Obtaining of Composite Cements with Addition of Fly Ash

Abstract

The potential for creating composite cements by incorporating fly ash is demonstrated. Analysis revealed that the fly ash examined consists of 69.66 wt. % silicon oxide, 21.34 wt. % aluminum oxide, 1.57 wt. % calcium oxide and 2.78 wt. % iron oxide. Fly ash mainly consists of quartz (SiO₂), goethite (FeO(OH)) and mullite (3Al₂O₃·2SiO₂). The properties of the cement composition containing 5 to 25 wt. % fly ash were studied. Incorporating fly ash enhances system dispersion, promotes mixture uniformity, and stimulates the pozzolanic reaction. Compositions of composite cements consisting of 90% CEM I 42.5 and 10% fly ash were developed. The cement stone based on the obtained composite cement had a compacted structure with a density of 2.160 g/cm³, which is 9.4% higher than the control sample. It is shown that when composite cement containing 10% fly ash interacts with water, hydration reactions of cement minerals (C₃S, C₂S, C₃A and C₄AF) begin first. This is accompanied by the formation of hydrate neoplasms, such as calcium hydroxide (Ca(OH)₂) and calcium hydrosilicates (C-S-H). Fly ash particles containing amorphous silica progressively participate in a pozzolanic reaction with Ca(OH)₂, leading to the formation of additional calcium hydrosilicates phases. This process enhances structural densification and reduces the porosity of the cement matrix. After 28 days of curing, the compressive strength of the resulting composite cements ranged from 42.1 to 54.2 MPa, aligning with the strength classes 32.5 and 42.5 as specified by GOST 31108-2020.

1. Introduction

Cement is both material- and energy-intensive to produce, as its primary component is Portland cement clinker. Clinker is obtained by burning a special mixture at a temperature of 1450 °C. Producing one ton of cement consumes around five tons of raw materials, additives, fuel, water, air, refractories, and other resources [1,2,3]. Various types of concrete are made on the basis of cement. While concrete does not release carbon dioxide (CO₂) during its curing process, its production involves cement—a major contributor to CO₂ emissions. As a result, the manufacturing of cement and concrete plays a substantial role in greenhouse gas emissions, particularly CO₂. Cement production emits approximately 4 billion metric tons of CO₂ annually, accounting for approximately 8% of global human-caused CO₂ emissions [4,5,6]. Significant advancements in addressing the global climate crisis can be achieved by lowering carbon emissions in these sectors. Consequently, decarbonizing the production of cement, concrete, and concrete products is essential for sustainable construction and climate change mitigation [7,8,9,10,11].

Sustainable socio-economic development of any country is unthinkable without taking into account environmental factors, which involves a thorough evaluation of human impact on the environment, including the climate system and the biosphere as a whole. Sustainable development means fulfilling current needs without compromising the ability of future generations to meet theirs. The core idea of sustainable development is to address economic, social, and environmental challenges in an integrated way [12].

In the XXI century, construction issues are beginning to be considered from a sustainability perspective. This primarily concerns the use of materials, which should be produced with the lowest carbon footprint. This means that materials should be obtained using the best available technologies. By “technologies,” we mean both the technologies themselves and design and planning methods [13].

The Organization for Economic Cooperation and Development (OECD) predicts that by 2060, material-intensive sectors will grow substantially: industry—3.2 times, construction—2.6 times, agriculture—1.8 times. Therefore, not only the deficit of resources necessary for economic development may grow but also the harmful effects on the environment. In the context of implementing the strategy of environmentally oriented growth, developed countries are developing green technologies at an accelerated pace. The pace of green construction is expanding. The concept of a green economy does not replace the concept of sustainable development, but scientists and practitioners recognize that achieving sustainability largely depends on the development of a green economy [14]. In green construction, not only are building materials reused but waste is also recycled, new insulating materials and alternative energy sources are used, waste warm air is used for heating, etc.

Sustainable construction is defined as a critical approach to mitigating environmental impacts while enhancing economic and social well-being. Sustainable construction is a revolutionary approach in the building sector that aims to cut down environmental impacts while improving economic and social aspects. The idea arose as a reaction to increasing worries about climate change, resource exhaustion, and social responsibility, aiming to find a balance among environmental conservation, economic sustainability, and social equity.

In the late XX century, the concept of “sustainability” in construction gained significant momentum under the influence of various international movements. This period was marked by the emergence of “green” building standards and certifications system such as LEED (Leadership in Energy and Environmental Design) and BREEAM (Building Research Establishment Environmental Assessment Method) [1], which have spread widely throughout the world.

Sustainable construction is inherently multifaceted, involving various stakeholders, including architects, engineers, contractors, clients, and policymakers. It covers a wide range of practices, from the selection of environmentally friendly materials to the implementation of energy-efficient technologies and waste reduction strategies. This interdisciplinary approach requires a combination of technical knowledge, innovative design and strategic planning.

In 2020, the cement and concrete industry introduced the concept of carbon neutrality through the Climate Ambition 2050 initiative [8,15]. Many methods and technical solutions have been developed to reduce CO₂ emissions. The simplest and easiest way is to reduce the clinker factor, i.e., the amount of clinker in cement.

Utilizing recycled materials and implementing circular economy principles can reduce the need for new cement production [16,17,18,19,20,21,22]. The circular economy (CEE), also called the circular economy [23], is an economic model focused on resource use and consumption that emphasizes sharing, renting, reusing, repairing, refurbishing, and recycling materials and products to extend their lifespan as much as possible [24,25,26]. The concept seeks to tackle global issues like climate change, biodiversity loss, waste, and pollution by emphasizing three fundamental principles of a design-based model. These key principles for advancing a circular economy include preventing waste and pollution, prolonging the use of products and materials, and restoring natural ecosystems. This approach lessens the strain on natural resources and lowers the environmental impact linked to raw material extraction and processing.

