Study on Effect of Nano-CaCO3 on Properties of Phosphorus Building Gypsum

Phosphogypsum is an industrial by-product from the wet preparation of phosphoric acid. Phosphorus building gypsum (PBG) can be obtained from phosphogypsum after high-thermal dehydration. Improving the mechanical properties of PBG is of great significance to extending its application range. In this paper, PBG was modified by adding nano-CaCO3. Specifically, this study, conducted on 0.25–2% nano-CaCO3-doped PBG, tested effects on the fluidity, setting time, absolute dry flexural strength, absolute dry compressive strength, water absorption and softening coefficient of PBG, followed by its microscopic analysis with SEM and XRD. The experimental results showed that, with an increase in nano-CaCO3 content, the fluidity and setting time of PBG-based mixes were decreased. When the content was 2%, the fluidity was 120 mm, which was 33% lower than that of the blank group; the initial setting time was 485 s, which was 38% lower than that in the blank group; the final setting time was 1321 s, which was reduced by 29%. Nano-CaCO3 evidently improved the absolute dry flexural strength, absolute dry compressive strength, water absorption and softening coefficient of PBG to a certain extent. When the content was 1%, the strengthening effect reached the optimum, with the absolute dry flexural strength and absolute dry compressive strength being increased to 8.1 MPa and 20.5 MPa, respectively, which were 50% and 24% higher than those of the blank group; when the content was 1.5%, the water absorption was 0.22, which was 33% lower than that of the blank group; when the content approached 0.75%, the softening coefficient reached the peak of 0.63, which was 66% higher than that of the blank group. Doping with nano-CaCO3 could significantly improve the performance of PBG, which provides a new scheme for its modification.


Introduction
Phosphogypsum is an industrial by-product from the wet preparation of phosphoric acid. Its major component is CaSO 4 ·2H 2 O, which is usually gray-white or gray-black. About 4~5 t of phosphogypsum is produced per 1 t of phosphoric acid. At present, the annual discharge of phosphogypsum in China is about 30 million t [1][2][3], which is one of the largest solid wastes from the chemical industry. Phosphorus building gypsum (PBG) can be obtained from phosphogypsum after high-thermal dehydration. The major component is CaSO 4 ·1/2H 2 O. When PBG is used as a building material, it exhibits low strength and poor water resistance due to a large number of pores, which makes it difficult to be more widely applied [4][5][6][7].
To improve various properties of PBG, other materials are usually introduced as reinforcement. Wu [8][9][10] studied the effects of short-cut basalt fiber, glass fiber and polypropylene fiber on the properties of PBG and obtained the best mixing ratio of these different fibers. Liang [11] explored the influences of ordinary silicate cement on the mechanical properties of PBG, with the results showing that the addition of this cement could effectively promote the late strength of PBG. Ma [12] investigated the doping effects of the amount of recycled brick powders and the type of activators on the compressive strength and water resistance of PBG, with the results exhibiting that the introduction of brick powders could raise the compressive strength of PBG. Li [13] researched the impacts of doping limes and cements on the strength and water resistance of PBG-based cementitious materials and found these properties of the modified materials were both significantly increased in the later stages. Ji [14] applied multi-wall carbon nanotube materials to modify the PBG to make its internal structure more dense and its mechanical properties stronger. Zhao [15] adopted fly ashes and silica fumes to modify PBG-based mortar, which also improved its performance.
Nano-materials are ultra-fine, with particle sizes less than 100 nm [16,17]. This type of nano-admixture has already been broadly applied to, and has significantly improved the various properties of, building materials [18][19][20][21][22]. Given that the ultra-refinement of nano-CaCO 3 particles changes the crystal structure and surface electronic structure, many characteristics have been generated [23,24]. The research of Kawashima [25] showed that, after doping with nano-CaCO 3 , the initial setting and final setting times of the cement were both shortened, while its hydration rate was significantly improved. Camiletti [26] studied the effect of nano-CaCO 3 on the early properties of ultra-high performance concrete, with the test results exhibiting that nano-CaCO 3 could improve the early mechanical properties of cement-based materials. The research of Liu [27] presented that the addition of nano-CaCO 3 could promote both the compressive strength and flexural strength of cement-based materials at the optimal content of 1%. Detwiler [28] found that for the hydration product, C-S-H gel, of cements, the action of nano-CaCO 3 crystal nuclei, accelerated its formation rate on the particle surface. The research results of Qian [29] demonstrated that nano-CaCO 3 was able to fill the pores of cement-based materials, making the concrete structure more compact, improving its mechanical properties.
Despite these study results, there remains a lack of research on the modification of PBG by nano-CaCO 3 . In this study, nano-CaCO 3 was added into PBG as a reinforcing material to study its influence on fluidity, setting time, absolute dry flexural strength, absolute dry compressive strength, water absorption and softening coefficient of PBG, explore its optimal content and analyze its influence mechanism. The properties of PBG can be improved effectively by adding nano-CaCO 3 into PBG, so that PBG can be widely used. At the same time, nano-CaCO 3 is a type of environmental protection material. The combination of nano-CaCO 3 and PBG and their application in the building materials industry not only meet the requirements of green building materials, but also achieves waste utilization. Therefore, using nano-CaCO 3 to modify PBG is a problem worth studying.

