Effect of Polycarboxylate Admixture on the Performance of Fluorogypsum-Based Self-Leveling Material

: The study explores the inﬂuence of polycarboxylate admixture (PCE) on ﬂuorogypsum-based self-leveling material (FSLM) performance. Wev conducted an array of tests to assess workability and mechanical properties, and utilized XRD, TG-DSC, SEM, and MIP techniques for microscopic analysis. The fresh state results showed that PCE enables FSLM to achieve good workability at lower water demand. It was found that PCE partially inhibited plaster hydration from anhydrite to dihydrate, as observed by X-ray diffraction analysis and thermogravimetric analysis, and with the increasing in PCE, the mass loss of samples reached 15.66% at 28 days. The mechanical properties and the microstructure studies proved that the optimal PCE doping level is 0.14%. At this doping level, there is an enhancement in the denseness of the hardened structure, a reduction in porosity—especially when the pores are more than 200 nm, optimization of pore size distribution, an increase in crystal aspect ratio, an enhancement in effective intergranular overlap, and a signiﬁcant improvement in the 28-day ﬂexural and compressive strength to 7.2 MPa and 36 MPa, respectively. The FSLM prepared under these conditions demonstrates good performance and meets the primary performance index requirements of the Chinese Industry Standard JC/T 1023-2021 (gypsum-based self-leveling ﬂoor compound), thereby promoting the comprehensive utilization of ﬂuorogypsum.


Introduction
Gypsum resources primarily comprise natural gypsum and industrial by-product gypsum, including fluorogypsum.Fluorogypsum is generated during the production of hydrofluoric acid.In China, every ton of hydrofluoric acid produces 4-5 tons of fluorogypsum [1].Its main physical phase is CaSO 4 , which contains a small amount of unreacted CaF 2 impurities compared with natural anhydrite.The physical and chemical properties of fluorogypsum vary widely, exhibiting poor hydration and hardening ability, low early strength, and other defects [2].Because of the low utilization rate [3], a significant amount of fluorogypsum is deposited as solid waste in piles or landfilled.However, some studies have demonstrated that fluorogypsum has a higher potential activity compared to natural anhydrite [4].In recent years, numerous scholars have effectively stimulated the potential activity of fluorogypsum through various modifications, such as pulverization treatment, various salt excitations [5][6][7][8], and further enhancements using active minerals like slag, fly ash, silica fume, and Portland cement [9][10][11][12][13][14].These modifications have led to the production of high-strength cementitious materials that are more widely used in the field of building materials.For instance, Mridul Garg [15] developed a high-strength composite cementitious material using fluorogypsum, slag, and Portland cement.Similarly, Huang Xuquan [16] and Jang Junbong [17] used a fluorogypsum-based cementitious agent to cure sludge.Self-leveling materials, which are now more widely used, can be categorized into cement-based and gypsum-based materials based on their properties [18].Although Appl.Sci.2023, 13, 12802 2 of 15 cement-based self-leveling mortars possess excellent mechanical properties, they are prone to shrinkage and cracking.On the other hand, gypsum-based self-leveling mortars not only offer advantages such as sound and heat insulation, fire resistance, moisture regulation, and resistance to shrinkage and cracking, but they can also reduce carbon emissions by 20% [19,20].Numerous studies have demonstrated that gypsum can be utilized to produce self-leveling mortars [21][22][23][24].For instance, Zhang used phosphorus II anhydrous gypsum as the primary cementing material and modified phosphogypsum with modified steel slag, β-hemihydrate gypsum, and calcium aluminate cement.The resulting self-leveling mortar was performed in accordance with JC/T 1023-2021 [25,26].However, due to the requirements of gypsum-based self-leveling mortar for low fluidity loss over time and high early strength, high-strength alpha-hemihydrate gypsum is primarily used as the cementitious material for gypsum-based self-leveling mortar [27,28].In contrast, the poor hydration and hardening ability, low early strength, and poor fluidity of fluorogypsum limit its application in preparing gypsum-based self-leveling mortars.
Superplasticizers are crucial admixtures that enhance the fluidity of self-leveling materials under low water-cement ratio conditions.They are widely used in cement-based materials, but it has also been shown that superplasticizers can improve the properties of gypsum-based materials by increasing their fluidity while reducing water consumption and porosity, thereby increasing their strength and water resistance [29][30][31].Silva [32] produced gypsum-based self-leveling mortars by adjusting the polycarboxylate admixture (PCE) content to achieve a suitable consistency for self-leveling, using hemihydrate gypsum as raw material.Moreover, compared to other types of superplasticizers, PCE has a less inhibitory effect on the growth and hydration of hemihydrate gypsum crystals [33].The water reduction rate of hemihydrate gypsum slurry reached 18-20% with a PCE dosage of 0.6%, and the structure of the hardened body was denser, with the compressive strength increasing up to 72% compared to the blank sample [34].However, the structure, dosage, and compatibility between the superplasticizers and the raw material collectively affect the enhancement of the superplasticizers [35].At the same time, the dosage of the superplasticizer significantly affects the fluidity of the gypsum slurry, thus creating a contradiction between fluidity and strength development.The chemical adsorption caused by the addition of PCE impairs the setting and hardening of gypsum, leading to poor mechanical properties and a low hydration rate, even though the fluidity requirements are met.While there are numerous studies on PCE in hemihydrate gypsum-based self-leveling materials, there are fewer studies on the effect of PCE on anhydrous gypsum-based selfleveling mortars, especially the impact of PCE on the performance of FSLM.For this reason, this study focuses on analyzing the effect of PCE on the performance of FSLM, tests the workability and strength of FSLM, and also examines the influence mechanism using test methods.First, the influence of PCE on the phase composition and thermal change of the fluorogypsum was characterized by XRD and TG-DSC.Then, MIP pore structure analysis was used to investigate the change in pore size distribution with PCE doping increasing.Finally, the hydration morphology with different dosage of PCE was analyzed by SEM.

