Cement Carbonation Under Fermentation Conditions as a Tool for CO₂ Emission Management—Technological, Environmental and Economic Analysis

Abstract

The aim of this study is an interdisciplinary assessment of the potential of cement pastes to permanently bind carbon dioxide (CO₂) under anaerobic digestion conditions, considering technological, microstructural, environmental, and economic aspects. The research focused on three types of Portland cement: CEM I 52.5N, CEM I 42.5R-1, and CEM I 42.5R-2, differing in phase composition and reactivity, which were evaluated in terms of their carbonation potential and resistance to chemically aggressive environments. The cement pastes were prepared with a water-to-cement ratio of 0.5 and subjected to 90-day exposure in two environments: a reference environment (tap water) and a fermentation environment (aqueous suspension of poultry manure simulating biogas reactor conditions). XRD, TG/DTA, SEM/EDS, and mercury intrusion porosimetry were applied to analyze CO₂ mineralization, phase changes, and microstructural evolution. XRD results revealed a significant increase in calcite content (e.g., for CEM I 52.5N from 5.9% to 41.1%) and the presence of vaterite (19.3%), indicating intense carbonation under organic conditions. TG/DTA analysis confirmed a reduction in portlandite and C-S-H phases, suggesting their transformation into stable carbonate forms. SEM observations and EDS analysis revealed well-developed calcite crystals and the dominance of Ca, C, and O, confirming effective CO₂ binding. In control samples, hydration products predominated without signs of mineralization. The highest sequestration potential was observed for CEM I 52.5N, while cements with higher C₃A content (e.g., CEM I 42.5R-2) exhibited lower chemical resistance. The results confirm that carbonation under fermentation conditions may serve as an effective tool for CO₂ emission management, contributing to improved durability of construction materials and generating measurable economic benefits in the context of climate policy and the EU ETS. The article highlights the need to integrate CO₂ sequestration technologies with emission management systems and life cycle assessment (LCA) of biogas infrastructure, supporting the transition toward a low-carbon economy.

1. Introduction

Biogas production through anaerobic fermentation of organic matter is a technology that integrates waste management with renewable energy generation. The main products of this process are methane (CH₄), which accounts for approximately 60–70% of the biogas volume, and carbon dioxide (CO₂), which is present at around 30–40%. While methane is a valuable fuel, CO₂, as a greenhouse gas, requires appropriate management strategies due to the need to reduce greenhouse gas emissions [1,2]. In this context, CO₂ sequestration in cement pastes emerges as a promising technology, as these materials demonstrate the ability to chemically bind CO₂ into stable mineral forms. Anaerobic methane fermentation takes place under mesophilic conditions (~35 °C) and includes the stages of hydrolysis, acidogenesis, acetogenesis, and methanogenesis. During these processes, CO₂ is produced and should be separated from methane to enhance the energy value of biogas [3,4]. Instead of being released into the atmosphere, an alternative approach is mineral CO₂ sequestration in cement-based materials, either within existing biogas plant infrastructure (e.g., concrete tanks) or in dedicated blocks or infill materials [5].

Cement pastes contain calcium hydroxide (Ca(OH)₂, known as portlandite) and CSH phases (calcium silicate hydrates), which undergo carbonation reactions with CO₂, leading to the formation of calcium carbonate (CaCO₃). This process not only contributes to the reduction of greenhouse gas emissions but also strengthens the microstructure of the cement paste by densifying it [6,7,8]. Under normal exposure conditions, all cement hydration products are subject to carbonation; however, the rate and efficiency of this process depend on several factors, including the paste composition and environmental conditions. In the initial stage of carbonation, CO₂ dissolves in water to form aqueous CO₂ (CO₂(aq)), which reacts with water to produce carbonic acid (H₂CO₃). This acid subsequently dissociates into H⁺, HCO₃⁻, and CO₃²⁻ ions. As the CO₂-saturated water diffuses into the cement matrix, Ca(OH)₂ dissolves in the acidic solution, leading to the migration of Ca²⁺ ions and precipitation of CaCO₃ [9,10].

The pore solution in cement paste has a high pH (~12), but carbonation causes a gradual decrease in pH. When the pH drops to around 10.5, CO₃²⁻ ions become dominant, and CaCO₃ forms a stable and durable phase, which contributes to reduced porosity. Initially, carbonation affects Ca(OH)₂, but as it becomes depleted, the process progresses to include C–S–H, calcium aluminates, and calcium sulfo-aluminates [11]. In the advanced stage of carbonation, when alkaline buffering phases are exhausted, the pH drops further, HCO₃⁻ ions dominate, and previously formed CaCO₃ dissolves into calcium bicarbonate. This leads to leaching of Ca²⁺ ions from the cement matrix, and the C–S–H phases are converted into amorphous silica gel (SiO₂·nH₂O), resulting in strength loss, increased porosity, and loss of microstructural integrity [11,12].

The main product of carbonation is calcite, although polymorphic forms vaterite (early stage) and aragonite (intermediate phase) may also form. The phase composition depends on environmental conditions, pressure, temperature, and salinity [13].

The methane fermentation environment can be chemically aggressive to the cement matrix. The presence of fatty acids, ammonium ions, and dissolved CO₂ accelerates the degradation of cement hydration products. Proper modification of the paste composition for example, by using CEM III cements with ground granulated blast-furnace slag or fly ash, can improve the material’s resistance to aggressive conditions while simultaneously enhancing its CO₂ binding capacity [14]. Studies indicate that carbonation under anaerobic conditions and in the presence of CO₂ leads to the formation of compact CaCO₃ layers in the near-surface zone of cement pastes, which reduce their permeability. Previous research has shown that the porosity of pastes may decrease by 5–9% within several weeks, and strength can increase by up to 17%, depending on the presence of supercritical CO₂ compared to its gaseous form [14].

In real-world systems, such as deep geological reservoirs, an additional corrosive factor is brine, highly saline water dominated by Na⁺ and Cl⁻ ions. Their presence significantly accelerates the corrosion and carbonation of cement pastes. Chlorides penetrate the cement structure and react with the C₃A phase to form Friedel’s salt (3CaO·Al₂O₃·CaCl₂·10H₂O), and partially bind with the CSH phase [15]. In zones subjected to prolonged CO₂ exposure, the disappearance of portlandite, an increase in CaCO₃ content, and the presence of silica gel are observed. Studies conducted on samples taken from great depths have shown carbonation levels reaching up to 45% and the occurrence of all polymorphic forms of CaCO₃ calcite, aragonite, and vaterite. The permeability of the surface layer decreases significantly, which may protect deeper layers from further degradation; however, once the alkaline buffering capacity is exhausted, carbonation can continue to progress inward [16].

Physicochemical parameters of carbonation, such as CO₂ partial pressure, temperature, moisture content, and salinity, not only influence the rate of the process but also its extent and the stability of the reaction products. From a durability perspective, it is crucial to maintain a balance between CaCO₃ formation and its potential dissolution in the presence of excess HCO₃⁻.

CO₂ sequestration in cement pastes can be carried out either passively, by exposing concrete elements to a biogas environment, or actively, under controlled conditions using gas mixture saturation. This approach enables precise monitoring of the carbonation level and optimization of the material’s mechanical properties [17].

From the perspective of EU climate policy and the Emissions Trading System (ETS), each ton of stable CaCO₃ that binds CO₂ translates into measurable financial savings. Integrating the methane fermentation process with mineral CO₂ sequestration technology can improve the profitability of biogas installations, reduce emissions, and increase the durability of technical infrastructure [18]. Mineral CO₂ sequestration in cement pastes under methane fermentation conditions represents an innovative and practical method of reducing greenhouse gas emissions. This technology combines sustainable development with material efficiency by leveraging the chemical potential of the cement matrix to bind CO₂ in stable mineral forms. Taking into account environmental factors, such as chemical aggressiveness, the presence of brines and chlorides, and microstructural changes in the paste, it is possible to design durable and functional solutions. The integration of this technology with biogas systems offers real added value from both environmental and economic perspectives.

