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Article

A Comprehensive Study on the Microstructure and Mechanical Behavior of Glycoluril–Formaldehyde Polymer-Modified Cement Paste

by
Nakarajan Arunachelam
1,*,
S. K. M. Pothinathan
2,
C. Chella Gifta
3 and
N. P. Vignesh
4
1
Department of Civil Engineering, Mepco Schlenk Engineering College, Sivakasi 626005, Tamil Nadu, India
2
Department of Civil Engineering, Kalasalingam Academy of Research, KrishnanKovil 626126, Tamil Nadu, India
3
Department of Civil Engineering, National Engineering College, Kovilpatti 628503, Tamil Nadu, India
4
Department of Civil Engineering, SRG Engineering College, Namakkal 637017, Tamil Nadu, India
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(10), 1598; https://doi.org/10.3390/buildings15101598
Submission received: 10 March 2025 / Revised: 4 May 2025 / Accepted: 6 May 2025 / Published: 9 May 2025
(This article belongs to the Section Building Materials, and Repair & Renovation)
Browse Figures
Versions Notes

Abstract

:
Concrete is popular in construction due to its strong performance and low maintenance. However, some structures become unsafe over time due to poor maintenance and design flaws. As demand for maintenance grows, restoring older structures is a cost-effective option. Advanced repair techniques aim to extend service life and improve concrete properties, with a focus on eco-friendly solutions. Recent trends have highlighted the potential of incorporating polymers into repair methods, but the use of glycoluril–formaldehyde, a polymeric material known for its hydrogen bonding capacity, remains unexplored in repairing existing structures. This research investigates the effects of glycoluril–formaldehyde in simple matrices like cement paste and mortar to understand its impact. By examining the chemical reactions between glycoluril–formaldehyde with cement paste, this study delves into the fresh, mechanical, and microstructural characteristics. To evaluate the influence of glycoluril–formaldehyde, cement paste specimens were subjected to various tests, including consistency, initial and final setting time, and miniature slump flow tests. Cement mortar specimens were then subjected to compression strength tests conducted at various ages. The results demonstrate that a 3% addition of glycoluril–formaldehyde in concrete offers optimum performance, ensuring improved mechanical strength and microstructure. The microstructural investigation using optical microscopy, an X-ray diffraction, and SEM analysis confirms the polymerization of glycoluril–formaldehyde and the formation of a denser microstructure. The thermogravimetric (TG) and differential thermogravimetric (DTG) analysis provides crucial insights into the thermal stability of the cementitious system, aiding its characterization for high-temperature applications.

1. Introduction

Concrete structures are often damaged by mechanical and chemical factors. The demolition of damaged structures is not the only solution, as repairing and rehabilitating them is often more sustainable. This is because rebuilding damaged structures can lead to the emission of carbon dioxide and the disposal of demolition waste, which can harm the environment [1]. The premature degradation of concrete structures is a significant issue that can be attributed to various factors, including scaling, disintegration, erosion, reinforcement corrosion, delamination, spalling, alkali–aggregate reaction, and cracking. Scaling is the loss of the outer layer of concrete, which can be caused by freeze–thaw cycles, chemical attacks, or poor concrete quality. Disintegration is the breakdown of concrete into smaller pieces, which can be caused by fire, sulfate attack, or alkali–aggregate reaction. Erosion is the gradual wearing away of concrete by water, sand, or other materials.
Corrosion of reinforcement is the chemical reaction between steel reinforcement and concrete, which can cause the reinforcement to rust and expand. This can lead to the cracking and spalling of the concrete [2]. Delamination is the separation of the concrete into layers, which can be caused by the corrosion of reinforcement, cracking, or poor concrete quality. Spalling is the sudden detachment of a large piece of concrete, which can be caused by corrosion of reinforcement, delamination, or other factors. The alkali–aggregate reaction is a chemical reaction between the concrete and the aggregate, which can cause the concrete to expand and crack [3]. Cracks can be caused by a variety of factors, including drying shrinkage, thermal expansion, and overloading. The severity of concrete deterioration depends on the specific factors that are causing it. However, even minor deterioration can weaken a concrete structure and make it more likely to fail. The average lifespan of concrete structures, estimated between 75 to 100 years [4], is significantly reduced due to environmental deterioration, improper maintenance, and design deficiencies, necessitating major structural interventions. In India, where a significant proportion of buildings exhibit structural defects and dampness, retrofitting not only addresses structural deficiencies but also mitigates environmental concerns associated with the generation of construction and demolition waste (CDW) [5]. The accumulation of CDW in landfills poses significant environmental threats, emphasizing the need for sustainable retrofitting solutions [6].
Polymers are a good material for repairing concrete. They can improve the strength and durability of concrete, as well as its resistance to chemicals and freezing–thawing cycles [7]. Polymers are large molecules made up of one or more monomers. They are used in a wide range of applications, including making fibers, foams, bottles, rubber, containers, machine parts, paints, adhesives, and damp-proof membranes. Polymers are also used as construction materials due to their low permeability, high mechanical strength, resistance to abrasion, waterproofness, quick setting time, and less hardening time [8]. There are five main types of polymer composites used in concrete repair: polymer concrete (PC), polymer-impregnated concrete (PIC), polymer cement concrete (PCC), and polymer-modified concrete (PMC). Polymer concrete (PC) is made by combining mineral aggregates with a monomer. The monomer polymerizes in situ to form a strong, durable concrete [9]. Polymer-impregnated concrete (PIC) is made by injecting a monomer into hardened concrete. The monomer polymerizes in the pores of the concrete, filling them and improving the strength and durability of the concrete [10]. Polymer cement concrete (PCC) is made by adding a monomer to cementitious materials. The monomer polymerizes in the concrete, improving the strength and durability of the concrete, as well as its resistance to water and chemicals [11]. Polymer-modified concrete (PMC) is made by adding a polymer to cementitious materials. The polymer improves the workability, strength, durability, and resistance to water and chemicals of the concrete. Polymers are a valuable tool for extending the lifespan of concrete structures and preventing premature failure [12]. A key advantage of PMC is its ease of use. The preparation process closely resembles that of regular concrete, making it familiar to construction crews and requiring minimal adaptation [13,14]. This approach offers PMC a significant advantage over other polymerization methods.
Latex and epoxy are widely used as polymers in concrete repair. Epoxy improves the workability and mechanical properties of the concrete, but it is highly flammable and has poor thermal resistance [15]. Latex in concrete can impair the mechanical properties of concrete, and it requires special treatment to improve the strength-carrying capacity of concrete [16]. To overcome the drawbacks of existing polymers, a new polymer is required to improve the performance of concrete. In this study, a new polymer named glycoluril–formaldehyde was investigated for its effect on cement paste and mortar. The results suggest that glycoluril–formaldehyde is a promising new construction chemical that could be used to improve the performance of concrete structures.