The simplest way is to replace part of the clinker with so-called secondary binding materials, such as granulated blast furnace slag, calcined clays, fly ash and shale [27,28]. Exploring alternative binding materials such as fly ash, slag, and silica presents opportunities to decrease reliance on conventional cement and lower the overall carbon footprint of concrete production [29,30].

It is obvious that it is impossible to completely remove CO₂ using only primary methods. However, enhancing resource efficiency in production and adopting circular economy strategies represent important steps toward addressing this challenge. In this context, incorporating natural or industrial waste into clinker and cement production is highly relevant, as it contributes to mitigating environmental issues and reducing greenhouse gas emissions.

In the Republic of Kazakhstan, as in many other countries, increasing attention is being given to issues of reduction in human impact on the environment and climate system—particularly through lowering greenhouse gas emissions in key sectors such as the cement industry [3,31]. Decarbonization promotes innovation and technological progress. The search for alternative cement materials, such as fly ash or slag, stimulates research and development in the construction sector. This drives the advancement of innovative production methods, carbon capture and storage technologies, energy-efficient practices, and sustainable construction approaches. These developments not only contribute to reducing carbon dioxide emissions but also support economic growth, job creation, and technological innovation [32,33].

In the context of rapid development of the construction sector and increasing requirements for the environmental friendliness of building materials, special attention is drawn to the possibility of using technogenic waste in the production of composite cements [34,35,36,37,38,39,40]. One of such widely available and promising materials is fly ash, a huge amount of which accumulates in Kazakhstan. In the modern world, there are three main types of ash [41]: fly ash—a by-product formed during the burning of coal in thermal power stations; rice hulls ash—a product of burning rice husks; wood ash and sugar cane ash.

Fly ash is a finely dispersed mineral additive to cement that not only allows for the disposal of significant volumes of waste but also significantly impacts the performance characteristics of composite cements [42].

Thomas et al. noted that fly ash exhibits significant pozzolanic activity by reacting with calcium hydroxide formed during cement hydration to form additional calcium silicate hydrate (C-S-H) [43]. Siddique demonstrated that the addition of 15–30% fly ash to cement increases long-term compressive strength (28–90 days) with only a slight decrease in early strengths [44]. Hemalatha and Ramaswamy, in an extensive review, confirmed that the high dispersion and glassy structure of fly ash ensure its effective participation in pozzolanic reactions, which leads to compaction of the microstructure of cement stone [45].

The studies of Mehta and Monteiro [46] showed that fly ash helps to reduce capillary porosity and increase resistance to penetration of aggressive agents such as chlorides and sulfates. Cordeiro et al. [47] emphasized that the reduction in free calcium hydroxide content due to the pozzolanic reaction reduces the risk of leaching and carbonation.

Kumar [48] notes that cements with the addition of mineral ash additive show high resistance to sulfate attack and alkali-silica reaction (ASR), which makes them promising for durable building structures.

Research by Biernacki and Chaudhari [49] showed that the inclusion of fly ash reduces the overall heat release during hydration, which is especially important in mass concreting. This reduces the likelihood of thermal cracks and shrinkage deformations in the early stages of hardening.

According to Chindaprasirt et al. [50], the optimum proportion of cement substitution by fly ash is between 20 and 35% depending on the activity class of the ash and the required characteristics of the final product. The importance of the fineness of grinding of the ash and its chemical composition (especially the content of reactive SiO₂ and loss on ignition) is also emphasized.

In recent years, the combined use of fly ash with other mineral additives such as microsilica and slags has been actively explored to synergistically improve the properties of cementitious composites (Zhang et al.) [51]. In addition, work is underway on the activation of fly ash using alkaline activators for the production of geopolymer materials (Provis and Van Deventer) [52].

As of 2020, Kazakhstan has accumulated about 2.7 billion tons of waste, a significant portion of which is fly ash waste. In particular, the Karaganda region has accumulated 745.7 million tons of hazardous waste, including fly ash and others [53]. Fly ash waste generated by burning coal in thermal power plants makes up about 10% of all industrial waste in the country. Its annual volume is about 19 million tons, and by 2024, more than 300 million tons of this refuse had accumulated in dumps, occupying about 8500 hectares. However, the processing of fly ash waste in Kazakhstan is at a low level. As of 2024, only 0.7% of the total volume of this waste had been processed [21].

The aim of this research work is to study the influence of fly ash on the physicomechanical and operational properties of composite cements of the CEM II class A-Z (6–20% fly ash) and CEM II class B-Z (21–35% fly ash) grades, as well as to determine the optimal ratios of components that provide the best technical and environmental performance of the material.

The practical significance of the work on the use of fly ash in composite cements is high, since it is aimed at solving environmental, technical and economic problems at the same time. The use of fly ash allows for the effective disposal of industrial waste, reducing the load on ash dumps and minimizing environmental pollution; reducing the cost of cement products due to the partial replacement of energy-intensive Portland cement clinker; improving the performance characteristics of cement and concrete, including durability, density and resistance to aggressive environments; expanding the raw material base of the cement industry, especially in coal regions with excess fly ash; stimulating the development of sustainable construction technologies and the implementation of the principles of the “green” economy.

The use of fly ash in composite cements is a relevant trend in environmentally friendly, technologically advanced and cost-effective production of building materials. This direction corresponds to global trends in sustainable construction and is actively developing both in the scientific community and in industry. In recent years, growing attention has been focused on recycling waste and utilizing industrial by-products, driven by the global movement toward sustainable development and minimizing environmental impact. Within this framework, fly ash is recognized as a promising material for the manufacture of composite cements [54].

