
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

72315
10.1038/s41598-024-72315-0
Article
The influence of selected grain size fractions of coal fly ash on properties of clay-cement mortars used for the flood levees construction
Delihowski Jurij ydeli@agh.edu.pl

1
Izak Piotr 1
Wojcik Łukasz 1
Stempkowska Agata stemp@agh.edu.pl

2
Jarosz Marcin 3
1 grid.9922.0 0000 0000 9174 1488 Faculty of Materials Science and Ceramics, AGH University of Kraków, Krakow, Poland
2 grid.9922.0 0000 0000 9174 1488 Faculty of Civil Engineering and Resources Management, AGH University of Kraków, Krakow, Poland
3 COMEX, Polska sp.Zoo, Krakow, Poland
14 9 2024
14 9 2024
2024
14 2148514 3 2024
5 9 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
This study examines the influence of different grain size fractions of coal fly ash on the properties of clay-cement mortars used in flood levee construction. Dry aerodynamic separation and mesh sieving were used to obtain ultrafine, fine, and medium fractions of high-calcium and silica fly ash. The experimental results reveal that the rheological properties of fresh mortars are significantly influenced by these fractions. High-calcium fly ash mortars exhibit high reactivity and rapid increase in viscosity, with finer fractions showing the highest reactivity. Silica ashes show increased reactivity in the later stages of suspension hardening. Their spherical shape contributes to reducing internal friction during flow in initial technological operations. Furthermore, the compressive strength of hardened mortars improves as the particle size decreases for both ashes, resulting in a dense and uniform microstructure. The separation and fractionation of fly ashes contribute to the obtaining of fractions that influence the parameters of clay-cement suspension application on different scales. The results show the potential benefits of ash separation, which can bring advantages in terms of economic viability, engineering performance, and ecological sustainability.

Keywords

Clay-cement suspensions
Coal fly ash
Compressive strain
Particle size distribution and properties
Rheology
Subject terms

Civil engineering
Mechanical engineering
Environmental sciences
Engineering
Materials science
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Coal combustion for energy purposes generates combustion by-products, where fly ash constitutes up to 70–90% of the total by-products1,2. Fly ash consists of incompletely burnt organic residues and thermally transformed mineral matter present in feedcoal. Pounds and disposals, where these ashes are stored, occupy large natural territories and can have a harmful impact on surrounding areas, underground water, and the atmosphere3–6. Globally, fly ash utilisation rates vary: India used 66% of its 233 million tons in 20217, China applied most of its 550 million tons produced in 2018 to construction and cement8, and Australia used 47% of 14 million tons in 20199. A few countries such as Denmark, Italy, and Netherlands achieve 100% utilisation rate10, but the average global rate is around 60%11. Due to the necessity of using incredible amounts of those by-products, numerous applications across various industrial sectors have incorporated fly ashes into production processes. Some examples include its use in ceramics, composite engineering, the cement industry, and soil modification and stabilisation additives4,6,12,13.

The mineral and chemical characteristics of fly ash can vary significantly depending on factors such as type and origin of the coal, combustion process, and precipitation techniques2,3,14. A better understanding of the tendencies and variations in the properties of fly ash can help to design efficient utilisation strategies and incorporate ashes into new industries.

Fly ash processing and separation are commonly used to enhance ash properties and extract various grain fractions with the desired parameters3,4. Concerning this study, the separation due to the grain size will be briefly discussed. In general, fine-grained fractions are associated with increased levels of certain phases, e.g. alumo- and calcium-silicates. These phases are known to be important for the pozzolanic properties of fly ash, which enhance the strength and durability of concrete15–17. In terms of chemical composition, these fine ash fractions tend to be enriched in certain oxides, eg sulphur, chlorine, and potassium, as well as some trace elements, eg Mn, Mg, Zn, Cr, Ni, and Pb4.

Middle-sized and coarse fractions generally have different mineralogical and chemical characteristics compared to fine fractions3,15. Coarse fractions tend to be enriched in crystalline phases, including significant levels of mullite and quartz as well as unburned feedcoal grains. These mineral grains can act as supplementary inert unreactive fillers in constructions and concrete. By subjecting coarse fractions to additional pretreatment (mechanical, chemical, or thermal), their reactive potential can be activated, which, in turn, transforms them into active pozzolanic additives18–21.

Middle-sized fractions are also known to have some pozzolanic properties22,23. The chemical composition of middle-sized fractions generally contains lower levels of active elements compared to fine fractions and a higher amount of amorphous fractions compared to a coarse fraction. Notable is a high level of porosity and the highest level of presence of cenospheres in this fraction24–26. This can make them a more attractive option for certain specific applications where some restrictions to building materials are required, eg, lightweight building materials, thermal insulators, etc.6,12,13.

Among many other applications, fly ash in clay-cement mixtures has emerged as a promising approach in geotechnical engineering, particularly for applications such as soil stabilisation, embankment construction, and flood levee reinforcement27,28. High water resistance is one of the most crucial parameters for materials used in flood levees and protective structures, as these structures need to withstand prolonged exposure to water without degrading or losing their integrity29,30. Those sealing suspensions consist of a base clay-water suspension and cement, forming a thixotropic fluid that can be pumped and introduced into embankments, gaining strength over time. They provide the necessary strength, stability, and water resistance for levees. In these suspensions, clay provides cohesive and impermeable properties, while cement acts as a binder that improves strength in the later stages of curing. The interactions between clay platelets, cement hydration products and water create a complex microstructure that resembles a continuous 3D network with mechanical durability and infiltration properties31–33.