Raw Materials
Phosphogypsum: light-yellow powder from a phosphogypsum yard of Yunnan Yuntianhua Co., Ltd. (Kunming, China). Its chemical composition is shown in Table 1. This phosphogypsum was washed with water, neutralized with lime and then dried at 130 • C for 6 h to obtain PBG. Nano-CaCO 3 : produced by Jiangxi Bairui Calcium Carbonate Co., Ltd. (Yichun, China). The SEM and particle size distribution are shown in Figure 1 and the technology parameters are shown in Table 2.

Experimental Design
Different contents of nano-CaCO3 were added into PBG according to Table 3, stirred and dispersed evenly, then added to water, mixed evenly and poured into a 40 mm × 40 mm × 160 mm mold; mixtures then underwent vibration for 30 s on the vibration table. For each mixture proportion, the fluidity of specimens was measured using the gypsum consistency testing meter and the initial and final setting times of specimens were measured using the Vicat apparatus, according to the Chinese national standard "Gypsum plasters Determination of physical properties of pure paste" (GB/T17669.   [30].

Absolute Dry Flexural Strength and Absolute Dry Compressive Strength
For each mixture proportion, three specimens were molded for the tests. Specimens were demolded after curing at 25 °C , 50 ± 5% RH for 24 h and cured at the same environment for 7 days. Then, all the specimens were dried at the temperature of 45 °C in an electric thermostatic drying oven until the weight was constant. The absolute dry compressive strength and absolute dry flexural strength of specimens were tested in accordance with the Chinese national standard "Determination of Mechanical Properties of Building Plaster" (GB/T 17669.  [31].

Water Absorption and Softening Coefficient
Three specimens were immersed in water for 24 h and the saturated specimens were prepared by wiping off the water on their surfaces with towels. The masses of the specimens were measured by an electronic balance with 0.01 g accuracy. The breaking load of the specimens, F, was measured according to the standard GB/T 17669.3-1999. The water

Experimental Design
Different contents of nano-CaCO 3 were added into PBG according to Table 3, stirred and dispersed evenly, then added to water, mixed evenly and poured into a 40 mm × 40 mm × 160 mm mold; mixtures then underwent vibration for 30 s on the vibration table. For each mixture proportion, the fluidity of specimens was measured using the gypsum consistency testing meter and the initial and final setting times of specimens were measured using the Vicat apparatus, according to the Chinese national standard "Gypsum plasters Determination of physical properties of pure paste" (GB/T17669.   [30].