Raw Materials
The X-ray diffraction pattern and chemical composition of the fluorogypsum used in this experiment are depicted in Figure 1 and Table 1, respectively.The fluorogypsum (FG) was milled to a specific surface area of 657.61 m 2 /kg.The primary physical phase was anhydrous calcium sulfate with a small amount of calcium fluoride, as indicated in the XRD pattern (Figure 1).White cement (PW) and K 2 SO 4 was used as an activator for fluorogypsum; the X-ray diffraction pattern and chemical composition of the white cement used are depicted in Figure 1 and Table 1, respectively.Other admixtures included a polycarboxylate admixture (PCE), a water-retaining agent (WR), and a defoaming agent (DF), all of which were in powdered form.The commercially available polycarboxylate admixture (PCE) was used to adjust the fluidity of self-leveling materials; the water-retaining agent (also commercially available) was used to reduce water loss and prevent water secretion; the defoaming agent (commercially available as well) was used to decrease the number of air bubbles in self-leveling materials.pa ern (Figure 1).White cement (PW) and K2SO4 was used as an activator for fluorogypsum; the X-ray diffraction pa ern and chemical composition of the white cement used are depicted in Figure 1 and Table 1, respectively.Other admixtures included a polycarboxylate admixture (PCE), a water-retaining agent (WR), and a defoaming agent (DF), all of which were in powdered form.The commercially available polycarboxylate admixture (PCE) was used to adjust the fluidity of self-leveling materials; the water-retaining agent (also commercially available) was used to reduce water loss and prevent water secretion; the defoaming agent (commercially available as well) was used to decrease the number of air bubbles in self-leveling materials.The sum of MoO3, RuO4, Rh2O3, P2O5, TiO2, ThO2, WO3, Au, MnO, Yb2O3, Cr2O3, etc.

Mixing Ratio and Sample Preparation
The molding ratios of the five groups of specimens are presented in Table 2, all by mass ratio.The effect of PCE on the performance of fluorogypsum-based self-leveling materials was determined through experimental studies of groups FSLM1, FSLM2, FSLM3, FSLM4, and FSLM5.The admixtures of the defoamer and water-retaining agent were 0.06 wt% and 0.02 wt% of the gelling material, respectively.The water-cement ratio used was 0.24.According to the Chinese standard JC/T 1023-2021, all dry materials were mixed with low-speed stirring for 60 s before adding water.Then, water was added with low-speed stirring for 60 s, left for 30 s, followed by high-speed stirring for 60 s, and then paused for 60 s before immediate molding.Some images of the preparation are shown in Figure 2.

Mixing Ratio and Sample Preparation
The molding ratios of the five groups of specimens are presented in Table 2, all by mass ratio.The effect of PCE on the performance of fluorogypsum-based self-leveling materials was determined through experimental studies of groups FSLM1, FSLM2, FSLM3, FSLM4, and FSLM5.The admixtures of the defoamer and water-retaining agent were 0.06 wt% and 0.02 wt% of the gelling material, respectively.The water-cement ratio used was 0.24.According to the Chinese standard JC/T 1023-2021, all dry materials were mixed with low-speed stirring for 60 s before adding water.Then, water was added with low-speed stirring for 60 s, left for 30 s, followed by high-speed stirring for 60 s, and then paused for 60 s before immediate molding.Some images of the preparation are shown in Figure 2.

Se ing Time
The Chinese Standard GB/T 17669.4-1999[36] was referred to for testing the se ing time of gypsum slurry using the Vicat apparatus.After pouring the stirred slurry into the truncated cone mold, a metal scraper was used to remove the excess slurry.The initial se ing time was determined when the initial se ing needle first sinks without touching the bo om, while the final se ing time was when it sinks no more than 2 mm.

