Effect of borax on the hydration and hardening of β-hemihydrate gypsum at high water–plaster ratio

The existing methods of preparing lightweight gypsum blocks are to make hollow slats or to make foamed blocks, both of which are defective and fail to meet the standards. In order to prepare lightweight gypsum blocks, this paper investigates the method of increasing the proportion of moisture to reduce the weight of gypsum blocks. To further understand the performance relation between β-hemihydrate gypsum (β-hemihydrate phosphogypsum and β-hemihydrate flue-gas desulfurization gypsum) and its products, the effect of borax on the hydration and hardening of β-hemihydrate gypsum at high water–plaster ratio was studied. The results showed that with an increase in borax dosage, the setting time of β-hemihydrate phosphogypsum (β-HPG) was evidently prolonged; the initial setting time increased from 15 to 62 min, and the final setting time increased from 22 to 93 min. The difference between the initial and final setting times also increased, and the fluidity of the gypsum slurry was improved. When the borax dosage reached 0.5%, the flexural strength of β-hemihydrate flue-gas desulfurization gypsum (β-HFGD) increased from 5.2 to 6.3 MPa and the compressive strength increased from 4.7 to 9.3 MPa after 28 d. By analyzing the changes in phase transition, hydration degree, infrared spectrum, particle size, and crystal microstructure during the hydration of β-hemihydrate gypsum, it was found that β-HPG was more sensitive to borax than β-HFGD at high water–plaster ratio and β-HFGD showed superior mechanical properties. The study findings will provide a theoretical basis for the application of β-hemihydrate gypsum products under humid conditions and expand the application range of gypsum products.


Introduction
Gypsum is divided into natural gypsum and industrial byproduct gypsum based on its source [1,2].Industrial byproduct gypsum is a solid waste produced through industrial production processes with hydrated calcium sulfate as the primary component.Phosphogypsum and flue-gas desulfurization (FGD) gypsum are the main types of industrial byproduct gypsum [3,4].Phosphogypsum is produced during the preparation of phosphoric acid using the wet method [5,6], FGD gypsum is produced during FGD in thermal power plants [7][8][9].The industrial by-product gypsum solid waste contains many pollutants, and long-term storage is extremely harmful to the environment [10].Therefore, it is urgently required to examine the recyclability of gypsum solid waste, which is not only conducive to alleviating the shortage of gypsum resources in China but also can reduce the operating costs of the gypsum industry [11][12][13][14].
Building gypsum with β-hemihydrate gypsum as the primary component is obtained by calcining phosphogypsum and FGD gypsum, which is the most common and mature technology for the use of gypsum [15].Building gypsum poses little threat to human health and the natural environment [16][17][18], and can be used to produce gypsum blocks and gypsum boards [19].It is recyclable and environmentally friendly; thus, it can be widely used in the construction industry [20].However, β-hemihydrate gypsum is challenged by its heavy weight when produced into building materials such as panels.In order to reduce weight, the traditional process is to make gypsum blocks into hollow core laths or into foamed blocks.Although the hollow slats meet the relevant standards in terms of weight and strength, they are weak in terms of nail, saw and impact resistance due to the fact that the sheets are hollow, while the foamed blocks can be well controlled in terms of weight, and are strong in terms of nail, saw and impact resistance, but their compressive strength does not reach the standard higher than 3.5 MPa.In order to overcome the above defects, this paper proposes a method to reduce the weight of gypsum by increasing the water-plaster ratio.Through the preliminary research, it is found that the gypsum prepared by this method can meet the requirements of lightweight gypsum blocks in terms of weight and mechanical properties.It is an energy-saving and environmentally friendly method.
However, there are obvious defects in β-hemihydrate gypsum blocks with large water-plaster ratio as building materials.For example, it coagulates easily after mixing with water, the early strength changes greatly [21][22][23], and it cannot meet the needs of normal construction operations [24].Therefore, before its use in construction, the method of adding gypsum retarder is adopted to prolong the setting time of the gypsum slurry [25].However, because of the influence of raw material characteristics, impurities, and the preparation process of calcined gypsum products, the setting and hardening properties of β-hemihydrate gypsum products change greatly.Different retarders have applicability in adjusting the setting time of gypsum, which limits the application of gypsum retarders [26].
Common gypsum retarders include organic acids and their soluble salts, alkaline phosphates, and proteins.Many scholars have studied the influence of common retarders on the setting performance of gypsum [27].Marcos Lanzón et al [28] studied the effect of citric acid concentration on the setting properties of βhemihydrate gypsum at a fixed water-binder ratio.The setting time of gypsum was prolonged by adding a low concentration of citric acid.When the concentration of citric acid was 3000 ppm, the initial setting time of gypsum was 81 min and the loss of flexural and compressive strength was more than 60%.Zhang et al [29] studied the retarding effects of various retarders, including citric acid, tartaric acid, sodium tartrate, salicylic acid, melamine, sucrose, and cement, on building gypsum.The results showed that the retarders had different degrees of influence on the building gypsum strength, among which the effect of citric acid was the most significant.Magallanes-Rivera et al [30] studied the effects of temperature, water-binder ratio, and other conditions as well as the addition of citric acid and malic acid on the hydration process of hemihydrate gypsum.It was found that borax is an ideal retarder for gypsum, which not only increases the early mixing workability of gypsum blocks, but also improves the mechanical properties of gypsum blocks.In addition, previous research on the hydration and hardening of gypsum primarily focused on the water consumption of standard consistency, and there were only a few studies on the hydration and hardening of gypsum at high water-plaster ratio.Studies on the hydration process, microstructure, and mechanical properties of building gypsum at high water-plaster ratio are still lacking.Therefore, it is of great significance to study the effect of a retarder at different dosages on the setting time and strength of building gypsum at high water-plaster ratio.
In this study, the effect of different borax dosages on the hydration and hardening process of β-hemihydrate gypsum and the retarding effect of borax were studied at high water-plaster ratio through experiments.The changes in the mineral phase, hydration degree, infrared spectrum, particle size, and crystal microstructure in the hydration process of β-hemihydrate phosphogypsum (β-HPG) and β-hemihydrate FGD gypsum (β-HFGD) with different borax dosages were observed and compared.The feasibility of using borax as a retarder for building gypsum was discussed to meet the operational requirements of industrial production and provide technical support for the realization of high value-added resource use of building gypsum.

