Sources, hazards and prevention of chloride ions in silica fume

In modern concrete engineering, silica fume With its ultrafine particles effect and exceptional volcanic ash activity, it is known as a "supervitamin" to improve the strength and durability of concrete.However, if not properly controlled, the black-gray powder can easily carry a deadly impurity - chlorine ions （Cl⁻）.

Chlorine ions have been called a "cancer trigger" for reinforced concrete. When the chloride content of silica ash exceeds the limit and is mixed into the concrete system, it will no longer be a supplement, but an "invisible killer" lurking inside the structure.

Based on engineering practices and materials science principles, this article will deeply analyze the deep logic and response to the excessive chloride ion in silicon ash.

I. Tracing the source: Where did the chloride ions "slipped" in?

The main component of silica fume is silicon dioxide (SiO2), which in theory should not contain chloride ions. However, as a by-product of industrial smelting, its complex chain of production provides a ready opportunity for chloride ions to invade. Chlorine is not the "native" of silicon ash, but an "invader" accidentally introduced in the process of production and circulation.

The following diagram clearly shows the full path of the chloride ion that could invade the silica ash, which we will dismantle one by one:

1. The "native carriage" of raw materials for production (the fundamental source)

Silicon ash is a volatile gas produced in a thermal furnace at an iron alloy plant when it is used to smelt silicon iron or industrial silicon. The purity of the source determines the color of the final product.

· Dangers of alloying raw materials: Although the main component of silica (quartz) for smelting is silicon dioxide, some sources are accompanied by trace chloride impurities.

· Pollution from reducing agents: More common are reducing compounds (such as petroleum coke, refined coal, charcoal, etc.). In particular, some of the lower quality coal or petroleum coke has a higher natural chlorine content.In the high-temperature environment of more than 2000 ° C in the arc furnace, chlorine can not be destroyed, but volatilized into the flue gas, and eventually enriched on the surface of micro-silica particles.

· Supplementary materials: High-temperature consumables such as electrode pastes and binders are also sources of pollution that cannot be ignored if they are not strictly dechlorinated.

2. Secondary pollution of processes (critical links)

This is the link that is most easily overlooked but often causes a surge in chloride levels.

· Deadly "quench water": SiO-containing gas discharged from the furnace is extremely hot and must be cooled rapidly to condense into amorphous microspheres. This process typically involves water cooling or spray cooling. The core risk lies in the water supply - if factories use untreated river water, salty groundwater, or even industrial reclaimed water for cost savings, the chlorine salt in the water will evaporate at high temperatures and crystallize directly onto the silica fume particles. This is the most common man-made cause of serious over-limits of silica fume chloride ions.

· Dangers of dust collection systems: In the dusting process of cloth bags, if the manufacturing process of the filter bags uses chlorine-containing chemical preparations, or the blowing system introduces polluted air, trace amounts of secondary pollution will also be caused.

3. "Environmental penetration" in the storage and transportation chain

Silica fume has a large specific surface area (15000-30000 m2 / kg), which means that it has a very strong adsorption.

· Sea salt aerosols: If the storage warehouse is located in coastal areas, and the sealing is not good, the air rich in sea salt particles (NaCl) will be absorbed by the silica fume like a sponge.

· Contact pollution: After the packaging breaks, the chloride content increases exponentially if it comes into contact with ground contaminated with de-ice salt or is dampened.

⚠️ Part 2 : How do microscopic particles destroy macroscopic structures ？

The reason why chloride ion excess is so scary is that its destructive effects are hidden, persistent and irreversible. When silica ash, which contains excess chloride ions, is added to the concrete, a microscopic "chemical war" begins on the steel surface.

· Break the line of defense: Destroy the passivation film Normally, the high alkalinity （pH > 12.5） of the concrete pore solution forms a dense oxide passivation film （γ-Fe₂O₃） on the steel surface, which is the "bulletproof vest" of the steel。However, chloride ions have extremely strong characteristics of small penetrating radius and high activity. It can adsorb where the passivation film is defective, replace oxygen ions, and form soluble iron chloride, causing the local rupture of the passivation membrane.This process is called "de-passivation."

