Why Refractory Still Shatters Even With Anti-Explosion Fibers? Wrong Calcium Aluminate Cement Nullifies Anti-Explosion Fiber

Premature failure of anti-explosion fibers stems not only from inherent defects in the fibers themselves but also from the improper selection of the aluminate cement grade, which renders the fiber addition ineffective. These two contributing factors often interact with each other.

Anti-Explosion Fiber: Working Mechanism and Intrinsic Causes of Failure

The primary function of anti-explosion fibers is to form countless micro-venting channels within castables through melting, shrinkage, or combustion during the early baking stage (typically between 65°C and 220°C). Improper fiber grade, dimensions, or addition rate can result in functional failure.

Mismatched melting point:

The melting point of the fiber should be lower than the temperature at which moisture vaporizes and generates high internal pressure (normally above 200°C). A lower melting point allows venting pores to form earlier and helps reduce the risk of explosion, whereas fibers with a higher melting point cannot function effectively in time.

Improper dimensions:

Excessively long fibers impair the flowability of the castable, while overly short fibers cannot interconnect to form a three-dimensional network. Fibers that are too thick result in an insufficient number of pores, whereas ultra-fine fibers create excessive resistance to water vapor discharge. All of these conditions lead to poor anti-explosion performance.

Unreasonable addition rate:

The standard fiber dosage ranges from 0.1% to 0.2%. An insufficient dosage results in inadequate pore formation, whereas excessive addition increases overall porosity and reduces the mechanical strength and bulk density of the refractory material.

Uneven dispersion:

Poor premixing with ultra-fine powders can cause fiber agglomeration, leaving localized areas completely devoid of effective anti-explosion protection.

Core Issue: Why the Grade of Aluminate Cement Plays a Decisive Role

Incorrect selection of cement grade can render anti-explosion fiber addition completely ineffective. According to the Chinese National Standard GB/T 201-2015, aluminate cement is classified into four grades based on alumina (Al₂O₃) content: CA-50, CA-60, CA-70, and CA-80. The Al₂O₃ content directly determines the refractoriness of the material and influences its hydration products and microstructure-two factors closely related to the synergistic effect of anti-explosion fibers.

Cement grade dictates material permeability (a key factor):

Hydration of different cement grades produces distinct colloidal and crystalline structures, resulting in significant variations in intrinsic permeability. Research indicates that the permeability of castables bonded with aluminate cement can be 6 to 115 times higher than that of products using alternative binders. If a cement grade with very low permeability is used, water vapor cannot escape through the dense matrix, even when venting pores are formed by fibers, leading to continuous internal pressure buildup. This issue is common with certain cement grades whose pores are completely filled by hydration gels.

Cement grade affects curing behavior and pore channel formation:

Different curing temperatures alter the hydration rate and the resulting products of aluminate cement, which in turn influence the final matrix permeability. CA-70 and CA-80 are among the most widely used grades. While anti-explosion fibers generally improve overall permeability during curing, improper cement selection can severely impair this synergy: prefabricated fiber channels may become blocked by hydration products or damaged by early-stage internal shrinkage of the castable.

This failure often occurs during oven drying and anti-explosion treatment of refractory castables. The underlying mechanism is as follows:

Basic principle: Conventional organic anti-explosion fibers (mainly polypropylene) prevent explosions by melting or decomposing between 100–200°C to form abundant interconnected micro-channels inside castables. These channels allow high-pressure steam, generated from water vaporization, to escape smoothly, preventing burst damage caused by excessive internal pressure.

Despite the use of anti-explosion fibers, castable cracking or bursting can still occur. In addition to improper cement selection, other frequent causes include:

1. Improper fiber specification or insufficient dosage:

Standard fiber addition ranges from 0.1% to 0.3%. Under-dosage leads to inadequate venting channels. Fibers with melting points above 200°C remain intact when water vapor is generated in large quantities, failing to form the required pores. Poor mixing can also cause fiber agglomeration, preventing the formation of a uniform, interconnected exhaust network and reducing anti-explosion effectiveness.

2. Unreasonable oven-drying schedule (the most common on-site cause):

Even with suitable fibers, a rapid temperature rise and insufficient thermal insulation during the low-temperature venting interval (50–300°C) shorten the time available for venting. Rapidly accumulated internal steam pressure can exceed both the capacity of pre-formed pores and the mechanical strength of the castable, causing cracking.

3. Defective mix design or poor construction workmanship:

Excessive water-cement ratio increases the total free water content, generating more steam than fiber-derived pores can release. In thick monolithic linings, long vapor migration paths from the inner core to the surface encourage pressure buildup. Over-vibration can densify the castable surface, blocking external vapor escape and causing explosive spalling.

Wrong Selection of Aluminate Cement Grade May Render Anti-Explosion Fiber Addition Completely Ineffective

Aluminate cement grades, including CA-50, CA-70, CA-80, and pure calcium aluminate cement, differ significantly in their hydration characteristics and post-hardening pore structures. Improper grade selection can negate the anti-explosion function of the fibers.

When a cement grade with high water demand is mistakenly used, additional mixing water is required to achieve the desired workability and flowability, resulting in a significant increase in total water content. The limited micro-channels formed by the fibers cannot effectively discharge the excessive water vapor generated, thereby directly compromising their anti-spalling performance.

Certain aluminate cement grades produce large amounts of hydration gel after curing, resulting in an excessively dense matrix. The hydration gel fills the interstitial voids around the fibers and the interconnected gaps between them. As a result, even after the fibers melt, continuous venting channels cannot be formed, rendering the fibers ineffective in preventing explosive spalling.

Improper use of ultra-fast-setting cement may cause the castable to harden completely before residual free water can naturally migrate and escape after casting. The excessive amount of trapped free water may exceed the venting capacity provided by the anti-explosion fibers, ultimately leading to explosive spalling.

Comprehensive Summary and Troubleshooting Recommendations

Accordingly, explosive spalling in fiber-containing castables usually results from a mismatch among cement grade, fiber properties, and construction practices. If such defects occur, the following inspection steps are recommended:

Recheck Cement Selection

Verify whether the selected aluminate cement grade complies with the technical design requirements.

Review Fiber Specifications

Check whether the fiber melting point, aspect ratio, and dosage are compatible with the selected cement grade and service conditions.

Optimize the Construction Process

Inspect the mixing and curing procedures to ensure uniform fiber dispersion, and evaluate whether the oven-drying and heating schedule is appropriate.