denseness of castable refractories. When coexisting with Al2O3, it is easier to generate mullite phase, which enhances its high temperature strength and thermal vibration resistance.
Scope of application:
Commercial concrete, high-strength concrete, self-leveling concrete, indefinite refractory materials, dry-mixed (ready-mixed) mortar, high-strength non-shrink grout, wear-resistant industrial flooring, repair mortar, polymer mortar, insulation mortar, impermeable concrete, concrete compacting agent, concrete anticorrosion agent, cement-based polymer waterproofing agent; rubber, plastic, unsaturated polyester, paint, coating and other polymer materials reinforcement, modification of ceramic products, etc.
Application areas:
1﹑Used in mortar and concrete.
High-rise buildings, harbor terminals, reservoir dams, water conservancy, culverts, railroads, highways, bridges, subways, tunnels, airport runways, concrete pavement and coal mine roadway anchor spray reinforcement, etc.
2、In the material industry.
(1) High-grade high-performance low-cement refractory castables and prefabricated parts, service life is three times that of ordinary castables, refractoriness increased by about 100 ℃, high temperature strength and thermal shock resistance are significantly improved. It has been commonly used in: coke oven, iron making, steel making, steel rolling, non-ferrous metal, glass, ceramics and power generation industries.
(2) Large iron ditch and steel ladle material, breathable brick, coated repair material, etc.
(3) Self-flowing refractory casting materials and dry and wet jet construction applications.
(4) Oxide bonded silicon carbide products (ceramic kiln furniture, flame separator, etc.)
(5) High temperature type calcium silicate lightweight heat insulation materials.
(6) Corundum mullite push plate for electric porcelain kilns.
(7) High-temperature wear-resistant materials and products.
(8) Corundum and ceramic products.
(9) Cylon bonded products. At present, in addition to the common use in the cast refractory materials, in the capacitor and sintered refractory materials also get a lot of applications.
3﹑New wall materials, finishing materials.
(1) Wall insulation with polymer mortar, insulation mortar, interface agent.
(2) Cement-based polymer waterproofing materials.
(3) Light aggregate insulation and energy-saving concrete and products.
(4) Putty powder processing for interior and exterior wall construction.
4﹑Other uses.
(1) Raw material of silicate brick.
(2) Production of water glass.
(3) Used as reinforcing material for organic compounds. Because its composition is similar to that of silica produced by fumed method. It can be used in rubber, resin, paint, paint, unsaturated polyester and other polymer materials as filling reinforcing material.
(4) Used as anti-caking agent in fertilizer industry.