Recent studies have shown that fly ash type and particle size distribution significantly influence mixture properties. Černý et al.34 confirmed the usability of fly ash-clay mixtures for water-protective dikes, achieving hydraulic conductivity coefficients of 10−8 to 10−10 m/s. Studies Mir35 Das and Yudhbir36 demonstrated that calcareous fly ash (Class C) was more effective than silicious fly ash (Class F) in improving strength and reducing swelling of expansive clay. Yoon et al.37 reported California Bearing Ratio (CBR) values of 19–29% for fly ash-bottom ash mixtures, which is within the range of many natural soils used in construction. This suggests that ash mixtures can provide adequate support for overlying structures while potentially offering other benefits such as reduced weight and improved workability. One notable advantage of fly ash embankments is their consolidation behavior. Chai et al.38 observed that fly ash fills tend to settle more rapidly than those composed of natural soils. This faster consolidation can be beneficial in reducing long-term settlement issues and allowing for quicker construction timelines.

The rheological properties of ash mixtures are particularly interesting due to their complex, concentration-dependent behavior. Das et al.36 found that fly ash suspensions undergo a transition from Newtonian behavior at low concentrations (below 15 wt%) to non-Newtonian behavior with yield stress at higher concentrations. Above 35 wt%, fly ash suspensions exhibit a two-step yielding behavior, indicating the formation of complex internal structures.

Bentonite suspensions, on the other hand, show non-Newtonian behavior and yield stress even at low concentrations due to their swelling nature. Shakeel et al.39 observed single-step yielding in bentonite suspensions across a wide range of concentrations (3–20 wt%). When fly ash and bentonite are mixed, the resulting suspensions can exhibit either single-step or two-step yielding depending on the mixing ratio, allowing for fine-tuning of rheological properties.

Das and Yudhbir36 reported significant differences in compressive strength between low and high calcium fly ashes. Low calcium fly ashes typically achieved 0.5–3 MPa after 28 days, while high calcium fly ashes reached over 20 MPa. Mixing 20% high calcium fly ash with alluvial silty clay increased 7-day compressive strength from 0.2 to 4 MPa.

Increasing fly ash content decreased maximum dry density while increasing optimal moisture content during compaction. Li et al.40 observed that fly ash initially increased soil mixture permeability, which decreased over time due to pozzolanic reactions. Martin et al.41 reported Class F fly ash embankments reached 0.14–0.28 MPa after 28 days, with continued strength gain over time.

The significant heterogeneity of ash properties, such as grain size distribution, chemical composition, and mineralogy, presents both challenges and opportunities in its applicability. Although the potential of fly ash as a substitute material in the construction industry has been recognised, the hidden value in different ash fractions often remains unexplored. By extracting and using appropriate fractions, we can optimise desired properties for specific use cases while potentially repurposing other fractions for different industrial applications, thus maximising the overall utility of this abundant resource42–51.

In this study, the use of locally-sourced Polish fly ashes in sealing suspensions for flood levee construction. By examining how different fractions of fly ash size affect clay-cement suspensions, we aim to optimise their use in flood protection infrastructure. This interdisciplinary approach not only advances our understanding of fly ash-clay-cement systems but also promotes sustainable construction practices. Our research has implications for the development of efficient and cost-effective sealing suspensions while contributing to the principles of the circular economy in geotechnical engineering. Furthermore, this work underscores the importance of studying individual cases to unlock the full potential of fly ash as a substitute material which promotes its utilisation.

Materials and methods

Materials

Fly ash

Two different types of fly ashes from Polish power plants were used:

S1—High calcareous fly ash from lignite combustion (S1.O), obtained from the Belchatów power plant, Poland, from brown coal combustion.

S2—Silicious fly ash from bituminous coal (S2.O), from the Kraków power plant, Poland.

Granulometric separation was used to obtain representative fractions. The chemical, mineral, and granulometric compositions of the obtained fractions are presented in the following parts.

Cement

Cement CEM I 42.5 R Odra Opole, produced by the cement plant "Odra" SA, is composed of Portland clinker in the range of 95–100% and secondary components such as gypsum in the range of 0–5%. The chemical composition is presented in Table 1.Table 1 Chemical composition of Cement CEM I 42,5 R Odra Opole.

Chemical composition (% by weight)	
SiO2	Al2O3	Fe2O3	CaO	MgO	Na2Oeq	
19.8	6.2	2.6	63	1.4	0.6	

Manual sieving using a 63 μm mesh sieve was performed to remove agglomerates and coarser particles, ensuring a high degree of homogeneity in the cement additive to minimise its impact influence of the cement grain size on the pore structure and, consequently, on the mechanical and durability properties of the material.

Clay

The mineral accompanying the brown coal deposits at the Bełchatów mine was used as clay raw material. The Bełchatów clay present in this paper is the most common clayey silt in this deposit. The chemical composition is presented in Table 2.Table 2 Chemical composition of Clay Bełchatów.

Chemical composition (% by weight)	
SiO2	Al2O3	CaO	MgO	Fe2O3	MnO	Na2O	K2O	TiO2	H2O	LOI	
58.65	14.55	4.75	0.72	3.46	0.72	1.02	1.05	0.42	5.52	9.72	

From a mineralogical point of view, this clay consists mainly of beidellite and quartz.

