Preparation of CaCO3-TiO2 Composite Particles and Their Pigment Properties

CaCO3-TiO2 composite particles were prepared with calcium carbonate (CaCO3) and TiO2 in stirred mill according the wet grinding method. The pigment properties, morphology, and structure of CaCO3-TiO2 composite particles and the interaction behaviors between CaCO3 and TiO2 particles were explored. In the CaCO3-TiO2 composite particles, TiO2 is uniformly coated on the surface of CaCO3 and the firm combination between CaCO3 and TiO2 particles is induced by the dehydration reaction of surface hydroxyl groups. CaCO3-TiO2 composite particles have similar pigment properties to pure TiO2. The hiding power, oil absorption, whiteness and ultraviolet light absorption of composite particles are close to those of pure TiO2. The application performance of CaCO3-TiO2 composite particles in the paint is consonant with their pigment properties. The contrast ratio of the exterior paint containing CaCO3-TiO2 composite particles is equivalent to that of the paint containing the same proportion of pure TiO2.


Introduction
CaCO 3 is an important inorganic mineral material. Especially, the CaCO 3 powder material prepared with various non-metallic minerals, such as aragonite, calcite, and some rocks with calcite as the main ingredient component, has many advantages, such as high purity, high whiteness, good compatibility with organic and inorganic matrices, and low cost [1,2]. Therefore, CaCO 3 has become the most widely used filler in many industrial products such as plastic, paint, and paper due to its obvious cost advantage compared to most non-metallic minerals [3,4]. In order to further increase the utilization value of CaCO 3 and achieve the efficient utilization of mineral resources, it is necessary to develop CaCO 3 -based composite with oxides, such as TiO 2 [5,6]. Recently, the composite pigment prepared by coating TiO 2 particles on the surface of CaCO 3 particles, had received wide attention [7]. The composite pigment can not only increase the utilization efficiency of pigment TiO 2 , but also reduce the consumption of pigment TiO 2 . The preparation of the composite pigment will enhance the comprehensive utilization of titanium resource [8][9][10].
TiO 2 -coated mineral materials are generally prepared by hydrolysis precipitation coating method [11,12]. Zhou [13] prepared the TiO 2 -coated barite composite pigments through the hydrolysis of TiOSO 4 on the barite surface. Ninness [14] and Lu [15] prepared TiO 2 -coated kaolin composite pigments and Gao prepared the anatase and rutile TiO 2 -coated mica composite pigments through the hydrolysis coating of TiCl 4 [16,17]. However, a strong acid is produced during the hydrolysis process of titanium salts. CaCO 3 is instable in acid media due to its poor acid fastness. Consequently, the CaCO 3 -based composite could not be prepared through the hydrolysis of titanium salts such as TiOSO 4 .

Preparation of the Construction Paint for Exterior Wall
The construction paints for exterior wall was prepared respectively with CaCO 3 -TiO 2 composite particles and TiO 2 particles as pigments. The raw materials were added into a high-speed mixer sequentially according to a certain proportion and then stirred to form a stable paint. In the paint, the TiO 2 or the CaCO 3 -TiO 2 composite particles were used as white pigment, they imparts opacity to the coating by bonding with the film-forming materials. The ground calcium carbonate was used as the main filler, playing a role in filling and reducing the cost of coatings. The total content of white pigment and ground calcium carbonate was 35.5 wt %. Besides, the acrylic emulsion was used as a film former, which allows the coating to be firmly adhered to the substrate to form a continuous film. Several additives including wetting agent, water, dispersant, pH regulator, defoamer, film-forming additive, leveling agent, and thickener were added to adjust the coating performance.

