Influence Of SiO₂ Crystallisation Properties in Siliceous Raw Materials On Clinker Firing

SiO₂ is the main component of silicate cement raw materials, and its crystallisation characteristics in these materials have a decisive influence on both the energy consumption during clinker firing and the quality of the resulting clinker. With the increasing use of industrial solid waste in production, the sources of SiO₂ in raw materials have become complex and diverse. Resources such as shale, sandstone, windblown sand, tailings, and slag are now widely used as siliceous raw materials. While this contributes to the stability of cement production, it also leads to higher energy consumption and presents challenges in optimising clinker firing based on the characteristics of siliceous raw materials.

1. The Physicochemical Role of SiO₂ in Siliceous Raw Materials During the Clinker Firing Process

In the clinker firing process, the SiO₂ component of siliceous raw materials participates in a series of reactions, including clay mineral decomposition and solid-phase reactions. The specific physicochemical reaction processes are as follows:

(1)When the temperature rises to 300–400 °C, clay minerals begin to lose structural water, with the reaction accelerating significantly between 500–600 °C.

(2)When the temperature exceeds 900 °C, clay minerals undergo decomposition and structural transformation. The complex Si–O anionic layers are released, creating conditions for the further depolymerisation of reactive [SiO₄] tetrahedra.

(3)Oxygen, provided through air or other oxidizing agents, dissolves into the Si–O complex anionic layers, promoting the depolymerisation of active [SiO₄] tetrahedra.

The physicochemical reaction process involved in clinker formation varies slightly depending on the type of siliceous raw material. During the clay mineral decomposition stage, different siliceous materials exhibit varying degrees of decomposition and different post-decomposition structures. For example, montmorillonite and illite retain a crystalline structure after dehydration, whereas kaolinite decomposes into amorphous metakaolin upon dehydration, making it more reactive.

In the solid-phase reaction stage, however, the SiO₂ in all siliceous raw materials chemically combines with CaO through diffusion reactions. This process leads to the formation of C₂S (dicalcium silicate), a key mineral phase in cement clinker, during sintering. Therefore, the ability of SiO₂ to combine with CaO during the solid-phase reaction stage is a critical factor in evaluating the reactivity of siliceous raw materials.

To produce calcium silicate minerals, the siliceous component must undergo depolymerisation of the reactive [SiO₄] tetrahedra during firing, enabling it to react with newly formed reactive CaO. Since the melting point of crystalline SiO₂ is higher than the clinker firing temperature, the [SiO₄] tetrahedra cannot melt under these conditions; they can only be activated through external forces or chemical dissolution. Thus, the crystalline chemical properties of SiO₂ in different siliceous raw materials significantly affect the depolymerisation of [SiO₄] tetrahedra, which in turn plays a crucial role in clinker formation and directly impacts both clinker quality and firing energy consumption.

2. The Raw Material Characteristics of Siliceous Raw Materials

Siliceous raw materials vary significantly in their characteristics depending on their form and source. These characteristics mainly include the type of Si–O structure in the main minerals, quartz content and particle size, quartz crystal type, and crystallinity-all of which directly affect the clinker firing process.

(1). Si–O Structure

According to crystallographic and mineralogical classifications, siliceous raw materials can be divided into silicate minerals and quartz minerals. Silicate minerals can be further classified into five structural types based on their Si–O configurations: island, ring, chain, layer, and framework structures, with increasing structural complexity.

Since the C₂S and C₃S phases in clinker possess island-like structures, all types of siliceous raw materials must depolymerise independent [SiO₄] tetrahedra to combine with CaO and form calcium silicate minerals. Silicate minerals with different Si–O structures vary in the difficulty of depolymerising reactive [SiO₄] tetrahedra during firing, and thus exhibit different reactivities in the clinker formation process.

Quartz minerals fall under the framework (also known as "tectosilicate") Si–O structural category. However, unlike framework silicate minerals such as feldspar, quartz has the structural formula [SiO₂]. This structure is highly stable due to the zero-valent electrovalence, making the depolymerisation of [SiO₄] tetrahedra more difficult than in other silicate minerals. As a result, quartz shows lower reactivity in the clinker firing process.

When high-temperature, rapidly cooled slag and similar materials are used as siliceous components in raw mixes, the Si–O bonds are disrupted, forming simpler Si–O structures. This enhances the burnability of the raw materials. Therefore, selecting silicate minerals with simpler Si–O structures-such as island, ring, or chain types-as siliceous raw materials in cement production can improve clinker quality and reduce the energy consumption during clinker firing.

(2). Effect of Quartz Content and Grain Size on Clinker Firing

Quartz content and grain size are key factors influencing the burnability of raw materials. A higher proportion of coarse quartz particles in siliceous raw materials leads to lower reactivity, which makes clinker firing more difficult. There is a strong correlation between the burnability of raw materials and the grain size of quartz crystals-the coarser the grains, the stronger this correlation.