Absolute Dry Flexural Strength and Absolute Dry Compressive Strength
For each mixture proportion, three specimens were molded for the tests. Specimens were demolded after curing at 25 • C, 50 ± 5% RH for 24 h and cured at the same environment for 7 days. Then, all the specimens were dried at the temperature of 45 • C in an electric thermostatic drying oven until the weight was constant. The absolute dry compressive strength and absolute dry flexural strength of specimens were tested in accordance with the Chinese national standard "Determination of Mechanical Properties of Building Plaster" (GB/T 17669.   [31].

Water Absorption and Softening Coefficient
Three specimens were immersed in water for 24 h and the saturated specimens were prepared by wiping off the water on their surfaces with towels. The masses of the specimens were measured by an electronic balance with 0.01 g accuracy. The breaking load of the specimens, F, was measured according to the standard GB/T 17669. . The water absorption was calculated by Formula (1) and the softening coefficient was calculated by Formula (2): where W is the water absorption, m 0 is the dry mass of the sample and m 1 is the mass of the sample saturated in water.
In addition, K is the softening coefficient, F 1 is the breaking load of the dry sample and F 2 is the breaking load of the saturated sample.

Microscopic Morphology of Gypsum Particles
A small number of samples were taken from the middle of the broken block, from which the micromorphology was observed under a scanning electron microscope after vacuum metal spraying. Table 4 shows the influence of different contents of Nano-CaCO 3 on properties of PBG. Firstly, the water requirement was adjusted to make the fluidity of PBG in the blank group reach 180 mm, then nano-CaCO 3 was added according to the mixing proportions in Table 3 to compare the effects of different contents of nano-CaCO 3 on the PBG fluidity. The comparison results were shown in Figure 2. It could be seen from Figure 2 that, with the increase in nano-CaCO3 content, the fluidity of PBG decreased continuously. When the content of nano-CaCO3 was 2%, the fluidity of PBG was reduced to 120 mm as the minimum, which was 33% lower than that of the blank group.

Effect of Nano-CaCO3 on Setting Time of PBG
With the water requirement of standard consistency of PBG being unchanged, nano-CaCO3 was added according to the mixing proportions in Table 3 to compare the effects of different contents of nano-CaCO3 on the initial and final setting times of PBG. The comparison results were shown in Figure 3.  It could be seen from Figure 3 that, with the increase in nano-CaCO3 content, the initial and final setting times of PBG were both continuously shortened. When the content of nano-CaCO3 reached 2%, the initial setting time of PBG fell to 485 s as the minimum, It could be seen from Figure 2 that, with the increase in nano-CaCO 3 content, the fluidity of PBG decreased continuously. When the content of nano-CaCO 3 was 2%, the fluidity of PBG was reduced to 120 mm as the minimum, which was 33% lower than that of the blank group.

Effect of Nano-CaCO 3 on Setting Time of PBG
With the water requirement of standard consistency of PBG being unchanged, nano-CaCO 3 was added according to the mixing proportions in Table 3 to compare the effects of different contents of nano-CaCO 3 on the initial and final setting times of PBG. The comparison results were shown in Figure 3. It could be seen from Figure 2 that, with the increase in nano-CaCO3 content, the fluidity of PBG decreased continuously. When the content of nano-CaCO3 was 2%, the fluidity of PBG was reduced to 120 mm as the minimum, which was 33% lower than that of the blank group.

Effect of Nano-CaCO3 on Setting Time of PBG
With the water requirement of standard consistency of PBG being unchanged, nano-CaCO3 was added according to the mixing proportions in Table 3 to compare the effects of different contents of nano-CaCO3 on the initial and final setting times of PBG. The comparison results were shown in Figure 3.  It could be seen from Figure 3 that, with the increase in nano-CaCO3 content, the initial and final setting times of PBG were both continuously shortened. When the content of nano-CaCO3 reached 2%, the initial setting time of PBG fell to 485 s as the minimum, It could be seen from Figure 3 that, with the increase in nano-CaCO 3 content, the initial and final setting times of PBG were both continuously shortened. When the content of nano-CaCO 3 reached 2%, the initial setting time of PBG fell to 485 s as the minimum, which was 38% lower than the 786 s of the blank group; also, the final setting time dropped to 1321 s as the minimum, which was 29% lower than the 1861 s of the blank group.