Sample preparation
The mass ratio of plaster powder to water of 1:1 was used to prepare the gypsum block.First, the appropriate amount of borax was dissolved in water to prepare a certain concentration solution and then the water and βhemihydrate gypsum powder required for the experiment were measured.The plaster powder was mixed with the water and borax solution, and the gypsum slurry was prepared by evenly stirring.The fluidity and setting time were assessed according to GB/T 17669.4-1999(Gypsum plasters-Determination of physical properties of pure paste) [31].Gypsum blocks of 40 mm × 40 mm × 160 mm were formed.After final solidification, we marked the surface of the gypsum blocks and removed the molds.When the natural curing reached the specified age, they were dried to dryness in an electric blast drying oven at 40 °C ± 5 °C.The compressive strength and flexural strength were assessed according to GB/T 17669.3-1999(Gypsum plasters-Determination of mechanical properties) [32].
To clearly understand the hydration process, the ratio of plaster powder to water used was 1:10, which could not only prolong the hydration process but also facilitate solid-liquid separation.Plaster powder was used in the same way as deionized water and stored at 20 °C.Deionized water (80 g) was weighed using a 250 ml glass beaker, and 8 g of plaster powder was added to it.The beaker was immediately placed on a magnetic stirrer at 800 r min −1 and 20 °C.Simultaneously, the time was recorded as the initial time.Hydration was stopped at each specific hydration time (5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 1 h, 2 h, 1 d).For accuracy, each sample was prepared separately.The samples were filtered through vacuum filtration, soaked in alcohol for 5 min, filtered again, circulated three times, and stored in a vacuum drying oven at 40 °C for testing.The powder   obtained at different hydration times was used for XRD phase change analysis, Fourier-transform infrared (FT-IR) spectroscopy, particle-size analysis, and hydration degree analysis through subtraction of drying difference [33].