2. Materials and Methods

2.1. Materials Characterization

The study utilized three types of Portland cements, differing in their chemical and mineralogical composition as well as in the rate of strength development:

• Portland cement CEM I 52.5N (Chełm Cement Plant; Chełm, Poland) is characterized by very high early strength (≥20 MPa after 2 days) and low tricalcium aluminate (C₃A) content, making it suitable for applications requiring rapid strength gain and enhanced durability.
• Portland cement CEM I 42.5R-1 (Górażdże Cement Plant; Górażdże, Poland) exhibits high initial strength (≥20 MPa after 2 days) and elevated C₃A content, which increases its reactivity but limits its resistance to chemically aggressive environments.
• Portland cement CEM I 42.5R-2 (Odra Cement Plant; Opole, Poland) also has high early strength (≥20 MPa after 2 days) but features a very high content of C₃A and alkalis, which enhances its carbonation potential but may reduce durability in chloride- and sulphate-rich environments.

The chemical and phase composition of the cements, presented in

Table 1. Oxide composition, phase content, and key properties of materials.

Oxides, Phases, Modules	Type of Cement
	CEM I 52.5N	CEM I 42.5R-1	CRM I 42.5R-2
SiO2	21.10	21.48	18.50
Al2O3	3.23	5.10	5.50
Fe2O3	4.51	2.75	2.70
CaO	64.90	64.20	64.00
MgO	1.13	1.44	1.60
SO3	3.08	2.53	2.10
K2O	0.43	0.83	1.00
Na2O	0.20	0.14	0.10
Cl−	0.047	0.08	0.028
Na2Oeq	0.48	0	0
Phase composition
C3S	62.27	61.4	62.50
C2S	13.86	13.5	6.22
C3A	0.94	8.9	10.02
C4AF	13.71	8.3	8.21
CaSO4	5.24	4.3	3.57
Modules
MN	99.51	97.59	108.58
MS	2.73	2.64	2.26
MG	0.72	1.85	2.04
Specific surface area [cm2/g]
	4550	3668	3460
Chemical composition of aqueous chicken manure extract
Element	Symbol	Content (mg/L)
Total nitrogen (ammonium, nitrates, etc.)	N	2750
Phosphorus (as P)	P	450
Potassium	K	2800
Calcium	Ca	320
Magnesium	Mg	110
Sodium	Na	220

and Figure 1, is crucial for understanding their physicochemical properties, durability, and capacity for binding carbon dioxide through the carbonation process. The analysis of the three types of Portland cement, CEM I 52.5N, CEM I 42.5R-1, and CEM I 42.5R-2, enables the formulation of key conclusions regarding their carbonation potential, chemical resistance, and suitability for environmental applications, including CO₂ sequestration under methane fermentation conditions.

The chemical composition of the reference cements was determined using the XRF method, while the quantitative phase composition was obtained using the Rietveld method. In addition, the aqueous solution of poultry manure was analyzed using ICP. The specific surface area of the cements was determined using the Blaine method.

These cements are characterized by a high calcium oxide (CaO) content exceeding 64%, which promotes the formation of portlandite (Ca(OH)₂), the primary carrier of CO₂ binding capacity in the cement matrix. The highest CaO content was observed in CEM I 52.5N (64.90%), slightly higher than in CEM I 42.5R-1 (64.20%) and CEM I 42.5R-2 (64.00%).

The content of silicon dioxide (SiO₂) is also an important factor, as it influences the amount of CSH phases formed, the main component responsible for the density and tightness of the cement paste microstructure. In this case, the highest SiO₂ content was recorded in CEM I 42.5R-1 (21.48%), while CEM I 42.5R-2 had the lowest content (18.50%), which may negatively affect the material’s long-term durability in aggressive environments.

Oxide components such as Al₂O₃ and Fe₂O₃ determine the presence of aluminate and ferrite phases, which affect the cement’s resistance to chlorides and sulphates. CEM I 42.5R-2 contains the highest amount of Al₂O₃ (5.50%) and the lowest amount of Fe₂O₃ (2.70%), suggesting a dominance of the C₃A phase, which promotes the formation of various chemical corrosion products. In contrast, CEM I 52.5N contains significantly less Al₂O₃ (3.23%) and more Fe₂O₃ (4.51%), which may indicate a higher proportion of the C₄AF phase, known to exhibit some resistance to sulfate attack. The MgO content in all the cements does not exceed 1.6%, indicating no risk of secondary expansion related to periclase transformations. Magnesium, present in CEM I 42.5R-2 at a level of 1.60%, may enhance the chemical resistance of the cement matrix in the presence of ammonium compounds and acidic metabolites.

Alkali oxides, potassium oxide (K₂O) and sodium oxide (Na₂O), also play a significant role, as they influence pH and may contribute to the alkali–silica reaction (ASR). CEM I 42.5R-2 exhibits the highest total alkali content (Na₂Oeq = 0.48%), which increases the risk of reaction with reactive silica, but also helps maintain an alkaline environment during the initial phase of service life.

The concentration of sulphur trioxide (SO₃) ranges from 2.10% (CEM I 42.5R-2) to 3.08% (CEM I 52.5N). Although these values fall within permissible standards, in the presence of moisture and fluctuating pH, the formation of secondary ettringite may occur, leading to expansion and microcracking within the structure. The chloride (Cl⁻) content is low in all cements (0.028–0.08%) yet, in methane fermentation environments rich in brines and chlorine compounds, the presence of chlorides can intensify corrosion processes—especially in cements with high C₃A content. The phase composition of the cements further clarifies their behaviour under operating conditions. All three cements contain over 61% alite (C₃S), which ensures intense hydration and the generation of large quantities of portlandite in the early stages. CEM I 42.5R-2 contains the highest amount of C₃S (62.50%) but the lowest amount of C₂S (6.22%), suggesting a limited capacity for long-term maturation of the cement matrix and a higher rate of carbonation. In comparison, CEM I 52.5N and CEM I 42.5R-1 contain 13.86% and 13.5% C₂S respectively, which favours gradual improvement of mechanical properties and greater microstructural stability over time. The variation in C₃A content is significant: CEM I 42.5R-2 and CEM I 42.5R-1 contain 10.02% and 8.9% respectively, whereas CEM I 52.5N contains only 0.94%. This low C₃A content makes CEM I 52.5N particularly suitable for applications requiring increased resistance to chemical corrosion.

The C₄AF phase, important for resistance to, for example, sulphates, is present in all samples in similar amounts (8.21–13.71%). Meanwhile, the content of calcium sulphate (CaSO₄), which acts as a setting time regulator, ranges between 3.57% and 5.24%. Modular ratios, namely the saturation factor (MN), silica modulus (MS), and alumina modulus (MG), offer a better understanding of the physicochemical properties of the studied cements. CEM I 42.5R-2 is characterized by the highest saturation factor (MN = 108.58), indicating a dominance of CaO over other oxides, which favours carbonation. However, this cement also has the lowest silica modulus (MS = 2.26), suggesting a limited content of calcium silicates (CSH), potentially impacting the long-term durability of the matrix. The alumina modulus (MG) is also the highest for CEM I 42.5R-2 (2.04), which corresponds with its high C₃A content. On the other hand, CEM I 52.5N has the highest MS value (2.73) and the lowest MG (0.72), which indicates a greater proportion of stable CSH phases. The specific surface area, which affects hydration rate and material reactivity, is the highest in CEM I 52.5N (4550 cm²/g), promoting rapid setting and early strength development—but also accelerating the carbonation process in its initial stage. The lowest surface area is found in CEM I 42.5R-2 (3460 cm²/g), which may limit the reaction rate but favours slower development of microstructures that are more resistant to CO₂ diffusion. The cements used (CEM I 52.5N, CEM I 42.5R-1 and CEM I 42.5R-2) differ significantly in terms of chemical and phase composition as well as in their modular ratios, which may influence their capacity for carbonation and their durability in chemically aggressive environments. CEM I 42.5R-2 demonstrates the greatest potential for CO₂ binding due to its high CaO and C₃S content and favourable modular ratios. However, its high content of C₃A and alkalis may limit its durability in the presence of chlorides and sulphates. CEM I 52.5N, with a lower proportion of reactive aluminate phases, shows greater long-term stability and potentially better microstructural resistance. The selection of cement for use in CO₂ sequestration technologies should therefore take into account both its ability to bind carbon dioxide and its resistance to specific environmental factors associated with methane fermentation, such as high salinity and variable pH. The chosen cements allowed for a comparison of their resistance to biological environments and their capacity for permanent carbon dioxide (CO₂) binding. The cement compositions were analysed with regard to their content of CaO, SiO₂ and Al₂O₃ oxides, which are responsible for forming the main hydration phases (C–S–H, Ca(OH)₂ and C₃A) that are key in carbonation and degradation processes under chemically and biologically aggressive conditions.