The main scientific assumptions and prerequisites for obtaining composite cements with the addition of fly ash are as follows:

• Fly ash contains amorphous silicon dioxide (SiO₂), which can react with calcium hydrates formed during cement hydration and subsequently participate in the formation of calcium hydrosilicates, which increase the strength of cement and durability of concrete solution;
• Finely dispersed particles of fly ash with a size of less than 45 μm improve the structure of the cement mortar, reducing porosity and increasing density;
• Utilizing fly ash as a partial substitute for Portland cement clinker decreases both the energy demand and the environmental impact of cement manufacturing;
• Replacing the clinker component with up to 30% fly ash can significantly reduce CO₂ emissions, making cement production more sustainable.

The scientific novelty of this study is as follows:

Based on the study of fly ash from Kazakhstan’s State District Power Plant power plants, it has been established that this ash is very acidic, with a silicon, aluminum, and iron oxide content of 91.1–93.8% by mass.

For the first time, the possibility of producing an effective composite cement using highly acidic fly ash as a mineral additive in an amount of 5 to 25% wt. % was substantiated in the Republic of Kazakhstan.

This study showed that adding fly ash improves the dispersion and homogeneity of the cement mixture and activates pozzolanic reactions. This leads to additional formation of calcium hydrosilicates (C-S-H) and compaction of the structure of the cement stone.

It was shown that the obtained composite cements have high compressive strength (42.1–54.2 MPa on the 28th day of hardening), corresponding to the requirements of GOST 31108-2020 for classes 32.5 and 42.5, which confirms the feasibility of using acidic fly ash as an active mineral additive. At the same time, the introduction of up to 25 wt. % acidic fly ash does not lead to a decrease in strength, compared to cement without additives.

2. Basic Research Methods

2.1. Materials

In this scientific work, the following components were used to obtain composite cements: Portland cements of the CEM I class 32.5 and CEM I class 42.5 grades, fly ash from the Ekibastuz State District Power Plant-2 (Ekibastuz City, Kazakhstan). The quality of the resulting cements was assessed for compliance with the specifications outlined in the state standard GOST 31108–2020—“General Construction Cements” [55].

2.2. Methods of High-Precision Instrumental Diagnostics

The following methods were used to conduct this study: X-ray fluorescence spectroscopy (XRF), X-ray diffraction analysis (XRD), microscopic analysis, determination of the ash activity index and determination of the physicomechanical properties of the obtained composite cement.

2.2.1. Chemical Analysis

Chemical analysis of cement and fly ash was carried out according to GOST 5382-2019 “Cements and Materials for Cement Production. Chemical Analysis Methods” [56].

2.2.2. Analysis by X-Ray Fluorescence Spectroscopy (XRF)

Element concentrations in the samples were determined using X-ray fluorescence analysis (XRF) on an AxiosmAX vacuum spectrometer from PANalytical (Panalytical, Almelo, the Netherlands). The dried samples were ground in a vibratory mill. The resulting powders were pressed into 32 mm diameter tablets, which were then placed in the instrument. Measurements were then taken, and the resulting data was displayed on the instrument’s display.

2.2.3. X-Ray Diffraction (XRD) Analysis

X-ray diffraction analysis was performed on a DRON-3 automated diffractometer (NPP Burevestnik, St. Petersburg, Russia) with Cu_Ka radiation and a β-filter. Powders of the analyzed materials were pressed into 32 mm tablets. After placing the tablets in the diffractometer, the instrument recorded the intensity of diffraction reflections as a function of the viewing angle. Phase identification was performed using ICDD data [57].

2.2.4. Microscopic Analysis

Electron microscopic studies were conducted using a JSM-6490LV scanning electron microscope (JEOL, Tokyo, Japan). A thin layer of silver was sputtered onto dried samples measuring 2–5 mm. The prepared sample was then placed in the instrument, and the resulting structure was recorded at different magnifications [58].

2.2.5. Research of the Ash Activity Index

To determine the activity index, three standard samples measuring 40 × 40 × 160 mm are formed for each test period. Control samples are made of cement and sand according to GOST 30744 [59]. For the test samples, some of the cement is replaced with ash. Compressive strength is determined at 28 and 90 days. To determine the activity index, general construction cement according to GOST 31108 without mineral additives (type CEM I) of strength class 42.5 and higher was used. The activity index was calculated using the following formulas:

I_A28 = Ro₂₈/Rk₂₈·100%;

I_A90 = Ro₉₀/Rk₉₀·100%.

where I_A28—the ash activity index at the age of 28 days;

• I_A90—the ash activity index at the age of 90 days;
• Ro₂₈ and Ro_90—the compressive strength of the base composition mortar at the age of 28 and 90 days, respectively;
• Rk₂₈ and Rk₉₀—the compressive strength of the control composition mortar at the age of 28 and 90 days, respectively.

2.2.6. Physical and Mechanical Tests

The fineness of the cement grinding was estimated based on the residue on sieve No. 008. The following testing tools were used:

• pneumatic sieving device
• scales (error no more than 0.01 g).

The specific surface area of the material was determined using the PSKh-12 device (LLC “Own Technologies”, Moscow, Russia), designed to study and control the dispersion processes of solids. All measurements and calculations in the device are automated and visualized on the display of the connected laptop. The range of specific surface area measurements is from 200 to 50,000 cm²/g, and the range of average particle size is from 0.5 to 250 μm. The instrumental error of the device does not exceed 1%. The results obtained using the PSKh-12 comply with the requirements of ISO [60] and ASTM [61] standards. The analysis requires 10–15 g of powdered material [62].