Equipment and characterization
The setting time of the gypsum slurry was assessed using a setting time tester (ISO new standard Vicat instrument, Shanghai Luda Experimental Instrument Co., Ltd).The flexural and compressive strengths of hardened gypsum pastes at different hydration ages were tested using a microcomputer-controlled flexural and compressive testing machine (YAW-300B, Zhejiang Yingsong Instrument and Equipment Manufacturing Co., Ltd).The phase compositions of the powder samples obtained at different hydration times were analyzed through x-ray diffraction (XRD) (Germany-Brook-BRUKER D8 ADVANCE).The samples with different hydration time were characterized by Fourier transform infrared spectrometer (FT-IR) (Japan-Shimadzu-IRTracer 100).The particle-size distributions of powder samples at different hydration times were assessed using a laser particle-size analyzer (British-Malvin-Mastersizer 2000).The microstructure of the powder samples after 1 d of hydration was assessed by scanning electron microscope (SEM) (Germany-Zeiss Sigma 300).
According to the Chinese standard GB/T 5484-2012 (Methods for chemical analysis of gypsum) [34], the chemical binding water content at different hydration times (5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 1 h, 2 h, 1 d) was measured through subtraction of drying difference, and the hydration degree of the samples was then calculated.Approximately 1-g (m 1 ) powder samples that had been dried to a constant quantity varying in the range of 0.001 g at 40 °C were dried at 230 °C ± 5 °C in a drying oven for 2 h.Then, the samples were removed and cooled to room temperature and weighed (m 2 ).We repeated the drying process until the weight of the sample remained constant (the weight change of the last two weighings was limited to 0.001 g).The content of crystal water, , was calculated according to equation (1).It is assumed that the powder sample in the raw material was pure calcium sulfate hemihydrate and the calcium sulfate hemihydrate hydrated into calcium sulfate dihydrate, resulting in an increase of 1.5 crystal water.Therefore, the calcium sulfate hemihydrate involved in the reaction can be calculated by increasing the crystal water [35], and the hydration degree, D , CS is calculated according to equation (2):

Results and discussion
3.1.Effect of borax on the setting times of β-hemihydrate gypsums Figure 3 shows the effect of different borax dosages on the setting times of β-HPG and β-HFGD at high waterplaster ratio.In figure 3(a), without adding borax, the initial setting time of β-HPG was 15 min and the final setting time was 22 min.When the dosage of borax was 0.1%, the initial setting time of β-HPG was 23 min and the final setting time was 36 min, indicating that the setting time of β-HPG was prolonged when the dosage of borax was negligible.When the dosage of borax was 0.5%, the initial setting time of β-HPG reached 62 min and the final setting time reached 93 min.The initial and final setting times of β-HPG gradually increased with increasing borax dosages, and the differences between the initial and final setting times also showed an increasing trend.In figure 3(b), when the dosage of borax increased from 0.1% to 0.4%, the initial and final setting times of β-HFGD almost did not change compared to those without borax.The initial setting time of β-HFGD was extended from 20 to 24 min, and the final setting time was extended from 26 to 31 min when the borax dosage was 0.5%.This indicated that only when the dosage of borax was 0.5%, the setting time of β-HFGD was affected.In summary, borax had a more notable effect on the setting times of β-HPG compared with those of β-HFGD.

Effect of borax on the fluidity of β-hemihydrate gypsums
The effects of different borax dosages on the fluidity of β-hemihydrate gypsum at high water-plaster ratio are shown in figure 4. Evidently, when the dosage of borax was 0.1%, the fluidities of β-HPG and β-HFGD increased, compared to those without borax.In figure 4(a), with an increase in borax dosages, the fluidity of β-HPG also increased linearly from 480 to 500 mm.In figure 4(b), when the borax dosage was 0.5%, the fluidity of β-HFGD increased from 500 to 503 mm.Borax improved the fluidity of β-hemihydrate gypsums to a certain extent, and its effect on that of β-HPG was more obvious.

Effect of borax on the strength of β-hemihydrate gypsums
The flexural and compressive strengths of β-hemihydrate gypsums at different hydration ages (1, 3, 7, and 28d) at high water-plaster ratio and different borax dosages are shown in figures 5 and 6.With the increase in borax dosages, the flexural and compressive strengths of β-HPG decreased gradually.The compressive strength decreased from 4.2 to 3.5 MPa after 28 d.The flexural and compressive strengths of β-HFGD were improved compared to those without borax.The compressive strength increased from 4.7 to ∼9.3 MPa.When the dosage of borax was less than 0.5%, the changes in strength were relatively stable and the flexural strength and compressive strengths of β-HFGD were better than those of β-HPG.
Overall, the influence trends of borax on the flexural and compressive strengths of β-HPG and β-HFGD were consistent.From the perspective of strength development, the early strengths (1-, 3-, and 7-d strengths) of the hardened gypsum after adding borax were lower than those without borax.β-HPG performed more obviously at this point; however, it could catch up with or even exceed the strength of the hardened gypsum without borax after 7 d.At this time, the strength of β-HFGD was better, and the strength after 28 d was obviously higher than that without borax.This may be due to the high water-plaster ratio.In the early strength tests (1, 3, and 7 d), the water in the hardened gypsum had not been completely discharged, resulting in the relatively stable early strength of gypsum.When the hydration age reached 28 d, the water in the hardened gypsum was mostly discharged and the strength was much improved.At high water-plaster ratio, the strength of the hardened gypsum was better in the middle and late hydration stages, and borax is more applicable for β-HFGD.