2.2. Methods

Cement pastes were prepared in accordance with the PN-EN 196-1:2016-07 standard [19], using a water-to-cement ratio (w/c) of 0.5. After casting and initial curing, the samples were exposed to two types of liquid media for a period of 90 days: tap water (pH = 7.4) as the reference environment, and an aqueous suspension of chicken manure (pH ≈ 11.2) as a biologically active medium with fermentative and corrosive properties. The chicken manure suspension was prepared at a 1:2 ratio (m/m) by diluting the organic material with tap water. Prior to the start of the exposure period, a chemical analysis of both media was carried out. The suspension was found to contain significant concentrations of nitrogen, potassium, phosphorus, calcium and magnesium compounds, as well as trace elements such as chromium and lead.

After the 90-day exposure period, the cement paste samples were subjected to a series of laboratory analyses aimed at assessing structural, phase and mechanical changes, with particular emphasis on carbonation effects:

• The phase composition of the samples was analyzed using X-ray diffraction (XRD) with an apparatus consisting of a power supply unit stabilizing the operation of the PW1140/00/60 X-ray tube and a vertical goniometer PW1050/50 (Philips, Eindhoven, The Netherlands). The device was equipped with a vertically mounted PHILIPS X-ray tube with a copper anticathode (Cu) and a wavelength of Kα = 1.54178 Å, using a nickel filter. A PW2216/20 “fine focus” X-ray tube with a power of 1.2 kW was applied; the operating power was 1 kW, corresponding to a lamp voltage of 40 kV and a cathode filament current of 25 mA. The use of a narrow radiation beam and appropriate adjustment of the diffractometer settings improved the accuracy of the measurement results [20].
• Cement pastes were examined using differential thermal and thermogravimetric analysis (DTA–TG). Measurements were performed with a STA 449 F3 Jupiter analyzer (Netzsch, Kraków, Poland) coupled with a QMS 403C Aeolos quadrupole mass spectrometer (Netzsch, Kraków, Poland). Samples of approximately 75 mg were placed in alumina crucibles (Al₂O₃). The tests were carried out in a synthetic air atmosphere with a gas flow rate of 40 mL/min and a heating rate of 15 °C/min. The temperature range was from 30 °C to 1000 °C. Based on the thermogravimetric (TGA) results concerning mass losses in characteristic temperature ranges, the content of individual components in the tested samples was estimated. The amounts of Ca(OH)₂ and CaCO₃ in the initial samples were determined from the mass loss values on the TG curves and the corresponding stoichiometric coefficients [21]
• The morphology of the samples was examined using an ultra-high-resolution scanning electron microscope (FEI Nova NanoSEM 200, Philips, Eindhoven, The Netherlands), equipped with a thermal field emission electron gun (FEG-Schottky emitter, Philips, Eindhoven, The Netherlands). The measurements were carried out at an accelerating voltage of 18 kV. The elemental composition of the tested samples was determined using an energy-dispersive X-ray spectrometer (EDS, EDAX Genesis XM) coupled with the scanning electron microscope [22].
• Porosity and pore size distribution were examined using a mercury intrusion porosimeter PoreMaster 60 (Quantachrome Instruments, Boynton Beach, USA), enabling measurements in the pressure range from 0.2 psia to 60,000 psia. This range corresponds to the analysis of pores with diameters from approximately 1100 µm to 0.0036 µm. The measurement was conducted in two stages: a low-pressure range (0.2–50 psia), which allowed the determination of macropores, and a high-pressure range (up to 60,000 psia), enabling the analysis of mesopores and micropores. Data analysis was performed using the dedicated PoroWin software (version 8.0+), which allowed the determination of parameters such as total porosity, pore volume, specific pore surface area, and pore size distribution [23].

3. Results

3.1. XRD Analysis

XRD diffractometric analysis conducted on cement paste samples after 90 days of hydration in two different environments, namely tap water (denoted as W) and an aqueous solution of chicken manure (denoted as B), allows for the evaluation of the impact of organic and inorganic compounds present in the biomass on the mineral phase composition of cements CEM I 52.5N, CEM I 42.5R-1 and CEM I 42.5R-2. Based on the qualitative and quantitative comparison of individual crystalline phases presented in

Table 2. Mineral phase content in three types of cement (CEM I 52.5N, CEM I 42.5R-1, CRM I 42.5R-2) after exposure to water and chicken manure (XRD analysis).

Name	Symbol XRD	Formula	Cement Type
			CEM I 52.5N		CEM I 42.5R-1		CRM I 42.5R-2
			(W)	(B)	(W)	(B)	(W)	(B)
Calcite	C	CaCO3	5.9	41.1	9.3	22.7	6.8	23.1
Vaterite	G	CaCO3	-	19.3	-	-	-	-
Portlandite	P	Ca(OH)2	24.2	11.2	6.1	10.4	18.4	19.9
Belite	S	C2S	9.5	-	8.2	-	7.8	6.3
Unidentified	-	-	-	-	-	-	-	-

, and the intensity of diffraction peaks shown in Figure 2, Figure 3 and Figure 4, significant differences in hydration processes and secondary phase transformations can be identified as a result of the exposure medium. For the CEM I 52.5N cement, the presence of chicken manure has a marked influence on mineralogical transformations. The most significant change is a substantial increase in the content of calcite (CaCO₃), from 5.9% in the sample hydrated in water to as much as 41.1% in the sample exposed to chicken manure.

Such a transformation may be the result of accelerated carbonation, caused by an increased presence of bicarbonate ions and carbon dioxide in the biodegradable environment. In addition, the appearance of vaterite as a new polymorphic form of calcium carbonate, found exclusively in sample B (19.3%), indicates specific crystallisation conditions that may favour its formation in an organic environment rich in nitrates. The presence of vaterite, the thermodynamically least stable form of CaCO₃, suggests a rapid carbonation process in the initial stage, before this phase transforms into calcite. The portlandite (Ca(OH)₂) content decreased from 24.2% in the water-based environment to 11.2% in the presence of chicken manure. This may indicate a more intensive carbonation process in the presence of CO₂ ions and precipitation of CaCO₃ at the expense of Ca(OH)₂ decomposition, as well as potentially greater consumption of portlandite in secondary reactions with organic acids present in the manure.

Belite (C₂S), the main phase responsible for long-term strength development, also undergoes transformation. In sample B, it was not identified, which may suggest either its complete hydration or strong overlap with hydration or carbonation products. For CEM I 42.5R-1 cement, an increase in calcite content was also observed in the presence of chicken manure, from 9.3% to 22.7%. Although not as pronounced as in CEM I 52.5N, it still represents a significant change, indicating active carbonation processes. An interesting aspect is the absence of vaterite in both samples, which may be related to the different chemical composition of the cement or a different microstructure of the hydrated phases that does not favour its stabilisation. The portlandite content in sample W was only 6.1% after 90 days of hydration. In the sample exposed to the aqueous chicken manure solution, this value increased to 10.4%, which may indicate inhibition of carbonation or secondary recrystallisation of Ca(OH)₂ in the presence of components found in chicken manure. It is also possible that, in an environment with reduced pH, some reactions may lead to the reprecipitation of portlandite from previously released calcium ions. The belite content did not change significantly, remaining at 8.2% in sample W, while its absence in sample B may indicate that more complete hydration of this phase occurred in the presence of manure or that it was masked by secondary products. In the case of CEM I 42.5R-2, the calcite content in sample W was 6.8%, whereas in sample B it increased to 23.1%. This difference is consistent with previous observations and again suggests that the organic environment intensifies the carbonation process. In this case, vaterite was not identified, which may be attributed to greater mineralogical stability or a different character of the hydration process.

