Research on the Thermal Conductivity and Water Resistance of Foamed Phosphogypsum

Abstract

Phosphogypsum is a kind of solid waste which pollutes the environment without reasonable treatment. The application of phosphogypsum in the field of building materials provides an ideal method to solve this problem. Meanwhile, the phosphogypsum mixed with foam can be used for the thermal insulation of wall materials. This paper aims to study the influence of foam volume and cement content on the thermal conductivity, water resistance coefficient and the mechanical strengths of foamed phosphogypsum. The volume of foam in this study ranged from 0% to 60%. Moreover, the influences of humidity and cement content on the mechanical strengths of phosphogypsum are investigated. Scanning electron microscope and the mercury intrusion porosimetry are used for the microscale research and analysis. Results indicate that the relationships between the parameters (thermal conductivity, water resistance coefficient and mechanical strengths) and foam volume fit well with a negatively correlated linear function. The addition of cement can improve the compactness of phosphogypsum and decrease the diameter and volume of pores. Therefore, the thermal conductivity, water resistance coefficient and mechanical strengths of phosphogypsum are increased by the addition of cement.

1. Introduction

Phosphogypsum is a byproduct of wet process phosphoric acid. Each ton of phosphoric acid produced produces 5 tons of phosphogypsum. Generally, phosphogypsum is produced and stored as industrial waste in the yard. The main component of phosphogypsum is CaSO₄·2H₂O. If this kind of material is properly treated, then the phosphogypsum is a rich source of gypsum. However, impurities such as phosphorus, sulfate, fluoride, residual acid and heavy metals have affected the normal utilization of phosphogypsum. According to some statistical information, the utilization rate of global phosphogypsum is only about 15% [1,2,3].

Nowadays, many studies on the reuse of phosphogypsum at home and abroad have been reported [4,5,6]. The semihydrated gypsum can be used to prepare building gypsum and related products by using the cementitious properties [7,8]. Moreover, the phosphogypsum with sulfate activity under alkaline conditions can be used to manufacture the composite materials [9,10]. Li et al. [11] studied the influence of fast burning on the mechanical properties and water resistance of phosphogypsum cementitious materials and found that the burning speed is beneficial to the mechanical strength of gypsum. Lin et al. [12] has prepared the desulfurized gypsum fly ash composite cementitious material with strength higher than pure gypsum and water resistance coefficient higher than 0.85. Ma et al. [13] pointed out that the soluble phosphorus and fluorine impurities in phosphogypsum can inhibit the diffusion and activity of SiO₂ and Al₂O₃. Moreover, the temperature and alkalinity can affect the activity of SiO₂ and Al₂O₃ [14]. As obtained in Huang’s research that the compressive of composite material composed of 45% of phosphogypsum, 10% of steel slag, 35% of slag powder and 10% of limestone powder can reach 40 MPa [15]. Zhou et al. [16] found that the unburned bricks prepared by pressure molding of undisturbed phosphogypsum, river sand, cement and lime can reach the strength grade of MU25 according to GB11945-1999 Chinese standard.

Gypsum is porous with low mechanical strength, poor water resistance and good thermal insulation. The environmental humidity may result in vast turnarounds to the mechanical properties of gypsum. Although many studies on the mechanical performance and durability of gypsum products have been reported, little research is focused on the mechanical strength of phosphogypsum under different humidities [17,18,19]. Additionally, phosphogypsum with foam can be applied for the preparation of thermal insulation wall materials.

In this paper, the thermal conductivity, water resistance coefficient and mechanical strengths (compressive and flexural strengths) of phosphogypsum mixed with foam and cement content ranging from 0% to 35% were studied. Moreover, the mechanical strengths were determined in various humidity environments. A scanning electron microscope was selected to observe the micromorphology of the phosphogypsum. Finally, the mercury intrusion porosimetry was used for the research of size distribution of pores in phosphogypsum.

2. Experimental

2.1. Raw Materials

The phosphogypsum used in this study is a kind of light-gray powder, which is produced by Guizhou Honghai Gypsum Powder Industry Co., Ltd., Guiyang, China. This phosphogypsum is mainly composed by CaSO₄·2H₂O. Moreover, some phosphorus oxide and fluoride exist in the phosphogypsum. Pungent odor gas is produced after the phosphogypsum is mixed with water. Ordinary Portland cement is used as another cementitious material.