Effect of borax on the hydration process of β-hemihydrate gypsums
To understand the effect of borax on the hydration of β-hemihydrate gypsums, the mineral phase change, hydration degree, infrared spectrum, particle size, and crystal microstructure of β-HPG and β-HFGD with different borax dosages during hydration were compared and studied at high water-plaster ratio.

Phase changes
The phase changes of hemihydrate to dihydrate gypsum at different β-HPG hydration times with different borax dosages were analyzed by XRD at high water-plaster ratio.As shown in figure 7, the typical diffraction peak (2θ = 14.82°,PDF card No. 41-0224) of the hemihydrate was outstanding in the early hydration time.When borax was not added, the hemihydrate peak disappeared in the sample after ∼1 h.When the borax dosage was 0.1%, it took 2 h for the hemihydrate peak to disappear.When the borax dosage was 0.5%, a weak hemihydrate peak could still be observed after 1 d of hydration.Accordingly, without borax addition, the intensity of the diffraction peak (2θ = 11.68°,PDF card No. 33-0311) of dihydrate gypsum increased slightly in the first 30 min, which may be due to the multistep nucleation process in the early stage of the hydration period.After 1 h of hydration, the peak of dihydrate gypsum was considerably enhanced.In contrast, when the borax dosage was 0.5% (figure 7(c)), the weak peak of dihydrate gypsum persisted much longer and eventually strengthened with longer hydration time, which confirmed that crystallization during β-HPG hydration was also affected by the increase in borax dosage.
Moreover, the phase changes of hemihydrate to dihydrate gypsum at different β-HFGD hydration times with different borax dosages were analyzed by XRD at high water-plaster ratio.As shown in figure 8, compared with β-HPG, the hemihydrate diffraction peak (2θ = 14.82°,PDF card No. 41-0224) of β-HFGD was not prominent at the initial hydration time.Before borax addition, the hemihydrate peak disappeared in the sample after ∼20 min.When the borax dosage was 0.1%, the disappearance of the hemihydrate peak took only 30 min.When the borax dosage was 0.5%, the hemihydrate peak was not observed after 1 h of hydration.Correspondingly, without borax addition, the diffraction peak (2θ = 11.68°,PDF card No. 33-0311) strength of dihydrate gypsum was outstanding after 30 min of hydration, and after 1 h, the peak of dihydrate gypsum was enhanced.On the contrary, at 0.5% borax (figure 8(c)), the dihydrate gypsum peak began to increase after    30 min of hydration, which also confirmed that only when the dosage of borax was 0.5% or higher, the crystallization process of β-HFGD hydration would be affected.This conclusion is consistent with the effect of borax on the setting time of β-HFGD.

Hydration degree
To quantitatively analyze the phase transition of gypsum, figure 9 shows the hydration degree of β-HPG and β-HFGD samples without borax at high water-plaster ratio.The two types of hemihydrate gypsum began to hydrate at ∼10 min.At this time, the hydration degree of β-HPG was 1.80% and the hydration degree of β-HFGD was 3.66%.After 15 min, the hydration degree of β-HPG was 4.67% and the hydration degree of β-HFGD reached 59.49%.Further, after 30 min, the hydration degree of β-HFGD reached 62.68% and the hydration process was complete.When the degree of hydration was 50.24%, β-HPG hydrated completely after 2 h.Obviously, the hydration rate of β-HFGD was much faster than that of β-HPG.β-HFGD powder particles have a large internal specific surface area and slit pores, resulting in rapid hydration.
The effect of borax on the hydration process of β-HPG and β-HFGD samples at high water-plaster ratio is displayed in figure 10.With the addition of borax, the transformation time of hemihydrate to dihydrate gypsum in the β-HPG sample was considerably delayed (figure 10(a)).After 1 h, the hydration degree of the sample with 0.1% borax was 19.61%, whereas the sample with 0.5% borax was barely hydrated.After 2 h, the samples with different borax dosages had begun to hydrate.The degree of hydration decreased with increasing borax dosages, which was consistent with the phase change results of β-HPG in the hydration process (figure 7).In contrast, in figure 10(b), although borax was added, β-HFGD began to hydrate after 10 min and completed hydration after ∼1 h.This indicates that β-HFGD is not sensitive to the retarding effect of borax, perhaps because of the existence of fine slit pores inside the β-HFGD powder particles.In some pores, water molecules with smaller molecular weights can enter, whereas retarder molecules with larger molecular weights cannot, which limits the effect of borax.