The sample was placed in a measuring cuvette, compacted with a plunger, after which the specific surface area and average particle size were determined using the built-in software. The parameters are displayed on the device display as a final protocol. Physicomechanical tests were carried out in accordance with GOST 310.1–81 [63] and GOST 310.3–81 [64]. Cement paste of normal consistency was mixed in a ball bowl, and cement mortar—using an ALS-5 laboratory automated mixer (Testing machine plant “ZIM TOCHMASHPRIBOR”, Armavir, Russia). The normal consistency of the cement paste was determined using a Vicat device (ELE International, Milton Keynes, UK), which includes a needle, pestle, ring and plate. The water-cement ratio (cement mortar consistency) was determined using a cone mold with a centering device and a shaking table.

For physicochemical studies, samples measuring 20 × 20 × 20 mm were prepared, and for testing strength characteristics according to GOST 30744–2001—standard samples measuring 40 × 40 × 160 mm [59]. The compressive strength test of the control cement (without additives) was carried out according to GOST 310.4–81 [65] after 28 days of hardening on a small-sized hydraulic press PGM-100 MG-4A (LLC “SKB Stroypribor”, Chelyabinsk, Russia), with a load limit of up to 100 kN [66].

3. Results and Discussion

3.1. Composition of the Source Materials Used in This Research

The process of obtaining composite cements using fly ash includes a number of basic stages, starting with the preparation of source materials (fly ash grinding, ash drying, chemical composition analysis), dosing of components, combined grinding, homogenization of the composition and subsequent hardening of the cement.

Chemical analysis of the composition of the source materials used was carried out in the Central Laboratory for Certification Testing of Building Materials, Almaty). The results of the chemical analysis of fly ash from the Ekibastuz State District Power Plant-2 are presented in

Table 1. Chemical analysis of fly ash from the Ekibastuz State District Power Plant-2.

Material	Chemical Composition, wt. %
	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Na2O	K2O	TiO2	MnO	P2O5	Loss on Ignition	Total
Fly ash	69.66	21.34	2.78	1.57	0.39	0.23	0.45	0.54	0.96	0.06	0.33	1.69	100.0
	64.28	22.85	4.69	1.34	0.63	1.47	0.44	0.69	1.03	0.18	0.39	2.01	100.0
	64.94	21.49	4.71	1.13	0.72	1.77	0.59	0.69	1.30	0.28	0.39	1.99	100.0

.

According to the analysis results, the silicon dioxide content in the fly ash exceeds 64–69%, which indicates the acidic nature of the material. The material is enriched with SiO₂ and has pozzolanic properties. The total content of alkaline oxides (Na₂O + K₂O) is 0.99–1.28%, which does not exceed the permissible value of 2.0%. The magnesium oxide (MgO) content is 0.39–0.72%, which is also within the norm (no more than 5.0%). The permissible loss on ignition value for the fly ash is up to 7.0%, which meets the regulatory requirements. The mass fraction of reactive CaO in acidic fly ash should be less than 10.0%. The CaO content of the Ekibastuz fly ash is 1.13–1.57%, which meets the requirements of GOST 31108-2020 “General Construction Cements. Technical conditions”. Chemical analysis showed the stability of the fly ash composition in different batches.

X-ray diffraction analysis of the fly ash from the Ekibastuz State District Power Plant-2 is shown in Figure 1.

Figure 1 shows that the main minerals in the fly ash sample from Ekibastuz State District Power Plant-2 are quartz (SiO₂), goethite (FeO(OH)) and mullite (3Al₂O₃·2SiO₂). The X-ray diffraction patterns characteristic of these minerals appear:

• SiO₂ d/n spectra—4.24; 3.36; 1.59 Å;
• 3Al₂O₃·2SiO₂ d/n spectra—5.34; 2.87; 2.68; 2.53; 2.28; 2.20; 2.11; 1.59; 1.52 Å;
• FeO(OH) d/n spectra—7.37; 2.28; 1.38 Å.

The quartz content in the ash reaches 28%, mullite 38%, goethite 9%, the rest is 25% glass phase. The type of diffraction pattern indicates that part of the material is in amorphous form. Forsgren J. [67] and van Riessen [68] also note that in the XRD spectrum of fly ash, along with crystalline reflections, a wide amorphous hump is present.

Analysis of the phase composition of the fly ash showed that only the glass phase, which contains microspheres, has pozzolanic and hydraulic activity. In addition to the glass phase, the fly ash contains mullite (3Al₂O₃·2SiO₂), quartz (SiO₂) and goethite (FeO(OH)). In addition, the ash contains trace elements As, Cr, Ga, Ge, Mn, Ni, P, Pb, Sc, Ti, Zr, W, which do not form independent compounds, but are part of minerals and the glass phase.

The basicity modulus (i.e., the ratio) ${\displaystyle \frac{(CaO+MgO)\%}{(Si{O}_2+A{l}_2{O}_3)\%}}$ is about 0.02, and the activity modulus (i.e., ${\displaystyle \frac{(A{l}_2{O}_3)\%}{(Si{O}_2)\%}}$) is 0.5.

This indicates that the fly ash from the Ekibastuz State District Power Plant-2 is extremely acidic (i.e., superacidic) and has the lowest activity. The granulometric composition by fractions is distributed as follows: up to 0.5 mm—0.14%; 0.45 mm—2.26%; 0.25 mm—3.6%; 0.1 mm—25.8%; 0.09 mm—0.84%; 0.08 mm—12.12%; 0.06 mm—4.5%; 0.05 mm—21.46%; 0.045 mm—21.38%; 0.04 mm—7.9%. The fly ash grinding process was performed in a QM-5L laboratory ball mill (TENCAN, Changsha, China). The total grinding time was 30 min. The specific surface area of the fly ash was 2900 cm²/g.

The structure and local X-ray fluorescence analysis of fly ash are shown in Figure 2.

