Influence of silica crystallinity and fineness in clinker raw meal on the formation of tri-calcium silicate polymorphs

Abstract

This study aims to deepen the understanding of the formation of tricalcium silicate (alite) polymorphs in synthetic clinker, produced under controlled conditions that simulate industrial scenarios, with the goal of contributing to more sustainable practices in Portland cement production. The innovation of this research lies in the combined evaluation of the effects of crystallinity and particle size of silica—in the forms of crystalline quartz and amorphous silica gel, across different granulometric ranges—on the nucleation and stabilization of the M1 and M3 monoclinic polymorphs of alite. Using advanced characterization techniques such as laser granulometry, X-ray fluorescence, X-ray diffraction, and optical microscopy, it was demonstrated that increasing the silica particle size, regardless of crystallinity, reduces the alite content and promotes an increase in the content of free lime and belite. Furthermore, the silica particle size directly influences the size of alite crystals, while the interaction between crystallinity and particle size has a significant impact on the stabilization of alite polymorphs. These findings are essential for optimizing the clinker manufacturing process, potentially reducing energy consumption and promoting the production of more reactive forms of cement, thereby contributing to sustainability in the cement industry.

Keywords:
Alite; Crystallinity; Rietveld Refinement; Quartz

1. INTRODUCTION

Having celebrated its 200th anniversary, the Portland cement industry, a strong indicator of the development of modern society, faces its greatest global challenge. Especially since the 1990s, reducing CO₂ emissions has become, undeniably, a priority for improving life cycle performance. The global contribution of the Portland cement industry is approximately 5% of anthropogenic CO₂ emissions on the planet [¹]. Although the significance and implications of this percentage may be debated, Portland cement could potentially account for even greater emissions in the future given the necessity of improving infrastructure worldwide. Despite various studies presenting new products as substitutes, there are no signs of a material being developed that can match Portland cement in terms of production volume, low cost, ease of application, short-term and long-term mechanical strength development, and durability. With this scenario, the sector has implemented a set of measures that have allowed for a systematic reduction of these emissions, a fact confirmed by official emission data for the sector. While average global emissions were around 800 Kg CO₂/t. cement in the 1990s, they were around 670 Kg CO₂/t. cement in 2015 [²].

Among some strategic actions towards sustainability, attention has been given to using alternative fuels, co-processing, and clinker replacement materials. The effect of those actions demands in-depth consideration. For instance, minor constituents affect the crystal structure of clinker components, especially alite, which can have adverse consequences for production and durability issues [³].

The exploration of such strategies also brings about complex technological challenges, opening possibilities for adjustments in manufacturing processes to maximize the production of the most reactive phases. For instance, it is imperative to understand how reducing the thermal consumption of kilns, decreasing electrical consumption in all stages of the production process, and replacing fossil fuels with significant calorific power sources affect some parameters such as the combustion dynamics, furnace residence time, and raw meal chemical and physical characteristics.

As far as the raw meal characteristics are concerned, the presence of silica (c.a. 12–14%) is crucial to the formation of the main clinker crystals [⁴]. Indeed, its reactivity is reported to depend on the degree of crystallinity of the original mineral, geological characteristics, and particle size, which significantly influence the manufacturing process. This influence extends from the grinding of the raw mix to homogenization and burning, as well as the wearing due to abrasion in industrial mills, electrical consumption in raw mix grinding, and thermal consumption of the kiln [⁵]. Nevertheless, the effect of the silica source’s physical and crystallinity on the clinker’s crystalline structure is still lacking in systematic studies, as it influences the cement grinding performance, development of mechanical strengths, and other cement characteristics. The silicate sources are predominantly crystalline phases such as quartz and feldspar. They play a very important role as they are present in virtually all raw mixes for clinker manufacture, representing in most cases over 40–60% of the silica content.

The physical characteristics of the phases of raw materials can affect the solidification of clinker phases. It has been reported that calcite grains with diameters greater than 125 µm formed regions of free lime with a diameter of approximately 100 µm, whereas quartz grains with diameters greater than 44 µm generated compact zones of belite belite with a diameter also around 100 µm, when subjected to firing at 1,400°C for 30 minutes [⁶,⁷,⁸,⁹,¹⁰]. Reducing the maximum size of silicon grains in the raw material from 100 µm to 75 µm results in a decrease in the average size of alite alite from 30 µm to 20 µm and of belite from 28 µm to 22 µm, thereby improving the grindability of the clinker by up to 15% [¹¹]. However, other researchers, attribute the growth of alite and belite crystals exclusively to process variables such as kiln residence time, temperature, and dimensional characteristics of the kiln [¹²]. As for the belite crystals, an average diameter between 20 and 40 µm is characteristic of the normal residence time of material inside the kiln [⁵].