Structure and Property Characterization
We observed the morphology of CaCO 3 , TiO 2 , and CaCO 3 -TiO 2 composite particles by scanning electron microscope (SEM, S-3500N, HITACHI, Tokyo, Japan) under an energy dispersive spectroscope at 5.0 kV and transmission electron microscope (TEM, FEI Tecnai G2 F20, Portland, OR, USA) under an acceleration voltage of 300 kV. For morphological observation, the powder samples were dispersed in ethanol solvent and sonicated for 10 min, and then the suspensions were directly dropped to the conductive adhesive or copper net for natural drying. The phase analysis were carried out on the X-ray diffractometer (XRD, D8 ADVANCE, BrukerAXS GmbH, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5406 Å) generated at 40 kV and 15 mA and a scanning rate of 8 • /min. The Fourier-transform infrared (FT-IR) spectra were recorded on an infrared spectroscope (Spectrum 100, PerkinElmer Instruments (Shanghai) Co., Ltd., Shanghai, China) in a scanning range of 4000-400 cm −1 . The particle size was tested with centrifugal sedimentation (BT-1500, Dandong Bettersize Instrument (Liaoning) Co., Ltd., Liaoning, China). The whiteness was tested with a whiteness meter (SBDY-1, Shanghai Yuet Feng Instrument Co., Ltd., Shanghai, China). The ultraviolet (UV)-vis spectra of the prepared composite materials were obtained in the range of 200~800 nm on a TU-1901 double beam spectrophotometer (Beijing Presee General Instrument Co., Ltd. Beijing, China).

Property Tests of the CaCO 3 -TiO 2 Composite Particles and the as-Prepared Paint
The pigment properties of the CaCO 3 -TiO 2 composite particles were evaluated in terms of oil absorption, hiding power, and relative hiding power and the previous testing methods [23,24] were adopted.
Oil absorption is an important index of pigments. According to the China National Standard GB/T 5211. 15-2014 [25], the index refers to the minimum amount of varnish (linseed oil) required for completely wetting 100 g pigment.
Hiding power is another important index of pigments. It refers to the minimum amount of pigment required for completely covering a black and white checkerboard. The hiding power of a pigment can be tested according to the China National Industrial Standard HG/T 3851-2006 [26] (the Test Method of Pigment Hiding Power).
The relative hiding power (R, %) is defined as the ratio of the hiding power of the composite particles to that of pure TiO 2 pigment. The R value can be calculated as: where H T (g/m 2 ) and H CT (g/m 2 ) are respectively the hiding power values of TiO 2 and the CaCO 3 -TiO 2 composite particles.
The value of ∆R calculated by Equation (2) represents the increase in the hiding power of TiO 2 caused by the combination with CaCO 3 : where R 0 is the mass ratio of TiO 2 to the composite particles. According to the China National Standard GB/T 23981-2009 [27], the contrast ratio of the construction paint for exterior wall (C) can be tested according to the following method. A coating film with a certain thickness was firstly coated on the standard black-and-white plate and then the reflectivity values of black (R B , %) and white regions (R W , %) were tested with a reflectivity measuring instrument (C84-III, Tianjing Jinke Material Testing machine Co., Ltd., Tianjing, China). Finally, the contrast ratio of coating film can be obtained by Equation (3) [23]: The total color difference (∆E) between the paint containing CaCO 3 -TiO 2 composite particles and the paint containing TiO 2 pigment can be calculated by Equation (4): where the L T *, a T *, and b T * represent the chromaticity values of coating films containing TiO 2 particles; L CT *, a CT *, and b CT * represent the chromaticity value of coating films containing CaCO 3 -TiO 2 composite particles. All the chromaticity values were measured with a portable integrating sphere spectrophotometer (X-Rite Sp60, X-Rite (Shanghai) International Trade Co., Ltd., Shanghai, China).

Morphology of CaCO 3 -TiO 2 Composite Particles
The uniformity and completeness of TiO 2 coating on the surfaces of minerals are important influencing factors of the pigment properties of minerals-TiO 2 composite particles. Therefore, we investigated the morphology of CaCO 3 and TiO 2 raw materials, as well as CaCO 3 -TiO 2 composite particles with different mass ratios of TiO 2 . Corresponding SEM and TEM images are shown in Figure 1.