Fluidity
Firstly, the inside of the barrel and glass plate of the fluidity test mold (inner diameter was 30 ± 0.1 mm, height was 50 ± 0.1 mm) was wiped and kept wet, and the barrel was placed vertically on the glass plate.The water was poured into the container.A total of 100 g of the specimen was weighed and quickly poured into the water.It was mixed with an electric mixer for 3 min to obtain a uniform gypsum slurry.The slurry was then quickly injected into the barrel of the fluidity test mold, and the overflowing slurry was scraped off with a scraper to flush the slurry surface with the top surface of the barrel.The barrel of the fluidity test mold was quickly and evenly lifted.After 4 min, the diameter of the test cake in the two vertical directions of the slurry expansion was measured, and its arithmetic mean was calculated, which represented the initial flow.
The initial slurry was left in the stirrer for 30 ± 0.5 min, then rapidly stirred for 1 min.The fluidity was retested and recorded as the 30 min fluidity.The difference between this flow rate and the initial flow rate was the 30 min flow rate loss.

Flexural and Compressive Strength
The water was weighed according to the required amount and poured into the mixing pot.The well-mixed powder was then weighed and poured into the water for stirring.The slurry was immediately poured into the 40 mm × 40 mm × 160 mm triple mold after stirring.The time from adding water to pouring the powder into the mold was not more than 5 min.
The size of the specimen was 40 mm × 40 mm × 160 mm, with a total of three specimens.The paste specimens were maintained at a temperature of 20 °C and a humidity of 60%.After reaching the specified age, the strength test was conducted immediately.

Test Method 2.3.1. Setting Time
The Chinese Standard GB/T 17669.4-1999[36] was referred to for testing the setting time of gypsum slurry using the Vicat apparatus.After pouring the stirred slurry into the truncated cone mold, a metal scraper was used to remove the excess slurry.The initial setting time was determined when the initial setting needle first sinks without touching the bottom, while the final setting time was when it sinks no more than 2 mm.

Fluidity
Firstly, the inside of the barrel and glass plate of the fluidity test mold (inner diameter was 30 ± 0.1 mm, height was 50 ± 0.1 mm) was wiped and kept wet, and the barrel was placed vertically on the glass plate.The water was poured into the container.A total of 100 g of the specimen was weighed and quickly poured into the water.It was mixed with an electric mixer for 3 min to obtain a uniform gypsum slurry.The slurry was then quickly injected into the barrel of the fluidity test mold, and the overflowing slurry was scraped off with a scraper to flush the slurry surface with the top surface of the barrel.The barrel of the fluidity test mold was quickly and evenly lifted.After 4 min, the diameter of the test cake in the two vertical directions of the slurry expansion was measured, and its arithmetic mean was calculated, which represented the initial flow.
The initial slurry was left in the stirrer for 30 ± 0.5 min, then rapidly stirred for 1 min.The fluidity was retested and recorded as the 30 min fluidity.The difference between this flow rate and the initial flow rate was the 30 min flow rate loss.

Flexural and Compressive Strength
The water was weighed according to the required amount and poured into the mixing pot.The well-mixed powder was then weighed and poured into the water for stirring.The slurry was immediately poured into the 40 mm × 40 mm × 160 mm triple mold after stirring.The time from adding water to pouring the powder into the mold was not more than 5 min.
The size of the specimen was 40 mm × 40 mm × 160 mm, with a total of three specimens.The paste specimens were maintained at a temperature of 20 • C and a humidity of 60%.After reaching the specified age, the strength test was conducted immediately.

XRD
After the specimens reached the age of 1 d, 3 d, and 28 d, they were first made into small pieces and placed in anhydrous ethanol.They were then dried in a vacuum drying oven before the test, ground finely by an agate mortar, and sieved through 200 mesh.Instrument: PANalytical X'Pert Powder type powder X-ray diffractometer-Panalytical B.V., the Netherlands.

TG-DSC
After maintaining the specimens to the specified age (1 d, 3 d, and 28 d), intermediate samples were taken.The samples were first cut into small pieces and placed in anhydrous ethanol for preservation.The specimens were then dried in a vacuum drying oven before the test.Subsequently, the dried specimens were finely ground using an agate mortar to prepare them for measurement.The instrument used for the analysis was the NETZSCH STA 449F3 (Germany) integrated thermal analyzer.Approximately 3 mg of the specimen was taken and placed into an Al 2 O 3 crucible.The analysis was performed under an air atmosphere, with the temperature ramped up from 40 to 1000 • C at a rate of 10 • C/min.

SEM
After the specimens reached the specified age (28 d), middle samples were extracted.These samples were shaped into small squares and immersed in anhydrous ethanol to halt the hydration process.Subsequently, they were removed from the ethanol, dried, and prepared for testing.The dried samples were affixed to the sample table using conductive adhesive and then coated with a layer of gold for measurement.The instrument used for the measurement was the Quattro S type environmental scanning electron microscope by Thermo Fisher Inc. (Waltham, MA, USA).The magnification used for the analysis was 5000× and 10,000×.

MIP Pore Structure Analysis
After maintaining the specimens for a period of 28 days, internal particles were extracted and shaped into block samples measuring approximately 5 mm in length.These samples were then placed in anhydrous ethanol to stop the hydration process.Following that, they were removed from the ethanol and dried in a vacuum drying oven before being prepared for testing.The instrument used for this testing is the AUTOPORE IV9500 V1.04 mercury pressurizer by Micromeritics Inc. (Norcross, GA, USA).The instrument operates by applying pressure within the range of 0.5-50,000 psia and holding it for 10 s at each pressure level.The low-pressure test range is 0.5-30 psia, while the high-pressure range is 30-50,000 psia.