Methods

Chemical composition analysis XRF

The WD-XRF S8 TIGER spectrometer from Bruker was used for the analysis. The measurements were performed using the vacuum method with the built-in Quant Express reference standard.

Mineralogical analysis XRD

The phase composition of the ashes was determined using a PANalytical Empyrean X-ray diffractometer. Measurements were made using monochromatic radiation with a wavelength corresponding to the copper K(α1) emission line (1.54178 Å), in the angular range of 5–90° in 2θ scale, with a goniometer step size of 0.008 degrees. The qualitative analysis of the phase composition was carried out using the X'Pert HighScore Plus computer programme developed by PANalytical. The reference databases: PDF-2 (2004) and FIZ Karlsruhe ICSD database (2012).

Lost of ignitions (LOI)

LOI tests were performed according to the ASTM D7348 procedure. In the first step, the sample was weighted and heated at 110 °C for 1 h. The sample was then placed in a desiccator to cool for 60 min before being reweighted. The weight loss in this step was recorded as the moisture content. In the second step, dried fly ash was placed in a furnace and heated in a stepped schedule for 2 h to reach 950 °C. The fired sample was cooled down to room temperature in a desiccator and then weighted. Weight loss associated with the firing of the sample is known as loss of ignition (LOI).

Density analysis

Density analysis was performed using the volumetric method in a discontinuous manner with the ASAP Micromeritics 2010. The sample was degassed at 350 °C for 24 h under a vacuum of 10–3 mmHg. The BET multi-point method was used to determine the specific surface area at relative pressures ranging from 0.05 to 0.30. The apparatus performed the volume calibration prior to the actual measurement. Nitrogen with a purity of 99.999% was used as the adsorbate, and the measurement was conducted at the temperature of liquid nitrogen.

Rheological measurements

For rheological studies, a Brookfield DV-III + rheometer with a coaxial cylinder system was used, which allows for the measurement of shear stress or viscosity of liquids at different shear rates.

The first part of the rheological investigation focused on studying the short-time reactivity of fly ashes, both S1 and S2, and their fractions. The tests were performed using water-fly ash suspensions, where the ash-to-water ratio was adjusted (w/a). For fly ashes S1, the w / a ratio was established at 10/3 and for fly ashes S2 at 1/1. These ratios were chosen based on noticeable differences in water demand and suspension workability, with S1 ashes exhibiting greater reactivity compared to S2 ashes. The measurement was performed at a constant shear rate of 20 rpm immediately after the preparation of the suspension. The test duration time was 40 min. Obtained changes in suspension viscosity provided insights into the short-time reactivity of the fly ashes concerning their ability to react with water immediately after mixing and their ability to form a spatial structure.

The second part of the rheological tests involved measuring the rheological properties of various mixtures of fly ash-clay-cement. Viscosity measurements were performed using an algorithm in which the shear rate was increased by 2.0 rpm every 30 s. The rheometer started at 2.0 rpm and data was collected at each speed change, up to a maximum of 40.0 rpm. Then, a second stage of measurements takes place and the shear rate decreases until 2.0 rpm. For each data point, apparent viscosity, shear stress, and time were measured.

Mixtures for 20%, 30% and 40% fly ash additives were prepared by weight. Where for S1 ashes the base suspension density of 1.13 g/cm3 was selected and for S2 ashes 1.20 g/cm3. Cement in all mixtures is 10% by weight. Such recipes allow for the most appropriate measurement.

Mechanical measurements

Compressive strain tests were conducted using the ZwickRoell Tira Test 2300 mechanical test machine with Senga software. Cylindrical samples measured 45 × 45 mm were subjected to a curing process submerged in water and tested after 14, 28, and 90 days of curing. Mixtures for 20%, 30% and 40% fly ash additives were prepared by weight. Where for S1 ashes the base suspension density of 1.13 g/cm3 was selected, and for S2 ashes 1.20 g/cm3. Cement in all mixtures is 10% by weight. Such recipes allow for the most appropriate measurement.

The proportions of the mixture of the tests performed are gathered in the form of a table (Table 3).Table 3 Mixture proportions of performed tests.

Name of the test	Mixture proportions	
Rheological	Tested fractions	Base dencity	Water	Ash	Cement	Curing time	Results in	
Water-ash short time reactivity	S1	O, C, M, F, UF	n.a	10	3	n.a	directly after mixing	Figure 3a	
S2	O, C, M, F, UF	n.a	1	1	n.a	directly after mixing	Figure 3b	
Sealing suspensions rheology	S1 and S2	O, UF	1.20 g/cm3	n.a	20%	10%	directly after mixing	Figure 4	
S1	O, M, F, UF	1.13 g/cm3	n.a	20, 30, 40%	10%	directly after mixing	Figure 5a	
S2	O, M, F, UF	1.20 g/cm3	n.a	20, 30, 40%	10%	directly after mixing	Figure 5b	
Mechanical	Tested fractions	Base dencity	Water	Ash	Cement	Curing time	Results in	
Compressive strain	S1	O, C, M, F, UF	1.13 g/cm3	n.a	20, 30, 40%	10%	14, 28 and 90 days	Figure 5a	
S2	O, C, M, F, UF	1.20 g/cm3	n.a	20, 30, 40%	10%	14, 28 and 90 days	Figure 5b	
n.a. not applicable.