Binding Properties of CaCO3 and TiO2 Particles
3.2.1. XRD Analysis Figure 2 shows the XRD pattern of CaCO3, TiO2, and CaCO3-TiO2 composite particles with the TiO2 mass ratio of 50%. There were only calcite and rutile diffraction peaks in the XRD pattern of CaCO3-TiO2 composite particles, indicating that the composite particles were still composed of caclite In Figure 1a, the bulk CaCO 3 particles with the size of 1-3 µm were obvious and the surfaces of CaCO 3 particles were smooth, without covering. In Figure 1b, the granular TiO 2 particles with the particle size of 0.2-0.3 µm showed good dispersivity. However, after CaCO 3 and TiO 2 particles were ground together (Figure 1c-e), many fine particles were uniformly coated on the surfaces of CaCO 3 , and the smooth surface of CaCO 3 became rough. As indicated by the Energy Dispersive Spectrometer (EDS) results (Figure 1d), the particles coated on the surfaces of CaCO 3 should be TiO 2 . Apparently, the CaCO 3 -TiO 2 composite particles were characterized by the uniform TiO 2 coating on the surface of CaCO 3 . Consequently, the CaCO 3 -TiO 2 composite particles should possess the properties of TiO 2 , such as the similar pigment properties to that of TiO 2 .
The uniformity and completeness of the TiO 2 coating on the surfaces of CaCO 3 particles increased accordingly when the mass ratio of TiO 2 increased from 30% to 50% (Figure 1c-e). Particularly, when the mass ratio of TiO 2 increased to 50%, the surfaces of the CaCO 3 particles were almost completely covered by TiO 2 particles. Additionally, as shown in the TEM image (Figure 1f), the CaCO 3 -TiO 2 composite particles are composed of the particles characterized by uniform and dense TiO 2 coating on the surface of CaCO 3 . Figure 2 shows the XRD pattern of CaCO 3 , TiO 2 , and CaCO 3 -TiO 2 composite particles with the TiO 2 mass ratio of 50%. There were only calcite and rutile diffraction peaks in the XRD pattern of CaCO 3 -TiO 2 composite particles, indicating that the composite particles were still composed of caclite and rutile TiO 2 , and no new phase was produced in the preparation process of the composite particles. Meanwhile, the complete crystal phases of the CaCO 3 and TiO 2 materials remained without any changes. Therefore, it can be inferred that the binding of CaCO 3 and TiO 2 particles should occur at the interfacial region of the particles, whether the binding is of a chemical or physical nature.  Figure 2 shows the XRD pattern of CaCO3, TiO2, and CaCO3-TiO2 composite particles with the TiO2 mass ratio of 50%. There were only calcite and rutile diffraction peaks in the XRD pattern of CaCO3-TiO2 composite particles, indicating that the composite particles were still composed of caclite and rutile TiO2, and no new phase was produced in the preparation process of the composite particles. Meanwhile, the complete crystal phases of the CaCO3 and TiO2 materials remained without any changes. Therefore, it can be inferred that the binding of CaCO3 and TiO2 particles should occur at the interfacial region of the particles, whether the binding is of a chemical or physical nature.

Infrared Spectral Analysis
In addition to the coating behaviors of TiO2, including completeness and orderliness, on the surfaces of mineral particles, the binding properties and binding strength between mineral particles and TiO2 particles are also important influencing factors of the properties of the TiO2-coated composite particle pigments. In order to investigate the binding properties between TiO2 and CaCO3 particles, the infra-red (IR) spectra of TiO2, CaCO3, and CaCO3-TiO2 composite particles were tested