2. Materials and Methods

2.1. Cement

Ordinary Portland cement (OPC) of 53 grade, conforming with IS: 12269–1987 [17], was used in this research. To ensure uniformity, the total required quantity of cement for the entire research work was calculated and purchased. The cement bags were stored in a suitable location to prevent moisture from the atmosphere from reacting with the cement due to its hygroscopic nature. The properties of the cement were determined using standard tests. The specific gravity of the cement is 3.17, which is within the range of 3.10 to 3.25 specified by IS: 4031-1988, Part 11 [18]. The standard consistency of the cement is 32%, which is also within the range of 25% to 35% specified by IS: 4031-1988, Part 4 [19]. The initial setting time of the cement is 30 min, and the final setting time is 340 min, which are both within the specified ranges of 30 to 60 min and 300 to 600 min, respectively. The chemical and physical compositions of the cement used are listed in Table 1 and Table 2, respectively. The normal and microscopic images of the cement used in this study are shown in Figure 1.

2.2. Fine Aggregate

The river sand used as the fine aggregate throughout this study was locally sourced. The physical properties of the river sand, such as specific gravity, water absorption, fineness modulus, and density, were tested in accordance with IS: 2386, Part 3 [20], and tabulated in Table 3. The grading of the fine aggregate was carried out according to IS: 383, and the gradation curve is presented in the Figure 2 [21].

2.3. Water

Water conforming to potable standards was used to prepare and cure the concrete in this study.

2.4. Glycoluril

Glycoluril and its derivatives are used as monomers to synthesize macrocyclic cucurbituril polymers, which can bind to a variety of neutral and anionic species [1]. Glycoluril was synthesized in the laboratory following the procedure shown in Figure 3. Normal and microscopic images of the glycoluril are shown in Figure 4.

2.5. Formaldehyde

Formaldehyde, a simple chemical composed of hydrogen, oxygen, and carbon, is renowned for its preservative and antibacterial properties, making it a versatile compound utilized in a variety of value-added products [4]. The formaldehyde used in this study is a 37% solution with a methanol content of 1–10%. It is a white liquid with a strong pungent odor. The pH of the solution is between 2.8 and 4.0, and the specific gravity is between 1.070 and 1.135. The iron content is less than 5.0 PPM.