Table 1. Main chemical compositions of phosphogypsum/wt.%.

Types	Na2O	MgO	Al2O3	SiO2	P2O5	SO3	K2O	CaO	Fe2O3	BaO	F	Cl
Cement	0.13	1.73	5.47	22.17	/	2.66	0.35	62.23	3.94	1.32	/	/
Phosphogypsum	0.077	0.059	0.471	4.129	0.865	48.618	0.063	44.891	0.179	0.207	0.335	0.018

shows the main chemical compositions of phosphogypsum and ordinary cement. The plant foaming agent is Type JD-2 foaming agent containing 99% vegetable proteins produced by Zhengzhou Xinniu Chemical Products Co., Ltd., Zhengzhou, China, and is a light-yellow liquid. The gas production of this foaming agent is 10,000 mL/g. The decomposition temperature of the gas production is 80 °C. The high-range water-reducing agent with 40% water-reducing rate produced by Henan Lichuang lier Technology Co., Ltd. is used in this study.

Table 2. Particle passing percentage of the raw materials/wt.%.

	Particle Size/um	0.3	0.6	1	4	8	64	100	360
Types
Cement		0	0.33	2.66	15.01	28.77	93.59	100	100
Phosphogypsum		0.4	5.71	11.21	26.32	47.62	81	100	100

shows the particle size distribution of cementitious materials. The volume of foam in this study ranged from 0% to 60%. The water–binder ratio in this study is 1.0. The ratio of cement in this study ranged from 0% to 35% by mass of the total binder. The aggregates used in this study were river sand (fine aggregate) with fineness modulus of 2.35.

2.2. Samples Preparation and Measurement

The samples are prepared as follows:

The foaming agent is mixed and with water uniformly in a ratio of 1:30, and then the well-mixed foam solution is poured into a foaming machine to form foam. The phosphogypsum is mixed with water and stirred for 1 min to obtain a uniform fresh paste. Next, the fresh paste is mixed with the prepared foam to manufacture foamed phosphogypsum. The slump flow of the fresh paste is adjusted to 270 mm~280 mm by adding different dosage of water-reducing agent. The mixed foamed phosphogypsum is poured into the oiled mold to form specimens with size of 40 × 40 × 160 and 100 × 100 × 100. Three specimens are selected for this experiment, and the standard deviation is ±10%.

After being demolded, all specimens are treated in the 101-2A electrothermal blowing drying oven (with maximum drying temperature of 400 °C) manufactured by Tianjin Taisite Instrument Co., Ltd., Tianjin, China at temperature of 140 °C for 9 h. After drying, all specimens are cured in the room environment (40% relative humidity and the temperature of 20 °C) for 28 d.

TC3000E portable thermal conductivity tester was produced by Xi’an Xiaxi Electronic Technology Co., Ltd., Xi’an, China with the coefficient of thermal conductivity ranging from ~0.001–10 W/(m·K). Specimens with size of 100 × 100 × 100 are used for the measurement of thermal conductivity and water resistance coefficient. The mechanical strengths are measured according to the Chinese Standard GB/T 17671-1999. Some specimens with size of 100 × 100 × 100 are immersed in water until the weight is constant. The water resistance coefficient is the ratio of the compressive strength of specimen before immersed in water to the specimen under saturated condition. Automatic universal testing machine equipped with a relative humidity controllable box is provided to determine the mechanical strength of the specimens mixed with 10%, 20% and 30% cement and 30 % foam. The relative humidities in this study are 30%, 40%, 50%, 60%, 70% and 80%. All specimens are cured for 28 d in room environment and kept in this environment for 6 h before testing. The microstructure and crystal types of hydration products are determined by the JSM-6360LV scanning electron microscope (Japan Electron Optics Laboratory, Tokyo, Japan).

3. Results and Discussion

3.1. Thermal Conductivity

Figure 1 shows the thermal conductivity (k_x) of specimens with different cement content varying with the foam volume (V).

Table 3. The fitting results of the thermal conductivity and foam volume.

