A complete guide to the application of silica fume (microsilica): from material cognition to engineering proficiency
yellow river microsilica
In the evolutionary history of modern engineering materials, few waste products have accomplished such a gorgeous counterattack as silica fume (microsilicon powder). Once the soot that smelters were eager to deal with, it is now the indispensable soul of high-performance concrete, oilfield wells and top-notch refractory materials.
Industry insiders call it "industrial MSG," which means that only a little bit of it can make a qualitative leap in performance. However, many people do not know that this flavor of "MSG" may even become "arsenic" of engineering quality if it is delivered at the wrong time and in the wrong dose. Today, we're going to dispel the fog, from microscopic mechanics to macroscopic reality, to dismantle this "literal book" on silicon ash for you.
Core Value Decoding
Silica fume is not a simple physical filler; its intervention is a sophisticated chemical battle that takes place at the micron level. To understand its value, we must first understand its "three weapons."
1. Extreme micro-filling
It 's a game about " occupying space . " The average particle size of silica fume is only 0.1-0.3 microns, which is about one percent of the diameter of ordinary cement particles。 If concrete particles are likened to basketballs, silica fume is glass beads scattered in cracks.This physical level of extreme density is the first line of defense against the penetration of harmful substances.
2. Strong volcanic ash reaction
It 's a chemical trick that turns enemies into friends . The high-activity amorphous silica of silica fume, like a predator, quickly captures the calcium hydroxide （CH） produced by the hydration of cement, forming a high-strength hydrated calcium silicate （C-S-H） gel。 This process not only eliminates weak links, but also achieves a double leap in strength and durability.
3. Contradictory morphological effects
It 's a ball . It 's a sponge . The silica fume particles are spherical and can theoretically act as "ball bearings."However, due to its huge surface area, it has extremely strong water adsorption. Without intervention, it can lead to a sharp increase in viscosity.Therefore, "ball effect" must be activated in combination with high-efficiency water-reducing agent .
Five dimensions of comprehensive evaluation of quality
In the market, the quality of silica ash is very different. How to avoid buying "fake silicon ash" or inferior ash? Please hit the following dimensions:
Dimension 1: Chemical purity (hard indicator)
The content of silica （SiO2） is the source of activity。 For high-grade concrete or UHPC, be sure to lock in high-purity products with SiO2 ≥ 92% 。 The higher the purity, the more complete the volcanic ash reaction.
Dimension 2: Burn loss (invisible killer)
The loss on ignition mainly represents the unburned carbon。 Carbon will be crazy adsorption of water reducer and water, resulting in slump loss very fast. The loss of quality silica must be controlled below 3 % , and more than 4 % is inferior .
Dimension 3: Physical Dimension (Filling Force)
Judging by the residue of the 45 micron square pore screen.The filter residue for top products is usually < 2.0%. The lower the filter residue, the better the microfill effect.
Dimension 4: Water Demand Ratio (Economic Account)
This is the key to evaluating side effects on fluidity.The lower the water demand ratio (national standard ≤ 125%), indicates that the less water (or water reduction agent) is added to achieve the same flow level, which is directly related to cost and construction convenience.
Dimension 5: Safety (red line)
For reinforced concrete structures, chloride content is a touchy subject。 It must be strictly tested to prevent steel bars from rusting.
Formulation ideas for different application scenarios
Scenario 1: High Performance and UHPC Concrete
Objective : To pursue strength （ > 150MPa ） and durability limit .
Strategy: The admixture content is usually 20% to 25% of the total cementitious material。The key lies in compounding - it must form an iron triangle with polycarboxylic water-reducing agent and steel fiber, and be combined with thermal curing.
Scenario 2: An oil field bunker
Objective : Respond to high temperature and high pressure to prevent formation pressure leakage .
Strategy: The dosage can be increased to more than 30%。 At this time, silica fume and floating beads work together to lock the slurry density at 1.60-1.75 g / cm3, ensuring that the slurry does not separate underground.
Scenario 3: Fire-resistant materials (imformated pouring)
Objective : High-temperature bond strength and high-temperature refractoriness .
Strategy: High purity requirement （SiO2 ≥ 95%）。The commonly used dosage is 3% - 8%. It reacts with alumina at high temperature to form mullite whiskers, which reinforce the matrix like steel bars.
Practical tips and a guide to avoiding pitfalls
Silicon ash is good, but bad use is a disaster. 90% of engineering accidents die in detail.
Golden Rule: No water reduction agent, no silica ash
Increases in water use can never be relied upon to regulate mobility. It must, certainly and absolutely be used with high-efficiency water reduction agents to open the flushing structure in order to realize the filling effect.
Additive trap: Don't be superstitious "the more the better."Excessive amounts can make the plasma thick and difficult to pump.It is usually the most cost-effective between 5% and 10%.
Fatal " self-drying " : silica fume concrete has almost no efflorescence , and the internal water consumption is very large . Spray or film curing must be carried out immediately after pouring , the traditional " curing after final solidification " is too late
Mixing process : It is recommended to mix dry or mix with aggregate first , and extend the mixing time by 30-60 seconds to prevent silica dust from forming a " cancer . "