2.6. New Approach in Glycoluril–Formaldehyde Polymer Concrete

Despite the widespread use of epoxy- and latex-based polymers in construction, they exhibit several drawbacks, such as poor fire resistance, low workability, and minimal improvement in mechanical strength. Glycoluril–formaldehyde, a less common polymeric material, is an effective cement accelerator [14]. This resin is composed of NH groups, which refers to an amine group, which consists of a nitrogen atom bonded to a hydrogen atom. It is a functional group commonly found in organic compounds, which facilitate hydrogen bonding [15]. The structural orientation of glycoluril–formaldehyde allows it to covalently bond with more electronegative atoms, which contributes to its hydrogen bonding capacity. The glycoluril–formaldehyde structural orientation is illustrated in Figure 5. Glycoluril–formaldehyde is a polymeric resin formed from the condensation of glycoluril (C4H6N4O2) and formaldehyde (CH2O) and chemically written as follows:
C4H6N4O2 + CH2O = C5H8N4O3.

2.7. Mix Proportions

A variety of cement mixtures were prepared and evaluated to investigate the effect of different additives on their properties. Sixteen cement mixtures were designed, including a control mix. Each of the three sets of cement pastes contained five distinct mixtures. Set one comprises cement and glycoluril with dosage levels ranging from 1 to 5%. Set two comprises cement and formaldehyde with dosage levels ranging from 1 to 5%. Set three comprises cement, glycoluril with varying dosage levels (1 to 5%), and formaldehyde added at a fixed concentration of 2%. The minimum water demand of 23% consistency was observed for CF mix with a 2% addition of formaldehyde. The formaldehyde in cement mixtures reduces the surface attraction and repulses the cement particles that require less water to fill the voids and improve the fluidity of the cement paste. As a result, the amount of water required for the chemical interaction between cement and water was decreased. The cement mixture’s consistency increased when the formaldehyde content was increased beyond 2%. This is due to the auto-polymerization reaction of formaldehyde during the rise in temperature on the hydration of cement. Hence, the glycoluril was added at various dosage levels from 1 to 5%, and the formaldehyde addition was kept as 2% constants with respect to the weight of the binder. The mixture proportions used in this study are presented in Table 4.

2.8. Tests on Cement Mortar

2.8.1. Fresh Cement Mortar Properties

Fresh properties of the cement mortar, such as the consistency and the initial and final setting time of the mortar, were studied on the cement mortars admixed with glycoluril and formaldehyde as per the IS: 4031 (1945)–1988 [19]. These properties help to determine the behavior of the mortar, ensuring compliance with the standards followed during the general construction. Furthermore, the compressive strength was evaluated on cement mortar rather than paste, in accordance with IS: 4031, Part 6, 1988 (first revision), reaffirmed in 2019, which specifies the method for determining the compressive strength of hydraulic cement using a 1:3 ratio of cement to fine aggregate [22].

2.8.2. Compressive Strength on Mortar Cubes Mix Proportions

The compressive strength of the cement mortar was evaluated using cube specimens measuring 70.6 mm × 70.6 mm × 70.6 mm in accordance with IS: 10086-1982 [23]. This study incorporated glycoluril in varying concentrations (1, 2, 3, 4, and 5%) relative to the binder’s weight. The binder-to-fine aggregate ratio was maintained at 1:3, while the water-to-binder ratio was fixed at 0.45, as shown in Table 5. Standard hand mixing was employed during the mixing process, and the cement mortar was compacted in three layers within the mould (Figure 6). Following a 24 h room temperature drying period, the specimens were demolded. In this study, the decision was made to keep the specimens submerged in formaldehyde for three days to initiate the curing process. This was followed by an additional curing period in water lasting for 4, 11, or 24 days, resulting in total curing durations of 7, 14, or 28 days. This approach was chosen because the water permeability DIN 1048 [24] test measures the resistance of concrete to water penetration under a pressure of 500 kPa over a period of three days. Hence, it was decided to submerge the specimens in formaldehyde under atmospheric pressure (100 kPa) on all sides for the three-day curing period.

2.8.3. Test on Microstructural Behavior

This study used optical microscopy, X-ray diffraction, and scanning electron microscopy (SEM) to investigate the microstructural behaviour of the samples.
Optical microscopy was used first to check for the formation of a polymerized gel between glycoluril and formaldehyde and to study the effect of glycoluril on the cement paste. Hardened cement mortar samples were extracted after 28 days of curing for XRD and SEM analysis. XRD analysis was performed on the extracted cement mortar samples, which contained various%ages of glycoluril as an additive (0%, 1%, 2%, 3%, and 4%). SEM analysis was also performed on all extracted cement mortar samples.

X-Ray Diffraction Analysis

XRD analysis was performed on a Siemens/Bruker D-5000 X-Ray diffractometer, (made in Germany) using powdered cement mortar samples that were cured for 28 days and sieved through a 90 µm sieve. The specifications of the Siemens D-5000 XRD meter are given in Table 6.