The portlandite content is particularly interesting. In the sample exposed to water, it reaches a value of 18.4%, whereas in the chicken manure environment it increases to 19.9%. Similar to CEM I 42.5R-1, this increase may indicate an unusual course of secondary mineralisation processes, including the possible release of Ca²⁺ ions from more soluble phases and the subsequent reprecipitation of Ca(OH)₂. Meanwhile, the belite content decreases from 7.8% to 6.3%, which may suggest further hydration of this phase under conditions rich in chemically active substances. A comparative analysis of the three types of cement shows that the hydration environment has a significant effect on phase composition, and that the extent of mineralogical transformations depends on the characteristics of the specific cement. CEM I 52.5N demonstrates the highest carbonation potential, as evidenced by the increase in calcite content and the presence of vaterite, which indicates rapid carbonate transformations. CEM I 42.5R-1 is marked by the lowest portlandite content in water-based conditions, which may affect its durability and chemical resistance. CEM I 42.5R-2 also undergoes intense carbonation processes, which may influence its long-term performance in environments containing CO₂ and organic components. Based on the intensity of the diffraction peaks visible in the diffractograms, it is evident that the presence of chicken manure leads to a clear weakening of the signals characteristic of portlandite and belite, while simultaneously enhancing the intensity of peaks corresponding to calcite.

This confirms the dominant effect of the organic environment on secondary mineralisation of hydration products and their transformation into more stable carbonate phases. It also points to the potential use of such environments as a factor for modifying the cement microstructure, which could have practical relevance, especially in the context of CO₂ sequestration or the transformation of organic waste into useful engineering materials. The XRD analysis provides a comprehensive view of how the hydration environment affects the mineralogy of cement pastes. The presence of chicken manure as a reactive medium leads to intensified carbonation processes and transformation of hydration products, particularly portlandite, into more stable phases such as calcite and vaterite. This effect is strongly dependent on the type of cement, which highlights the need for careful selection of materials according to operating conditions and the desired level of durability. These results also indicate the potential of fermentation environments as a factor supporting CO₂ mineralisation, which can be utilised in sustainable development strategies and circular economy practices.

3.2. Thermal Analysis

Based on the data presented in

Table 3. Quantitative phase composition of cement pastes based on thermal analysis (TG/DTG/DTA).

Phase	Phase Composition of the Paste	Cement Type
		CEM I 52.5N		CEM I 52.5N		CEM I 52.5N
		(W)	(W)	(W)	(W)	(W)	(W)
Calcite	CaCO3	6.95	26.3	10.97	19.75	5.70	19.61
Vaterite	CaCO3	-	8.82	-	-	-	-
Portlandite	Ca(OH)2	21.40	6.60	7.80	19.16	22.90	20.90
CSH, CAH	Dehydration	71.65	55.58	81.23	61.09	71.40	59.49

and in the Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10 of the attached document, a detailed analysis was carried out of the phase composition of cement pastes determined by thermal analysis (DTA) after 90 days of hydration in two environments: water (W) and chicken manure (B). The phase composition was established on the basis of mass losses corresponding to the decomposition of calcite (CaCO₃), portlandite (Ca(OH)₂), and the dehydration of calcium silicate and calcium aluminate hydrate products (CSH, CAH). In one case, the presence of vaterite, a less stable polymorphic form of calcium carbonate, was also identified.

In the case of CEM I 52.5N cement, after 90 days of hydration in water, the calcite content was found to be 6.95%, portlandite 21.4%, and with the highest share among all samples for the CSH and CAH phases, amounting to 71.65%. The presence of calcite indicates partial carbonation of portlandite due to reaction with atmospheric CO₂, although the extent of this process was relatively limited. On the other hand, the high content of portlandite suggests intensive hydration of the main clinker phases, particularly alite (C₃S), which dominates in 52.5-grade cements. The very high proportion of hydrated phases (CSH, CAH) confirms the effectiveness of the hydration process and the formation of CSH gel, which is responsible for the development of the material’s strength.

In the corresponding sample hydrated in the presence of chicken manure, significant changes were observed. Calcite reached as much as 26.3%, portlandite dropped to 6.6%, and the CSH and CAH phases decreased to 55.58%. The high calcite content, along with the presence of vaterite (8.82%), suggests intense carbonation, which may have been facilitated by the presence of CO₂ and ammonia in the biological medium. The drastic reduction in portlandite indicates extensive decomposition and its transformation into calcium carbonate. At the same time, the noticeable decrease in the content of CSH and CAH phases may result from disrupted hydration processes or partial degradation of previously formed CSH gel due to the aggressiveness of the organic environment.

For CEM I 42.5R-1 cement, different results were obtained. In the sample hydrated in water, the calcite content was 10.97%, portlandite 7.8%, and the CSH and CAH phases reached the highest value among all analysed samples at 81.23%. Such a high content of dehydrated hydration products may result from more advanced development of the CSH gel, possibly due to a favourable ratio between the aluminate and silicate phases in this type of cement. At the same time, the relatively low portlandite content may indicate that most of the Ca(OH)₂ formed had already undergone further secondary reactions, such as partial carbonation.

In the sample hydrated in chicken manure, a noticeably higher portlandite content of 19.16% was recorded. This may suggest a lower intensity of secondary transformations of Ca(OH)₂ in the presence of organic matter, possibly due to limited access to CO₂ or disruption of the microstructure by nitrogen compounds. The calcite content in this sample was 19.75%, which also indicates significant carbonation. The CSH and CAH phases reached 61.09%, clearly lower than in the water-based sample. This may reflect the adverse effect of the organic environment on the hydration process, limiting the formation of CSH gel or causing its partial degradation. In the case of CEM I 42.5R-2, which is likely of modified composition, the results of thermal phase analysis after hydration in water showed a calcite content of 5.7%, portlandite at 22.9%, and dehydrated phases (CSH and CAH) at 71.4%. The high portlandite content indicates advanced hydration of alite and a relatively low level of secondary carbonation reactions. The small amount of calcite suggests good stability of the water environment and limited exposure to CO₂. After 90 days of hydration in chicken manure, the calcite content increased to 19.61%, portlandite content decreased slightly to 20.9%, and the CSH and CAH phases declined to 59.49%. As in the other samples, hydration in the presence of chicken manure promoted carbonation while also reducing the formation or stability of CSH gel. The observed reduction in dehydrated phases may result from changes in pH and the presence of aggressive nitrogen, phosphorus, or sulphur compounds, which interact with the cement matrix, destabilising hydration products and initiating secondary mineralogical transformations.

Thermal analysis provides clear evidence of the influence of the hydration environment on the phase composition of cement pastes. Hydration in water results in a higher content of portlandite and CSH/CAH phases, indicating efficient hydration processes that lead to the formation of a stable cement structure. In contrast, the environment containing chicken manure significantly alters the hydration pathway. Intense carbonation occurs (evidenced by the increased content of calcite, and in some cases vaterite), along with a reduction in the proportion of dehydrated phases and variation in portlandite content depending on the cement composition. The presence of vaterite exclusively in the CEM I 52.5N sample hydrated in chicken manure is particularly noteworthy. It may point to highly dynamic phase transformations or the specific influence of certain organic compounds on the direction of mineralogical changes. From the perspective of cement material durability, the observed phenomena have significant practical implications. A reduction in gel phase content may lead to a weakening of mechanical properties, while intensive carbonation may have both positive effects (such as microstructure densification) and negative effects (such as a loss of alkalinity and increased susceptibility to reinforcement corrosion).

The varied behaviour of individual cement types in the presence of organic matter suggests that selecting an appropriate type of cement may be crucial when designing cement composites intended for contact with such environments containing biological or organic substances, as is the case in biogas plants, composting facilities, or wastewater treatment systems. Higher-strength cements (such as CEM I 52.5N) tend to consume portlandite more rapidly and undergo more advanced carbonation in the presence of chicken manure. This may limit their suitability for such conditions, despite initially superior mechanical properties. On the other hand, modified cements may offer greater resistance to chemically variable environments, although this comes at the cost of slower hydration and lower content of strengthening phases. This analysis highlights the need for further research into the mechanisms of interaction between organic environments and the cement matrix, particularly in the context of long-term durability and stability of construction materials.