Scanning electron microscopy analysis, shown in Figure 2, shows that:

• the ash particles are spherical, glassy and hollow, ranging in size from 1 µm to 50 µm;
• large particles contain smaller spherical particles within their cavities (shown by the arrow);
• tiny loose beads are typically firmly “glued” to the surface of large particles.
• The mechanism of fly ash particle formation can be represented as follows:
• the ash carried out of the furnace is at a high temperature; therefore, upon rapid cooling (during interaction with water), tiny glass beads are formed;
• very hot small glass beads stick together upon collision, trapping air bubbles with them; this can form large beads containing many smaller beads;
• balls of different sizes cool at different rates, so hotter small balls attach to the surface of colder large balls and create a dense outer shell.

In ref. [69], it was established that, based on the chemical and mineralogical composition, the small balls inside the large ones are mullite and α-quartz [69].

The ash activity index is a characteristic of fly ash obtained during fuel combustion, which shows its ability to interact with cement stone in concrete. This index affects the strength and other properties of concrete made using ash as an additive.

The fly ash activity index is shown in

Table 2. Activity index.

№	Name of Indicators	Designation of Regulatory Documentation for Test Methods	Standards for Regulatory Documentation	Actual Results	Note
1	Activity index at the age of 28 days, %, not less	GOST 25592-91 [70] GOST 30744-2001	75	72.0	Fly ash from the Ekibastuz State District Power Plant-2
2	Activity index at the age of 90 days, %, not less	GOST 25592-91 GOST 30744-2001	85	87.7

.

According to the specifications of GOST 31108–2020, fly ash may be incorporated into the following types of composite cements, in the specified percentages, %:

• CEM II class A-Z—6–20%;
• CEM II class B-Z—21–35%;
• CEM V class A—18–30%;
• CEM V class B—31–49%.

Portland cements of the CEM II class 32.5 and CEM I class 42.5 grades from the ALACEM cement plant were used as binders. The results of the chemical analysis of Portland cements are presented in

Table 3. Chemical analysis of Portland cements.

Material	Chemical Composition, wt. %
	SiO2	Al2O3	Fe2O3	CaO	MgO	SO3	Na2O	K2O	TiO2	MnO	P2O5	Loss on Ignition	Total
CEM I 42.5	20.79	3.96	3.70	63.88	0.55	2.56	0.29	0.63	0.10	0.16	0.10	3.48	100.0
CEM II 32.5	18.25	5.84	3.27	62.40	0.59	2.30	0.36	0.69	0.48	0.10	0.10	5.82	100.0

.

The general results of the physical and mechanical studies of Portland cements CEM II 32.5 and CEM I 42.5 are presented in

Table 4. General results of physical and mechanical studies of Portland cements CEM II 32.5 and CEM I 42.5.

№	Name of Tests	Requirements According to GOST 30744-2001	CEM II 32.5	Requirements According to GOST 30744-2001	CEM I 42.5
1	Fineness of grinding by passing through sieve No. 008, %	not standardized	97.0	not standardized	97.4
2	Normal density, %	not standardized	27.5	not standardized	28.0
3	Start of setting, minutes, not earlier than	75	210	60	220
4	Uniformity of volume change (expansion), mm, not more than	10	not changes	10	not changes
5	Compressive strength, MPa, at the age of 7 days, not less than	16	17.0	10	29.7
6	Compressive strength, MPa, at the age of 28 days,		41.0		45.3
	not less than	32.5		42.5
	not more than	52.5		62.5
7	Ultimate compressive strength after heat treatment, MPa	more than 25.5	28.0	more than 27.0	29.3
8	Specific effective activity of natural radionuclides, Bq/kg	up to 370	103.1	up to 370	100.7
9	Tricalcium silicate (C3S), %	not standardized	54.19	not standardized	59.09
10	Dicalcium silicate (C2S), %	not standardized	16.65	not standardized	10.74
11	Tricalcium aluminate (C3A), %	not standardized	7.37	not standardized	9.93
12	Calcium alumoferrite (C4AF), %	not standardized	15.76	not standardized	15.02
13	Free calcium oxide (CaO), %	to 2%	1.44	to 2%	0.41
14	Periclase (MgO), %	to 5%	1.43	to 5%	1.47
15	Gypsum (CaSO4·2H2O)	not standardized	3.16	not standardized	3.34

.

XRD analysis of cements CEM II class 32.5 and CEM I class 42.5 are shown in Figure 3.

According to the results of X-ray phase analysis, the following minerals were recorded in CEM II 32.5 cement: alite d/n—1.76; 2.18; 2.78; 3.03 Å, belite d/n—1.63; 2.61; 2.73 Å, C₃A d/n—1.54; 1.93; 4.11 Å, C₄AF d/n—1.48; 1.72; 2.27; 2.39; 3.16 Å, CaSO₄·2H₂O d/n—2.29; 2.44 Å.

According to the results of X-ray phase analysis, the following minerals were recorded in CEM II 42.5 cement: alite d/n—1.76; 2.18; 2.78; 3.03 Å, belite d/n—1.63; 2.61; 2.74 Å, C₃A d/n—1.53; 3.86 Å, C₄AF d/n—1.48; 2.39 Å, CaSO₄·2H₂O d/n—2.29; 2.44 Å.

The results of scanning electron microscopy and X-ray fluorescence analysis of Portland cements grades are shown in Figure 4.

As a result of microscopic analysis, it was found that the arrangement of C₃S and C₂S minerals in the micrograph of cement is uniform. The crystallization of minerals is very clear and dense. Alite crystals are shown in the tetragonal and rhombohedral images with a right angle. Tricalcium aluminate occurs in orthorhombic and cubic forms. Alite crystals have sizes from 10 to 20 μm. Belite is presented in the image in a round and oval shape and different sizes. The intermediate phase consists of tetracalcium alumoferrites and tricalcium aluminates (C₃A + C₄AF).

3.2. Properties of Composite Cements

Based on the aforementioned raw materials, composite cement formulations were developed with fly ash content ranging from 5% to 25% (

Table 5. Composition and properties of composite cements CEM II 32.5.