SEM and microstructure

Scanning electron microscopy (SEM): measurements were made on the fly ash fractions and hardened mortars obtained after 14, 28, and 90 days of curing. The Thermoscientific Fisher Phenom XL SEM equipped with an Energy Dispersive Spectroscopy (EDS) attachment was used. The EDS attachment allows for the efficient determination of the point chemical composition of the tested samples.

Results and discussion

Fly ash fractions characteristic

Granulometry and microstructure characteristic

The dry aerodynamic separation process resulted in the obtaining of different fractions according to the grain size distribution. The ACX separator produced by COMEX Polska sp.zoo was used52. These fractions include:Ultra Fine fraction (UF): particles with a grain size range of 0–10 μm. It consists of the finest particles obtained.

Fine fraction (F) has a grain size distribution from around 5–20 μm. This fraction contains particles slightly larger than those in the UF fraction.

Middle fraction (M) was obtained by manual sieving of the aerodynamic separation residues using a 100 μm mesh sieve, resulting in a grain size range of approximately 20–100 μm, and represents particles of intermediate range size.

Coarse fraction (C) consists mainly of grains larger than 100 μm and represents the residues on top of the 100 μm sieve.

Figures 1 and 2 presents SEM images of obtained S1 and S2, respectively.Fig. 1 SEM images of fly ash fractions: (a) S1.C; (b) S1.M; (c) S1.F; (d) S1.UF.

Fig. 2 SEM images of fly ash fractions: (a) S2.C; (b) S2.M; (c) S2.F; (d) S2.UF.

The parameters d90, d50 and d10 parameters in Table 4 were used to determine the size of the fractions, representing percentiles denoted by the letter d followed by the percentage value. Therefore, d10 = 1.7 µm means that 10% of the particles are smaller than 1.7 µm, etc. Figure 3 presents the granulometric distribution of the fractions obtained as a cumulative volume graph.Table 4 Fly ash fractions granulometry.

	S1.O	S1.C	S1.M	S1.F	S1.UF	S2.O	S2.C	S2.M	S2.F	S2.UF	
Distribution (μm)	
 d10	1.7	69.6	22.7	1.17	0.5	3.1	73.8	17.6	1.8	0.4	
 d50	28.4	159.7	53.6	7.4	2.3	21.9	179.2	52.3	5.9	2.4	
 d90	140.3	349.8	101.8	19.9	9.8	142.6	338	102.6	21.2	10.5	

Fig. 3 Granulometric distribution of the fly ash fractions obtained: (a) S1 ashes; (b) S2 ashes.

Chemical and mineralogical composition

The chemical composition of the fractions obtained for the ashes of S1 and S2 is presented in Table 5.Table 5 Chemical composition of the fly ash fractions S1 and S2.

	Chemical composition (% wt)	
S1.O	S1.C	S1.M	S1.F	S1.UF	S2.O	S2.C	S2.M	S2.F	S2.UF	
SiO2	30.9	44.4	28.8	21.1	16.2	54.2	58.9	51.3	49.7	48.0	
Al2O3	30.1	33.1	28.8	26.0	24.1	24.5	23.7	23.0	29.5	30.1	
CaO	24.7	12.5	25.4	33.9	37.8	5.1	3.0	6.6	4.3	4.2	
SO3	0.2	2.1	3.4	4.7	6.4	0.4	0.3	0.4	1.0	1.3	
Fe2O3	9.5	5.7	10.7	10.6	10.4	8.3	7.0	9.1	6.1	5.9	
MgO	1.1	0.8	1.0	1.3	1.5	3.4	2.5	4.2	2.9	2.5	
TiO2	0.7	0.9	0.7	0.6	0.6	1.0	0.9	0.9	1.3	1.3	
P2O5	0.6	0.4	0.6	0.5	0.6	0.3	0.2	0.3	0.7	0.9	
K2O	0.2	0.2	0.1	0.1	0.2	2.6	2.7	2.6	3.3	3.2	
Na2O	–	–	–	–	–	1.0	0.6	1.5	2.3	2.4	
Total Ʃ	98.0	100.1	99.5	98.8	97.8	100.8	99.8	99.9	101.1	99.8	
LOI (% of total mass)	1.7	6.5	0.9	1.7	1.8	11.2	14.9	7.7	13.3	13.3	

The mineral composition is presented in Tables 6 and 7. The identified phases in each fraction are denoted using a ranking scale to indicate the relative abundance:Table 6 Mineralogical composition of S1 ash fractions.

Identified phase	S1.O	S1.C	S1.M	S1.F	S1.UF	
Calcite	−	−	−	+	+	
Eckermannite-gehlenite	++	+	++	++	++	
Quarts low	+	++	++	−	−	
Anhydrite	+	−	+	++	+++	
Hematite	+	−	+	++	+	
Mullite	−	+	−	−	−	
Lime	−	−	−	−	+	
Complex high-calcium oxides	+	−	++	+++	+++	
Amorphous	++	++	++	++	++	

Table 7 Mineralogical composition of S2 ash fractions.