Equation	Cement Volume	a	b	R2
kx = aV + b	0%	−0.00600	0.408	0.981
	5%	−0.00569	0.430	0.988
	10%	−0.00534	0.452	0.992
	15%	−0.00517	0.484	0.997
	20%	−0.0052	0.541	0.992
	25%	−0.00495	0.576	0.983
	30%	−0.00422	0.643	0.988
	35%	−0.00446	0.714	0.974

shows the fitting results of thermal conductivity and the foam volume. It can be observed from Figure 1 that the thermal conductivity of specimens decreases linearly with the increasing foam volume. This is attributed to the fact that the foams in specimens can block the heat transmission, thus decreasing the thermal conductivity of specimens [20,21]. Moreover, the addition of cement leads to increasing the thermal conductivity of specimens. This can be ascribed to the fact that the structure of cement stone is more compact than that of phosphogypsum [22,23]. Finally, as obtained from Figure 1 and Table 3, the fitting degrees of these fitting functions are higher than 0.97, indicating that the thermal conductivity is a highly consistent linear function with the foam volume.

3.2. Water Resistance

Figure 2 shows the water resistance coefficient (K) of specimens with different cement content varying with the foam volume (V). As shown in Figure 2, the water resistance coefficient decreases with linear function. It can be observed from Figure 2 that the foam volume demonstrates negative effects on the water resistance of phosphogypsum. This is attributed to the fact that the increased diameter and number of pores induced by the increased foam volume make more water enter the phosphogypsum, thus leading to the destruction of its structures and the decline in its mechanical strength [24,25]. However, the addition of cement results in increasing the water resistance coefficient of specimens due to the improved compactness of the phosphogypsum, thus preventing the immersion of water and improving the water resistance coefficient of phosphogypsum [26,27]. Moreover, as obtained from Figure 2 and

Table 4. The fitting results of water resistance coefficient and foam volume.

Equation	Cement Volume	a	b	R2
kx = aV + b	0%	−6.21 × 10−4	0.215	0.960
	5%	−6.61 × 10−4	0.222	0.968
	10%	−7.14 × 10−4	0.228	0.968
	15%	−6.21 × 10−4	0.234	0.981
	20%	−6.43 × 10−4	0.248	0.874
	25%	−5.39 × 10−4	0.252	0.846
	30%	−4.64 × 10−4	0.271	0.965
	35%	−1.21 × 10−3	0.342	0.988

, the fitting degree of the fitting function is higher than 0.846; therefore, the relationship between water resistance coefficient and foam volume fits well with a linear function.

3.3. Mechanical Strengths

Figure 3 and Figure 4 show the mechanical strengths (flexural and compressive strengths) of phosphogypsum varying with different foam volume. It can be observed from Figure 3 and Figure 4 that the mechanical strengths decrease linearly with the increasing foam volume. This is attributed to the fact that the increased foam volume can lead to increasing the internal porosity of phosphogypsum, thus decreasing the mechanical strengths [28,29]. However, the addition of cement can improve the compactness of the microstructures of phosphogypsum, thus improving the mechanical strengths [30,31].

Table 5. The fitting results of the flexural strength (f_t) and foam volume (V).

Equation	Cement Content	a	b	R2
ft = aV + b	0%	−0.0683	5.190	0.982
	5%	−0.0644	6.432	0.985
	10%	−0.0607	6.698	0.988
	15%	−0.0573	7.528	0.993
	20%	−0.0579	8.638	0.995
	25%	−0.0554	9.052	0.988
	30%	−0.0472	9.796	0.986
	35%	−0.0500	10.599	0.958

and

Table 6. The fitting results of the compressive strength (f_cu) and foam volume (V).

Equation	Cement Content	a	b	R2
fcu = aV + b	0%	−0.158	12.014	0.992
	5%	−0.149	12.604	0.991
	10%	−0.141	13.220	0.990
	15%	−0.136	14.081	0.994
	20%	−0.139	15.680	0.996
	25%	−0.133	16.672	0.991
	30%	−0.113	18.456	0.987
	35%	−0.120	20.381	0.962

show the fitting results of flexural and compressive strengths of phosphogypsum and the foam volume, respectively. As expressed in Table 4 and Table 5, the fitting degrees of all fitting curves are higher than 0.95; therefore, the mechanical strengths and the foam volume fit well with linear function.