Effect of Nano-CaCO 3 on Absolute Dry Flexural Strength of PBG
With the water requirement of standard consistency being unchanged, the content of nano-CaCO 3 was varied to compare the corresponding changes in absolute dry flexural strength of PBG. The comparison results were shown in Figure 4. which was 38% lower than the 786 s of the blank group; also, the final setting time dropped to 1321 s as the minimum, which was 29% lower than the 1861 s of the blank group.

Effect of Nano-CaCO3 on Absolute Dry Flexural Strength of PBG
With the water requirement of standard consistency being unchanged, the content of nano-CaCO3 was varied to compare the corresponding changes in absolute dry flexural strength of PBG. The comparison results were shown in Figure 4. It could be seen from Figure 4 that, with the increase in nano-CaCO3 content, the absolute dry flexural strength of PBG showed a trend of first increasing then decreasing. When the content of nano-CaCO3 was 1%, the absolute dry flexural strength of PBG reached 8.1 MPa as the maximum, which was 50% higher than that of the blank group; when the content of nano-CaCO3 exceeded 1%, the absolute dry flexural strength of PBG began to decrease. When the content of nano-CaCO3 was 2%, the absolute dry flexural strength of PBG was 5.2 MPa, which was 4% lower than that of the blank group.

Effect of Nano-CaCO3 on Absolute Dry Compressive Strength of PBG
With the water requirement of standard consistency being unchanged, the content of nano-CaCO3 was varied to compare the corresponding changes in absolute dry compressive strength of PBG. The comparison results were shown in Figure 5. It could be seen from Figure 4 that, with the increase in nano-CaCO 3 content, the absolute dry flexural strength of PBG showed a trend of first increasing then decreasing. When the content of nano-CaCO 3 was 1%, the absolute dry flexural strength of PBG reached 8.1 MPa as the maximum, which was 50% higher than that of the blank group; when the content of nano-CaCO 3 exceeded 1%, the absolute dry flexural strength of PBG began to decrease. When the content of nano-CaCO 3 was 2%, the absolute dry flexural strength of PBG was 5.2 MPa, which was 4% lower than that of the blank group.

Effect of Nano-CaCO 3 on Absolute Dry Compressive Strength of PBG
With the water requirement of standard consistency being unchanged, the content of nano-CaCO 3 was varied to compare the corresponding changes in absolute dry compressive strength of PBG. The comparison results were shown in Figure 5. It could be seen from Figure 5 that, with the increase in nano-CaCO3 content, the absolute dry compressive strength of PBG showed a trend of first increasing then decreasing. When the content of nano-CaCO3 was 1%, the absolute dry compressive strength of PBG rose to 20.5 MPa as the maximum, which was 24% higher than that of the blank It could be seen from Figure 5 that, with the increase in nano-CaCO 3 content, the absolute dry compressive strength of PBG showed a trend of first increasing then decreasing. When the content of nano-CaCO 3 was 1%, the absolute dry compressive strength of PBG rose to 20.5 MPa as the maximum, which was 24% higher than that of the blank group; when the content of nano-CaCO 3 exceeded 1%, the absolute dry compressive strength of PBG started to decrease. When the content of nano-CaCO 3 reached 2%, the absolute dry compressive strength of PBG dropped to 15.9 MPa, which was 4% lower than that of the blank group.

Effect of Nano-CaCO 3 on Water Absorption of PBG
With the water requirement of standard consistency being unchanged, the content of nano-CaCO 3 was varied to compare the corresponding changes in PBG's water absorption. The comparison results were shown in Figure 6. It could be seen from Figure 5 that, with the increase in nano-CaCO3 content, the absolute dry compressive strength of PBG showed a trend of first increasing then decreasing. When the content of nano-CaCO3 was 1%, the absolute dry compressive strength of PBG rose to 20.5 MPa as the maximum, which was 24% higher than that of the blank group; when the content of nano-CaCO3 exceeded 1%, the absolute dry compressive strength of PBG started to decrease. When the content of nano-CaCO3 reached 2%, the absolute dry compressive strength of PBG dropped to 15.9 MPa, which was 4% lower than that of the blank group.