Studies on the influence of silica particle size on the diameter of alite crystals demand systematic investigation. ONO [¹³] developed a methodology for the microscopic evaluation of crystal size and other process variables. The Brazilian Cement Association [¹⁴] adopted this methodology to evaluate the characteristics of clinker crystals and some manufacturing process variables, a method widely disseminated in Brazil. It has been concluded that the average dimensions of alite crystals are controlled by the burning temperature and residence time in the kiln. Longer flames result in slow heating of the material inside the kiln, leading to prolonged clinkerization time, causing larger crystals of alite and belite [⁸]. Heating rate is the most important factor in determining the size of alite crystals, as a high heating rate would increase alite crystal nucleation, resulting in a decrease in the growth rate of these crystals [¹⁵]. However, the methodology does not consider the influence of silica particle size. Additionally, the impact of silica particle size present in the raw mix on alite polymorphism remains a matter for in-depth investigation. As far as the polymorphism of alite phase is concerned, STANĚK and SULOVSKÝ [¹⁶] studied the influence of MgO and SO₃ on the stabilization of T3, M1, and M3, alite polymorphs, stating that the mechanical strengths of cement produced from clinker with alite M1 were 10% higher compared to cement produced from clinker with alite M3. The same authors suggest that a smaller grain size in the raw mix should favour the stabilization of polymorph M3, while raw mixes with larger grain sizes should favour the stabilization of polymorph M1. Also, LI et al. [¹⁷] studied the influence of MgO and SO₃ on the stabilization of alite polymorphs M1 and M3 but stated that an increase in SO₃ content hinders alite formation, and this effect can be mitigated by increasing MgO content [¹⁷].

KRIVOBORODOV and SAMCHENKO [¹⁸] presented positive results regarding the burning of the raw mix, increased alite content, and gains in the mechanical strength of cement when the particle size of the raw mix is reduced. However, no more detailed data is provided on the particle size distribution of raw mix components or any specific relationship with silica granularity and no reference was made to the particle size distribution of silica. Thus, the influence of particle size, as well as its crystallinity on alite polymorphism, still needs to be investigated.

In spite of the physical aspects of the raw meal as in above mention studies, the solidification of clinker phases is also dependent on the rheology of the liquid phases. TIMASHEV [¹⁹] conducted a study demonstrating important aspects of the rheology of the liquid phase, studying the importance in the formation of belite and alite crystals, and how the introduction of minor chemical elements, such as S, MgO, K, Na, etc., affects this rheology and interferes in clinker crystal solidification. TAYLOR [⁴] state that sulfur (S) plays a crucial role in clinker crystal formation, affecting nucleation and growth rates, favouring alite growth and stabilizing morphological form M1. On the other hand, magnesium (Mg) predominantly promotes nucleation, thus reducing alite crystal size while stabilizing the morphological form M3. Consequently, these two elements have opposite effects on the rheology, kinetics and nucleation of clinker formation reactions.

Therefore, although the published literature indicates the chemical and physical aspects governing the solidification of clinker phases, there is still a lack of knowledge on the particle dimensions of silica, specifically the crystalline phases such as quartz. The objective of this study is to evaluate the influence of the size of silica particles, as well as their crystallinity, on the solidification of the alite phase and its polymorphs.

2. MATERIALS AND METHODS

2.1. Preparation of raw meal samples

Table 1 presents the chemical compositions of the raw meals, determined by X-ray fluorescence (XRF), formulated based on the average raw mix used by a cement manufacturer in Brazil. This formulation takes into account the enrichment of volatile elements commonly observed during the clinker production process. Analytical-grade reagents were used in the preparation of the raw meals, with the following purities: CaCO₃ (99.0%), CaSO₄ (99.6%), MgCO₃ (99.1%), Fe₂O₃ (98.8%), Al₂O₃ (98.8%), Na₂O (98.2%), and K₂O (98.9%).

Table 1
Standard chemical composition of the raw meals.

SAMPLES	Q1	Q1A	Q2	Q3	Q4	Q5	AS6	AS6A	AS7	AS8
> 53 µm (%)	0.23	0.23	40.46	70.31	94.19	98.1	0.15	0.15	76.85	96.13
PF	35.39	35.28	35.28	35.19	35.25	35.13	35.42	35.5	36.25	35.9
SiO2	12.59	12.92	12.92	12.65	12.67	12.64	12.76	12.58	12.73	12.8
Al2O3	2.94	2.84	2.93	3.04	3.05	2.94	2.91	2.9	2.89	2.9
Fe2O3	1.81	1.92	1.78	1.81	1.81	1.78	1.8	1.88	1.82	1.89
CaO	40.5	40.89	40.5	40.62	40.68	40.53	40.72	40.5	40.39	40.55
MgO	3.39	3.17	3.36	3.48	3.49	3.36	3.15	2.99	3.01	3,00
SO3	2.35	1.61	2.32	2.48	2.43	2.32	2.49	3.31	2.4	2.32
K2O	1.27	1.32	1.27	1.3	1.29	1.26	1.27	1.23	1.27	1.22
Na2O	0.13	0.1	0.1	0.11	0.13	0.14	0.12	0.1	0.13	0.1
FSC	101.53	100.34	100.89	101.09	101.05	101.27	100.96	101.59	100.38	100.15
MS	2.65	2.73	2.7	2.61	2.61	2.68	2.71	2.63	2.7	2.67
MA	1.62	1.47	1.65	1.68	1.69	1.65	1.62	1.54	1.59	1.53

For the preparation of the raw mix samples, pure chemical compounds (analytical grade) were used in the form of carbonates, oxides, and sulfates; crystalline silica in the form of quartz, and amorphous silica in the form of silica gel. The chemical composition of the silica sources is presented in Table 2, and their respective diffractograms are shown in Figure 1. The quartz and amorphous silica were crushed using a jaw crusher and subsequently ground in a ring mill. After grinding, the quartz and amorphous silica were separated into fractions using standardized sieves, creating five distinct fractions for the raw mix compositions with quartz and three fractions for those with amorphous silica.