Infrared Spectral Analysis
In addition to the coating behaviors of TiO 2 , including completeness and orderliness, on the surfaces of mineral particles, the binding properties and binding strength between mineral particles and TiO 2 particles are also important influencing factors of the properties of the TiO 2 -coated composite particle pigments. In order to investigate the binding properties between TiO 2 and CaCO 3 particles, the infra-red (IR) spectra of TiO 2 , CaCO 3 , and CaCO 3 -TiO 2 composite particles were tested and analyzed ( Figure 3). Figure 3a displays several peaks in the range of 400~700 cm −1 , which are ascribed to the stretching vibrations of Ti-O-Ti bonds. These peaks were characteristic peaks of TiO 2 [28,29]. In Figure 3b, the absorption peaks at 880 cm −1 and 718 cm −1 were ascribed to the characteristic absorption peaks of CO 3 2− in CaCO 3 [30,31]. Meanwhile, the peak at 3420 cm −1 was ascribed to the hydroxyl groups formed by the reaction between H 2 O and the ions on CaCO 3 surface such as Ca 2+ and CO 3 2− . Several changes were observed in the IR spectrum of CaCO 3 -TiO 2 composite particles ( Figure 3c), compared to that of the raw materials. Firstly, the absorption peak of CO 3 2− in CaCO 3 appeared at 1465 cm −1 , showing a shift and broadening phenomenon compared with the absorption peak at 1440 cm −1 in the infrared spectrum of CaCO 3 . The shifted absorption peak indicated that the chemical environment of CO 3 2− changed due to the reaction with other materials, and the broadened absorption peak showed that the association degree between CaCO 3 particles increased. Secondly, compared to the characteristic peak of hydroxyl groups at 3420 cm −1 in CaCO 3 raw materials (Figure 3b), the characteristic peak of water and hydroxyl groups at 2892~3300 cm −1 in Figure 3c [32] was broadened and shifted in the direction of the low wave number. Thirdly, there is a clear absorption peak at 1030 cm −1 in Figure 3c, indicating that there might be a chemical bonding behavior related to the formation of Ti-O-Ca bond. According to the above analysis, it can be inferred that the combination of CaCO 3 and TiO 2 should be realized by the chemical interaction between hydroxyl groups on particle surfaces, so this combination should be firm. Undoubtedly, based on the uniform coating of TiO 2 on the surface of CaCO 3 , the strong combination strength between CaCO 3 and TiO 2 largely determines the good pigment properties of CaCO 3 -TiO 2 composite particles.
Materials 2018, 11, x FOR PEER REVIEW 6 of 11 and analyzed ( Figure 3). Figure 3a displays several peaks in the range of 400~700 cm −1 , which are ascribed to the stretching vibrations of Ti-O-Ti bonds. These peaks were characteristic peaks of TiO2 [28,29]. In Figure 3b, the absorption peaks at 880 cm −1 and 718 cm −1 were ascribed to the characteristic absorption peaks of CO3 2− in CaCO3 [30,31]. Meanwhile, the peak at 3420 cm −1 was ascribed to the hydroxyl groups formed by the reaction between H2O and the ions on CaCO3 surface such as Ca 2+ and CO3 2− . Several changes were observed in the IR spectrum of CaCO3-TiO2 composite particles (Figure 3c), compared to that of the raw materials. Firstly, the absorption peak of CO3 2− in CaCO3 appeared at 1465 cm −1 , showing a shift and broadening phenomenon compared with the absorption peak at 1440 cm −1 in the infrared spectrum of CaCO3. The shifted absorption peak indicated that the chemical environment of CO3 2− changed due to the reaction with other materials, and the broadened absorption peak showed that the association degree between CaCO3 particles increased. Secondly, compared to the characteristic peak of hydroxyl groups at 3420 cm −1 in CaCO3 raw materials ( Figure  3b), the characteristic peak of water and hydroxyl groups at 2892~3300 cm −1 in Figure 3c [32] was broadened and shifted in the direction of the low wave number. Thirdly, there is a clear absorption peak at 1030 cm −1 in Figure 3c, indicating that there might be a chemical bonding behavior related to the formation of Ti-O-Ca bond. According to the above analysis, it can be inferred that the combination of CaCO3 and TiO2 should be realized by the chemical interaction between hydroxyl groups on particle surfaces, so this combination should be firm. Undoubtedly, based on the uniform coating of TiO2 on the surface of CaCO3, the strong combination strength between CaCO3 and TiO2 largely determines the good pigment properties of CaCO3-TiO2 composite particles.

Mechanism Analysis
The hydration behavior of unsaturated ions on the cleavage planes of CaCO3 and TiO2 can explain the formation of surface hydroxyl groups. The calcite CaCO3, which belongs to the tripartite crystal system, is completely cleaved on the plane (10⎯11), and the unsaturated Ca 2+ and CO3 2− ions are exposed. The unsaturated Ca 2+ on CaCO3 surface will undergo the following hydration reactions: Ca 2+ + H2O→Ca(OH) + + H + Ca(OH) + + H2O→Ca(OH)2 + H + The above hydration reactions of Ca 2+ are related to the pH value. The larger the pH value is, the stronger the hydration effect is [33,34]. Therefore, the hydration reaction of CaCO3 is intense. Obviously, there should be a certain amount of hydroxyl groups on the surfaces of CaCO3 generated