Identified phase	S2.O	S2.C	S2.M	S2.F	S2.UF	
Calcite	−	−	−	+	−	
Mullite	++	+	++	++	++	
Quartz low	++	++	++	+	+	
Hematite	+	−	+	+	+	
Lime	−	−	+	−	−	
Amorphous	++	++	++	+++	+++	

"−" indicates the absence or very low phase presence;

"+" denotes a minor amount of the phase;

"++" signifies the phase presence in moderate quantities;

"+++" represents a high amount of the phase compared to other fractions.

In Table 8, the density and obtained specific surface area of fractions are presented.Table 8 Density and specific surface area (SSA) of the fly ash fractions S1 and S2.

	Density [g/cm3]	SSA [m2/g]		Density [g/cm3]	SSA [m2/g]	
S1.C	2.9	8.9	S2.C	2.3	5.5	
S1.M	2.6	67.4	S2.M	2.3	3.5	
S1.F	3.0	8.1	S2.F	2.5	6.7	
S1.UF	3.1	13.1	S2.UF	2.7	7.2	

The XRD analysis of S1 fly ash samples highlights a variety of complex high-calcium oxides with increasing concentrations in finer fractions S1.F and S1.UF. Those are calcium sodium aluminium oxide, calcium aluminium oxide sulphate, calcium iron aluminium oxide, and calcium magnesium aluminium silicate. These oxides are characterised by structurally modified forms with variable chemical compositions, which also affects their reactivity. Compounds involving calcium, aluminium, and sulphate ions can participate in various chemical reactions when exposed to water. For instance, calcium sodium aluminium oxide might form aluminium hydroxide and sodium hydroxide, releasing heat in the process when exposed to water. Calcium aluminium oxide sulphate is the anhydrite-/gypsum-like phase that can undergo relatively rapid surface hydration reactions with water, increasing viscosity. The presence of these minerals in finer fractions suggests faster hydration reactions due to larger surface areas and fineness of grains53–57.

It is worth noticing that the mineral distribution across grain sizes in fly ashes indicates a "disintegration effect"58,59. Hard minerals like quartz show a lower tendency to be affected by mechanical separation, typically remaining in the coarser fractions. On the contrary, softer minerals, such as anhydrite, appear more frequently in the finer fractions, because of their greater susceptibility to breakdown during processing. Such low-durable minerals concentrated in fine fractions strongly influence short-time reactivity, thus causing an increase in viscosity. Gehlenite, a mineral containing calcium and silica, can react with water and contribute to a rapid increase in viscosity in the suspension. Anhydrite can interact with water to form gypsum and the potential formation of ettringite from calcium aluminium oxide sulphate. Fly ashes S1 contain an increased amount of these and other compounds, which are unevenly distributed throughout the particle size range, contributing to the difference in reactivity between fractions (Table 4)60–63.

The observed mineralogical distribution finds a response in the results of the chemical composition of fly ash samples S1 and S2, with a focus on their finer fractions, suggesting that the alumosilicate glass present is likely to exhibit higher levels of modification compared to the coarser fractions57,64. In finer fractions, the glass phase is expected to be less siliceous and more modified, potentially enriched with network modifiers such as calcium, magnesium, sodium, and potassium, according to the results of the chemical composition analysis. These modifiers disrupt the glass structure, making it more reactive. This contrasts with the coarser fractions, where the glass is anticipated to be more siliceous and less modified, thus more stable and slower in participating in pozzolanic reactions. The composition of the glass phase, including the balance of network formers and modifiers, plays a critical role in the reactivity of fly ash, influencing the dissolution rates and the formation of strength-enhancing products within the cement matrix.

Rheological tests

Short-time reactivity

For both ashes in water suspensions (Fig. 4), the viscosity increased with a measure of time was observed. This suggests that some ash-water reactions start immediately after mixing. S1 ashes, with their higher calcium content, demonstrate faster reactivity, while the more siliceous nature of S2 results in slower reactivity.Fig. 4 Influence of the ash fraction on the viscosity of water-ash suspension viscosity; fly ash. Water/ash (w/a) ratio: (a) S1-10/3; (b) S2-1/1.

In particular, the coarse S1.C fraction, despite having less reactive mineral composition than the S1.UF and S1.F groups, shows a tendency for a quicker viscosity development in the initial stage (2–3 min). This behaviour can be attributed to the presence of large angular particles that result in interlocking and/or rapid sedimentation of coarse grains, resulting in a shear-thinning behaviour similar to that of sand65.

However, it is observed that the viscosity increases in fine S1.F and S1.UF fractions are lower compared to the S1.M fraction (Fig. 4a). This can be explained by the dominant factor influencing the viscosity increase immediately after mixing for these ashes being the surface area development and thus free water absorption. The S1.M fraction has the highest surface development compared to the other fractions, 67 m2/g, resulting in the absorption of free water and an accompanying increase in viscosity, while for S1.UF and S1.F states 13.1 m2/g and 8.2 m2/g (Table 7).

For all S1 samples, the system stabilises within 15–30 min after mixing, ensuring the stability of the rheological measurements at a constant level. Stabilisation refers to the equilibrium of the phenomena of structure formation and shear-thinning structure destruction.