Scanning Electron Microscopy

Scanning electron microscopy (SEM) was performed on cement mortar samples after 28 days of curing. The samples were cut with a saw cutter, and their surfaces were coated with evaporated copper before examination. SEM analyses were conducted at a maximum magnification of 10.0× using the CARL ZEISS EVO18 Scanning Electron Microscope (SEM), which has a resolution of 3.5 nm.

3. Results and Discussion

3.1. Consistency of Cement Paste

To determine the minimum water requirement for the chemical reaction between cement and water, a consistency test was conducted. This study also examined the effects of glycoluril and formaldehyde on water demand in cement paste. Sixteen samples were evaluated, and the results are presented in Figure 7. The consistency of a control specimen (CS) served as a benchmark for evaluating the impact of modified cement paste. The proportion of glycoluril in the CG mix was changed between 1 and 5% of the cement weight. The test findings revealed that the CG mix with 5% glycoluril addition exhibited the greatest water demand, reaching 35%, which is 3% higher than the control specimen. Due to its fine particle size and high surface area, glycoluril absorbs water, increasing the water demand [25,26,27,28].
The addition of formaldehyde in cement mixtures was investigated by varying its dosage from 1 to 5% by weight of cement. The minimum water demand for a consistency of 23% was observed for the CF mix with a formaldehyde addition of 2%. Formaldehyde’s repulsive effect on cement particles reduces their surface attraction, resulting in lower water consumption and improved fluidity of the cement paste [29,30]. Consequently, the amount of water needed for the hydration reaction between cement and water was diminished. However, increasing the formaldehyde content beyond 2% led to a rise in the consistency of the cement mixture. The auto-polymerization reaction of formaldehyde during the temperature increase associated with cement hydration [31].
In the third set of experiments (CGF), glycoluril was introduced at various dosage levels ranging from 1 to 5%, while the formaldehyde addition was maintained at a constant 2% by the weight of the binder. The results of CGF revealed that the consistency of the cement mixture varied between 26 and 29%, which falls within the normal consistency range of OPC, which is between 25 and 30% [32]. This improvement in the CGF mix is attributed to the synergistic effect of formaldehyde’s repulsive action on cement particles [23] and glycoluril’s water absorption property [21].
The consistency test indicated that glycoluril in the cement mixtures increases water demand, while formaldehyde reduces water requirements [33]. The combination of glycoluril and formaldehyde in the cement mixture showed no significant changes in cement paste consistency. This confirms that the addition of glycoluril–formaldehyde in cement mixtures influences water demand. The setting time test was further conducted to determine the effect of GF on cement mixtures.

3.2. Initial and Final Setting Time of Cement

This study examines the influence of glycoluril and formaldehyde on the setting time of cement mixtures. The setting time of cement combined with glycoluril and formaldehyde was evaluated. The initial setting time (IST) and the final setting time (FST) were determined for 16 samples using the procedures described in Section 2.8.1. The results are presented in Figure 8 and Figure 9.
The addition of glycoluril (CG) resulted in a 15% reduction in initial setting time (IST) and a 9% reduction in final setting time (FST) compared to the control specimen. The IST of the CG mix decreased from 32 min to 27 min, and the FST decreased from 5.7 h to 5.2 h. This reduction in setting time is attributed to the water absorption capacity of glycoluril powder, which leads to faster drying of the cement paste [31].
Conversely, the incorporation of formaldehyde (CF) caused a significant delay in both IST and FST. The IST of the CF mix increased from 40 min to 100 min, while the FST extended from 9.2 h to 15 h. This delayed setting is attributed to the dispersion of cement particles by formaldehyde during the auto-polymerization reaction triggered by temperature rise. Also, it retards the hydration process, particularly affecting the hydration of tricalcium silicate (C3S) and tricalcium aluminate (C3A). This retardation can lead to a more controlled hydration process, potentially improving the microstructure of the cement paste [25]. The auto-polymerization reaction may replenish water molecules, thereby hindering cement hydration [26].
The CGF mix demonstrated an intermediate setting behaviour, with the IST decreasing from 53 min to 44 min and the FST reducing from 9 h to 7 h. This improved setting performance is attributed to the polymerization reaction between glycoluril and formaldehyde (GF), which enhances the viscosity and workability of the cement mixture and accelerate the reaction in the cement [34]. Further investigations using the miniature slump cone test are warranted to elucidate the precise impact of glycoluril–formaldehyde on the flow characteristics of the cement mixture.