From a quantitative perspective, for every 1% increase in coarse quartz content, the amount of free calcium oxide in the clinker increases by approximately 0.93%. A grain size of 45 μm is generally considered the critical threshold beyond which the influence of quartz on clinker firing becomes more pronounced. Once this size is exceeded, the firing quality of the clinker gradually deteriorates as quartz grain size increases.

It is generally believed that the mechanism by which quartz grain size affects clinker firing lies in its impact on belite formation during the solid-phase reaction stage. Specifically, quartz grain size influences the rate of belite formation at lower temperatures but does not significantly affect the formation rate of alite at higher temperatures.

At around 1200 °C, coarse quartz grains react with CaO to form three different types of belite clusters. These clusters have dense surfaces and undergo significant volume expansion-up to 7–8 times the original volume of the quartz grains. As the reaction progresses, the belite clusters continue to grow and eventually interconnect, forming continuous belite zones. These zones hinder the subsequent reaction between belite and CaO, thereby slowing the formation of alite and increasing the content of free calcium oxide in the final clinker.

(3). Effect of Quartz Crystal Type and Crystallinity on Clinker Firing

Quartz exhibits various crystal forms at different temperatures. Although the transformation between these crystalline forms does not alter the framework Si–O skeleton structure, and theoretically the difficulty of depolymerising independent [SiO₄] tetrahedra remains nearly the same, differences in reactivity among different quartz crystal types have still been observed.

The reactivity of different forms of SiO₂ with CaO generally increases in the following order:

quartz < chalcedony < α-quartz < α-phosphorous quartz < SiO₂ in mica < SiO₂ in clay < amorphous SiO₂.

The crystallinity of quartz also affects the reactivity of SiO₂ during clinker firing. Amorphous SiO₂ exhibits higher reactivity with CaO than crystalline quartz. However, quartz glass, despite being non-crystalline, has even lower reactivity, which negatively impacts the sintering performance of clinker.

During the crushing and milling processes, structural defects in quartz crystals can also influence their reactivity. Grinding tends to disrupt the crystal structure, generating numerous unstable, newly formed surfaces that increase reactivity. For example, in ball-milled raw materials, quartz grains typically remain intact without visible microcracks. In contrast, raw materials processed by vertical mills often show microcracks within quartz grains and increased lattice defects, which facilitate the diffusion of CaO into the quartz. This promotes the formation of C₂S and C₃S, improving burnability and enhancing the clinker quality.

3. Methods for Adjusting the Reactivity of SiO₂ in Quartz Raw Materials

Due to the chemical inertness of quartz raw materials, their SiO₂ reactivity can be improved based on the factors affecting the reactivity of siliceous raw materials. The primary methods include mechanical activation and chemical activation.

Mechanical activation mainly involves controlling the fineness of the siliceous raw material so that the quartz grains are smaller than the critical particle size.

Chemical activation enhances reactivity by doping with specific ions, adding mineralising agents, or supplying oxygen. These approaches work by disrupting the complex Si–O structure or transforming quartz into a more reactive crystal form, thereby increasing the reactivity of SiO₂.

Mineralising agents are commonly used to lower the temperature at which the liquid phase appears and to reduce its viscosity. Certain mineralising agents can also alter the crystalline structure of SiO₂, improving its reactivity and, consequently, the quality of clinker firing.

For example, fluorite, a common mineralising agent containing Ca²⁺ and F⁻ ions, can break down complex Si–O structures and depolymerise independent [SiO₄] tetrahedra, thereby increasing the reactivity of silicate minerals. Carbonates and borates can facilitate the transformation of quartz in sandstone into more reactive forms, such as tridymite, thus enhancing quartz reactivity.

The addition of alkali metal ions such as K⁺, Na⁺, and other ions like Ca²⁺ can also promote the depolymerisation of the complex Si–O structure and increase SiO₂ reactivity. Additionally, OH⁻ ions can replace part or all of the oxygen atoms in the [SiO₄] tetrahedra, breaking the long-chain polymers with high viscosity and reducing their viscosity, which further enhances reactivity.

4. Conclusion

For cement enterprises, improving the quality of clinker firing and reducing energy consumption in cement production can be significantly achieved by preferentially controlling the quality of siliceous raw materials in the following ways:

Select appropriate siliceous materials: High-temperature, rapidly cooled slag and similar materials should be prioritized as siliceous raw materials. The Si–O bonds in these materials are broken, forming simpler Si–O structures that enhance raw material burnability.

Control quartz particle size: Ensure that the quartz content in the raw material's sieve residue is less than 45 μm, which improves the burnability of the raw mix.

Prioritize siliceous materials based on reactivity: The preferred order of selection is:

amorphous SiO₂ > SiO₂ in clay > SiO₂ in mica > α-phosphorous quartz > α-quartz > chalcedony > quartz.

Use efficient grinding methods: Vertical mill grinding or roller press grinding should be prioritized to enhance the effectiveness of raw material preparation.

Introduce impurity ions: The addition of specific impurity ions can enhance the reactivity of siliceous materials and improve the burnability of the raw mix.