3.3. Scanning Electron Microscopy

3.3.1. Cements Exposed to Water—General Images

The SEM micrographs presented in Figure 11 (magnification 1000×) show the surface of the same three Portland cements (CEM I 52.5N, CEM I 42.5R-1, and CEM I 42.5R-2) after exposure to a water environment. In comparison with the samples exposed to chicken manure leachate solution, the images reveal a significantly lower density of secondary mineral deposits and the absence of well-developed calcium carbonate (CaCO₃) crystallites. The surface of the pastes displays a noticeably smoother profile, dominated by a gel-like matrix and an undeveloped porous structure. A few small deposits with irregular morphology are visible, but without the presence of sharp-edged crystals characteristic of calcite. Particularly in the CEM I 52.5N and CEM I 42.5R-1 samples, the surface resembles a thin hydration layer, with occasional shrinkage cracks, indicating the progress of hydration processes, but with no significant carbonate mineralisation. Only in the CEM I 42.5R-2 sample are a few irregular mineral grains visible; however, their form and distribution suggest random adsorption or initial stages of crystallisation rather than fully developed carbonation.

The absence of characteristic calcite morphologies and the low intensity of surface deposits confirm that the lower concentration of CO₃²⁻ ions in the solution significantly limited the ability to bind CO₂ within the microstructure of the cement paste.

In comparison with previous observations, it can be clearly concluded that the presence of a source of carbonate ions (for example, a biodegradable environment such as chicken manure leachate) has a substantial effect on the process of secondary calcium carbonate mineralisation. Under conditions of limited CO₃²⁻ availability, the mechanism of CO₂ sequestration within the cement microstructure is considerably less effective, which may influence both the environmental potential of the material and its long-term sealing capacity and durability.

3.3.2. Cements Exposed to Water—Detailed Images

The SEM micrograph presented in Figure 12, taken at a magnification of 10,000×, shows the microstructure of a cement paste with clearly different morphology compared to the previously analysed samples, in which calcium carbonate crystallites were dominant. The image reveals a dense, irregular microstructure with distinctly marked fibrous formations, characteristic of cement hydration products, particularly the ettringite (AFt) phase.

The presence of silicon in each of the analysed points indicates the co-existence of the CSH phase, although its contribution is secondary to the extensive network of needle-like structures. In point 3, iron (Fe) was also detected, which may indicate the presence of fine mineral admixtures of clinker origin or the migration of ions from the solution.

Carbon was observed in all points; however, its intensity was significantly lower compared to samples in which active carbonation had occurred. This confirms the limited presence of calcium carbonate in the analysed microstructure. Both the morphological characteristics and chemical composition of the sample clearly indicate that the dominant processes were hydration reactions rather than carbonate mineralisation. The absence of well-developed CaCO₃ crystallites and the low intensity of carbon peaks in the EDS spectra suggest a limited capacity for permanent CO₂ binding within the microstructure of this material. Although the presence of phases such as ettringite and CSH may contribute positively to the sealing of the microstructure and the reduction of ion and gas migration, their ability to directly sequester CO₂ is minimal. Thus, the analysed sample represents a case of cement paste with low carbonation potential, where hydration products dominate over secondary carbonate phases.

The next SEM micrograph (Figure 13) shows a highly developed, cracked and folded surface structure, characterised by the presence of numerous irregular forms with a texture resembling gels or colloidal layers. The microstructure of this sample indicates a predominance of products with low crystallinity, likely corresponding to CSH-type phases (calcium silicates with amorphous or poorly ordered structures), as well as possible organic or biotic residues.

Also visible are small clusters of granular forms, which may suggest the co-occurrence of crystalline secondary admixtures. However, their quantity and degree of development appear to be significantly limited.

The general impression of softness, delamination, and the absence of sharp-edged forms typical of CaCO₃ clearly indicates that no significant carbonate mineralisation occurred in this sample. Instead, a gel-like structure dominates, which most likely represents a residue from the hydration of cement mineral phases, possibly formed in the presence of a limited amount of CO₂ or under conditions unfavourable to its effective binding.

This interpretation is supported by the results of surface microanalyses performed using the EDS method at three different points on the sample. In all spectra, the dominant elements were calcium (Ca), oxygen (O), carbon (C), and silicon (Si), with the presence of potassium (K), aluminium (Al), sodium (Na), and magnesium (Mg) also noted.

The spectrum from point one shows the greatest elemental diversity, suggesting the presence of complex phases typical of hydration products such as CSH with potassium admixtures. The presence of carbon does not clearly indicate the presence of CaCO₃; its level is similar to that of typical trace amounts in gel phases formed in environments with moderate CO₂ availability. The spectrum from point two shows distinct peaks for Ca and O, along with Mg, Al, and Si, which also corresponds to a structure dominated by hydration products, without clear evidence of a well-formed carbonate phase. Point three, the most chemically straightforward, also indicates a dominance of Ca, C, and O with contributions from Mg and Si, which suggests the presence of poorly crystalline calcium–silicate phases, without a prevailing presence of CaCO₃. Both morphological and chemical composition analyses indicate that no effective CO₂ sequestration in the form of carbonate mineralisation occurred in this sample. Instead, a highly developed gel structure was observed, most likely the result of intense hydration under limited availability of carbonate ions or pH conditions too low for CaCO₃ precipitation. Although such a structure may partly contribute to improved sealing of the cement paste and may retain some CO₂ molecules in a dispersed form, the absence of distinct crystallites indicates low durability of this binding and limited potential of this microstructure as a medium for long-term CO₂ sequestration.

In the final SEM micrograph (Figure 14), a highly developed and complex surface microstructure of the cement paste is visible. This structure is characterised by considerable porosity, irregular morphology, and the presence of numerous amorphous or semi-crystalline forms resembling layered or flaky aggregates. Noticeable also are numerous flake-like features and rough accumulations, which suggest a dynamic hydration environment with significant participation of secondary products.

Against this background, dispersed clusters of secondary carbonate phases are visible, exhibiting various degrees of development. Some appear as small, unevenly distributed crystalline aggregates, while others may occur in amorphous or gel-like forms. This suggests the presence not only of calcite but also of potentially less-ordered calcium carbonate forms, such as vaterite, although their definitive identification would require supplementary XRD analysis at this location. To confirm the nature of these phases, point-specific chemical microanalysis (EDS) was performed. All three analysed points showed a very strong presence of calcium (Ca), carbon (C), and oxygen (O), which clearly confirms the presence of carbonation products, particularly calcite (CaCO₃).

In point 1, a range of additional elements was also identified, including Si, Mg, and Al, which may indicate the partial coexistence of CSH or residual hydration products in the background of the analysed crystallite. Point 2 exhibited an even higher proportion of Ca and lower levels of admixtures, suggesting that the analysis site was located directly on a well-formed, pure calcite crystal. The very low presence of Si and Al further confirms the limited contribution of silicate phases. In point 3, the elemental ratios closely resemble those in point 1, with the additional presence of Fe, which may result from inclusions of mineral admixture microparticles or corrosion products. Nevertheless, the dominance of Ca, O, and C continues to indicate the carbonate nature of the primary phase.

The observed morphological features and chemical composition clearly confirm the active occurrence of secondary carbonation. The presence of numerous carbonate structures, their irregular morphology, and varying degrees of crystallinity point to a heterogeneous nature of CO₂ sequestration, alongside a high sorption potential of the material. In particular, the presence of calcite in the form of extensive aggregates with significant specific surface area may contribute to the effective binding of CO₂ within the microstructure’s pores, promoting the long-term stabilisation of carbon in the form of durable mineral products.

3.3.3. Cements Exposed to an Aqueous Solution of Chicken Manure—General Images

Figure 15 shows SEM micrographs (magnification 1000×), the morphology of the tested Portland cements after exposure to an aqueous environment of organic origin (chicken manure leachate) is presented. The aim of the observations was to assess the potential for carbonate mineralisation occurring as a result of the interaction between calcium ions released from the hydrating cement pastes and carbonate ions present in the reactive medium, as a model process of CO₂ binding within the material’s microstructure. In all three cases, numerous secondary crystallites with morphology typical of calcium carbonate (CaCO₃), particularly in the form of calcite, were observed. These crystals form a layer covering the surface of the cement grains and fill the pore spaces, providing clear evidence of ongoing carbonation.