Table 2
Particle size classification of crystalline quartz and amorphous silica gel.

CHEMICAL COMPOSITIONS OF THE SILICAS
	LI	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	K2O	Na2O
Quartz	0.38	98,30	0.50	0.16	0.04	0.02	0.04	0.02	0.00
Amorphous Silica	0.42	97,98	0.48	0.39	0.01	0.00	0.01	0.07	0,03

Figure 1:
Silica diffractograms.

The crystalline and amorphous silica fractions were tested using the SILAS Laser Granulometer, in an alcoholic medium, to establish particle size distribution curves. From the five quartz fractions, five raw mix samples were prepared, and from the three amorphous silica fractions, three samples were prepared.

Note: Additionally, two more raw mix samples were prepared from the finest silica fractions—one with quartz and low SO₃ content, and another with amorphous silica and high SO₃ content.

The chemical composition of the samples was determined by X-ray fluorescence. The standard used was ABNT NBR 14656 – Portland Cement and Raw Materials – Chemical Analysis by X-ray Spectrometry. Chemical analyses of all samples — quartz, amorphous silica, raw mix, and clinker — were performed in triplicate, with the final result expressed as the average of each triplicate. Other standards used for chemical analyses of both the raw mixes and the produced clinkers included:

• ABNT NBR NM 13 – Portland Cement – Chemical Analysis – Determination of Free Lime by Ethylene Glycol;

• ABNT NBR NM 18 – Portland Cement – Chemical Analysis – Determination by Loss on Ignition.

The equipment used for chemical analysis was the Axius-100 – Panalytical, using fused pellets and NIST standards.

2.2. Clinker synthesis

The samples of the raw mixtures were moistened with deionized water, manually nodulized into diameters ranging from 0.5 to 1.5 cm, then dried in an oven at 100°C for 60 minutes and placed in a desiccator before being fired in the kiln. The nodules were placed inside a preheated muffle furnace at 1,450°C, where they remained for 20 minutes, and were then cooled with an air flow to room temperature, at a cooling rate similar to that of an industrial grate cooler, thereby simulating typical industrial process conditions. The firing procedure was carried out in an EDG muffle furnace with a maximum temperature of 1,700°C.

2.3. Optical microscopy

The clinker samples were analyzed using optical microscopy with the ONO [¹³] method- ology. The samples were prepared with phenolic resin under vacuum; then, they were ground with sandpapers ranging from 200 to 1200 grit and polished with diamond paste with a diameter of 0.25 µm. The equipment used was the OLYMPUS BX51M reflected light microscope, supported by the Stream Essentials 1.9 software for data recording and analysis. In order to determine the particle distribution, a set of 10 images of different magnifications were used per each mix.

2.4. X-ray diffraction

X-ray Powder Diffraction (XRPD) data for the silica and produced clinker samples were collected using a Bruker D2 PHASER diffractometer, operating in reflection geometry with Cu Kα radiation. The data acquisition was performed over a 2θ range of 5° to 70°, with a step size of 0.02° and a counting time of 0.5 seconds per step. The samples were ground in a ring mill until reaching a granulometry of 100% passing 38 µm, then submitted to the equipment using an oriented silica sample holder, which is ideal for minimizing background noise effects in phase quantification.The refindment quality parameters used were Rwp and GOF listed for each spectrum and the cystral structure models were selected as follows, Table 3 e 4.

Table 3
Parameters X-ray powder diffraction.

FASE/POLIMORFO	C3S/M1	C3S/M3	C2Sβ	C3A CÚBICO	FERRÍTA	PERICLÁSIO
ICSD	Willian/2018-UFPB	94742	81096	1841	9197	9863

Table 4
XRD refinement results.

AMOSTRAS	Q1	Q1a	Q2	Q3	Q4	Q5	AS6	AS6a	AS7	AS8
RWP	7,760	7,830	8,080	7,850	9,170	9,290	7,280	7,250	7,820	8,860
GOF	2,080	2,010	2,130	2,120	2,440	2,520	2,000	2,030	2,170	2,410

3. RESULTS AND DISCUSSIONS

3.1. Analysis of the particle size distribution of silica

Figure 2 shows the particle size distribution of quartz and amorphous silica in each of the five selected particle size ranges for quartz and three for amorphous silica.

Figure 2:
Cumulative fraction (%) vs Diameter (µm).

Note that Figure 2 presents us with two important pieces of evidence. The granulometric fractions of silica Q1 and AS6 have very similar particle distributions, showing a very low percentage of particles > 53 µm. For this reason, the study will focus on the effect of increasing the percentage of silica particles > 53 µm on the structure and crystalline composition of the clinker, as well as the effect of the average diameter of silica particles in each fraction on the average diameter of alite. In the clinker samples, for comparative purposes between the two systems—crystalline and amorphous—the analysis will be conducted between those with granulometric compatibility, i.e., samples containing Q1 and AS6; Q3 and AS7; Q5 and AS8. This granulometric equivalence is evidenced in Figure 2.