Setting Time and Fluidity
Figure 3a illustrates the impact of PCE on the setting time of FSLM.The results indicate that PCE has a noticeable effect on delaying the setting of the plaster, with the setting time increasing as the superplasticizer dosage increases.Specifically, when the superplasticizer dosage was 0.2%, the initial setting time extended to 330 min, and the final setting time reached 450 min, which were 4.5 and 2.8 times longer than the blank group, respectively.This effect can be attributed to the complexation between the carboxylic group of the superplasticizer and the calcium ions of gypsum, as well as the chemisorption on the nucleation clusters.As the concentration of the superplasticizer increases, the adsorption of gypsum crystals on the superplasticizer also increases, thereby hindering the formation of gypsum crystals [33,37].In summary, the alteration in setting time can be attributed to the modification of the crystallization rate of gypsum crystals by PCE [29].Figure 3b presents the trends of initial slump, 30 min slump, and 30 min slump loss of FSLM with PCE doping.It can be observed that the group doped with 0.08% PCE exhibits no fluidity at a water-cement ratio of 0.24, with a 30 min slump of 57 mm.As the superplasticizer dosage increases, the adsorption of the superplasticizer on the surface of gypsum particles increases, resulting in increased inter-particle bilayer repulsion and enhanced fluidity.When the superplasticizer dosage reaches 0.14%, the 30 min slump of the plaster increases to 142 mm.Further increasing the superplasticizer dosage to 0.2% leads to a 30 min slump of 159 mm for the plaster.For PCE dosages of 0.08%, 0.11%, 0.14%, 0.17%, and 0.20%, the 30 min slump losses of the samples are 25 mm, 16 mm, 4 mm, 3 mm, and 1 mm, respectively.The improvement effect of PCE on 30 min slump loss strengthens with increasing dosages.This is because PCE possesses a comb-like molecular structure, and its dispersion effect is primarily achieved through the spatial dislocation effect provided by its long side chains.As a result, the fluidity of gypsum plaster is enhanced, and the plaster's fluidity loss is reduced while improving dispersibility [22,38].When combined with the effect on the se ing time, the dispersion mechanism of PCE on gypsum powder is realized by influencing the se ing time and hydration reaction between gypsum and water [39].The higher the PCE admixture, the greater the adsorption on the surface of anhydrous gypsum, resulting in higher fluidity and be er dispersion retention of the plaster.
The dashed line in Figure 3b represents the baseline requirement for the 30 min slump of gypsum-based self-leveling mortar according to JC/T 1023-2021.It can be observed that the 30 min slump of FLSM samples meets the standard requirement when the PCE admixture is ≥0.14%.Additionally, as the superplasticizer reaches 0.14%, the initial slump of the plaster shows a lesser tendency to increase with the increase in superplasticizer dosage, indicating that the appropriate PCE dosage is 0.14%.

Strength
Figure 4a,b shows the results of the effect of PCE on the strength of FSLM; the dashed line in Figure 4 represents the minimum strength requirement for the gypsum-based selfleveling mortar according to JC/T 1023-2021.From Figure 4a, it can be seen that the flexural and compressive strengths of FSLM show an overall trend of increasing first and then decreasing with the increase in the superplasticizer dosing, and the peak strength at each age corresponds to 0.14% of the superplasticizer dosing.When the PCE dosage is less than 0.14%, the 28 d strength of the specimen is improved, the effect on 1 d and 3 d strength is smaller, and the strength growth is slow.When the PCE dosage is greater than 0.14%, the compressive strength of the specimen at each age starts to show a linear decreasing trend and reaches the minimum value when the PCE dosage is 0.2%.When the PCE dosage increased from 0.14% to 0.2%, the strength at each age decreased significantly, and the Figure 3b presents the trends of initial slump, 30 min slump, and 30 min slump loss of FSLM with PCE doping.It can be observed that the group doped with 0.08% PCE exhibits no fluidity at a water-cement ratio of 0.24, with a 30 min slump of 57 mm.As the superplasticizer dosage increases, the adsorption of the superplasticizer on the surface of gypsum particles increases, resulting in increased inter-particle bilayer repulsion and enhanced fluidity.When the superplasticizer dosage reaches 0.14%, the 30 min slump of the plaster increases to 142 mm.Further increasing the superplasticizer dosage to 0.2% leads to a 30 min slump of 159 mm for the plaster.For PCE dosages of 0.08%, 0.11%, 0.14%, 0.17%, and 0.20%, the 30 min slump losses of the samples are 25 mm, 16 mm, 4 mm, 3 mm, and 1 mm, respectively.The improvement effect of PCE on 30 min slump loss strengthens with increasing dosages.This is because PCE possesses a comb-like molecular structure, and its dispersion effect is primarily achieved through the spatial dislocation effect provided by its long side chains.As a result, the fluidity of gypsum plaster is enhanced, and the plaster's fluidity loss is reduced while improving dispersibility [22,38].When combined with the effect on the setting time, the dispersion mechanism of PCE on gypsum powder is realized by influencing the setting time and hydration reaction between gypsum and water [39].The higher the PCE admixture, the greater the adsorption on the surface of anhydrous gypsum, resulting in higher fluidity and better dispersion retention of the plaster.
The dashed line in Figure 3b represents the baseline requirement for the 30 min slump of gypsum-based self-leveling mortar according to JC/T 1023-2021.It can be observed that the 30 min slump of FLSM samples meets the standard requirement when the PCE admixture is ≥0.14%.Additionally, as the superplasticizer reaches 0.14%, the initial slump of the plaster shows a lesser tendency to increase with the increase in superplasticizer dosage, indicating that the appropriate PCE dosage is 0.14%.