Effect of Nano-CaCO3 on Water Absorption of PBG
With the water requirement of standard consistency being unchanged, the content of nano-CaCO3 was varied to compare the corresponding changes in PBG's water absorption. The comparison results were shown in Figure 6. It could be seen from Figure 6 that, with the increase in nano-CaCO3 content, the water absorption of PBG first decreased then stabilized. When the content of nano-CaCO3 reached 1.5%, the water absorption of PBG was 0.22, which was 33% lower than that of It could be seen from Figure 6 that, with the increase in nano-CaCO 3 content, the water absorption of PBG first decreased then stabilized. When the content of nano-CaCO 3 reached 1.5%, the water absorption of PBG was 0.22, which was 33% lower than that of the blank group; when the content of nano-CaCO 3 exceeded 1.5%, the water absorption of PBG gradually stabilized.

Effect of Nano-CaCO 3 on Softening Coefficient of PBG
With the water requirement remaining unchanged, the content of nano-CaCO 3 was varied for comparing the corresponding changes of PBG's softening coefficient. The comparison results were shown in Figure 7.
It could be seen from Figure 7 that, with the increase in nano-CaCO 3 content, the softening coefficient of PBG first increased then decreased, before tending to stabilize. When the content of nano-CaCO 3 was 0.75%, the softening coefficient of PBG rose to 0.63 as the maximum, which was 66% higher than that of the blank group; when the content of nano-CaCO 3 exceeded 0.75%, the softening coefficient of PBG began to decrease; when the content of nano-CaCO 3 reached 1.5%, the softening coefficient of PBG gradually stabilized. At that moment, the softening coefficient remained at 0.56, which was 47% higher than that of the blank group. the blank group; when the content of nano-CaCO3 exceeded 1.5%, the water absorption of PBG gradually stabilized.

Effect of Nano-CaCO3 on Softening Coefficient of PBG
With the water requirement remaining unchanged, the content of nano-CaCO3 was varied for comparing the corresponding changes of PBG's softening coefficient. The comparison results were shown in Figure 7. It could be seen from Figure 7 that, with the increase in nano-CaCO3 content, the softening coefficient of PBG first increased then decreased, before tending to stabilize. When the content of nano-CaCO3 was 0.75%, the softening coefficient of PBG rose to 0.63 as the maximum, which was 66% higher than that of the blank group; when the content of nano-CaCO3 exceeded 0.75%, the softening coefficient of PBG began to decrease; when the content of nano-CaCO3 reached 1.5%, the softening coefficient of PBG gradually stabilized. At that moment, the softening coefficient remained at 0.56, which was 47% higher than that of the blank group.

Microanalysis
The microstructures of PBG prototypes and mixtures with nano-CaCO3 were separately observed by scanning electron microscopy (SEM). The results were shown in Figure 8.

Microanalysis
The microstructures of PBG prototypes and mixtures with nano-CaCO 3 were separately observed by scanning electron microscopy (SEM). The results were shown in Figure 8. It could be seen from Figure 7 that, with the increase in nano-CaCO3 content, the softening coefficient of PBG first increased then decreased, before tending to stabilize. When the content of nano-CaCO3 was 0.75%, the softening coefficient of PBG rose to 0.63 as the maximum, which was 66% higher than that of the blank group; when the content of nano-CaCO3 exceeded 0.75%, the softening coefficient of PBG began to decrease; when the content of nano-CaCO3 reached 1.5%, the softening coefficient of PBG gradually stabilized. At that moment, the softening coefficient remained at 0.56, which was 47% higher than that of the blank group.