The visible morphological diversity—ranging from fine aggregates to clearly developed, sharp-edged forms—indicates the dynamic nature of this process and the varying reactivity of the individual cements. In the case of the CEM I 52.5N sample, the highest density and uniformity of carbonate deposits were observed. This may suggest greater reactivity of this cement and increased release of calcium ions, resulting in a higher potential for CO₂ binding. The CEM I 42.5R-1 and CEM I 42.5R-2 samples display more varied surface textures and the presence of fine-grained deposits, which point to carbonation occurring under conditions of slower diffusion or lower supersaturation with Ca²⁺ and CO₃²⁻ ions. The process of secondary CaCO₃ mineralisation is important not only in terms of the material’s durability, but above all as a mechanism for the permanent binding of carbon dioxide within the cement structure. Secondary carbonation products may reduce the porosity of the paste and contribute to sealing the microstructure, thereby limiting ion and gas transport and improving the material’s resistance to aggressive environmental factors. These observations clearly confirm that a well-selected cement composition and appropriate exposure conditions can favour effective CO₂ sequestration within carbonate phases at the microstructural scale.

3.3.4. Cements Exposed to an Aqueous Solution of Chicken Manure—Detailed Images

SEM micrograph Figure 16 shows the morphology of mineral phases in the analysed sample, revealing the presence of numerous crystallites with well-defined edges and varied morphology, predominantly in the lower and central parts of the image. The crystallites mostly exhibit an isometric or slightly elongated habit with distinct growth planes, which may indicate their formation under conditions of moderate supersaturation with Ca²⁺ and CO₃²⁻ ions, leading to spontaneous nucleation and further crystallisation.

Finer aggregates and deposits with more irregular morphology are also present between the larger grains, which may suggest the presence of secondary phases or gel-like products. In the upper part of the micrograph, a zone with a fine-grained, porous structure can be observed, resembling a biofilm-type or amorphous mineral matrix, which may result from bioprecipitation or hydration processes.

Local chemical composition analyses using EDS, conducted at three marked points (1, 2, and 3), allow the identification of the main elements present in the examined crystallites:

Point 1 refers to a grain of relatively irregular shape, located in the transitional zone between larger crystallites and the porous matrix in the upper part of the image. The spectrum indicates the presence of calcium (Ca), silicon (Si), oxygen (O), carbon (C), as well as potassium (K), magnesium (Mg), and aluminium (Al). Such a broad elemental composition may suggest a complex mixture of mineral phases containing components typical of calcium silicate hydrates (CSHs), as well as potential remnants of the reactive medium or bioorganic structures, which may indicate incomplete crystallisation or the presence of transitional products. Point 2 concerns a well-formed crystal with a regular, polyhedral form. The spectrum is dominated by calcium and carbon signals, with lower levels of oxygen and silicon. This composition clearly indicates the presence of calcium carbonate, most likely in the form of calcite, which constitutes the main mineral phase of this crystallite. The presence of Si may result from minor inclusions of adjacent CSH phases or remnants of gel deposits. Point 3 represents a distinctly isolated crystal with a sharp-edged, columnar morphology. The spectrum shows a strong dominance of calcium along with carbon and oxygen, with a very low silicon content. This spectral profile confirms the high purity of the phase, clearly identifying it as well-crystallised calcite. The absence of other elements may suggest favourable growth conditions that enabled the formation of a pure CaCO₃ phase without admixtures. Both the crystallite morphology and chemical composition display characteristic features indicative of carbonate mineralisation, with the involvement of secondary silicate phases and potentially organic components. The presence of CSH and elements typical of the cementitious environment may also indicate an early stage of hydration and carbonation. This phase combination may be significant in terms of durability, microstructure, and application potential, particularly in systems for biological or mineral CO₂ sequestration.

As part of the continued investigation into the micromorphology and chemical composition of the analysed material, SEM micrograph Figure 17 is presented compared with the previous observations, the visible structure exhibits higher porosity and less distinctly developed crystallites. The image reveals the presence of irregular aggregates with fragmented and poorly defined geometry, which may represent products of secondary crystallisation or remnants of gel phases. Numerous grains display irregular shapes, with a tendency to form spheroidal or lumpy clusters. Such morphology may be typical of low-crystallinity transitional phases precipitated from supersaturated solutions under dynamic or biological conditions.

In many areas, the structure resembles amorphous deposits, and the presence of thin layers with fibrous or flaky morphology may indicate the presence of CSH gels or biotic structures (e.g., biofilm residues). To complement the morphological observations, local analyses using the EDS method were carried out in three marked points. Point 1 represents an area of distinctly fine-crystalline character. The EDS spectrum reveals the presence of calcium (Ca), carbon (C), oxygen (O), silicon (Si), aluminium (Al), and magnesium (Mg). Such a diverse elemental composition indicates the presence of a mixture of phases, probably secondary in nature or remnants of the hydration and mineralisation process. The presence of Si, Al and Mg suggests a CSH-type phase with additional ionic inclusions or deposits from the reactive medium (biochemical environment). The presence of carbon, on the other hand, indicates partial saturation of these phases with carbonation products.

Point 2 shows a dominance of calcium along with significant amounts of carbon and oxygen, and the presence of silicon and magnesium. The chemical composition suggests a mixed phase in which both carbonation products (CaCO₃) and secondary inclusions or remnants of the CSH phase are present. The relatively high content of Mg may indicate its incorporation into the structure of hydration products, which is possible in the presence of Mg²⁺ ions in the reaction environment. Point 3 shows a very similar composition to point 2, with calcium and carbon dominating, along with silicon, magnesium and trace amounts of aluminium. This elemental profile confirms the presence of a carbonate phase, most likely calcite, coexisting with small amounts of secondary hydration products and compounds derived from the cement matrix. SEM observations and EDS analyses clearly indicate the presence of a complex mineral system dominated by carbonation products (mainly calcite), coexisting with calcium-silicate residues and deposits of chemical or biological origin. The irregular morphology of crystallites, the wide range of detected elements and the presence of inclusions suggest an ongoing process of secondary mineralisation, typical of dynamic or biologically active environments. Such a structure may be relevant for engineering applications, particularly in the context of reactivity, chemical resistance and potential for long-term CO₂ sequestration. In the SEM micrograph shown in Figure 18, a well-developed crystalline surface structure of the cement paste is visible, indicating advanced mineralisation. The image shows numerous crystallites with clearly developed rhombohedral or pseudo-isometric habit, featuring sharp edges and smooth growth faces. This morphology is clearly characteristic of calcite (CaCO₃), the primary product of secondary carbonation. These crystallites are densely distributed and, in some places, partially overgrow one another, which may indicate a high supersaturation of the solution with Ca²⁺ and CO₃²⁻ ions and an intense progression of carbonate mineralisation.

To confirm the chemical composition of the analysed structures, three spot EDS microanalyses were carried out. The spectra for all points display a very similar elemental profile, with a clear dominance of calcium (Ca), carbon (C) and oxygen (O), confirming the presence of calcium carbonate. In point 1, additional signals of sulphur (S), aluminium (Al) and magnesium (Mg) were recorded, although their proportion is minor and likely related to trace inclusions of hydration products or residual reaction environment (e.g., traces of CSH or AFt phases). Despite their presence, the proportions of Ca, C and O clearly indicate that the main phase is well-crystallised calcite. Point 2 shows an almost identical profile, although the concentration of minor elements is slightly lower, which may suggest a slightly higher purity of the analysed crystallite. Additionally, traces of Mg and Si are visible, possibly originating from adjacent secondary phases. Point 3 confirms the presence of the dominant CaCO₃ phase, and the presence of trace amounts of Mg and Si does not significantly affect the interpretation. This elemental signature confirms that the analysed crystal is composed almost entirely of calcium carbonate. These observations clearly indicate effective CO₂ sequestration in the cement paste microstructure in the form of crystalline calcite. The well-developed morphology of the crystals and their high purity reflect a stable precipitation environment and a high capacity for CO₂ binding. The presence of calcite in such a form favours long-term carbon retention within the material structure, while simultaneously sealing the microstructure, which additionally improves the cement paste’s resistance to aggressive agents and gas migration.