FT-IR analysis
The hydration process of β-hemihydrate gypsums was further characterized by FT-IR.At high water-plaster ratio, the FT-IR spectra of β-HPG with different borax dosages at different hydration times are shown in figure 11.In figure 11, without borax addition, the infrared absorption bands at 3613.9 and 1008.3 cm −1 correspond to the vibration absorption bands of OH in hemihydrate crystalline water and the vibration absorption bands at 3405.9 and 1688.8 cm −1 correspond to the vibration absorption bands of OH in dihydrate gypsum crystalline water.The absorption bands at 3558.1 and 1622.1 cm −1 are the characteristic absorption bands of crystalline water shared by hemihydrate and dihydrate gypsum, the strong and slightly wider absorption band at 1127.6 cm −1 is the characteristic absorption band of S-O, and the narrow absorption bands at 663.3 and 600.1 cm −1 are the characteristic absorption bands of SO 4 2− .Therefore, the hydration process of β-HPG can be traced according to the changes in the characteristic bands of 3613.9 and 3405.9 cm −1 .The band at 3613.9 cm −1 slowly weakened during early hydration and disappeared after 30 min of hydration, indicating that the hemihydrate content in the system decreased.When the borax dosage was 0.1%, the band at 3613.9 cm −1 disappeared after 2 h of hydration.When the borax dosage was 0.5%, the band disappeared after 1 d of hydration.Moreover, without borax addition, the band at 3405.9 cm −1 appeared after 10 min of hydration and then slowly increased.After 30 min of hydration, the absorption band was greatly enhanced, indicating that hydration had intensified and a large number of dihydrate gypsum crystals were generated.When the borax dosage was 0.5%, the band at 3405.9 cm −1 was more prominent after 1 d of hydration.This indicated that with the increase in borax dosage, the hydration process of β-HPG gypsum had been successfully delayed.These results are consistent with the XRD phase change results.
Similarly, the hydration process of β-HFGD was also characterized by FT-IR.At high water-plaster ratio, the FT-IR spectra of β-HFGD with different borax dosages at different hydration times are shown in figure 12.In  figure 12(a), without borax addition, the infrared absorption bands at 3614.1 and 1005.9 cm −1 corresponded to the vibration absorption bands of OH in hemihydrate crystalline water and those at 3407.4 and 1687.3 cm −1 corresponded to the vibration absorption bands of OH in dihydrate gypsum crystalline water.Figure 12(b) shows that when the borax dosage was 0.1%, the bands at 3614.1 and 3407.4 cm −1 were relatively unchanged, indicating that the lower dosage of borax had little effect on the hydration process of β-HFGD.In figure 12(c), when the borax dosage was 0.5%, the band at 3614.1 cm −1 slowly weakened after 30 min of hydration and disappeared after 2 h of hydration.This indicates that the content of hemihydrate in the system was considerably reduced.The band at 3407.4 cm −1 was more prominent after 2 h of hydration, and a large number of dihydrate gypsum crystals were generated at this time.The results showed that when the borax dosage was 0.5%, it affected the hydration process of β-HFGD.Compared with β-HPG, β-HFGD was less sensitive to the retarding effect of borax.These results are also consistent with the XRD phase change analysis.