Mechanism Analysis
The hydration behavior of unsaturated ions on the cleavage planes of CaCO 3 and TiO 2 can explain the formation of surface hydroxyl groups. The calcite CaCO 3 , which belongs to the tripartite crystal system, is completely cleaved on the plane (1011), and the unsaturated Ca 2+ and CO 3 2− ions are exposed. The unsaturated Ca 2+ on CaCO 3 surface will undergo the following hydration reactions: Ca(OH) + + H 2 O → Ca(OH) 2 + H + The above hydration reactions of Ca 2+ are related to the pH value. The larger the pH value is, the stronger the hydration effect is [33,34]. Therefore, the hydration reaction of CaCO 3 is intense. Obviously, there should be a certain amount of hydroxyl groups on the surfaces of CaCO 3 generated in the hydration reactions of Ca 2+ and CO 3 2− . As for TiO 2 , the hydration reactions of Ti 4+ are intense.
Hydrolysis reactions and corresponding constants [35] are provided as follows: Therefore, a large amount of hydroxyl groups are formed on the TiO 2 surface due to the intense hydrolysis of Ti 4+ . Consequently, it can be inferred that CaCO 3 and TiO 2 are combined together by the reaction of hydroxyl groups on their surfaces.
Based on the above analysis, the preparation mechanism of CaCO 3 -TiO 2 composite particles by the mechanochemical method can be summarized as follows ( Figure 4). First, through the high speed stirring of raw materials, the CaCO 3 and TiO 2 particles can be further dispersed, and then the hydration, dehydration, and other reactions between particles in aqueous media can be promoted because the slight distortion of the lattice of mineral particles and the activation of the particle surface during the grinding process enhances the binding between the surfaces [36]. Second, the high energy imported in the co-grinding process can increase the chance of the collision between CaCO 3 and TiO 2 particles, overcome the energy barrier of the repulsive interaction between the two particles, and reduce the distance between particles below the range allowing the interaction between functional groups on particle surface. Finally, the firm combination of CaCO 3 and TiO 2 particles can be achieved by the dehydration reaction among the hydroxyl groups on particle surface. Therefore, a large amount of hydroxyl groups are formed on the TiO2 surface due to the intense hydrolysis of Ti 4+ . Consequently, it can be inferred that CaCO3 and TiO2 are combined together by the reaction of hydroxyl groups on their surfaces.
Based on the above analysis, the preparation mechanism of CaCO3-TiO2 composite particles by the mechanochemical method can be summarized as follows ( Figure 4). First, through the high speed stirring of raw materials, the CaCO3 and TiO2 particles can be further dispersed, and then the hydration, dehydration, and other reactions between particles in aqueous media can be promoted because the slight distortion of the lattice of mineral particles and the activation of the particle surface during the grinding process enhances the binding between the surfaces [36]. Second, the high energy imported in the co-grinding process can increase the chance of the collision between CaCO3 and TiO2 particles, overcome the energy barrier of the repulsive interaction between the two particles, and reduce the distance between particles below the range allowing the interaction between functional groups on particle surface. Finally, the firm combination of CaCO3 and TiO2 particles can be achieved by the dehydration reaction among the hydroxyl groups on particle surface.  Figure 5 shows the pigment properties of CaCO3, TiO2 raw materials, and the CaCO3-TiO2 composite materials with different TiO2 mass ratios, including hiding power, relative hiding power (R), the increased ratio of hiding power (ΔR), oil absorption, and whiteness.

Pigment Properties
As shown in Figure 5a, CaCO3-TiO2 composite particles exhibited much better hiding properties than CaCO3. When the mass ratio of TiO2 in the CaCO3-TiO2 composite was only 30%, its hiding power reached about 23 g/m 2 , which shows much stronger hiding properties than the CaCO3 raw  Figure 5 shows the pigment properties of CaCO 3 , TiO 2 raw materials, and the CaCO 3 -TiO 2 composite materials with different TiO 2 mass ratios, including hiding power, relative hiding power (R), the increased ratio of hiding power (∆R), oil absorption, and whiteness.