Figure 3 shows the average diameter of each silica fraction, corresponding to the quartz samples (Q) (squares) and the amorphous silica samples (AS) (circles). It can be observed that both silica sources have similar average diameters within the established study ranges.

Figure 3:
Average diameters v.s fraction range plot. (circle): amorphous silica; (square): quartz.

Table 5 provides a summary of the data from Figures 1 and 2, corresponding to the average diameters of each silica fraction and the percentage of particles > 53µm for each sample tested. The samples Q1a and AS 6a were introduced into the tests to evaluate the influence of sulfur.

Table 5
Granulometric data of samples: average particle diameter and percentage of particles larger than 53 µm.

SAMPLE	Q1	Q1a	Q2	Q3	Q4	Q5	AS6	AS6a	AS7	AS8
Average Diameter µm	14.78	14.78	49.78	60.79	82.23	115.64	16.47	16.47	64.85	120.82
Particles > 53 µm (%)	0.23	0.23	40.46	70.31	94.19	98.10	0.15	0.15	76.85	96.13

3.2. Overall chemical composition of oxides by XRF and fundamental control parameters

Table 6 presents the results of the chemical analyses of the clinker samples. It can be observed that all fundamental control chemical parameters were standardized for both the quartz and amorphous silica samples, which is essential for assessing the effect of silica particle size. The variation of sulfur in samples Q1a and AS 6a aimed to study the influence of this chemical element on the observed parameters.

Table 6
Chemical analyses of the clinkers.

SAMPLE	Q1	Q1a	Q2	Q3	Q4	Q5	AS6	AS6a	AS7	AS8
>53 µm	0.23	0.23	40.46	70.31	94.19	98.10	0.15	0.15	76.85	96.13
LI	0.60	0.60	0.55	0.58	0.57	0.52	0.60	0.60	0.55	0.51
SiO2	19.63	20.14	19.71	19.53	19.63	19.64	20.05	19.60	19.69	19.92
Al2O3	4.62	4.35	4.61	4.58	4.60	4.45	4.42	4.46	4.38	4.59
Fe2O3	2.63	2.83	2.60	2.61	2.64	2.61	2.71	2.78	2.73	2.71
CaO	63.01	63.61	63.04	62.42	62.71	62.80	62.95	62.78	62.73	62.89
MgO	5.08	4.75	5.07	5.20	5.07	5.00	4.91	4.82	4.97	4.90
SO3	2.65	1.81	2.69	2.99	2.81	2.50	1.88	2.56	1.82	1.87
K2O	1.54	1.60	1.49	1.82	1.66	1.55	1.68	1.70	1.73	1.79
Na2O	0.21	0.16	0.18	0.10	0.06	0.18	0.14	0.12	0.14	0.15
LSC	101.44	100.39	101.15	101.03	100.97	101.39	99.74	101.34	101.06	99.90
SM	2.71	2.81	2.74	2.71	2.71	2.78	2.81	2.71	2.77	2.73
AM	1.76	1.54	1.77	1.75	1.74	1.70	1.63	1.60	1.60	1.69
MgO/SO3	1.91	2.62	1.88	1.74	1.80	2.00	2.61	1.78	2.73	2.62

The only significant variation in chemical composition introduced in the sample series corresponds to the sulfur content in samples Q1a and AS 6a. This was intended to highlight the influence of this chemical element on the observed parameters, as referenced in the literature cited in this article, thereby emphasizing the strong influence of silica particle size.

3.3. Analysis of the crystalline phase of clinker by X-Ray powder diffraction and alite diameter via optical microscopy

Figure 4 presents the X-ray Powder Diffraction patterns (XRPD patterns) of the clinker samples.

Figure 4:
XRPD patterns of the clinker samples. (1): Ferrite (9197); (2): M1 Alite (FERNANDES et al. [³]); (3): M3 Alite (94742); (4): Belite (81096); (5): Aluminate (1841); (6): Lime (52783); (7): ericlase (104844); (8): Portlandite (15471).

The proportions of the phases obtained by the Rietveld refinement method and the results from the optical microscopy analysis are presented in Table 7. It is worth noting that only the phases evaluated in the study are described in the table.

Table 7
Proportions of phases present in clinker samples as determined by Rietveld refinement and optical microscopy.

OPTICAL MICROSCOPY
SAMPLE	Q1	Q1a	Q2	Q3	Q4	Q5	AS6	AS6a	AS7	AS8
Quantity of measured crystals	666	1218	719	436	801	732	551	823	440	512
Average D.C3S (μm)	26.70	15.32	35.23	40.15	51.92	59.65	13.22	22.50	24.56	34.19

Note that the increase in the percentage of silica particles > 53µm leads to significant changes in all analyzed phases of the clinkers, as detailed in the following analysis.

Considering the data from Table 6, it is observed that the variation in sulfur percentage in the Q1 and Q1a samples, as well as in the AS 6 and AS 6a samples, caused a significant change in the alite diameter. In the samples with quartz (Q1 and Q1a), the reduction in sulfur percentage led to a 42.62% decrease in the average alite diameter. On the other hand, in the samples with amorphous silica (AS 6 and AS 6a), the increase in sulfur percentage resulted in a 70.20% increase in the average alite diameter.

