The impact of andalusite on the quality of refractory materials is multifaceted.

The impact of andalusite on the quality of refractory materials is indeed multifaceted; when utilized appropriately, it can significantly enhance refractory performance, whereas improper application may lead to a deterioration in quality:
The positive effects of adding andalusite in appropriate quantities (typically controlled within the 10%–30% range) include:

1. Optimization of Microstructure: At high temperatures, andalusite undergoes a mullitization reaction, generating an *in-situ* network of interlocking acicular mullite crystals that serve a reinforcing function. Furthermore, this process increases the proportion of the mullite phase within the matrix while reducing the glassy phase, thereby optimizing the matrix's physicochemical properties. The SiO₂ glassy phase generated during this reaction can also fill pores, thereby enhancing the density of the final product. 

2. Enhancement of Core Service Performance: It improves the refractory material's creep resistance, flexural strength, thermal shock stability, and slag resistance, with refractoriness capable of exceeding 1800°C. Moreover, the irreversible volume expansion of approximately 4% that occurs during the mullitization process serves to counteract the refractory material's high-temperature shrinkage, thereby effectively improving its high-temperature volume stability.

Negative Effects of Excessive Andalusite Addition (Exceeding 30%) or Inappropriate Parameters:

1. Excessive addition leads to increased material porosity and reduced room-temperature strength; furthermore, the excessive formation of mullite can trigger internal stress concentrations, thereby degrading the overall structure and performance of the material. 

2. Inappropriate grain size also compromises quality: the finer the andalusite grain size, the faster the mullitization reaction proceeds—a phenomenon that, counterintuitively, results in reduced material density. Conversely, increasing the grain size corresponds to higher bulk density and lower apparent porosity; therefore, the appropriate grain size must be selected based on the specific requirements of the final product.

The following presents specific test data from existing research regarding the impact of andalusite on the quality of refractory materials:

1.  Fundamental Phase Transformation Characteristics:** Upon heating to 1300°C, andalusite begins to transform into mullite; above 1350°C, acicular mullite forms, aligned along the orientation of the original crystals. This transformation process generates an irreversible volume expansion of approximately 4%, which serves to counteract the high-temperature shrinkage of the refractory material; following this transformation, the material's refractoriness can exceed 1800°C.

2.  Performance Data Based on Varying Addition Levels:

•   In low-cement high-alumina castables, when andalusite is added at levels of 10%, 15%, and 20% (by mass fraction—hereinafter referred to as the same basis), increasing the andalusite content leads to a gradual rise in the load-softening temperature and high-temperature flexural strength of the castable, while its volume stability continues to improve.

•   The creep resistance of clay-based refractory materials improves as the andalusite content increases; industry practice typically limits this addition level to no more than 30%.

•   Within an appropriate range of addition, the SiO₂ glassy phase generated during the mullitization of andalusite can fill pores; consequently, the porosity of the refractory material gradually decreases as the andalusite content increases.

•   When the addition level exceeds 30%, it results in increased material porosity and a decline in room-temperature strength; the excessive formation of mullite triggers internal stress concentrations, thereby degrading the material's overall service performance.