For S2 ash fractions, the highest viscosity increase rate is observed for the coarse S2.C fraction (Fig. 4b). Similarly to the S1.C fraction, this can be related to sedimentation and interlocking effects of coarse angular grains. The medium S2.M fraction shows very similar characteristics to the original S2.O ash with a slight increase in viscosity after the 25th minute of measurement, which can suggest at the beginning of hydration reactions: the ultrafine S2.UF fraction does not show reactivity ability as it was in the fine fractions S1.F and S1.UF. Furthermore, the high spherical ashes of S2.F and S2.UF cause a decrease in suspension viscosity compared to the original ash of S2.O, probably due to the ball bearing effect66–68. Generally, due to the relatively high Si-Al composition and low Ca compounds of the bituminous ashes of S2, a low level of reactivity is observed in short periods.

In summary, the increase in viscosity immediately after mixing and its impact on the workability of the fly ash mixtures is mainly influenced by surface development and water absorption. Secondary influences include the presence of reactive minerals and compounds, as well as the shape of individual particles. The presence of spherical grains results in a reduction of internal friction, while coarse angular fractions cause an interlocking thickening effect. However, further analysis would be required to fully understand all of the processes that influence the changes in viscosity and the setting of the water ash suspensions.

Sealing suspensions rheology

Figures in this chapter present a change in sealing suspension viscosity as a function of the shear rate. The solid-filled markers present the part of the measurement for increasing shear rate speeds, and the non-filled markers for the decreasing shear rates. Figure 5 presents the comparison between the S1.O, S1.UF and S2.O, S2.UF samples with identical suspension proportions: fly ash 20%, cement 10%, base suspension density 1.20 g/cm3. Course fractions C, due to large grains, provide significant distortions to rheological measurement results, making them meaningless and causing a high probability of damaging the equipment. For these reasons, the coarse fraction studies were limited in this work.Fig. 5 Change in viscosity as a function of the shear rate for the S1.O, S1.UF, and S2.O, S2.UF fractions. Base clay suspension density 1.20 g/cm3, cement 10%, fly ash 20%.

S1 ashes, due to their high reactivity, show a rapid increase in viscosity. The S1.O sample shows shear thickening characteristics65,69. The addition of 20% of the high reactive S1.UF fraction to the 1.20 g/cm3 dense suspension of 1.20 g/cm3 results in the extension of a measurement scope after the first measurement point. Therefore, in further measurements for the S1 ashes samples, to obtain sufficient workability, the base suspension density of 1.13 g/cm3 was used.

The less reactive S2 ashes demonstrate shear thinning behaviour, whereas the curves for decreasing share rates lie lower than those for increasing share rates. The additive of the S2.UF fraction shows an increase in viscosity compared to the S2.O fraction, which is inconsistent with the results of the previous section (Fig. 4). It can probably be related to clay particles in suspensions that block or reduce the friction-thinning ball-bearing effect of ash grains. This also indicates that the behaviour of fly ash fractions differs when tested in pure water as compared to when tested in a clay-water suspension.

Influence of fly ash granulometry

The reduction of S1 fly ash particle size cases in an increase in the viscosity of clay-cement suspensions (Fig. 6). In the high shear rate region, the balance between the formation and destruction of the suspension structure is obtained. At lower shear rate areas, an increase in stress can be observed due to faster structure rebuilding, leading to an increase in viscosity. The fine fractions S1.F and S1.UF show the behaviour according to expectations and cases of a rapid increase in viscosity after mixing. For S1.UF suspension, the viscosity acquires values beyond the measurement range at a shear rate of 20 rpm. Similarly, the S1.F fraction results in a constant increase in viscosity followed by an exceeding of the scope.Fig. 6 The influence of fly ash fraction on suspension viscosity: (a) S1 20%, cement 10%, density 1.13 g/cm3; (b) S2 20%, cement 10%, density 1.20 g/cm3.

It should be noted that for the S1.M fraction, based on the results of the ash-water reactivity (Fig. 4), the highest viscosity development was expected in the clay-cementitious suspension. However, the viscosity behaviour of S1.M is not consistent with those expectations and the results obtained remain relatively low in a wide range of measurements. This indicates that S1.M ash does not possess reactive properties to the same extent as the S1.UF and S1.F fractions and the viscosity increase described in the S1.M water suspension (Fig. 4) was mainly caused by the absorption of free water on the surfaces of fly ash. Meanwhile, in the case of clay-based suspensions (Fig. 6), a relatively low viscosity level may be attributed to the following factors:Clay particles form a layer on the surface of the fly ash, acting as a surface agent and blocking the access of water molecules to the surface of the ash. This layer can act as a barrier, preventing water from reaching the pores, voids, and cracks on the surface of the ash where it could be adsorbed. This hinders the water absorption process so more free water remains in suspension and viscosity stays relatively low70;

Clay particles can form aggregates and structural networks in water-clay suspensions, and phenomena related to this ability are well studied in the literature65,70,71. These structures can block the movement of water molecules, reducing the availability of adsorption sites on the surface of the fly ash. This also slows down the water absorption by ash;

Clay particles contribute to the ability of water absorption on their surface and by their internal structure. If clay particles are present at high concentrations in a clay-water suspension, they can adsorb water, competing with fly ash. This can reduce the available surface area for water adsorption by fly ash particles, as water molecules are bound to clay minerals, thus maintaining free water in suspension and improving flowability, etc.65–67,72–74

For the S2 ashes, the addition of the S2.F fraction allows for a reduction in viscosity compared to the S2.M and S2.UF fractions. This can be related to the interaction between clay particles and ash grains, where the S2.F grains are large enough to cause a ball-bearing effect and reduce suspension internal friction. However, for ultra-fine S2.UF grains, the clay particles can cover ash surfaces and block their ball-bearing behaviour, which causes the suspension viscosity to increase. For the coarser S2.M fraction, as expected according to the short-time reactivity results of Fig. 4, the suspension viscosity reached the highest values. The addition of obtained fractions has increased the suspension viscosity compared to original S2.O ashes.