Pigment Properties
As shown in Figure 5a, CaCO 3 -TiO 2 composite particles exhibited much better hiding properties than CaCO 3 . When the mass ratio of TiO 2 in the CaCO 3 -TiO 2 composite was only 30%, its hiding power reached about 23 g/m 2 , which shows much stronger hiding properties than the CaCO 3 raw material (185 g/m 2 ). When the mass ratio of TiO 2 increases to 50%, the hiding power and relative hiding power of CaCO 3 -TiO 2 composite are 17.59 g/m 2 and 82.80% (R), which was 32.80% (∆R) higher than that of TiO 2 (R 0 ). As expected, the hiding power and relative hiding power of the CaCO 3 -TiO 2 composite respectively reached 15.79 g/m 2 and 92.27% when the TiO 2 mass ratio increased to 70%, indicating that the CaCO 3 -TiO 2 composite have obtained the excellent pigment properties equivalent to that of TiO 2 pigment. Obviously, the coating of TiO 2 on the surface CaCO 3 weakened the properties of CaCO 3 and induced the composite particles to exhibit the properties of TiO 2 . The abovementioned results indicated that the TiO 2 particles in the CaCO 3 -TiO 2 composite exhibited the higher utilization efficiency compared to the pure rutile TiO 2 pigment because the coating structure improved the dispersion of titanium dioxide and the synergistic effect of calcium carbonate as a carrier. Meanwhile, the whiteness of CaCO 3 -TiO 2 composite particles was also close to that of CaCO 3 and TiO 2 , and the oil absorption of CaCO 3 -TiO 2 composite was similar or less than that of TiO 2 .
Materials 2018, 11, x FOR PEER REVIEW 8 of 11 material (185 g/m 2 ). When the mass ratio of TiO2 increases to 50%, the hiding power and relative hiding power of CaCO3-TiO2 composite are 17.59 g/m 2 and 82.80% (R), which was 32.80% (ΔR) higher than that of TiO2 (R0). As expected, the hiding power and relative hiding power of the CaCO3-TiO2 composite respectively reached 15.79 g/m 2 and 92.27% when the TiO2 mass ratio increased to 70%, indicating that the CaCO3-TiO2 composite have obtained the excellent pigment properties equivalent to that of TiO2 pigment. Obviously, the coating of TiO2 on the surface CaCO3 weakened the properties of CaCO3 and induced the composite particles to exhibit the properties of TiO2. The abovementioned results indicated that the TiO2 particles in the CaCO3-TiO2 composite exhibited the higher utilization efficiency compared to the pure rutile TiO2 pigment because the coating structure improved the dispersion of titanium dioxide and the synergistic effect of calcium carbonate as a carrier. Meanwhile, the whiteness of CaCO3-TiO2 composite particles was also close to that of CaCO3 and TiO2, and the oil absorption of CaCO3-TiO2 composite was similar or less than that of TiO2.

Optical Property of CaCO3-TiO2 Composite Particles
The UV-vis absorption spectra of CaCO3-TiO2 composite particles, CaCO3 and TiO2 raw materials are shown in Figure 6. All samples almost showed no absorption of visible light in the wavelength range of 400-800 nm, reflecting their characteristics as white inorganic material. However, the three samples are different in the absorption of ultraviolet light in the wavelength range of 200-400 nm. CaCO3 raw materials exhibited almost no light absorption, but TiO2 and CaCO3-TiO2 composite particles exhibited strong light absorption. CaCO3-TiO2 composite particles exhibited less light absorption in the wavelength range of 200-350 nm than TiO2. Unlike CaCO3, the CaCO3-TiO2 composite particles obtained the same light absorption property as that of TiO2 and the result was in agreement with the previous report [5]. The abovementioned results not only indicated that the CaCO3-TiO2 composite particles obtained excellent UV resistance stability, but also reflected the mechanism that TiO2 particles coated on the surface of CaCO3 particles endowed the composite particles with the similar properties of TiO2.

Optical Property of CaCO 3 -TiO 2 Composite Particles
The UV-vis absorption spectra of CaCO 3 -TiO 2 composite particles, CaCO 3 and TiO 2 raw materials are shown in Figure 6. All samples almost showed no absorption of visible light in the wavelength range of 400-800 nm, reflecting their characteristics as white inorganic material. However, the three samples are different in the absorption of ultraviolet light in the wavelength range of 200-400 nm. CaCO 3 raw materials exhibited almost no light absorption, but TiO 2 and CaCO 3 -TiO 2 composite particles exhibited strong light absorption. CaCO 3 -TiO 2 composite particles exhibited less light absorption in the wavelength range of 200-350 nm than TiO 2 . Unlike CaCO 3 , the CaCO 3 -TiO 2 composite particles obtained the same light absorption property as that of TiO 2 and the result was in agreement with the previous report [5]. The abovementioned results not only indicated that the CaCO 3 -TiO 2 composite particles obtained excellent UV resistance stability, but also reflected the mechanism that TiO 2 particles coated on the surface of CaCO 3 particles endowed the composite particles with the similar properties of TiO 2 .
Materials 2018, 11, x FOR PEER REVIEW 8 of 11 material (185 g/m 2 ). When the mass ratio of TiO2 increases to 50%, the hiding power and relative hiding power of CaCO3-TiO2 composite are 17.59 g/m 2 and 82.80% (R), which was 32.80% (ΔR) higher than that of TiO2 (R0). As expected, the hiding power and relative hiding power of the CaCO3-TiO2 composite respectively reached 15.79 g/m 2 and 92.27% when the TiO2 mass ratio increased to 70%, indicating that the CaCO3-TiO2 composite have obtained the excellent pigment properties equivalent to that of TiO2 pigment. Obviously, the coating of TiO2 on the surface CaCO3 weakened the properties of CaCO3 and induced the composite particles to exhibit the properties of TiO2. The abovementioned results indicated that the TiO2 particles in the CaCO3-TiO2 composite exhibited the higher utilization efficiency compared to the pure rutile TiO2 pigment because the coating structure improved the dispersion of titanium dioxide and the synergistic effect of calcium carbonate as a carrier. Meanwhile, the whiteness of CaCO3-TiO2 composite particles was also close to that of CaCO3 and TiO2, and the oil absorption of CaCO3-TiO2 composite was similar or less than that of TiO2.