3.3.1. Influence of silica size on free lime

Figure 5 shows the behavior of free lime in relation to the increase in the percentage of silica particles > 53µm for samples with crystalline and amorphous silica, including samples Q1a and AS 6a. Points represented by circles refer to amorphous silica samples, while points represented by squares refer to quartz samples; circles and squares in red refer to samples AS 6a and Q1a, respectively.

Figure 5:
Free lime v.s % silica > 53µm plot. (square black): quartz; (circle gray): amorphous silica. (green icons): points Q1a and AS 6a.

Figure 4 shows that the free lime content in the clinker increases with the increase in the percentage of silica particles > 53µm for both quartz and amorphous silica samples. Even when considering the data from samples Q1a and AS 6a, which have low and high sulfur content, respectively, the observation remains consistent; indicating that, regardless of the crystallinity of the silica, the free lime in the clinker was always proportional to the size of the silica particles, provided that the chemical composition of the raw mixture, temperature, firing time, and cooling conditions are maintained.

3.3.2. Influence of silica particle size on alite size

Figure 6 shows the behavior of alite crystal growth about the growth of the average particle size of quartz and amorphous silica, excluding the Q.1a and A.S.6a samples.

Figure 6:
Variation of the average alite diameter about the average diameter of quartz (square) and amorphous silica particles (circle).

It is noted in both sets of samples the excellent correlation between the average diameters of the quartz and amorphous silica particles in relation to the average diameter of alite, proving the influence of the silica particle size on the growth of alite. However, the set of samples with quartz shows alite diameters noticeably higher than those with amorphous silica. Considering the equivalent samples from both sets, quartz (Q1, Q3, and Q5) and amorphous silica (A.S.6, A.S.7, and A.S.8), the quartz sample set shows an average alite size 75.77% higher than the amorphous silica sample set, which is significant. This fact seems to indicate an important influence not only of the silica particle size on the growth of alite but also of its crystallinity. It is worth noting that this influence becomes more significant as the size of the silica particles increases.

However, according to Table 4, the set of samples with quartz (Q1, Q2, Q3, Q4, and Q5) has an average SO₃ content of 2.73%, while the set of samples with amorphous silica (A.S.6, A.S.7, and A.S.8) has an average SO₃ content of only 1.86%. Studies by MAKI [¹⁵] and TAYLOR [⁴] reveal a strong influence of sulfur on the growth of alite crystals, and this may be the most relevant factor between the two sets, rather than their crystallinity. We can evaluate this issue by looking at Figure 6, which shows the effect of sulfur on the samples Q.1 (high sulfur), Q.1a (low sulfur), A.S.6 (low sulfur), and A.S.6a (high sulfur), as presented in Table 5 and Table 6.

The samples shown in Figure 6 have equivalent silica particle sizes. As presented in Table 3, the samples in Figure 7 were prepared with comparable silica particle sizes. When comparing samples Q1 and AS6a, both with high sulfur content, it is observed that the average size of the alite crystals is only slightly affected; the same is true when comparing the average size of the alite crystals in samples Q1a and AS6, both with low sulfur content.

Figure 7:
Average silica diameter v.s average alite diameter plot.(circle): amorphous silica; (square): quartz.

This indicates that the effect of sulfur is considerably greater than the effect of silica crystallinity on the growth of alite crystals. Therefore, the silica particle size, as shown in Figure 5, and the sulfur content have a significant effect on the size of the alite crystals, while the crystallinity of the silica appears to have only a minor influence

3.3.3. Phase quantification of samples

Figures 8 and 9 present the quantification diagram of the main phases, shown in Table 7, normalized to 100%, corresponding to the samples with quartz and amorphous silica, including the points for samples Q1a and AS 6a with special highlights in the diagram. The square icon represents the results for free CaO, the circular icon represents the results for belite, and the triangular icon represents the results for alite M3, with the phase above corresponding to alite M1.

Figure 8:
Diagram showing the phase propostions present in samples with crystalline silica. The green- colored icons refer to sample Q1a. The lightest gray region represents the amount of M1, the second lightest gray region represents the amount of M3, the second darkest gray region represents the amount of belite, and the darkest gray region represents the amount of free CaO.

Figure 9:
Phase quantification diagram of the samples with amorphous silica. The green-colored icons refer to sample AS 6a. The lightest gray region represents the amount of M1, the second lightest gray region represents the amount of M3, the second darkest gray region represents the amount of belite, and the darkest gray region represents the amount of free CaO.

The results presented in Figure 8 are in full agreement with those demonstrated by TIMASHEV [¹⁹], especially when we observe the behavior of sample Q1 in relation to Q1a. In these samples, the positive contribution of the increase in sulfur content to the quality of the clinker becomes evident, highlighting the importance of controlling the MgO/SO₃ ratio in the pursuit of maximizing the percentage of the M1 polymorph over M3 in alite. However, the study also adds evidence of the close relationship between the elimination of coarse silica particles larger than 53 µm and the maximization of alite content, particularly the M1 polymorph, while also reducing the content of belite and free lime.

This indicates that the diameter of silica is directly related to the kinetics of alite formation and its polymorphs. The quality of the clinker deteriorates as the percentage of silica particles > 53µm increases, which will certainly negatively impact the cement’s performance in terms of mechanical strength.