Results of the compressive strain and microstructure analysis

Compressive strain results

In the base clay-water suspension with a density of 1.13 g/cm3 for S1 ashes and 1.20 g/cm3 for S2 ashes, 10% cement and 20%, 30% and 40%wt of individual fly ash fractions were added and mixed. Differences in base suspension density are related to the workability of fresh mixtures. The results of the comprehensive strain tests at 14, 28, and 90 days of underwater curing are presented in Fig. 7.Fig. 7 Compressive strain for samples with S1 ash fractions: cement 10%wt, base suspension density 1.13 g/cm3; With S2 ahs fractions: cement 10%, base suspension density 1.20 g/cm3.

The obtained results show that decreasing the fineness of fly ash fractions increased the compressive strength obtained for both S1 and S2 ashes, resulting in 2.48, 2.65, 2.97, 4.70 and 6.93 MPa for samples S1.O, S1.C, S1.M, S1.F and S1.UF 40%; and 4.46, 1.71, 4.30, 5.75 and 5.95 MPa for samples S1.O, S2.C, S2.M, S2.F and S2.UF 40%, respectively. The additive of the ultrafine fraction S1.UF gave the maximum strain obtained from the tasted recipes showing an improvement of approximately 179% in strain compared to the original ash of S1.O, while for the 40% S2.UF sample this improvement states approximately 33% compared to S2.O.

These strain improvements can be attributed to several factors, among which is the chemical and mineral composition of the fractions with a higher concentration of reactive mineral phases in the finer fractions. In addition, particle fineness promotes more active participation of grains in structure-forming reactions, owing to the increased contact between the particle surfaces and water compared to coarser grains. On the other hand, the particle size of the F and UF fractions influences the suspension workability and structure arrangement. As a result, a more dense and compact microstructure is formed, with individual grains undergoing a more extensive reaction. This reduction in porosity and the improved interaction contribute significantly to the development of compressive strain. Microstructure analysis confirms that the addition of coarse grains causes the formation of a structure with large pores and cracks, while fine fractions result in a dense uniform structure. Figure 8 shows the influence of particle size on microstructure formations. For both ashes, there is a noticeable intensification of the deformation gain in later stages, which is consistent with the theory of delayed reactions in fly ashes16,23,75.Fig. 8 Microstructure differences depending on grain type: (a) coarse C grains, (b) middle M grains; (c) fine UF grains.

Notable is the variance in the densities of the base suspension used to achieve the optimal workability of the mixtures, with the S1 ashes having a density of 1.13 g/cm3 and the S2 ashes at 1.20 g/cm3. Due to this difference, direct comparisons of the two types of ash at the same replacement levels based on the module values of the final strain results are not appropriate. However, an analysis of the nature of strength development over time can be performed. The S1 ashes showed faster early compressive strain development in the 14–28 day period followed by slower later-stage strain gain compared to the S2 ashes. Instead, S2 ashes had low initial strength gain, but higher late-stage strength increased between 28 and 90 days. This could be attributed to the highly reactive mineralogical compounds in S1 lignite ashes that react at an early stage period, while for less reactive S2 bituminous ashes, these structure-forming reactions are shifted to later stages of curing. This can also be observed in SEM images Figs. 9 and 10 and will be partly discussed in a further chapter.Fig. 9 SEM images of microstructure evolution. S1.O (a–c) and S2.O (d–f) at 7, 28, and 90 days of curing. Fly ash 20%, cem.10%, 1.13 and 1.20 g/cm3 respectively.

Fig. 10 Grain reactivity in the function of chemical composition at 90 day with point EDS analysis: (a) high Al-Si grain; (b) medium Al-Si grain; (c) high Ca grain.

Although the coarser particles (C fraction) understandably had lower overall reactivity, especially in the early stages, a strong initial strain development can be observed at 0–14 days for the ashes of S1.C and S2.C. This can be related to the interlocking effect, where the angular shape of the coarse grains contributes to the mechanical interlocking between the particles, which manifested as reasonably high early strain test results17,76,77.

Medium fractions for both ash types S1.M and S2.M showed the overall lowest strength values, possibly because these particles were not large enough for a significant interlocking effect for the early-stage compressive strain gain, nor fine enough to contain sufficient reactive potential as in F and UF fractions. Additionally, for the S2 ashes, the influence of the interlocking effect on the structure formation processes is lower compared to S1, due to the nature of the grain shape, which is predominantly spherical and smooth for the bituminous ashes S2. Bituminous coal combustion processes occur at higher temperatures, resulting in an increase in the amount of melted particles and a higher average smoothness and sphericity of grains24,25.

Increasing the amount of ash additives beyond a certain limit may negatively impact the final strain results. This negative effect may appear as a result of the absorption of free water essential for the hydration microstructure formation processes; however, curing in underwater conditions might eliminate this factor. Additionally, an increase in ash content can result in a loss of workability, as confirmed by rheological tests, leading to an insufficient settlement of fresh mortars and incomplete structural continuity, promoting the formation of cracks, pores, and internal stresses. Furthermore, an excessive ash content in the matrix can result in increased nonreactive ash-to-ash interconnection clusters that impede strain development (Fig. 7), particularly in the early stages when coarse particles are involved78,79. This effect can be observed in the S2.C samples, where 40% ash addition results in a decrease in final strain compared to the S2.C 30%, resulting in 2.85 MPa and 1.71 MPa, respectively.