Particle-size distribution
The particle-size distribution changes of β-hemihydrate gypsums during hydration were monitored at high water-plaster ratio.The particle-size distribution of β-HPG with different borax dosages at different hydration times is shown in figure 13. β-HPG began to dissolve on contact with water, and the particle-size distribution shifted in the fine-grained direction.In figure 13(a), when borax was not added, the median particle size decreased from 8.990 to 4.828 μm after 20 min of hydration compared with the particle sizes after 10 min of hydration.Simultaneously, d 0.1 (fine particles) only decreased from 1.455 to 1.147 μm, and d 0.9 (coarse particles) decreased from 48.069 to 20.605 μm.This may be because small particles dissolve first and disappear during hydration, whereas large particles tend to break into small particles and then dissolve and disappear.Therefore, the large particle size fraction decreases sharply and resupplies the disappeared small particles, resulting in unnoticeable changes in small particle sizes, which is the dissolution stage in the hydration process.As hydration continues, the particle size is reduced further; when equilibrium is reached, hemihydrate is hydrated into dihydrate gypsum and the particle size increases.Compared with 30 min of hydration, the median particle size of β-HPG increased from 5.179 to 7.843 μm after 2 h of hydration, whereas d 0.1 and d 0.9 increased from 1.448 to 2.245 μm and from 16.487 to 20.467 μm, respectively.After 1 d of hydration, fine and coarse particles increased slightly; however, the overall particle size distribution did not change considerably compared to 2 h of hydration.The results showed that crystals of dihydrate gypsum generated by the hydration reaction were concentrated in the range of 2 to 25 μm and hydration was complete after 2 h.In figure 13(b), when the borax dosage was 0.1%, the median particle size decreased from 11.435 to 5.848 μm after 30 min of hydration, as compared to the decrease in the particle size after 10 min of hydration.Moreover, d 0.1 decreased from 1.884 to 1.256 μm and d 0.9 decreased from 55.986 to 17.129 μm.In figure 13(c), when the borax dosage was 0.5%, compared with the particle size after 10 min of hydration, the median particle size after 2 h of hydration decreased from 18.064 to 4.798 μm and d 0.9 decreased from 71.070 to 16.422 μm.This indicated that the large particles dissolved and disappeared.In summary, increasing borax dosages delayed the hydration of β-HPG, indicating that the retarding effect of borax on β-HPG was better, which was consistent with the analysis of the hydration degree.
Similarly, the particle-size distributions of β-HFGD with different borax dosages at different hydration times are shown in figure 14.Without borax addition (figure 14(a)), compared with the particle size after 10 min of hydration, the median particle size increased from 4.119 to 15.447 μm after 20 min of hydration.Furthermore, d 0.1 increased from 1.019 to 3.273 μm and d 0.9 increased from 21.423 to 32.115 μm.This indicated that β-HFGD completed the particle dissolution stage before 20 min and hydration was complete at this time.In figure 14(b), when the borax dosage was 0.1%, compared with the particle size after 10 min of hydration, the median particle size increased from 3.774 to 12.953 μm after 30 min of hydration and d 0.9 only increased from 25.911 to 29.204 μm.The particle size changed slightly after 30 min of hydration, as compared to when no borax was added.In figure 14(c), when the borax dosage was 0.5%, as compared to the particle size after 10 min of hydration, the median particle size increased from 4.115 to 14.850 μm and d 0.9 increased from 29.331 to 40.727 μm after 2 h hydration.The hydration of β-HFGD began to be affected only when the borax dosage was 0.5%, indicating that β-HFGD was not sensitive to the retarding effect of borax.This is consistent with the analysis of hydration degree.