3.4. Porosity Analysis

Porosity was investigated using both low-pressure (below atmospheric pressure) and high-pressure (above atmospheric pressure) porosimetry with the Quantachrome POROMASTER 60 instrument. Open porosity is one of the key microstructural parameters that determine the durability of cementitious materials, their permeability, and their susceptibility to environmental factors. In this study, cement paste samples made with various types of Portland cement were compared after being stored in two different environments—water and chicken manure—to determine the influence of exposure conditions on the development and characteristics of open porosity. The results of these measurements are summarised in

Table 4. Results of porosity tests of cement pastes after curing in water and chicken manure.

Cement	Vkum [mm3/g]	dpoz [g/cm3]	dr [g/cm3]	P [%]	W/C
Pastes exposed to water
CEM I 52.5N	162.4	1.59	2.29	32.12
CEM I 42.5-1	129.2	1.73	2.31	27.72	0.5
CEM I 42.5-2	107.7	1.84	2.36	29.87
Pastes exposed to chicken manure
CEM I 52.5N	159.9	1.93	2.24	21.98
CEM I 42.5-1	118.9	1.83	2.31	20.79	0.5
CEM I 52.5-2	98.8	1.94	2.34	22.98

V_kum—cumulative open pore volume normalised per 1 mm³ of the sample; P—open porosity; d_poz, d_r—apparent density and real density; these values are not of primary importance, as they are calculated by the software rather than measured directly. They should be treated as approximate estimates only.

and Figure 19.

The pore volume measurement covered a range from 3 nm to 10 μm, which corresponds to the dominant fraction of capillary pores and, partly, gel pores, both of which are responsible for the transport of liquids and gases within the paste structure.

In the case of samples stored in water, a distinctly higher open porosity was observed compared to the same cement pastes matured in chicken manure. For the paste made with CEM I 52.5N cement, the open porosity reached 32.12%, while for CEM I 42.5, depending on the sample, the values were 27.72% and 29.87%, respectively. When the same cements were matured in chicken manure, they exhibited significantly lower open porosity values: 21.98%, 20.79%, and 22.98%, respectively. A similar trend was observed for the cumulative open pore volume, although the differences were slightly less pronounced. It is worth noting that, although the Vcum for the CEM I 52.5N sample amounted to 162.4 mm³/g in the aqueous environment and 159.9 mm³/g in the chicken manure, the percentage porosity was considerably lower in the latter case. This may indicate differences in structural density and pore distribution. The observed reduction in open porosity in samples stored in chicken manure can be explained by several mechanisms. First and foremost, this environment is characterised by high alkalinity, the presence of both organic and mineral compounds (including phosphates, sulphates, and nitrogen compounds), as well as microbiological activity. Under such conditions, secondary mineralisation processes may occur, leading to pore closure, reduced pore connectivity, or the filling of microcracks with secondary reaction products. Particularly significant in this context is the carbonation of portlandite (Ca(OH)₂), which is a natural consequence of Portland clinker hydration. In the presence of CO₂ originating both from the atmosphere and from fermentation processes in the manure portlandite is converted into calcite (CaCO₃), which may precipitate within the capillary pores, reducing their volume and permeability.

It is also worth emphasising that, in a water environment, hydration conditions are relatively uniform and promote the even development of capillary and gel pores. Water ensures the availability of ions necessary for the formation of hydration products; however, it does not stimulate secondary mineralisation processes that could close or obstruct the existing pore network. Therefore, the higher open porosity values observed in samples stored in water are consistent with theoretical expectations and empirical observations regarding the classical course of cement hydration.

Although the cumulative pore volume values do not differ drastically between samples from both environments, significant differences in percentage open porosity may suggest changes in pore size distribution or connectivity. The pore structure may become sealed, and larger pores dominated by secondary products may lose contact with the capillary network, resulting in a reduction of total open porosity as measured by adsorption or porosimetry techniques. Differences in apparent and true density, although considered only as estimates, also indicate a greater degree of structural consolidation in the chicken manure samples. The results obtained allow the conclusion that the curing environment has a significant effect on the pore structure of cement pastes, and that chicken manure—due to its chemical and biological properties—leads to a reduction in open porosity. This phenomenon may have favourable practical implications, particularly in the context of using cement pastes in environments exposed to organic or biologically active substances, such as fertilised soils, biogas digesters or passive barriers for odour and pollutant emissions. Reduced porosity may result in improved material durability, decreased migration of aggressive ions, and slower degradation processes such as sulphate corrosion or calcium leaching.

The differences in open porosity between samples stored in water and those exposed to chicken manure result not only from the different progression of hydration but, above all, from secondary physicochemical processes which, in the case of an organic environment, lead to the modification and sealing of the pore structure. Further microstructural studies using techniques such as SEM/EDS, XRD or thermal analysis would allow for precise identification of these transformations and the assessment of their impact on the long-term durability and functionality of cement pastes.

3.5. CO₂ Sequestration in Concrete Used for a 500 kW Agricultural Biogas Plant Fermenter—Emission Reduction Potential and Economic Aspect

The construction of a fermenter in a medium-scale biogas plant with an installed capacity of 500 kW requires the use of a considerable quantity of concrete, which plays a key role in ensuring the durability, watertightness and structural strength of the fermentation tank. A typical fermenter in such a facility has a working volume in the range of 2000–3000 m³, which, given a standard geometry (e.g., 20 m diameter, 7 m height, 30 cm wall thickness and 50 cm base slab), results in a concrete requirement of approximately 320 m³. Assuming a concrete density of 2.4 t/m³, this corresponds to a total mass of 768 tonnes. In emission footprint analyses, increasing attention is being paid to the ability of concrete to bind CO₂ through the carbonation of cement hydration products. In the present case, it was assumed that high-strength Portland cement (CEM I 52.5N) was used, containing approximately 65% CaO. With a typical cement content of 320 kg/m³ of concrete, and assuming full carbonation of this calcium fraction, the potential CO₂ uptake can be calculated on the basis of the molar mass ratio between CaO and CO₂. In practice, however, carbonation does not proceed to completion. Laboratory analyses have shown that after 90 days of curing under operating conditions, the calcium carbonate (CaCO₃) content in the fermenter concrete reaches approximately 40% of the cement mass. This means that, from 102.4 tonnes of cement, 40.96 tonnes of CaCO₃ were formed. Given that the molar mass of CaCO₃ is 100.09 g/mol and that of CO₂ is 44.01 g/mol, the CO₂ content of the calcium carbonate corresponds to 43.96% by mass. This amount of sequestered carbon dioxide can be directly converted into an economic value when related to the current price of CO₂ emission allowances (EUA) in the European market. According to data from July 2025, the price of 1 tonne of CO₂ in the EU ETS is approximately EUR 75. This potential saving may be accounted for as a tangible environmental and economic benefit within the life cycle assessment (LCA) of the biogas facility or in ESG reporting. While the financial value represents only a minor fraction of the total investment cost of the fermenter, it nevertheless demonstrates the practical relevance of using concretes with high carbonation potential in the context of greenhouse gas emission balances. Moreover, considering that methane fermentation in a 500 kW biogas plant produces approximately 2.6–3.5 million m³ of biogas annually (containing on average 35–40% CO₂), the resulting combustion emissions range from 910 to 1225 tonnes of CO₂ per year. Against this backdrop, the sequestration of 18 tonnes of CO₂ in the fermenter concrete does not constitute macro-scale offsetting, but it can be regarded as a complementary mitigation measure, especially when low-emission cements or mineral additives (e.g., fly ash, slag, silica fume) are used to further reduce the embodied carbon footprint. Fermentation reactors made with concrete containing CEM I 52.5N cement can permanently sequester approximately 18 tonnes of CO₂ within the first three months of operation. Although the scale of this effect is limited, its significance increases in the context of investment planning aligned with sustainability principles, infrastructure carbon footprint calculations, and eligibility for green certification or support under EU funding programmes.