Microanalysis
The microstructures of PBG prototypes and mixtures with nano-CaCO3 were separately observed by scanning electron microscopy (SEM). The results were shown in Figure 8.  It could be seen from Figure 8a,b that, after adding nano-CaCO 3 , a lot of granular nano-CaCO 3 with tiny particle size appeared on the surface of PBG. This nano-CaCO 3 wrapped PBG crystals and filled the gaps between them, which reduced the total porosity and exerted the significant effect of micro-aggregation. However, when the added content became too high, CaCO 3 would become unevenly dispersed. It could be seen from Figure 8c that the unevenly dispersed CaCO 3 still wrapped the crystals of PBG. Although the porosity was further reduced, these unevenly dispersed parts generated stress concentration, which imposed an adverse impact on the strength of PBG.

Composition Analysis of Hydration Products
The EDS element mapping images and XRD patterns were characterized and shown in Figures 9 and 10, respectively. 8c that the unevenly dispersed CaCO3 still wrapped the crystals of PBG. Although the porosity was further reduced, these unevenly dispersed parts generated stress concentration, which imposed an adverse impact on the strength of PBG.

Composition Analysis of Hydration Products
The EDS element mapping images and XRD patterns were characterized and shown in Figures 9 and 10, respectively.  It could be seen from EDS and element mapping images (Figure 9a-f) that the main elements in the selected area were Ca (20.3%), S (18.1%) and O (60.5%), together with trace amounts of P (0.8%) and Si (0.3%). The P element in the sample was uniformly dispersed, while the Si element appeared as concentrated spots in very limited numbers, representing the quartz crystals detected by XRD ( Figure 10). There were no P-containing crystals detected by XRD, due to its high dispersion (Figure 9e). This also could be seen from Figure 10 that, after adding nano-CaCO3 into PBG, these two had not yet directly reacted with each other and the main product from the hydration of PBG was still CaSO4·2H2O [32][33][34]. It could be seen from EDS and element mapping images (Figure 9a-f) that the main elements in the selected area were Ca (20.3%), S (18.1%) and O (60.5%), together with trace amounts of P (0.8%) and Si (0.3%). The P element in the sample was uniformly dispersed, while the Si element appeared as concentrated spots in very limited numbers, representing the quartz crystals detected by XRD ( Figure 10). There were no P-containing crystals detected by XRD, due to its high dispersion (Figure 9e). This also could be seen from Figure 10 that, after adding nano-CaCO 3 into PBG, these two had not yet directly reacted with each other and the main product from the hydration of PBG was still CaSO 4 ·2H 2 O [32][33][34].