Strength
Figure 4a,b shows the results of the effect of PCE on the strength of FSLM; the dashed line in Figure 4 represents the minimum strength requirement for the gypsum-based selfleveling mortar according to JC/T 1023-2021.From Figure 4a, it can be seen that the flexural and compressive strengths of FSLM show an overall trend of increasing first and then decreasing with the increase in the superplasticizer dosing, and the peak strength at each age corresponds to 0.14% of the superplasticizer dosing.When the PCE dosage is less than 0.14%, the 28 d strength of the specimen is improved, the effect on 1 d and 3 d strength is smaller, and the strength growth is slow.When the PCE dosage is greater than 0.14%, the compressive strength of the specimen at each age starts to show a linear decreasing trend and reaches the minimum value when the PCE dosage is 0.2%.When the PCE dosage increased from 0.14% to 0.2%, the strength at each age decreased significantly, and the flexural strength decreased by 60.0%, 34.6%, and 25.7%, and the compressive strength decreased by 50%, 31.1%, and 26.3%, respectively, indicating that PCE has a greater negative impact on the mechanical properties of FSLM under the condition of large dosage, especially the early strength.
Appl.Sci.2023, 13, x FOR PEER REVIEW 7 of 15 flexural strength decreased by 60.0%, 34.6%, and 25.7%, and the compressive strength decreased by 50%, 31.1%, and 26.3%, respectively, indicating that PCE has a greater negative impact on the mechanical properties of FSLM under the condition of large dosage, especially the early strength.The aforementioned phenomenon suggests that PCE has a critical doping value of 0.14% for FSLM.When the PCE dosage is below this value, the superplasticizer chemically reacts with the calcium ions of gypsum to form complexes that can fill the pores, thereby improving the pore structure of the hardened body [29,40].As a result, the hardened body exhibits a dense structure and low porosity, which positively impacts strength enhancement.However, two main effects occur when the PCE dosage exceeds this value.Firstly, the pores formed due to the gas-entraining effect of PCE reduce the stacking between gypsum crystals [41].Secondly, the chelation and chemisorption of anionic groups with calcium ions in PCE diminish the excitation effect of the admixture, leading to a decrease in the hydration rate and affecting the strength development.

XRD
Figure 5 presents the XRD pa erns of FSLM3 and FSLM5 at different ages.The dominant crystalline compounds identified are gypsum and anhydrite, accompanied by portlandite and calcium fluoride traces, indicating a limited degree of hydration in FSLM.The characteristic peaks of gypsum gradually intensify as hydration progresses, while the characteristic peaks of anhydrite gradually weaken.This trend is particularly noticeable from 1 d to 3 d, indicating that hydration primarily occurs during the early stage.The slower hydration observed in the later stage may be a ributed to water evaporation under natural maintenance conditions.Furthermore, an increase in PCE doping results in a decrease in the diffraction peak intensity of anhydrite (e.g., d = 1.32 Å, d = 2.83 Å, d = 1.74 Å) and an enhancement of the diffraction peak of gypsum (d = 7.49 Å), particularly in the early stage.These findings demonstrate that the hydration of fluorogypsum in FSLM is predominantly concentrated in the early stage, and the presence of PCE doping influences the hydration products, suggesting that increased doping levels inhibit the hydration of fluorogypsum.The aforementioned phenomenon suggests that PCE has a critical doping value of 0.14% for FSLM.When the PCE dosage is below this value, the superplasticizer chemically reacts with the calcium ions of gypsum to form complexes that can fill the pores, thereby improving the pore structure of the hardened body [29,40].As a result, the hardened body exhibits a dense structure and low porosity, which positively impacts strength enhancement.However, two main effects occur when the PCE dosage exceeds this value.Firstly, the pores formed due to the gas-entraining effect of PCE reduce the stacking between gypsum crystals [41].Secondly, the chelation and chemisorption of anionic groups with calcium ions in PCE diminish the excitation effect of the admixture, leading to a decrease in the hydration rate and affecting the strength development.