Microstructure
To more intuitively study the effect of different borax dosages on the hydration process of β-hemihydrate gypsum at high water-plaster ratio, the microstructure of β-HPG was characterized by SEM after 1 d of hydration, as shown in figure 15.The hydration of hemihydrate transforms it into dihydrate gypsum, and its microstructure is composed of crystal morphology and lapping mode.It can be seen from figure 15(a) that when borax was not added, the dihydrate gypsum crystals of β-HPG were prismatic, containing few flaky crystals, and some prismatic crystals were stacked to form crystal clusters.Single prismatic dihydrate gypsum crystals, few flaky crystals, and crystal clusters randomly distributed, interspersed, and overlapped in a limited space to form a unique crystal structure so that the mechanical properties of the hardened gypsum body can be obtained.When the borax dosage was 0.1% and 0.3%, the dihydrate gypsum crystal clusters were not obvious, as shown in figures 15(b) and (c), respectively, with many single prismatic crystals overlapping with some flaky crystals.When the borax dosage was 0.5%, it can be clearly seen from figure 15(d) that in addition to the intersection and overlap of single prismatic crystals, there were many flocculent and incompletely developed dihydrate gypsum crystals adsorbed on the surface of large-sized crystals.These formations hindered the close overlapping of crystals to form a type of loose microstructure [36], which affected the strength of the hardened gypsum.In summary, with an increase in borax dosage, the hydration of β-HPG could get delayed.Moreover, the incomplete transformation of hemihydrate into dihydrate gypsum could result in poor mechanical properties of the hardened gypsum body.This conclusion is consistent with the results of the mechanical strength tests.
The SEM images of β-HFGD with different borax dosages at high water-plaster ratio after 1 d of hydration are shown in figure 16. Figure 16(a) shows that when borax was not added, the dihydrate gypsum of β-HFGD primarily possessed needle-rod-like crystals and there were dense plate-like crystals formed due to the aggregation of several individual crystals.When the borax dosage was 0.1% and 0.3%, it can be clearly seen in figures 16(b) and (c) that in addition to the aggregation of individual crystals, the dense plate-like crystals continued to aggregate and overlap with one another to form larger and denser crystal clusters, there was no obvious crystal interface between them, such as a thickened single crystal, which enhanced the crystal network   structure [37].When the borax dosage was 0.5%, several single crystals gathered on and near the surface of the dense crystal clusters (figure 16(d)) and the crystal clusters tended to increase in number and thickness.It can be imagined that if the hardened β-HFGD comprised crystal clusters or developed from a single crystal cluster, the compactness of the hardened gypsum would be improved and accompanied by a great improvement in mechanical properties.This can explain why the mechanical properties of β-HFGD were more advantageous than those of β-HPG.In contrast, the microstructure of β-HPG was obviously looser.Therefore, it was easy to see that when the borax dosage was within 0.5%, borax was beneficial in improving the mechanical properties of β-HFGD to a certain extent.

Conclusion
The hydration and hardening properties of β-hemihydrate gypsum with different borax dosages were studied at high water-plaster ratio.With the increase in borax dosages, the setting time of β-HPG was considerably prolonged.The initial setting time increased from 15 to 62 min, and the difference between the initial and final setting times increased.The initial setting time of β-HFGD was extended from 20 to 24 min when the borax dosage was 0.5%.To a certain extent, borax improved the fluidity of β-hemihydrate gypsums, and the effect on β-HPG was more obvious.From the perspective of strength development, the early strength (1-, 3-, and 7-d strengths) of hardened β-hemihydrate gypsum did not change considerably after adding an appropriate quantity of borax.When the hydration age reached 28 d, the strength improved and the compressive strength of β-HFGD increased from 4.7 to ∼9.3 MPa.The changes in the phase transition, hydration degree, infrared spectrum, particle size, and crystal microstructure of β-HPG and β-HFGD with different borax dosages were compared and studied at high water-plaster ratio.The results indicated that with the increase of borax dosages, the conversion time of hemihydrate crystals to dihydrate gypsum crystals during hydration was prolonged, which demonstrated the influence of hydration degree.Through the analyses of the infrared spectrum, particle size change, and crystal microstructure, it was found that the effect of borax on the hydration of β-HPG was more pronounced.The hydration and hardening process of β-HFGD was substantially affected only when the borax dosage was 0.5%.β-HPG was more sensitive to the effect of borax than β-HFGD at high water-plaster ratio, and β-HFGD showed better mechanical properties than β-HPG.

w
Where D CS is the hydration degree, H O are the crystal water contents of the samples before and after hydration, respectively.

Figure 13 .
Figure 13.Particle-size distribution of β-HPG crystals at different hydration times.(a) Reference, (b) Borax 0.1%, (c) Borax 0.5%.The insets in each graph are an expanded view of the shaded area.
•0.5H 2 O, and no obvious impurity peaks were observed.The chemical compositions of the two types of hemihydrate gypsum were analyzed by x-ray fluorescence (XRF), and the results are listed in table 1.The primary chemical components of β-HPG and β-HFGD are SO 3 and CaO, with contents of 56.09% and 36.03% and 58.03% and 38.09%, respectively.Scanning electron microscopic (SEM) images of the two hemihydrate gypsum powders are shown in figure 2. Analytical grade borax of 99.5% Na 2 B 4 O 7 •10H 2 O, supplied by Fuchen Chemical Reagent Co., Ltd, was used as the retarder.

Table 1 .
Chemical compositions (by mass %) of the raw materials.