For the tests of the samples with amorphous silica, Figure 9 and Table 6, the influence of the percentage of silica particles > 53µm on the main clinker phases is also observed. As the percentage of amorphous silica particles increases, a 50% reduction in the M1 polymorph content of alite is observed between samples AS 6 and AS 8; a 483% increase in belite content; and a 242% increase in free lime content. However, unlike the crystalline system, there is a 48% decrease in the content of the M3 polymorph between these samples. When comparing the amorphous silica sample AS 6 to its equivalent quartz sample, Q1, AS 6 shows 13% less M1 content; 43% more belite; and 76% more of the M3 polymorph. However, the AS 6, 7, and 8 samples were prepared with low sulfur content, maintaining a MgO/SO₃ ratio between 2.60 and 2.80, which might explain the discrepancy. The analysis of sample AS 6a, with an MgO/SO₃ ratio of 1.78, therefore, absolutely comparable to sample Q1, reveals behavior opposite to that of the quartz sample. Specifically, the decrease in M1 content is 21%, and the increase in M3 content is 172%, showing an inverse behavior compared to the quartz samples.

3.3.4. Behavior of the M1/M3 ratio and the % of silica

Figures 10 and 11 show the behavior of the ratio M1/M3 in relation to the percentage of silica particles > 53µm for the crystalline and amorphous systems. In Figure 9, the point marked in green corresponds to sample Q1a, while in Figure 10, the point marked in green corresponds to sample AS 6a.

Figure 10:
Average M1/M3 v.s average diameter quartz greater tran 53 µm plot.

Figure 11:
Average M1/M3 v.s average diameter amorphous silica greater tran 53 µm plot.

The data from Figures 10 and 11 further clarify the influence of crystallinity and the size of silica particles discussed in the analysis of Figures 8 and 9. For quartz, as the percentage of particles > 53µm increases, the M1/M3 ratio exhibits a linear behavior for samples Q1 through Q5, with high sulfur content and an MgO/SO₃ ratio between 1.70 and 2.00, as shown in Table 4. However, the introduction of sample Q1a, with an M1/M3 ratio of 2.52, once again reveals the strong influence of sulfur on stabilising the M1 and M3 polymorphs of alite. Note that sample Q1a has an M1/M3 ratio of a similar magnitude to sample Q5, which has 98% quartz > 53µm and high sulfur content, further highlighting the influence of sulfur, magnesium, and the size of crystalline silica particles on alite polymorphism.

In samples with amorphous silica, the behavior of the M1/M3 ratio in response to increasing silica particle size is considerably different. For samples AS 6 through AS 8, with low sulfur content and an MgO/SO₃ ratio between 2.60 and 2.80, regardless of silica fineness, the M1/M3 ratio remains between 3.80 and 4.70. However, sample AS 6a, with high sulfur content, shows an even lower M1/M3 ratio of 2.78, contrary to what was observed in the quartz samples.

3.3.5. Microscopic characteristics of the main clinker crystals

Figures 12 and 13 present microscopic images characterizing the main features of the clinker samples with quartz and amorphous silica, respectively. Next to the images, the size distribution of alite crystals obtained using the ONO [¹³] methodology is shown.

Figure 12:
Optical microscopy images of samples Q1, Q1a and Q5.

Figure 13:
Optical microscopy images of samples AS6, AS6a and AS8.

Table 7 and Figures 12 and 13 present the data from the measurements of alite crystals as well as their main morphological characteristics. The microscopic study of the clinkers, based on the optical microscopy technique using reflected light developed by ONO [¹³], is of great importance for analyzing the phenomena involving the size of silica particles and the sulfur content in the growth of alite crystals and their idiomorphism.

The Q1 sample shows a high content of predominantly sub-idiomorphic alite crystals, with smaller amounts of idiomorphic and xenomorphic crystals, having an average diameter of 26.70 µm. A notable presence of abnormally developed alite crystals with a pronounced level of inclusions, especially of belite, is observed. This is due to the strong presence of SO₃, which causes this disordered development in some crystals and contributes to the increase in the average diameter of alite [¹⁵]. In sample Q1a, with a low sulfur content, the characteristics of the alite crystals change. A reduction in the average diameter of alite to 15.32 µm is observed, with a predominance of idiomorphic crystals, secondarily sub-idiomorphic, and rare xenomorphic crystals with disordered growth. A significant reduction in the level of belite inclusions in the alite crystals is also observed, further demonstrating the effect of sulfur on the alite growth rate.

From sample Q2 (S1), there is an increasingly pronounced growth of alite crystals and a reduction in their contents, with increased percentages of belite, especially in zones, and free CaO. Samples Q4 (S1) and Q5 provide very revealing aspects regarding the effect of coarse silica grains. In these samples, compact zones of belite are observed close to concentrations of free CaO. At the edges of these zones, partially formed alite crystals can be seen, indicating a localized deficiency of CaO. This occurs due to the high concentration of silica in the form of coarse grains larger than 53 µm, which consume all the available CaO around the grains, resulting in only belite crystals being formed, as the diffusion coefficient of CaO [¹⁹] is insufficient to allow its migration from more distant regions to complete the reaction within the available time.