3.3. Miniature Slump Cone Test

The mini-slump test, a straightforward method, is employed to assess the workability of plastic cement pastes. The spread of the cement pastes is determined by measuring the bottom diameter of the miniature slump cone. The graphical representation of the test results can be found in Figure 10a. As illustrated in Figure 10c, the first set of tests (CG) yielded no alterations in rheological properties compared to the control specimen across all glycoluril dosage levels ranging from 1 to 5%. The minimal addition of glycoluril to the cement paste had no discernible impact on its rheological properties. The test findings reveal that the incorporation of formaldehyde (CF) into the cement mixture increases the spread from 72.5 to 137.8 mm, as depicted in Figure 10b. The increased flowability of the cement mixture may be attributed to the replacement of water particles with formaldehyde due to auto-polymerization, which, in turn, neutralizes the surface resistance of cement grains, leading to a reduction in viscosity [21] and an effect similar to that observed for setting time [25].
The slump flow test results indicate that the incorporation of glycoluril and formaldehyde into the CGF cement mixture reduces the spread from 61 mm to 53 mm. This reduction in spread is attributed to the polymerization reaction between glycoluril and formaldehyde, as observed in the consistency and setting time tests [35]. The CG and CGF mixes exhibit minimal impact on the spread. The CF mix, with a spread of 53 mm at 5%, demonstrates optimal workability. Additionally, the compression strength test conducted on modified cement mortar aimed to determine the optimal polymerization and curing method.

3.4. Compressive Strength of Cement Mortar

The compression test was performed on 70.6 mm × 70.6 mm × 70.6 mm cube specimens in accordance with IS: 10086-1982 [23] for all mixes. As per IS: 516, a total of nine cubes were cast for each mix, with three cubes tested at 7, 14, and 28 days, respectively, to determine the compressive strength [36]. The average compressive strength of these three cubes was considered the compressive strength for the corresponding mix. The compressive strength of the mixes varied with respect to glycoluril dosage levels ranging from 1 to 5%. The specimens were cured in formaldehyde for three days and then water-cured for 4, 11, and 24 days, as shown in Figure 11. The results indicate a continuous improvement in the compressive strength of specimens with glycoluril additions of up to 3% (CMG3) in the modified mortar. This led to an average increase in compressive strength of 21% at the seven-day test, 20% at the fourteen-day test, and 25% at the twenty-eight-day test compared to the control specimen. The increase in strength is attributed to the penetration of formaldehyde into the pore structure of the cement mortar at the initial curing stage, resulting in GF polymerization [37]. This polymerization reaction fills these pores and improves the concrete density [38,39,40,41,42,43,44]. The results also show that further increasing the glycoluril content in the cement mixture (CMG4 and CMG5) reduces the compressive strength. This decrease in strength is due to the excess polymer in the cement mixture affecting the binder hydrates [39,40,41,42].

3.5. Microstructural Study

Optical microscopy was used to investigate the polymerization between glycoluril and formaldehyde (Figure 11). XRD analysis was performed on GF-admixed cement paste (Figure 12). Finally, SEM analysis was used to highlight and show the pore structures and microcracks of noteworthy samples.

3.5.1. Microscopic Study

The samples were studied at the magnification of 40× to identify the character of the glycoluril and formaldehyde in the cement matrix. The different focal points for glycoluril and cement particles in Figure 12a show that glycoluril is finer than cement. During the initial period, polymerization between glycoluril and cement was not present in Figure 12b. At this stage, glycoluril particles were inert and showed no noticeable reaction with formaldehyde. At a later stage, glycoluril and formaldehyde tended to develop a polymerized gel and lower the porous structure of the cement mixture, as shown in Figure 12c,d.

3.5.2. XRD Analysis

The XRD pattern presented in the Figure 13 reveals the presence of multiple crystalline phases within the sample across the 2θ range of 5° to 80°. Prominent peaks are observed at around 10°, 29°, 33°, 36°, 47°, and 51°, indicating the formation of key hydration and cementitious phases. Peaks labeled “C” correspond to calcium-based compounds such as calcium hydroxide and calcium silicate hydrates (CSH), while “T” denotes the presence of tobermorite, a significant hydration product. The “M” peaks indicate mayenite (C12A7), commonly found in high-alumina systems. Peaks assigned as “A” suggest the presence of alumina or mullite phases, whereas “L” represents larnite (β-C2S), a known constituent in cement systems. The “D” peaks are attributed to dicalcium silicate (belite), and “G” suggests the occurrence of gehlenite. A minor peak, labeled “H,” may be related to hydrogarnet or other hydrated aluminosilicates. The pattern confirms the complex, multi-phase nature of the system and supports the presence of both hydration products and unreacted phases contributing to the material’s performance. Consistent with Figure 14, all 2-theta graphs exhibited a similar trend. However, the curve became increasingly sharp and intense as the GF content in the cement mixture was elevated [6]. This sharpening and intensification of the curve suggested an increase in calcium and aluminum oxide, with increasing GF content in the cement mixture. The optimum mix CG3 displayed the highest peak intensities for all phase formations, including alite, hilebrandite, larnite, maynite, mullite, and dmitryivanovite. As shown in the quantitative analysis of the 2-theta curve in Figure 15, CG3 also displayed the highest intensity between 17 and 18 degrees 2-theta. This finding suggests an increased presence of aluminum oxides in the cement mixture. This phase development corroborated the notion that adding 3% GF to the cement mixture enhanced its mechanical strength, which is mainly associated with the formation of phases such as mayenite and mullite [45].