For individual fly ash types and fractions, it is important to adjust the quantities and composition of additives to achieve the desired results, as they may exhibit varying behaviours, leading to different results.

Influence of fly ash granulometry

The microstructural studies maintain the compressive strain results, and a strong correlation was observed between the strain obtained and the microstructural results.

Comparative analysis of microscopic morphology (Figs. 9, 10, 11) demonstrates that in the early period of 14 days, the spherical fly ash particles were still smooth, indicating their relatively longer time reactivity, compared to grains with high Ca content and cement particles. The fly ash particles began to hydrate from the surface at later stages, so at 28 and 90 days the pits in the inert surface can be observed. It is worth noticing that the particle dissolution level of S2 ashes is lower compared to S1. This can be related to the higher Si content of S2 ashes, so those particles have a longer activation and dissolution time. Figure 10 presents the reactivity rate of the ashes at 90 days of curing depending on the composition of the EDS chemical content of the individual grain. The reactivity and solubility level increase with increasing content of structure modifiers in particles, so the high silicious S2 ashes contribute lower reactivity compared to calcaneus S1 ashes. Also, for this reason, the S1 sample structure is denser and more hydration products can be observed15,64,75,80.Fig. 11 (a) Cluster of fine grains inside a larger grain, sample S2.O at 90 day of curing; (b) Immobilized grains in "pockets".

There was also observed a difference in microscale structure formation for the S1 and S2 ashes, whereas, for the S1 sample, the highly dense needle crystals are abundant. These needle crystals are needle-shaped ettringite mineral crystals hydrated with Al-Ca-S, formed in the presence of high concentrations of SO4–2 and the presence of an elevated CaO content (as in the case of S1 ashes)81,82. The formation of ettringite is enhanced by the high containment of Al oxides, available from mineral decompositions and amorphous phase. This contributes to the formation of the crystalline phase, which affects the structure, as pores and free spaces become filled with needles. Ettringite formation can be responsible for the degradation of cementitious materials, through swelling of the material followed by diffusion of cracks in the structure81,83,84. Gesoğlu et al.85 show that for porous materials, such as fly ash clay-cement suspensions discussed in this study, the growth of ettringite crystals does not pose a risk of expansion cracking, because these crystals grow in pores and empty spaces in the structure, causing an increase in the final strain, the structure tightness, and potentially causing an increase in permeability properties86–88. So, the highly reactive S1 particles start to dissolve and intersect with the surroundings, in a result filling the pores and creating a more dense structure compared to S2. For the S2 samples, the cement hydration phases are mostly observed, while the ash particles remain unreacted.

The influence of agglomerates and 'pocket-enclosed' grains in the S1.O and S2.O ashes is worth noting (Fig. 11). These agglomerates cause the formation of regions in the structure, where clusters of unreacted ashes are present, providing anisotropy to the structure. Pocket-enclosed grains result in immobilisation of some potentially reactive grains. These effects cause the final strain to decrease for the suspensions of ash S1.O and S2.O. For other fractions, those effects have less impact because the aerodynamic separation processes performed cause a disintegration effect, where low-durable grains and agglomerates are broken down, and the ashes obtain a higher level of dispersion58,59. As a result, a higher number of individual particles can be released and participate in structure formation. Fractionation has a significant impact on the intensity of structure formation, hydration processes, and grain reactivity.

Conclusions

The separation of fly ash into different size fractions results in fractions with varying chemical, mineral, and physical characteristics. Finer fractions contain increased levels of reactive aluminum, calcium, and sulphur compounds compared to coarser with more silica-rich content.

Finer ash fractions possess higher reactivity, evidenced by rapid viscosity increases upon water mixing as well as enhanced early strength development. The ultrafine fraction induced flash setting within a few minutes after mixing minutes. In contrast, coarse fractions showed more gradual reactivity behavior over longer timescales.

Interactions between the ashes and clay suspensions led to complex rheological impacts dependent on particle fineness, shape, and mineralogy. High calcium S1 ashes resulted in more intense early hydration reactions compared to siliceous S2 ashes.

Microstructural analysis correlated the improved compressive strength of clay-cement mortars to the ability of fine ash fractions to accelerate the formation of denser, more uniform structures. The results of this work demonstrate the differences between individual fly ash fractions and their potential to engineer properties of suspension-based construction materials and systems.

The outcomes of this study not only advance our understanding of fly ash-clay-cement interactions but also provide a framework for optimizing the use of fly ash fractions in flood protection infrastructure. By unlocking the hidden potential in different fly ash fractions, this research opens up new possibilities for maximizing the utility of this abundant resource, potentially leading to more efficient and environmentally friendly construction practices.

Author contributions

JD contributed to conceptualization, methodology, validation, formal analysis, investigation, data curation, writing—original draft, and visualization. PI and ŁW, contributed to methodology, validation, formal analysis, investigation, data curation, resources, writing—review and editing, and supervision, AS—writing, editing and supervision, MJ—formal analysis.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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