For the samples with amorphous silica, Figure 13 shows that the phenomena related to alite crystal growth and other morphological characteristics, as observed through microscopy, are similar to those of the samples with crystalline silica. Figure 13 shows samples AS 6 and AS 6a exhibit characteristics comparable to those with crystalline silica. The average diameter of alite with low sulfur content is 13.22 µm, which increases to 22.50 µm in the sample with high sulfur content. Similarly, alite crystals show much more pronounced idiomorphism in the low sulfur content sample (AS 6) compared to the high sulfur content sample (AS 6a), reproducing the same disordered growth phenomenon of alite with increased SO₃ content, as observed in the quartz samples. For samples AS 7 (S1) and AS 8, the behavior is also similar to that of the corresponding samples with crystalline silica. The increase in silica diameter complicates alite crystal formation, accentuating the phenomenon of belite inclusions, increasing the percentages of belite, often in zones, and consequently also raising the residual free lime content, leading to a substantial loss in clinker quality.

3.3.6. Considerations on the effect on free lime

Free lime is a variable of significant importance in the industrial clinker manufacturing process. Therefore, the behavior observed in this study is of considerable practical importance, as it indicates that the diameters of silica particles in the raw mix play a crucial role in adjusting parameters to achieve better burning. In industrial processes, it is common to adjust the residual free lime content in clinker by reducing the LSF (Lime Saturation Factor), which means decreasing the supply of CaO for silica reactions in the formation of alite. However, while this measure is effective, it results in a reduction in alite content and an increase in belite content, thus compromising clinker quality. The evidence presented in this study offers an effective alternative without compromising the amount of alite. It is important to note that this involves reducing the size of silica particles.

3.4. Considerations on the influence of average alite crystal size

As a practical consequence for the industry, the data show that once the conditions of time, temperature, and chemical composition are established with the aim of reducing the diameter of the alite crystals in the clinker, reducing the size of the silica particles in the preparation of the raw mix may be the most important action to incorporate into the manufacturing process.

Regarding the phenomenon of alite crystal growth, [¹⁹] proposed that the growth of alite is governed by Equation 1.

(1)$$\chi =k·t$$

Where X is the diameter of the alite crystal in µm, t is the reaction time in seconds, and K is a constant in µm/s. The constant K is directly related to the percentage of the liquid phase and the temperature, and inversely related to the viscosity of the liquid phase. In the study presented by the author, there was no reference to the size of the silica particles and their influence on the size or growth rate of alite. Given the standardization of the sample composition and firing conditions in this study, a minimal variation in the average size of alite crystals was expected. However, the data show a very significant increase in the average size of alite as the average diameter of quartz and amorphous silica particles increases.

3.5. Considerations on the effect of particles on crystalline phases

NEVILLE [²⁰] shows that approximately 70% of alite hydrates within 28 days, while only 20% of belite achieves full hydration in the same period. Additionally, FERNANDES [³] demonstrates that the M1 polymorph of alite hydrates more quickly than the M3polymorph, strongly suggesting that M1 may contribute more significantly to the mechanical performance than M3. The results obtained in this research indicate the production of clinkers with these characteristics; therefore, reducing the diameter of silica particles significantly contributes to the firing of the raw mixture, improves the quality of the clinker, and consequently, its reactivity.

Regarding the influence of chemistry on alite polymorphs, MAKI [¹⁵] reported that increased SO₃ concentrations favor the stabilization of the M1 polymorph. On the other hand, higher MgO concentrations have been reported to positively affect the stabilization of the M3 polymorph of alite.

In this set of quartz samples, the introduction of sample Q1a aimed to assess this mentioned chemical influence. In samples Q1 to Q5, with high sulfur content, the MgO/SO₃ ratio was maintained between 1.70 and 2.00. Sample Q1a, with low sulfur content, highlighted in Figure 6, has an MgO/SO₃ ratio of 2.62. Compared to sample Q1, this reduction in SO₃ content led to a 32% decrease in M1 content and a 157% increase in M3 content; it also significantly increased the contents of belite and free lime.

This evidence is entirely consistent with MAKI [¹⁵] findings, as it confirms both the role of sulfur in stabilizing the M1 polymorph and the role of magnesium in stabilizing the M3 polymorph. Note that in samples Q2, Q3, Q4, and Q5, with high sulfur content, as the amount of coarse grains increases, there is a sharp reduction in M1 content and a more modest increase in M3 content, contrary to what occurred in sample Q1a. This leads us to consider that the phenomena related to the stabilization of alite M1 and M3 polymorphs have a physicochemical dependency on the raw mix, involving the MgO/SO₃ ratio and the size of quartz particles, as demonstrated in Figure 8.

The effect of sulfur can be better understood by observing what was postulated by TIMASHEV [¹⁹], showing that the introduction of acidic elements such as S, Cl, and F reduces the viscosity of the liquid phase, accelerating the diffusion coefficients of all components within the liquid phase, increasing the dissolution rate of CaO by 1.5 to 2 times, and causing a similar effect on the dissolution rate of belite (Table 8 and Figure 14).

Table 8
Diffusion coefficient and dissolution rate of elements.