Discussion
It can be seen from the above test data that, after nano-CaCO 3 was added into PBG, its fluidity and water absorption were both reduced, while its absolute dry flexural strength, absolute dry compressive strength and softening coefficient were all improved, with the absolute dry flexural strength seeing the largest improvement. The main reasons are as follows: Filling effect: Since the average particle size of nano-CaCO 3 is about 100 nm, which is far below that of PBG, a proper amount of nano-CaCO 3 can fill both the micropores of PBG and the internal pores of PBG's hydration product. Simultaneously, it can also improve the particle gradation and change the pore structure of PBG's hydration product by reducing macropores, increasing micropores and lowering the total porosity. The ability to fill the space between PBG particles is greatly promoted, the PBG's total porosity is reduced and the PBG's structure is denser, which improves the PBG's absolute dry flexural strength, absolute dry compressive strength and softening coefficient in all and reduces its water absorption as well.
Nucleation effect: The main component of PBG is CaSO 4 ·1/2H 2 O and its hydration reaction is mainly with water to produce CaSO 4 ·H 2 O. When PBG starts hydration, the hydrolysis of CaSO 4 ·1/2H 2 O releases a large amount of Ca 2+ . After nano-CaCO 3 is added to PBG, it does not directly participate in the hydration reaction, but, compared with ordinary CaCO 3 , the surface activity of nano-CaCO 3 is relatively high, which, therefore, adsorbs the Ca 2+ released by hydration. This causes the CaSO 4 ·2H 2 O around nano-CaCO 3 to nucleate in advance, which leads to the decrease in Ca 2+ concentration in the solution and the increase in Ca 2+ migration from CaSO 4 ·1/2H 2 O, thus accelerating the hydration efficiency of PBG. Based on the original structure, a new one is formed around nano-CaCO 3 as the crystal nucleus, which reduces both the internal surface area and porosity of PBG, increases its compactness and improves its various properties.
Pinning effect: Meanwhile, the existence of nano-CaCO 3 particles in PBG also generates a "pinning effect". This can be seen from Figure 8b, that some nano-CaCO 3 particles are embedded into the gaps of PBG's hydration product. When the hydration product is compressed to generate microcracks inside, their expansion is hindered by nano-CaCO 3 particles and their energy is consumed, which limits the crack propagation and improves various properties of PBG [35].
With the increase in nano-CaCO 3 content, the absolute dry flexural strength and absolute dry compressive strength of PBG both decrease. This is because the surface energy of nano-CaCO 3 is relatively large and, when the content becomes too high, nano-CaCO 3 agglomerates instead of evenly dispersing within PBG. While PBG is subjected to external force, these agglomerated nano-CaCO 3 particles have stress concentration, which affects both the absolute dry flexural strength and absolute dry compressive strength of PBG [36].
Moreover, the specific surface area of nano-CaCO 3 is extremely large. After mixing with water, this surface adsorbs a large amount of water, which reduces the water required to participate in the hydration reaction of PBG. Therefore, with the increase in nano-CaCO 3 content, PBG's fluidity and setting time both decrease. If excessive nano-CaCO 3 is added, PBG's strength is also affected [37]. However, when the softening coefficient is measured, part of the PBG without hydration continues to hydrate, resulting in the supplement to the wet strength of PBG and less reduction compared to the absolute dry strength, so the softening coefficient is improved [38].

Potential Applications and Prospects
The composition of the PBG is relatively complicated, with substantial impurities, which have a negative impact on its performance. According to the results of this paper, it can be seen that, after doping with nano-CaCO 3 , various properties of the PBG have all been improved, which would meet the application requirements for building gypsum. Compared to other materials, nano-CaCO 3 presents certain advantages, as listed in Table 5. Compared to other preparation methods, the method adopted in this paper also has advantages, in that there is no waste generated in the preparation process and it poses not only the merits of simplicity, environmental protection and low cost, but also the certain practical significance in production.

1.
Nano-CaCO 3 exerted a significant effect on the physical properties of PBG's paste with the increase in nano-CaCO 3 content: the fluidity and setting time of PBG were both decreased. Among them, the fluidity decreased by 33%, the initial setting time decreased by 38% and the final setting time decreased by 29%.

2.
Nano-CaCO 3 also presented a significant impact on the mechanical properties of PBG: with the increase in nano-CaCO 3 content, both the absolute dry flexural strength and absolute dry compressive strength of PBG first increased then decreased. When the content of nano-CaCO 3 was 1%, the absolute dry flexural strength of PBG increased by 50% and the absolute dry compressive strength increased by 24%. When the content of nano-CaCO 3 reached a certain level, it imposed a negative impact on the mechanical properties of PBG.

3.
Nano-CaCO 3 also had a significant influence on other properties of PBG: with the increase in nano-CaCO 3 content, the water absorption of PBG first decreased then stabilized gradually, decreasing by 33%, while the softening coefficient first increased, then decreased and finally tended to stabilize, with a maximum increase of 66%.

4.
After nano-CaCO 3 was added into PBG, it could fill the voids within the hardened body and improve PBG's pore structure. Meanwhile, based on the original structure, it could form a new one around nano-CaCO 3 as the crystal nucleus, which reduced both the internal surface area and porosity of PBG, increased PBG's compactness and improved PBG's various properties.