XRD
Figure 5 presents the XRD patterns of FSLM3 and FSLM5 at different ages.The dominant crystalline compounds identified are gypsum and anhydrite, accompanied by portlandite and calcium fluoride traces, indicating a limited degree of hydration in FSLM.The characteristic peaks of gypsum gradually intensify as hydration progresses, while the characteristic peaks of anhydrite gradually weaken.This trend is particularly noticeable from 1 d to 3 d, indicating that hydration primarily occurs during the early stage.The slower hydration observed in the later stage may be attributed to water evaporation under natural maintenance conditions.Furthermore, an increase in PCE doping results in a decrease in the diffraction peak intensity of anhydrite (e.g., d = 1.32 Å, d = 2.83 Å, d = 1.74 Å) and an enhancement of the diffraction peak of gypsum (d = 7.49 Å), particularly in the early stage.These findings demonstrate that the hydration of fluorogypsum in FSLM is predominantly concentrated in the early stage, and the presence of PCE doping influences the hydration products, suggesting that increased doping levels inhibit the hydration of fluorogypsum.

TG-DSC
Information about the hydration products in FSLM can be derived from the thermogravimetric analysis.Figure 6 illustrates the DSC-TG curves of FSLM at 28 d, 3 d, and 1 d.The first heat absorption peak observed between 80-100 °C may be attributed to the dehydration of structural water in the samples [10,42].The decomposition of gypsum crystals occurs in two stages, involving the dehydration of CaSO4•2H2O to CaSO4•0.5H2O and further dehydration to CaSO4 [43,44].The noticeable weight loss region in the figure, occurring at 110-200 °C, corresponds to the removal of chemically bound water from the gypsum crystals.A peak of heat absorption is observed between approximately 130-145 °C, while another peak occurs between 400 and 500 °C, which can be attributed to the continuous removal of chemically bound water [33,45,46] and the decomposition of CH [47][48][49].

TG-DSC
Information about the hydration products in FSLM can be derived from the thermogravimetric analysis.Figure 6 illustrates the DSC-TG curves of FSLM at 28 d, 3 d, and 1 d.The first heat absorption peak observed between 80-100 • C may be attributed to the dehydration of structural water in the samples [10,42].The decomposition of gypsum crystals occurs in two stages, involving the dehydration of CaSO 4 •2H 2 O to CaSO 4 •0.5H 2 O and further dehydration to CaSO 4 [43,44].The noticeable weight loss region in the figure, occurring at 110-200 • C, corresponds to the removal of chemically bound water from the gypsum crystals.A peak of heat absorption is observed between approximately 130-145 • C, while another peak occurs between 400 and 500 • C, which can be attributed to the continuous removal of chemically bound water [33,45,46] and the decomposition of CH [47][48][49].

TG-DSC
Information about the hydration products in FSLM can be derived from the thermogravimetric analysis.Figure 6 illustrates the DSC-TG curves of FSLM at 28 d, 3 d, and 1 d.The first heat absorption peak observed between 80-100 °C may be attributed to the dehydration of structural water in the samples [10,42].The decomposition of gypsum crystals occurs in two stages, involving the dehydration of CaSO4•2H2O to CaSO4•0.5H2O and further dehydration to CaSO4 [43,44].The noticeable weight loss region in the figure, occurring at 110-200 °C, corresponds to the removal of chemically bound water from the gypsum crystals.A peak of heat absorption is observed between approximately 130-145 °C, while another peak occurs between 400 and 500 °C, which can be attributed to the continuous removal of chemically bound water [33,45,46] and the decomposition of CH [47][48][49].The total weight loss of the samples decreases with increased PCE doping, indicating that excess PCE partially hinders the conversion of free water to bound water, thus inhibiting the hydration of fluorogypsum in FSLM.The slight mass loss of FSLM samples and the relatively significant percentage of early (3 d) mass loss suggest that the hydration of fluorogypsum in FSLM is limited and primarily occurs during the early stages.This finding is consistent with the results obtained from XRD analysis.Additionally, two heat-absorption phenomena related to decarburization are observed in the 500-800 °C interval [50], which may be a ributed to the presence of carbonates in the raw materials or formed during carbonization processes [1,47].

MIP
MIP pore structure analysis was performed on the FSLM 28 d hardened body with various PCE doping to investigate its effect on the material's porosity and pore size distribution.The results are shown in Figure 7.The total weight loss of the samples decreases with increased PCE doping, indicating that excess PCE partially hinders the conversion of free water to bound water, thus inhibiting the hydration of fluorogypsum in FSLM.The slight mass loss of FSLM samples and the relatively significant percentage of early (3 d) mass loss suggest that the hydration of fluorogypsum in FSLM is limited and primarily occurs during the early stages.This finding is consistent with the results obtained from XRD analysis.Additionally, two heat-absorption phenomena related to decarburization are observed in the 500-800 • C interval [50], which may be attributed to the presence of carbonates in the raw materials or formed during carbonization processes [1,47].