The sensitivity analysis (

Table 5. Sensitivity analysis of CO₂ sequestration potential in fermenter concrete depending on carbonation degree and EUA price.

Carbonation Degree [%]	EUA Price [EUR/t]	CO2 Sequestration [t]	Economic Value [EUR]
20%	50	9.0	450
40% (baseline)	75	18.0	1350
60%	100	27.0	2700
80%	125	36.0	4500
100%	150	45.0	6750

) indicates that the CO₂ sequestration potential in the concrete of a 500 kW fermenter is strongly dependent on the degree of cement carbonation and the price of EUA allowances. In the baseline scenario, with 40% carbonation and an EUA price of EUR 75 per ton of CO₂, approximately 18 tons of CO₂ can be bound, corresponding to an economic value of around EUR 1350. Increasing the carbonation degree to 100% and the EUA price to EUR 150 per ton would enable the sequestration of 45 tons of CO₂, equivalent to EUR 6750, which is five times higher than in the baseline case. However, it should be emphasized that this analysis is illustrative, as chemical constraints on the maximum carbonation degree of cement pastes in this technology practically prevent the full utilization of the CO₂ binding potential.

These results demonstrate that the economic effect of sequestration in fermenter concrete is currently limited compared to the total investment costs, but its importance increases with rising EUA prices. At the same time, increasing the degree of carbonation significantly enhances both the environmental and economic potential, although in practice these values are constrained by operational conditions. While sequestration in the range of several to several dozen tons of CO₂ does not offset the annual emissions associated with biogas production, it represents an important complementary measure in the emission balance and can be utilized in ESG reporting and in the process of applying for green financing. Moreover, the use of low-emission cements and mineral additives may further enhance the environmental effect and improve the competitiveness of the technology in the context of sustainable development.

3.6. Management of CO₂ Emissions and Economic Analysis of Cement Carbonation Technology in Biogas Facilities

Contemporary challenges related to the reduction of greenhouse gas emissions require not only innovative technological solutions but also a systemic approach to environmental and resource management. Cement, as one of the most carbon-intensive construction materials, can simultaneously serve as a passive CO₂ absorber through the process of carbonation. Integrating this property into emission management strategies in biogas facilities exemplifies a synergy between materials engineering, environmental economics, and climate policy. In the analyzed case of a 500 kW biogas plant fermenter constructed using concrete with CEM I 52.5N cement, it was demonstrated that approximately 18 tons of CO₂ can be permanently sequestered within the first 90 days of operation. At the current price of CO₂ emission allowances in the EU ETS system approximately EUR 75 per ton (as of July 2025) this translates into savings of around EUR 1350. Although this amount does not cover the construction costs of the fermenter, it represents a significant component of the net emissions balance and can be included in ESG reports and life cycle assessments (LCA) of technical infrastructure. Research indicates that the use of low-emission or recycled cements can further reduce emissions by as much as 47–94% per ton of cement, making carbonation technology even more attractive from both economic and environmental perspectives [5,23]. Emission management in biogas installations should account not only for direct sources of emissions, such as methane fermentation, but also for opportunities for compensation through the use of materials with sequestration properties. Including carbonation data in environmental management systems (e.g., ISO 14001) enables more accurate investment planning and assessment of climate impacts. Additionally, the appropriate selection of cement taking into account its phase composition, CaO and C₃A content, as well as alkali levels, can significantly affect the material’s durability in chemically aggressive environments such as fermentative organic slurries. Research findings indicate that CEM I 52.5N cement, despite its high reactivity, may be more susceptible to degradation in environments with elevated CO₂ concentrations and the presence of organic compounds. In contrast, blended cements containing additives such as ground granulated blast furnace slag or fly ash offer greater chemical resistance, although they exhibit a slower carbonation rate. The efficiency of carbonation depends not only on the cement composition but also on environmental conditions, including CO₂ partial pressure, humidity, temperature, and salinity. From the perspective of infrastructure durability, secondary calcium carbonate mineralization leads to the sealing of the cement microstructure, reduction of porosity, and enhanced resistance to the ingress of aggressive ions [6]. However, excessive carbonation can result in a drop in pH, depletion of portlandite, and structural weakening of the material, which necessitates proper risk management.

In this context, microstructural analysis, porosity assessment, and phase composition evaluation of cement should be integral components of the design and operational processes of biogas fermenters. Cement carbonation technology can also serve as a component of sustainable development strategies by reducing net CO₂ emissions, enhancing infrastructure durability, and enabling the attainment of environmental certifications such as BREEAM or LEED. Integrating this technology into ESG reports and life cycle assessments (LCA) increases investment transparency and may improve stakeholder perception and evaluation. Properly designed carbonation systems can not only enhance the mechanical properties of construction materials but also contribute to achieving climate goals at both local and global levels [24]. CO₂ emission management in biogas facilities should take into account the sequestration potential of construction materials, particularly cement. The selection of an appropriate cement type, control over exposure conditions, and the integration of carbonation data into environmental management systems and economic analyses can significantly improve emission reduction efficiency, enhance infrastructure durability, and support the implementation of sustainable development strategies.

4. Conclusions

Based on the conducted research, it can be concluded that the ability of cement pastes to bind carbon dioxide (CO₂) under conditions of biological fermentation depends both on the type of cement and on the exposure environment.

The highest carbonation potential was observed for CEM I 52.5N cement, as confirmed by both X-ray diffraction (XRD) and thermal analysis (TG/DTA). In this case, the greatest increase in calcite content was recorded—from 5.9% to 41.1% after exposure to poultry manure along with the presence of vaterite, indicating intense carbonation processes. Cement CEM I 42.5R-2, despite its higher content of aluminate phases (C₃A) and alkalis, also showed a notable increase in CaCO₃ content, reaching 23.1%. CEM I 42.5R-1 displayed a moderate carbonation potential, with calcite increasing from 9.3% to 22.7%, but without any vaterite detected, which may be attributed to differences in mineral composition and hydration structure.

The use of an organic medium—an aqueous suspension of chicken manure—significantly intensified the carbonation process. The presence of carbonate, nitrate and other fermentation-related ions promoted the precipitation of calcium carbonate and induced secondary transformations, such as portlandite decomposition and partial degradation of gel phases (CSH, CAH). In the case of CEM I 52.5N, portlandite content dropped from 21.4% to 6.6%, and gel phases from 71.65% to 55.58%, indicating extensive transformation of the cementitious matrix under aggressive environmental conditions. For CEM I 42.5R-1 and CEM I 42.5R-2, changes were also observed that confirmed carbonation processes, although the scale was smaller compared to the 52.5-class cement.

Scanning electron microscopy (SEM) confirmed the presence of secondary carbonation products in the form of calcite crystals, especially on the surface of cement grains in samples exposed to the organic medium. The observations showed that CEM I 52.5N exhibited the highest density and uniformity of carbonate deposits, indicating higher reactivity and sorption potential. In comparison, water-exposed samples exhibited a more developed gel-like structure dominated by hydration products but with limited capacity for permanent CO₂ binding. The presence of ettringite, CSH and mineral admixtures in the microstructure suggested that, in neutral environments, hydration processes prevail, while carbonation remains limited.

These findings confirmed that the carbonation process affects not only the phase composition of the cement paste but also its microstructure and physicochemical properties. The precipitation of CaCO₃ contributes to matrix densification, reduction of porosity and improved resistance to the ingress of aggressive agents. On the other hand, excessive carbonation may lead to a drop in pH, depletion of portlandite and structural weakening of the material, which could have negative implications for durability.

CO₂ sequestration in cement pastes under methanogenic fermentation conditions is highly dependent on both the cement type and the reactive environment. Cements with high CaO content and low C₃A levels, such as CEM I 52.5N, exhibit the greatest CO₂ binding potential and better microstructural stability under fermentative conditions. In contrast, cements containing higher levels of alkalis and aluminates, while showing considerable carbonation potential, may be more susceptible to degradation in chemically aggressive environments. The results indicate that an appropriate selection of cement type and control of exposure conditions can significantly enhance the efficiency of CO₂ sequestration, while simultaneously improving material durability and reducing greenhouse gas emissions in biogas installations.