Figure 5 and Figure 6 show the mechanical strengths (flexural and compressive strengths) varying with the relative humidity. It can be observed from Figure 5 and Figure 6 that the mechanical strengths decrease linearly with the increasing relative humidity. This is attributed to the fact that the phosphogypsum is in contact with water more frequently when the relative humidity increases, leading eventually to the destruction of the phosphogypsum’s structure [32,33]. Consequently, the phosphogypsum is corroded by water seriously by water. Therefore, the mechanical strengths decrease with the increasing relative humidity. However, as illustrated in Figure 5 and Figure 6, the addition of cement demonstrates a positive effect on the mechanical strengths of phosphogypsum, due to the fact that the cement is able to improve the compactness of microstructure of the phosphogypsum [34,35]. Thus, the addition of cement leads to the improvement of mechanical strengths of the phosphogypsum. Finally, as obtained from Figure 5, Figure 6 and

Table 7. The fitting results of the flexural strength (f_t) and relative humidity (R_H).

Equation	Cement Content	a	b	R2
ft = aRH + b	0%	−0.0606	7.065	0.986
	10%	−0.0711	8.596	0.960
	20%	−0.0694	9.602	0.981
	30%	−0.0614	9.629	0.987

and

Table 8. The fitting results of the compressive strength (f_cu) and relative humidity (R_H).

Equation	Cement Content	a	b	R2
fcu = aRH + b	0%	−0.212	7.065	0.977
	10%	−0.199	8.596	0.980
	20%	−0.184	9.602	0.995
	30%	−0.201	9.629	0.978

, the fitting degree of the fitting function is higher than 0.97; therefore, the relationship between relative humidity and mechanical strengths fits well with linear function. Compared with the compressive strengths of phosphogypsum-based cemented backfill and the hemihydrate phosphogypsum-based foam insulation materials, phosphogypsum mixed with cement shows better compressive strength [36,37].

3.4. Microscopic Analysis

Figure 7 shows the scanning electron microscope (SEM) photos of phosphogypsum. As shown in Figure 7a, the phosphogypsum with no cement shows a large number of tiny needle-like hydration products and columnar lamellar around phosphogypsum. Figure 7b shows the scanning electron microscope (SEM) photos of phosphogypsum with 20% cement by mass to the total binder. It can be seen in Figure 7b that more sheet integral hydration products can be found in Figure 7. This is attributed to the fact that the cement can react with water, forming hydraulic hydration products. Therefore, as obtained from Figure 7, the addition of cement is able to improve the compactness of the hydration products, thus increasing the mechanical strengths of phosphogypsum.

Figure 8 shows the curves of the relationship between the pore diameter and the cumulative pore volume of phosphogypsum or phosphogypsum with 20% cement. As illustrated in Figure 8, the main pore diameter of phosphogypsum varies from 5.65 nm to 3.45 × 10⁵ nm. Meanwhile, the main pore diameter of phosphogypsum with 20% cement varies from 5.65 nm to 9.07 × 10⁴ nm. Therefore, the phosphogypsum shows larger pore diameter and cumulative pore volume than the phosphogypsum with 20% cement. Due to smaller pore diameter and cumulative pore volume, the phosphogypsum with 20% cement presents better water resistance and mechanical strengths.

4. Conclusions

Based on this research, the conclusions can be summarized as follows:

(1) The relationships between these parameters (thermal conductivity, water resistance coefficient and the mechanical strengths) and foam volume of phosphogypsum can be described as a linear function well. The addition of cement demonstrates positive correlation to the thermal conductivity, the water resistance coefficient and the mechanical strengths.

(2) The mechanical strengths (flexural and compressive strengths) decrease linearly with the relative humidity of testing environment. The consistency of linear relationship is high. The addition of cement demonstrates a positive effect on the mechanical strengths of phosphogypsum.

(3) The addition of cement is able to improve the water resistance of phosphogypsum. Moreover, the compactness of phosphogypsum is increased, and the diameter and volume of pores in phosphogypsum are decreased by adding the cement.