№	Composition of Composite Cements, %		Grinding Time, Minutes	Specific Surface Area, S, cm2/g	Residue on Sieve No. 008, %
	CEM II 32.5	Fly Ash Ekibastuz State District Power Plant-2
1	95	5	60	3227	11.37
2	90	10	60	3439	10.36
3	85	15	60	3532	10.22
4	80	20	60	3620	9.48
5	75	25	60	3738	8.77

and

Table 6. Composition and properties of composite cements CEM I 42.5.

№	Composition of Composite Cements, %		Grinding Time, Minutes	Specific Surface Area, S, cm2/g	Residue on Sieve No. 008, %
	CEM I 42.5	Fly Ash Ekibastuz State District Power Plant-2
1	95	5	60	4077	6.84
2	90	10	60	4169	6.39
3	85	15	60	4289	5.84
4	80	20	60	4373	5.19
5	75	25	60	4509	4.92

). The mixtures were ground in a ball mill for 60 min, targeting a residue of no more than 13% on sieve No. 008. Residue measurements on sieve No. 008 were conducted at 20 min intervals during the grinding process.

Based on the data presented in Table 5 and Table 6, it is clear that the incorporation of fly ash leads to a reduction in the residue on sieve No. 008. This effect is attributed to the finer particle size and smoother, spherical shape of fly ash compared to cement particles, which enhances the dispersion of the composite cement powder. The increased specific surface area resulting from the addition of fly ash enlarges the contact area between the hydrating cementitious system and water, thereby accelerating the hydration process. Furthermore, fly ash contains reactive amorphous phases—primarily silicon dioxide (SiO₂)—which participate in pozzolanic reactions with calcium hydroxide [Ca(OH)₂] produced during cement hydration. These reactions contribute to an increase in the overall reactivity of the cement, enhancing its strength development at later stages of hardening. As a result of 60 min of grinding, all samples exhibited a residue on sieve No. 008 below 13%. With fly ash additions ranging from 5% to 25%, a progressive decrease in residue was observed: from 11.37% to 8.77% for CEM II 32.5, and from 6.84% to 4.92% for CEM I 42.5. Additionally, the specific surface area of the ground composite cements varied from 3227 to 4509 cm²/g.

Due to its very small spherical particles (less than 1 µm), fly ash is characterized by a Blaine specific surface area of 2500–4200 cm²/g. Consequently, when 5–25% of clinker is replaced with fly ash, a reduction in the proportion of coarse particles retained on sieve No. 008 is expected, accompanied by an increase in the specific surface area of the cement blend. This increase in surface area facilitates the acceleration of the hydration process, as the expanded contact area between water and cement promotes the formation of hydration products, particularly calcium silicate hydrate (C-S-H). The finely dispersed nature of fly ash introduces numerous active sites and enhances the pore structure of the cement matrix, contributing positively to the development of long-term strength characteristics [71,72,73,74,75].

The authors have developed compositions of composite cements with the addition of fly ash from 5 to 25%. With an increase in the fly ash content from 5 to 25% in the composition of composite cements, an increase in the specific surface area is observed: from 3227 to 3738 cm²/g. A similar trend is observed for cement CEM I 42.5—from 4077 to 4509 cm²/g. This indicates a positive effect of fly ash on the fineness of grinding of composite cements.

The results of determining the fineness of grinding of the obtained cements show that with an increase in the amount of additives introduced into the cement, the grindability of composite cements increases compared to non-additive ones. The values of the residue on sieve No. 008 decrease, and the values of the specific surface increase, which has a good effect on the reactivity of the cement.

The physicomechanical characteristic of composite cements according to GOST 310.4-81 were studied on samples—cubes measuring 40 × 40 × 160 mm, hardened in a water chamber.

Table 7. Properties of cements without additives and with the addition of 5–25% fly ash.

Addition of Fly Ash, %	Water Cement Ratio	Setting Time, Hour–Minutes		Compressive Strength, MPa			Average Density, g/cm3
		Start	End	3 Days	7 Days	28 Days
CEM II 32.5
0	0.264	3–30	4–30	12.94	17.00	41.00	1.812
5	0.272	3–40	4–45	17.25	27.94	42.10	1.946
10	0.276	3–45	4–55	18.44	28.55	44.85	1.953
15	0.278	3–35	4–20	18.12	28.15	44.10	1.906
20	0.282	2–30	3–50	17.08	27.80	43.52	1.857
25	0.286	2–25	3–40	16.94	27.41	42.48	1.798
CEM I 42.5
0	0.274	3–40	4–35	16.60	29.70	45.30	1.974
5	0.276	3–45	4–40	20.46	33.74	52.36	2.133
10	0.279	3–50	4–50	20.87	34.45	54.18	2.160
15	0.284	3–25	4–10	19.94	33.20	53.84	2.108
20	0.288	2–15	3–45	19.67	33.08	49.81	2.057
25	0.295	2–10	3–30	18.99	32.85	45.93	1.994

presents the strength test results of composite cement samples.

The water requirement of cement is determined by the amount of water required to obtain cement paste of normal consistency. Normal consistency of cement paste is understood as such consistency of the paste, at which the cement paste is mobile and easy to place (soft consistency).

Normal consistency (water requirement) depends primarily on the fineness of the grinding. Increasing the fly ash content of cement to 25% increases the normal consistency from 0.264 to 0.286. The type of cement also affects the water requirement of the cement paste. Tricalcium aluminate requires the highest water content, while dicalcium silicate requires the lowest. To achieve the desired workability, add more water than is required for hydration reactions. Excess water will increase porosity and reduce strength. Therefore, it is always advisable to reduce the amount of water when mixing cement and concrete.