DIFFUSION COEFFICIENTS (1,450–1,525°C)
ELEMENT	DIFFUSION COEFFICIENT (m2/s)
Ca	(5.31–8.55) × 10−9
Fe	(5.70–14.2) × 10−10
Al	(2.35–7.10) × 10−10
Si	(4.73–15.80) × 10−11
DISSOLUTION RATE (1,450°C)
COMPOUND	DISSOLUTION RATE (m/s)
CaO	(6.10–8.34) × 10−8
C2S	(1.94–2.78) × 10−8

• Source: [¹⁹].

Figure 14:
Adapted from [¹⁹].

The results presented in Figure 8 are in full agreement with those demonstrated by TIMASHEV [¹⁹], especially when we observe the behavior of sample Q1 in relation to Q1a. In these samples, the positive contribution of the increase in sulfur content to the quality of the clinker becomes evident, highlighting the importance of controlling the MgO/SO₃ ratio in the pursuit of maximizing the percentage of the M1 polymorph over M3 in alite. However, the study also provides evidence of the close relationship between the elimination of coarse silica particles larger than 53 µm and the maximization of alite content, particularly the M1 polymorph, while also reducing the content of belite and free lime.

Observing what was presented in Section 3.3.3, we can infer that the size of silica particles, and especially their crystallinity, seems to play a significant role in stabilizing the M1 and M3 polymorphs of alite. Therefore, identifying the sizes and contents of crystalline and amorphous silica in raw mixes can help explain the variations in reactivity of industrial clinkers. This becomes even more relevant when considering that quartz and amorphous silica present in these mixes react oppositely under the influence of sulfur and magnesium. This behavior is being investigated in a subsequent study.

3.6. Analysis of equilibrium conditions in alite formation

Figure 15 presents the CaO-SiO₂ phase diagram [⁴], where point P1 (CaO = 73; SiO₂ = 27) represents all the samples of this study, considering the standardized chemical compositions and the controlled burning temperature (1,450 °C).

Figure 15:
The CaO − SiO₂ phase diagram. Adapted [⁴]

The results of this experiment, along with the analysis of point P1 on the diagram above, reinforce that the formation of belite and alite crystals is a physico-chemical process. Note that point P1 represents the chemical composition of all the samples involved in the study, in terms of the amounts of SiO₂ and CaO available for the formation of calcium silicates, and they were subjected to the same conditions of time and clinkerization temperature. However, the clinkers exhibit very diverse crystalline compositions, indicating that the size of silica particles and their crystallinity play a fundamental role in the kinetics of crystal formation. Sample Q1 stands out as the one that best represents the equilibrium condition at point P1 regarding the formation of alite, belite, and residual free lime.

In industrial processes, there is a constant pursuit to improve the reactivity of clinker in order to produce cements with higher additions, whether active or not, while reducing CO₂ emissions per ton. This study shows that controlling the coarse silica particles > 53µm considerably improves the burnability of the raw mix, promoting not only the formation of alite but also its more reactive polymorph, M1.

3.7. Considerations on clinker microscopy

In industrial clinker manufacturing processes, there is a strong tendency to aim for the production of idiomorphic alite crystals, as opposed to sub-idiomorphic and xenomorphic crystals. However, the data in Table 6 reveal a considerable increase in the percentage of the M1 polymorph for sample Q1 compared to sample Q1a, indicating better reactivity of the clinker for Q1. Therefore, we can infer that the idiomorphism of alite crystals is not necessarily associated with faster hydration. However, from the perspective of the reflected light optical microscopy technique, the growth behavior and idiomorphism of alite crystals in relation to the size of silica particles and the variation in sulfur content suggest that samples with crystalline silica and amorphous silica are equivalent. Nevertheless, the study reveals a considerable superiority in the quality of clinker with crystalline silica. This identification was only possible through the combination of LG (Laser Granulometer), XRF, Reflected Light Microscopy, and XRD techniques.

4. CONCLUSIONS

Based on the results obtained, the following conclusions can be drawn:

• The results highlight the importance of evaluating the particle size of silica and its crystallinity in the raw mix, as this affects the solidification of alite crystals, as well as the distribution of their monoclinic polymorphs.

• The direct influence of silica particle size on the size of alite crystals has been demonstrated and is independent of the crystallinity of the silica.

• The data show a strong correlation between the size of silica particles and the polymorphism of alite, such that the stabilization of the M1 polymorph is increasednby the crystallinity of the silica source, while the M3 polymorph did not exhibit the same behavior.

Regarding the chemical influence of the raw mix, previous studies on the effect of sulfur on alite growth and the stabilization of its M1 polymorph were confirmed. However, this effect was more pronounced for samples with crystalline silica and became more intense as the particle size of the silica increased.

5. AGRADECIMENTOS

The authors would like to thank CSN Alhandra Cement Plant for providing the samples used in this study. They also thank the Federal University of Paraíba (UFPB), especially the NEPEM group, for their support in conducting the experimental analyses. The technical support and institutional collaboration were essential for the development of this work. The authors also express their gratitude to Professor Caroline Kirk, from the University of Edinburgh, for her valuable partnership and collaboration in the development of this article.

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Publication Dates

• Publication in this collection
  01 Aug 2025
• Date of issue
  2025

History

• Received
  01 May 2025
• Accepted
  30 June 2025