3.5.3. SEM Analysis

The scanning electron microscopy (SEM) analysis report of cement mortar samples were examined. The test samples consisted of cement mortar with 0%, 1%, 2%, 3%, and 4% glycoluril.
Based on the scanning electron microscopy (SEM) analysis report, the following observations were made.
All specimens exhibited the hydration of cement, with C3S particles forming large irregular-shaped crystals and C2S particles forming small, rounded crystals. This indicates the formation of tobermorite gel [46], which reduces voids in cement mortar. In addition to the formation of calcium silicate hydrates, the SEM images in Figure 16, Figure 17, Figure 18 and Figure 19 show the formation of glycoluril–formaldehyde polymer. As the glycoluril content increases, it reduces voids and micro-cracks, resulting in a denser mix. Figure 16 and Figure 17 show a large number of voids, indicating that the mix was not dense enough to produce better compression strength than other specimens. Figure 18 shows a very small number of cracks and voids due to the pore-filling nature of calcium silicate hydrate and glycoluril–formaldehyde polymer. This may be the reason for the optimum strength obtained for this mix. Figure 19 also shows unreacted residual glycoluril particles. These particles remain inert in cement mortar and may contribute to the reduction in strength. The compression strength test results show that strength increases with up to 3% addition of glycoluril but decreases slightly with 4% addition of glycoluril due to the unreacted glycoluril components.

3.6. Possible Chemical Interaction Between GF and Cement Paste

When hydrogen atoms are present, they can readily bond with oxygen or nitrogen atoms [46]. This section explores the potential chemical interactions between GF and cement paste. Figure 20 illustrates the theorized structure of GF and its interaction with cement paste. Figure 20a depicts the chemical structure of glycoluril, a molecule containing NH groups. These NH groups can react with formaldehyde (CH2O) to form cross-linked aminoplast, which is the primary component of GF resin (Figure 20b). The nitrogen atom in the GF resin can then interact with oxygen atoms within the cement SiO2, potentially forming covalent bonds (Figure 20c) [47].

3.7. Thermogravimetric (TG) and Differential Thermogravimetric (DTG) Analysis

To study the thermal decomposition behavior of the sample admixed with glycoluril–formaldehyde, thermogravimetric (TG) and differential thermogravimetric (DTG) analysis was performed under a nitrogen atmosphere at a heating rate of 10 °C/min, and the results are presented in Figure 21.
From the TG curve, it can be observed that at a temperature 50–200 °C, the physically bound water is evaporated, and its corresponding mass loss was found to be 5.604%. Furthermore, there is a gradual decomposition of hydrated phases, such as such as calcium silicate hydrates (C-S-H) and ettringite, in the temperature range of 200–450 °C [48,49]. A significant mass loss observed in the range of 650–700 °C corresponds to the dehydroxylation of calcium hydroxide (Ca(OH)2).
The DTG curve exhibits a major decomposition peak around 700 °C, confirming the decarbonation of calcium carbonate (CaCO3) into calcium oxide (CaO) and CO2, which is further supported by an endothermic reaction in the heat flow curve [50]. The peak intensity was reduced, suggesting a shift in the thermal degradation pathway due to the presence of the polymeric phase. The final residue (~72.8% at 900 °C) primarily consists of stable oxides such as CaO and SiO2 [51,52]. These findings provide crucial insights into the thermal stability of the cement sample, aiding in its characterization for high-temperature applications. In comparison with conventional cement, dehydration sharply decreases, vanishing by 800 °C, indicating the irreversible loss of C-S-H phases. Dehydroxylation increases with temperature, peaking at 88.76% at 800 °C, reflecting portlandite breakdown and partial recrystallization. Decarbonation rises until 600 °C then declines after full carbonate decomposition at higher temperatures. These changes serve as reliable indicators of thermal exposure in cementitious materials [50].