According to the data in Table 7, it is noticeable that as the amount of fly ash increases, the water demand to achieve normal consistency in the cement also rises. Specifically, the water requirement increased by 8.3% for CEM II class 32.5 and by 7.7% for CEM I class 42.5.

Fly ash affects the setting time of composite cements—at a content of 5–10%, it slows down the setting. Especially at its content of 10%, which corresponds to the typical behavior of pozzolanic additives. Fly ash contains mainly amorphous SiO₂, which reacts weakly with water at early stages. In the microstructure of cement, the amount of hydrating phases is reduced—C₃S and C₂S, which are responsible for the initial reaction and heat release. As a result, the start of setting occurs later and the setting time increases. With an increase in the fly ash content from 15% to 25%, the setting time is significantly accelerated. This is due to the active pozzolanic effect and the influence on the heat release of the system. In addition, the effect of fly ash depends on the composition of the cement. The more active CEM I 42.5 reacts to the additive more sharply. Most researchers are convinced that the retardation of setting occurs due to the adsorption of the retarder on the surface of the cement grains and hydration products, as well as due to the formation of hydration products that create a protective film that prevents water from penetrating to the interior of the grains.

As can be seen from the data in Table 7, the strength of the obtained composite cements increases with increasing curing time. The addition of fly ash to the composition of composite cement varies from 5 to 25%.

When adding 5 to 25% fly ash to CEM II class 32.5 cement, the compressive resistance of the cement stone was 42.1–44.9 MPa. When adding 10–15% ash, the strength increases significantly both at the early and late stages. Optimal strength is realized with 10% of the additive and is 44.9 MPa after 28 days. With the addition of more than 15%, the strength decreases slightly, but remains higher than the control composition without the additive by 1.5–3.1 MPa. With the addition of 5 to 25% fly ash to CEM I class 42.5 cement, the compressive strength of the cement stone was 45.3–54.2 MPa. The addition of fly ash up to 10–15% also provides the maximum strength increase for composite cement CEM I class 42.5 grade. With 10% ash, the maximum strength value is achieved on the 28th day—54.2 MPa. With the addition of more than 15%, the strength decreases slightly, but remains higher by 0.6–8.5 MPa compared to the control composition without the additive.

The analysis of the obtained results shows that the optimum dosage of fly ash is 10% for both cements. The improvement in strength is due to the pozzolanic activity of the ash, where amorphous silica in the fly ash reacts with calcium hydroxide, forming additional calcium hydrosilicates, which improve the structure and strength of the cement stone. Small particles of fly ash fill the pores between the cement grains, promoting compaction of the structure and reducing permeability. The resulting composite cement has a compacted structure of cement stone of 2.160 g/cm³, which is 9.4% higher than the control sample. When mixing a composition with the addition of fly ash with water, the interaction of cement minerals (C₃S, C₂S, C₃A and C₄AF) with water begins first, accompanied by the formation of hydration products such as calcium hydroxide and calcium hydrosilicates. Fly ash particles containing amorphous silica gradually enter into a pozzolanic reaction with calcium hydroxide, forming additional calcium hydrosilicates phases, which contributes to the compaction of the structure and a decrease in the porosity of the cement stone. Chemical reactions of cement stone hydration with the addition of 10% fly ash, main reactions:

• Hydration of cement minerals alite and belite without ash:

2C₃S + 6H → C₃S₂H₃ + 3Ca(OH)₂

(1)

2C₂S + 4H → C₃S₂H₃ + Ca(OH)₂

(2)

2

Pozzolanic activity resulting from the inclusion of fly ash:

SiO₂ (amorphous) + Ca(OH)₂ + H₂O → C-S-H

(3)

3

Reaction with other components of fly ash, in particular Al₂O₃:

Al₂O₃ + Ca(OH)₂ + H₂O → C₃AH₆

(4)

4

Reaction of ettringite formation:

C₃A + 3CaSO₄·2H₂O + 26H₂O → C₆AS₃H₃₂

(5)

Hydration products: calcium hydrosilicate (C-S-H), hydroxide of calcium (Ca(OH)₂), hydroaluminate of calcium (C₃AH₆), ettringite (C₆AS₃H₃₂).

The decrease in strength at dosages over 15% and more occurs due to the diluting effect of fly ash. Excess ash reduces the total content of binders necessary for the formation of a strong structure. Pozzolanic reactions require a sufficient amount of Ca(OH)₂, which can be exhausted with a high ash content. In addition, the content of unburned carbon in the fly ash can interfere with hydration and reduce strength.

Thus, the results of tests of composite cements with fly ash show that the addition of fly ash in an amount of 5–15% has a positive effect on the strength properties of the resulting cements. The compressive strength of the cements after 28 days is 44.9 and 54.2 MPa. These indicators correspond to the cement grade for strength class 32.5 and 42.5 according to GOST 31108-2020.

Cement hydration is a chemical reaction of cement minerals with water to form crystalline hydrates. XRD and scanning microscopic analysis were used to establish the phase composition and study the structure of hydrated cement stone. Figure 5 displays the results of the XRD analysis of the hydrated cement stone.

On the XDR spectra of the hydrated cement stone samples (Figure 5a), intermediate points were revealed corresponding to the following minerals:

-

calcium hydroxide (Ca(OH)₂) d/n—4.9386; 3.1192; 2.6345; 1.9298; 1.7978; 1.4816 Å;

-

calcium hydrosilicates (C-S-H) d/n—7.8237; 5.2264; 2.8585; 1.7212; 1.6783; 1.617; 1.512; 1.4494 Å;

-

alite (C₃S—3CaO·SiO₂) d/n—3.0432; 2.9836; 2.7818; 1.9816; 1.8242; 1.7665; 1.5412 Å;

-

belite (C₂S—2CaO·SiO₂) d/n—3.2454; 2.7529; 2.288; 2.21; 2.1334; 2.0566 Å;

-

ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O) d/n—8.7513; 5.6607; 4.6988; 3.6671; 3.4901; 2.5654; 2.4769 Å;

-

tricalcium aluminate (C₃A—3CaO·Al₂O₃) d/n—6.2998; 4.0642; 2.3573 Å;

-

tetracalcium aluminopherite (4CaO·Al₂O₃·Fe₂O₃) d/n—4.3293; 1.878; 1.6468; 1.5776; 1.4195; 1.3892; 1.366 Å.