· Induced civil unrest: Electrochemical corrosion and pit corrosion Once the passivation film is penetrated, a potential difference is formed on the steel surface。 The exposed iron base becomes the anode, and the unbroken area becomes a cathode.A violent electrochemical reaction takes place in the presence of water and oxygen.

· Pit corrosion: Chloride-induced corrosion is often not uniform, but forms deep pits of localized corrosion。 This "point type" of damage to the reinforcement section of the weakening is fatal, often in the component surface has not seen obvious cracks, the internal reinforcement has been broken.

· Catalyst effect: Chloride ions are not consumed in the reaction process, it plays the role of "cargo", the cycle of accelerating the dissolution of iron。 Corrosion will not stop as long as chlorides are present.

· Physical disintegration : The volume of rust （ iron oxide and its hydrates ） expands by a factor of 2 to 7 A thousand times. This enormous volume expansion will create tensile stresses of up to 10 trillion Pa in concrete, far exceeding the tensile strength of concrete.The result is that the concrete cracks along the direction of the steel bar, the protective layer peels off, and finally the bearing capacity of the structure decreases, even causing collapse accidents.

🔬 III. Prevention and control: building a tight quality firewall

In the face of the threat of chloride ions in silica fume, the engineering community must establish a three-dimensional prevention and control system from standard to practical practice.

1. Strict implementation of testing standards

The latest national standard GB / T 27690-2023 "Silica fume for mortar and concrete" has clearly listed the chloride ion content as a mandatory detection indicator.

· Test method: usually based on GB / T 176 " Methods for chemical analysis of cement " for determination, using potentiometric titration or ion chromatography, to ensure the accuracy of the data.

· Limit awareness: Different concrete application scenarios have different tolerances for chloride ions. For pre-stress concrete, the standards are more stringent.

2. Source and Process Control Strategy

1. Supplier Access and Traceability - Point-of-Sale Sourcing: Preferentially sourcing silicon slag from large ferroalloy plants with mature processes and clean water sources。Water source verification: in the supplier inspection stage, focus on verifying the cooling water source of the quenching system, and firmly refuse to use seawater or high salinity groundwater for cooling.

2. Incoming Inspection " Zero Tolerance " - Batch Management: Samples must be taken in batches when purchasing。 In addition to the usual active index and water demand ratio, the chloride ion content must be a compulsory test and cannot be relied upon only by the manufacturer's warranty letter.- Cross-validation: For important projects, it is recommended to periodically send samples to a third-party authority for re-inspection.

3. Design and Calculation of Scientific Mix Ratio: This is the last line of defense。 When designing the concrete combination, it is necessary to perform an accounting of the total chlorine content of the concrete. According to JGJ 55 Regulations for General Concrete Composition Design and GB 50010 Regulations on Concrete Structure Design, pre-stress concrete: the chloride content shall normally not exceed 0.06% of the total colloidal material. Reinforced concrete: usually not more than 0.10% of the total coated material (dry environment) or less (wet environment).Engineers must bring the amount of chlorine ions into silica fume, with cement, sand, stone, admixtures and mixing water chlorine ions amount of superposition calculation, to ensure the total amount is below the safety red line.

4. Standardization of storage environment: The construction site shall establish a silica powder storage tank or a dry warehouse。 Avoid stacking in the open to prevent packaging damage.It is necessary to take measures to prevent moisture and salt fog in coastal areas or areas where salt and ice are used.

Conclusion: Silica fume is small, but it is related to the overall situation。Out-of-control chloride levels often stem from a lack of attention to detail. In the pursuit of high performance concrete, we must not only see the strength dividends provided by silica ash, but also be vigilant about the durability risks that may be hidden behind it.Through strict source control, precise entrance detection and scientific proportion accounting, we are fully capable of taming this "invisible killer" and protecting the cornerstone of the century-old project.