4. Conclusions

Although glycoluril itself is not soluble in water, when combined with other components in glycoluril–formaldehyde, it forms a highly hydrophilic polymer that aligns well with the cement’s hydrophilic properties. It is believed that the hydroxyls (OH groups) in glycoluril–formaldehyde may react with silica oxide (SiO2) to create a robust polymer matrix. These covalent bonds might be the reason for glycoluril–formaldehyde’s enhanced performance in various mechanical and bonding strength tests.
The addition of glycoluril increased water demand due to its high surface area, while the formaldehyde addition reduced it by decreasing particle attraction and improving fluidity. However, the combined effect maintained consistency within the normal range.
Individual additions of glycoluril (CG) accelerated setting, while formaldehyde (CF) significantly delayed it. Combined use (CGF) resulted in a setting time close to the control, possibly due to a counterbalancing effect between the water absorption by CG and the influence of glycoluril–formaldehyde (GF) polymerization on paste viscosity.
While formaldehyde increased fluidity in the cement mixture, the addition of glycoluril (either alone or combined with formaldehyde) had minimal impact on rheological properties.
The addition of up to 3% glycoluril (CMG3) led to a significant increase in compressive strength compared to the control, likely due to pore filling through glycoluril–formaldehyde polymerization. However, higher glycoluril content (CMG4 and CMG5) resulted in decreased strength, suggesting a negative impact on binder hydrates.
Microstructural analysis of cement paste using standard microscopy revealed that glycoluril particles were initially inert, with no noticeable reaction between cement and glycoluril. However, when formaldehyde was added, glycoluril and formaldehyde polymerized to form a gel.
X-ray diffraction (XRD) analysis showed that cement paste with glycoluril–formaldehyde admixture contained a high amount of C2S and C3S. The proportion of C2S increased with increasing glycoluril content. This improves the density of concrete and provides resistance against impact, crushing, and increased durability. This explains the increased durability with increasing glycoluril content.
Scanning electron microscopy (SEM) analysis clearly showed the presence of hydrated cement particles such as C3S and C2S in the cement paste. Additionally, the glycoluril–formaldehyde polymer formed along with tobermorite gel when glycoluril was added to the cement paste.
The SEM image of the control specimen showed a large number of unfilled pores. However, the number of pores decreased as the glycoluril content increased in the samples. Also, TG/DTG results show a minimal degradation is samples.
The results show that pore and microcrack filling was most effective with glycoluril additions of up to 3%. The SEM image of the concrete with 4% glycoluril shows the presence of unreacted glycoluril particles, which contributed to the reduction in strength.