On the XRD spectra of the hydrated cement stone samples (Figure 5b), intermediate points were revealed corresponding to the following minerals:

-

calcium hydroxide (Ca(OH)₂) d/n—4.9143; 3.1157; 1.9263; 1.7976; 1.4848 Å;

-

calcium hydrosilicates (C-S-H) d/n—7.2842; 1.6881; 1.5267; 1.4475 Å;

-

alite (C₃S—3CaO·SiO₂) d/n—3.0379; 2.9688; 2.7795; 1.8263 Å;

-

belite (C₂S—2CaO·SiO₂) d/n—3.2433; 2.7795; 2.7463; 2.2859; 2.2106 Å;

-

ettringite (Ca6Al2(SO4)3(OH)12·26H2O) d/n—8.1539; 5.5912; 3.8917; 2.6324 Å;

-

quartz* (SiO₂) d/n—4.2806; 3.453 Å.

* Note: Quartz peak was detected only in the sample with 10% fly ash addition.

X-ray analysis showed that addition of 10% fly ash leads to change in microstructure and mineralogical composition in comparison with samples based on Portland cement without additives. Introduction of fly ash into composition of composite cement binder has complex effect on course of hydration processes. At early stages of hardening (up to 7 days) fly ash exhibits inert properties. However, as hydration develops (especially after 14–28 days) pozzolanic reaction between amorphous silica-containing phase of ash and Ca(OH)₂, which is formed as a result of hydration of CaO·SiO₂, is activated. This results in the formation of additional C-S-H, which contributes to densify the structure of the cement stone. Structure of cement stone with fly ash becomes more uniform and dense due to in of porosity, especially capillary. Fly ash particles act as microfillers, improving the system packing. The mineralogical composition of hydrated products includes: calcium hydrosilicates (CSH), calcium hydroxide (in smaller quantities compared to conventional cement), and a small amount of ethringite (AFt phase). The characteristics of cement stone blended with fly ash in the long term (28 days) are improved due to the additional formation of CSH phases, which increase strength.

The experimentally obtained sample of composite cements was studied using scanning electron microscopy. Figure 6 presents micrographs of hydrated cement stone samples containing 5–25% fly ash.

Electron microscopic analysis of hydrated cement stone showed the following hydration products. Calcium hydrosilicates are presented in the form of needle-shaped minerals and rounded masses with protruding needles. Calcium hydroxides are in the form of bars and plate minerals. Ettringite crystallizes in the form of needles. Calcium carbonate is present in the form of spherical growths. In addition, a certain amount of unreacted clinker minerals is observed, the surface of which is surrounded by the forming hydrosilicates.

The addition of fly ash initiates a pozzolanic reaction, as a result of which a decrease in the amount of Ca(OH)₂ crystals is observed. At the same time, the volume of calcium hydrosilicates (C-S-H) increases, forming a denser and fine-pored structure. The formation of gel products in the pores helps to reduce the overall porosity of the cement stone. Fly ash particles, mainly rounded in shape, can act as an inert phase and do not always exhibit full reactivity. Addition of 10% fly ash leads to:

• Reduction in the amount of free Ca(OH)₂;
• Compaction of the microstructure due to crystallization of calcium hydrosilicates;
• Increased durability due to reduced porosity;
• Modification of the ash-gel and ash-cement matrix interfaces.

4. Conclusions

Based on the conducted research, the following conclusions can be made:

• Compositions of composite cements based on Portland cement of CEM I class 42.5 and CEM II class 32.5 grades with the addition of fly ash in the amount of 5 to 25% were developed. The optimal composition for composite cement is CEM I 42.5—90%, fly ash—10%.
• The determination of fly ash showed that fly ash has a fairly stable composition. The majority of the fly ash is present in the form of glassy spheres; small amounts of phases are present—quartz (SiO₂), mullite (3Al₂O₃·2SiO₂), and goethite (FeO(OH)). In addition to the primary oxides SiO₂ and Al₂O₃, the ash also contains trace elements including As, Ni, Cr, P, Sc, Mn, Pb, Ti, Zr, Ge, Ga, and W. These elements do not create separate compounds but are integrated within minerals and the glass phase.
• During the grinding process, the incorporation of 5–25 wt. % fly ash to the structure of the cement composition helps to improve the dispersion of the system, increase the homogeneity of the mixture and activate the pozzolanic reaction. The use of fly ash as a joint additive leads to an increase in the grindability of composite cements. The grindability of the obtained compositions improved compared to the control by 10.6–15.8%.
• When assessing the strength properties of the cements studied, it was observed that incorporating fly ash enhances the strength of the composite mixtures compared to cement without any additives. In particular, when adding 5% fly ash, the strength increases by 2.7–15.6% compared to cement without additives; 10%—by 9.4–19.6%; 15%—by 7.6–18.9%; 20%—by 6.1–10.0%; 25%—by 1.4–3.6%. The test results showed that the compressive strength of the produced composite cements after 28 days ranged from 42.1 to 54.2 MPa, which fully meets the requirements for cement grades 32.5 and 42.5 according to the strength class specified in GOST 31108-2020.
• In further work, particular emphasis should be paid to studying the processes of interaction of composite cements with fly ash with various modifying additives. First of all, this applies to plasticizers, which are actively used in concrete compositions.