MIP
MIP pore structure analysis was performed on the FSLM 28 d hardened body with various PCE doping to investigate its effect on the material's porosity and pore size distribution.The results are shown in Figure 7.
The differential curves of the pore structure in Figure 7a reveal interesting trends regarding the effect of PCE doping on FSLM.When the PCE doping amount is 0.14%, there is a noticeable shift of the peak to the left, indicating a reduction in the most available pore size and an increase in the content of small pores in FSLM.This suggests that PCE at 0.14% dosage can effectively modify the pore structure of the material.Additionally, the peak value corresponding to the most available apertures initially increases and then decreases with the increase in PCE doping, indicating that PCE has the ability to increase the number of small pores in the material.However, there is a limit to this effect, as the degree to which PCE increases the number of small pores reaches a maximum and then starts to decrease.The differential curves of the pore structure in Figure 7a reveal interesting trends regarding the effect of PCE doping on FSLM.When the PCE doping amount is 0.14%, there is a noticeable shift of the peak to the left, indicating a reduction in the most available pore size and an increase in the content of small pores in FSLM.This suggests that PCE at 0.14% dosage can effectively modify the pore structure of the material.Additionally, the peak value corresponding to the most available apertures initially increases and then decreases with the increase in PCE doping, indicating that PCE has the ability to increase the number of small pores in the material.However, there is a limit to this effect, as the degree to which PCE increases the number of small pores reaches a maximum and then starts to decrease.
In Figure 7b, the cumulative mercury intrusion curve provides further insights into the pore structure of FSLM.It shows that the cumulative mercury intrusion increases to some extent with the increase in PCE dosage.Specifically, when the PCE dosage is increased from 0.08% to 0.2%, the cumulative mercury intrusion increases by 18.4%, 10.2%, 28.2%, and 18.1%, respectively.This indicates that PCE influences the pore size distribution in FSLM, and the cumulative mercury intrusion data are consistent with the porosity data in Figure 7d.It can be inferred that the pore size distribution and porosity collectively affect the strength of FSLM.
To further understand the effect of different pore sizes on strength, Figure 7c classifies the pores into four categories: harmless pores (less than 20 nm), less harmful pores (20-50 nm), harmful pores (50-200 nm), and multiple harmful pores (greater than 200 nm) [51].At 0.14% PCE doping, the number of multiple harmful pores is noticeably reduced, resulting in the lowest total ratio of harmful pores to multiple harmful pores.Additionally, the number of small pores less than 50 nm is the largest in this case.Conversely, at 0.2% PCE doping, the total ratio of harmful pores to multiple harmful pores is the highest, In Figure 7b, the cumulative mercury intrusion curve provides further insights into the pore structure of FSLM.It shows that the cumulative mercury intrusion increases to some extent with the increase in PCE dosage.Specifically, when the PCE dosage is increased from 0.08% to 0.2%, the cumulative mercury intrusion increases by 18.4%, 10.2%, 28.2%, and 18.1%, respectively.This indicates that PCE influences the pore size distribution in FSLM, and the cumulative mercury intrusion data are consistent with the porosity data in Figure 7d.It can be inferred that the pore size distribution and porosity collectively affect the strength of FSLM.
To further understand the effect of different pore sizes on strength, Figure 7c classifies the pores into four categories: harmless pores (less than 20 nm), less harmful pores (20-50 nm), harmful pores (50-200 nm), and multiple harmful pores (greater than 200 nm) [51].At 0.14% PCE doping, the number of multiple harmful pores is noticeably reduced, resulting in the lowest total ratio of harmful pores to multiple harmful pores.Additionally, the number of small pores less than 50 nm is the largest in this case.Conversely, at 0.2% PCE doping, the total ratio of harmful pores to multiple harmful pores is the highest, and there are almost no harmless pores.This indicates that excessive PCE doping can negatively impact the pore structure of FSLM, leading to an increase in the number of large pores and a significant coarsening of pore size, thereby affecting the material's strength.
Figure 7d provides additional information on porosity and average pore size.It demonstrates that the porosity fluctuates with increasing PCE doping, and the average pore size initially decreases and then increases.Interestingly, both porosity and average pore size reach their minimum values at 0.14% PCE doping.This suggests that the optimal amount of PCE can effectively optimize the pore structure of FSLM and refine the pore size.
Therefore, with an increase in PCE dosing, the strength of FSLM tends to initially increase and then decrease.This trend can be attributed to the effective reduction in porosity and the prevention of excessive coarsening of the pore size at lower dosing, which positively impacts the material's strength.On the other hand, at higher dosing levels, the pore structure of FSLM begins to deteriorate, resulting in an increase in the number of large pores and the coarsening of pore size.This negative effect on strength is influenced by factors such as air-entrainment and the hydration process.In summary, from a strength perspective, there exists a strong correlation between the mechanical properties of FSLM and its pore structure.Materials with lower porosity and smaller pore size tend to exhibit higher strength.Therefore, optimizing the pore structure by controlling PCE dosing is crucial to achieving higher strength in FSLM.