Author Contributions

Conceptualization, N.A. and S.K.M.P.; methodology, S.K.M.P., N.A. and C.C.G.; investigation, S.K.M.P., N.P.V. and N.A.; validation, N.P.V.; data curation, S.K.M.P. and C.C.G.; writing—original draft preparation, S.K.M.P. and N.A.; writing—review and editing, C.C.G. and N.A; visualization, S.K.M.P. and N.P.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data are available within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Normal view and microscopic view of cement: (a) normal image; (b) microscopic image.
Figure 1. Normal view and microscopic view of cement: (a) normal image; (b) microscopic image.
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Figure 2. Particle size distribution of river sand.
Figure 2. Particle size distribution of river sand.
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Figure 3. Synthesis of glycoluril.
Figure 3. Synthesis of glycoluril.
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Figure 4. Normal view and microscopic view of glycoluril: (a) normal image; (b) microscopic image.
Figure 4. Normal view and microscopic view of glycoluril: (a) normal image; (b) microscopic image.
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Figure 5. Structural orientation of glycoluril–formaldehyde.
Figure 5. Structural orientation of glycoluril–formaldehyde.
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Figure 6. Cast mortar cube specimens.
Figure 6. Cast mortar cube specimens.
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Figure 7. Consistency of cement paste for various combinations.
Figure 7. Consistency of cement paste for various combinations.
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Figure 8. Initial setting time of cement paste for various combinations.
Figure 8. Initial setting time of cement paste for various combinations.
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Figure 9. Final setting time of cement paste for various combinations.
Figure 9. Final setting time of cement paste for various combinations.
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Figure 10. Miniature slump cone test: spread of glycoluril and formaldehyde: (a) spread diameter of cement paste for various combinations; (b) observed spread; (c) no spread observed.
Figure 10. Miniature slump cone test: spread of glycoluril and formaldehyde: (a) spread diameter of cement paste for various combinations; (b) observed spread; (c) no spread observed.
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Figure 11. Compressive strength of cement mixture.
Figure 11. Compressive strength of cement mixture.
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Figure 12. Microscopic images of glycoluril–formaldehyde-admixed cement paste at different time periods: (a) cement and glycoluril; (b) cement and formaldehyde reaction in 1 min; (c) glycoluril–formaldehyde reaction after 10 min; (d) glycoluril–formaldehyde reaction in 30 min.
Figure 12. Microscopic images of glycoluril–formaldehyde-admixed cement paste at different time periods: (a) cement and glycoluril; (b) cement and formaldehyde reaction in 1 min; (c) glycoluril–formaldehyde reaction after 10 min; (d) glycoluril–formaldehyde reaction in 30 min.
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Figure 13. XRD analysis of GF-admixed cement paste.
Figure 13. XRD analysis of GF-admixed cement paste.
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Figure 14. XRD analysis of GF-admixed cement paste with various dosages.
Figure 14. XRD analysis of GF-admixed cement paste with various dosages.
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Figure 15. Quantitative analysis of XRD curve.
Figure 15. Quantitative analysis of XRD curve.
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Figure 16. SEM image of cement paste with 0% glycoluril.
Figure 16. SEM image of cement paste with 0% glycoluril.
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Figure 17. SEM image of cement paste with 1% glycoluril.
Figure 17. SEM image of cement paste with 1% glycoluril.
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Figure 18. SEM image of cement paste with 2% glycoluril.
Figure 18. SEM image of cement paste with 2% glycoluril.
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Figure 19. SEM image of cement paste with 4% glycoluril.
Figure 19. SEM image of cement paste with 4% glycoluril.
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Figure 20. Possible chemical interaction between GF and cement paste.
Figure 20. Possible chemical interaction between GF and cement paste.
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Figure 21. TG and DTG curves showing the thermal decomposition of cement paste with glycoluril–formaldehyde admixture.
Figure 21. TG and DTG curves showing the thermal decomposition of cement paste with glycoluril–formaldehyde admixture.
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Table 1. Physical properties of cement.
Table 1. Physical properties of cement.
Physical PropertiesResults
Mean diameter of particle (µm)Less than 30
Blaine surface area (cm2/gm)3420
Loss of ignition 3.17%
Table 2. Chemical properties of cement.
Table 2. Chemical properties of cement.
Chemical Composition Results
Calcium oxide (CaO)63.23%
Silicon dioxide (SiO2)21.41%
Aluminium oxide (Al2O3)4.98%
Sulphur trioxide (SO3)2.25%
Ferric oxide (Fe2O3)3.18%
Magnesium oxide (MgO)1.03%
Other mineral Oxides 1.78%
Table 3. Properties of river sand used.
Table 3. Properties of river sand used.
Properties of River SandResults
Specific gravity2.64
Percentage of voids44.23%
Fineness modulus2.73
Water absorption2.64%
Bulk density1.43 kg/m3
Sieve analysisZone II
Table 4. Mixture proportions of cement paste.
Table 4. Mixture proportions of cement paste.
SetMix IDCement
(gm)		Glycoluril
(gm)		Formaldehyde
(gm)
CS4000-
Set-1CG14004-
CG24008-
CG340012-
CG440016-
CG540020-
Set-2CF1400-4
CF2400-8
CF3400-12
CF4400-16
CF5400-20
Set-3CGF140048
CGF240088
CGF3400128
CGF4400168
CGF5400208
Table 5. Mix proportion of modified cement mortar.
Table 5. Mix proportion of modified cement mortar.
Mix IDCement
(g)		FA
(g)		Water
(ml)		Glycoluril
(g)
CS6001800270-
CMG160018002700.006
CMG260018002700.012
CMG360018002700.018
CMG460018002700.024
CMG560018002700.03
Table 6. Specifications of Siemens D-5000 for XRD analysis.
Table 6. Specifications of Siemens D-5000 for XRD analysis.
Cu K-Beta Radiation with 2Ɵ Scanning Is Used
Step size0.020
Measuring time10.00 Deg/min
Voltage40 kV
Current15 Ma
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Arunachelam, N.; Pothinathan, S.K.M.; Gifta, C.C.; Vignesh, N.P. A Comprehensive Study on the Microstructure and Mechanical Behavior of Glycoluril–Formaldehyde Polymer-Modified Cement Paste. Buildings 2025, 15, 1598. https://doi.org/10.3390/buildings15101598

AMA Style

Arunachelam N, Pothinathan SKM, Gifta CC, Vignesh NP. A Comprehensive Study on the Microstructure and Mechanical Behavior of Glycoluril–Formaldehyde Polymer-Modified Cement Paste. Buildings. 2025; 15(10):1598. https://doi.org/10.3390/buildings15101598

Chicago/Turabian Style

Arunachelam, Nakarajan, S. K. M. Pothinathan, C. Chella Gifta, and N. P. Vignesh. 2025. "A Comprehensive Study on the Microstructure and Mechanical Behavior of Glycoluril–Formaldehyde Polymer-Modified Cement Paste" Buildings 15, no. 10: 1598. https://doi.org/10.3390/buildings15101598

APA Style

Arunachelam, N., Pothinathan, S. K. M., Gifta, C. C., & Vignesh, N. P. (2025). A Comprehensive Study on the Microstructure and Mechanical Behavior of Glycoluril–Formaldehyde Polymer-Modified Cement Paste. Buildings, 15(10), 1598. https://doi.org/10.3390/buildings15101598

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