SEM
Figure 8 presents the microstructure of FSLM3 and FSLM5 at 28 days.The micrographs reveal that the dihydrate gypsum crystals in FSLM predominantly exhibit a short prismatic structure.This morphology is attributed to the inhibitory effect of PCE doping on the temperature increase during the acceleration phase, which is crucial for the growth of dihydrate gypsum crystals along the c-axis direction.This finding aligns with the observations made by Guan [52]. Figure 8a shows that the FSLM3 consists of a large number of rod-like calcium sulfate dihydrate crystals and a small number of needle-like ettringite.In addition, Figure 8c shows that the FSLM5 is assembled by a small number of short-column or plate-shaped dihydrate and un-hydrated gypsum crystals.This is because the excessive PCE is adhered to the gypsum crystal surface by physical or chemical adsorption, and the free water is difficult to convert into bound water so that there is a difference in the number and size of the FSLM crystals, causing the change in crystal composition and structure which leads to the difference in strength; it also proves that the excessive PCE inhibits the hydration of fluorogypsum to a certain extent.A comparison between Figure 8a,c indicates that the 0.14%-PCE-doped group exhibits a significantly higher number of crystals.These crystals also possess a larger aspect ratio, with increased interlapping and a greater number of contact points.Furthermore, a small amount of needle-like ettringite can be observed within the crystals.In contrast, the 0.2%-PCE-doped group exhibits coarse, short columnar, or plate-like crystals with a less compact structure and reduced interlapping.These factors contribute to a decrease in strength.
The enhancement of FSLM by PCE is primarily influenced by porosity size, pore size distribution, and crystal structure.By reducing the porosity of the hardened body, decreasing the number of macropores, refining the pore size, refining the grain size, and promoting effective interlapping of crystals, the strength development is facilitated.The results demonstrate that an appropriate amount of PCE doping improves the workability of FSLM and effectively enhances the strength at 28 days.The enhancement of FSLM by PCE is primarily influenced by porosity size, pore size distribution, and crystal structure.By reducing the porosity of the hardened body, decreasing the number of macropores, refining the pore size, refining the grain size, and promoting effective interlapping of crystals, the strength development is facilitated.The results demonstrate that an appropriate amount of PCE doping improves the workability of FSLM and effectively enhances the strength at 28 days.

Conclusions
This study investigated the effects of PCE on the se ing time, fluidity, and mechanical properties of FSLM.The hydration products, pore structure, and microstructure of FSLM were examined at different PCE doping levels, leading to the following conclusions: 1. Results showed that the optimal PCE doping level was 0.14%.By adding an appropriate dose of PCE, FSLM exhibits increased fluidity, and the 30 min slump of 142 mm and slump loss was reduced even at a lower water-cement ratio.FSLM prepared with 0.14% PCE demonstrates good performance and meets the main performance requirements outlined in JC/T 1023-2021 (gypsum-based self-leveling mortar).2. Optimal PCE doping leads to gypsum crystals with a larger aspect ratio and tighter intercrystalline lapping, improving the structural density and reducing porosity.This results in an optimized pore size distribution.However, excessive PCE hinders the above performances of fluorogypsum and thereby has a negative impact on strength.
According to the conclusions, these findings contribute to the comprehensive utilization of fluorogypsum.It has the potential to replace cement-self leveling materials, reducing the costs, carbon emission, and risk of cracking during large-scale use.However, it is

Conclusions
This study investigated the effects of PCE on the setting time, fluidity, and mechanical properties of FSLM.The hydration products, pore structure, and microstructure of FSLM were examined at different PCE doping levels, leading to the following conclusions: 1.
Results showed that the optimal PCE doping level was 0.14%.By adding an appropriate dose of PCE, FSLM exhibits increased fluidity, and the 30 min slump of 142 mm and slump loss was reduced even at a lower water-cement ratio.FSLM prepared with 0.14% PCE demonstrates good performance and meets the main performance requirements outlined in JC/T 1023-2021 (gypsum-based self-leveling mortar).

2.
Optimal PCE doping leads to gypsum crystals with a larger aspect ratio and tighter intercrystalline lapping, improving the structural density and reducing porosity.This results in an optimized pore size distribution.However, excessive PCE hinders the above performances of fluorogypsum and thereby has a negative impact on strength.
According to the conclusions, these findings contribute to the comprehensive utilization of fluorogypsum.It has the potential to replace cement-self leveling materials, reducing the costs, carbon emission, and risk of cracking during large-scale use.However, it is well known that the water resistance of gypsum is poor, so it is also an important aspect in future research to investigate and improve the water resistance of FLSM and determine whether PCE will have a positive or negative effect.

Figure 3 .
Figure 3.The setting time and fluidity of FSLM: (a) Setting time, (b) Fluidity

Figure 4 .
Figure 4. Influence of PCE on the strength of FSLM: (a) Flexural strength, (b) Compressive strength

Figure 7 .
Figure 7. Influence of PCE on the pore structure of FSLM for 28 days: (a,c) Pore size distribution, (b) Cumulative pore volume, (d) Porosity and average pore diameter

Figure 7 .
Figure 7. Influence of PCE on the pore structure of FSLM for 28 days: (a,c) Pore size distribution, (b) Cumulative pore volume, (d) Porosity and average pore diameter.