Optical Property of CaCO3-TiO2 Composite Particles
The UV-vis absorption spectra of CaCO3-TiO2 composite particles, CaCO3 and TiO2 raw materials are shown in Figure 6. All samples almost showed no absorption of visible light in the wavelength range of 400-800 nm, reflecting their characteristics as white inorganic material. However, the three samples are different in the absorption of ultraviolet light in the wavelength range of 200-400 nm. CaCO3 raw materials exhibited almost no light absorption, but TiO2 and CaCO3-TiO2 composite particles exhibited strong light absorption. CaCO3-TiO2 composite particles exhibited less light absorption in the wavelength range of 200-350 nm than TiO2. Unlike CaCO3, the CaCO3-TiO2 composite particles obtained the same light absorption property as that of TiO2 and the result was in agreement with the previous report [5]. The abovementioned results not only indicated that the CaCO3-TiO2 composite particles obtained excellent UV resistance stability, but also reflected the mechanism that TiO2 particles coated on the surface of CaCO3 particles endowed the composite particles with the similar properties of TiO2.   Figure 7 shows the property of the exterior paint containing CaCO 3 -TiO 2 composite particles (R-801, the mass ratio of TiO 2 is 70%) and rutile TiO 2 (BLR-699). As shown in Figure 7a, when the addition proportions of R-801 and BLR-699 in the paint were only 5%, the contrast ratios of the obtained paints respectively reached 0.94 and 0.95, and the two values were close. Moreover, the abovementioned contrast ratios of the paint were higher than the contrast ratio (0.93) of superior products required in China National Standard of Exterior Paints (GB/T 9756-2009) [37]), indicating that R-801 and BLR-699 exhibited excellent pigment properties when they were applied in the paint. When the addition proportions of R-801 and BLR-699 in the paint increased to 10% or more, the contrast ratio of the paint was increased slightly. The contrast ratio of the paint containing R-801 was always equivalent to that of the paint containing BLR-699, indicating that the application of CaCO 3 -TiO 2 composite particles as the pigment in the paint for exterior wall could achieve the equivalent covering effect to the paint containing the same proportion of TiO 2 . When the addition proportion of pigments in the paint is 5% or 10%, the whiteness of the paint containing R-801 was always lower than that of the paint containing the same proportion of BLR-699 ( Figure 7b). However, when the addition proportion of pigments increased to 15% and 20%, the whiteness of the paint containing R-801 was close to that of the paint containing BLR-699 and the color difference value (∆E) was reduced to about 0.7. As the addition proportion of pigment in the paint for exterior wall is generally high, the comprehensive performance of the paint containing R-801 is equivalent to that of the paint containing BLR-699. As shown in the SEM images of the dry paints respectively containing R-801 and BLR-699 (Figure 8), the two samples all present a continuous form composed of solid particles, such as the polymer formed after emulsion evaporation, binder, pigments, and fillers, indicating that both the R-801and BLR-699 have good compatibility with emulsion.  Figure 7 shows the property of the exterior paint containing CaCO3-TiO2 composite particles (R-801, the mass ratio of TiO2 is 70%) and rutile TiO2 (BLR-699). As shown in Figure 7a, when the addition proportions of R-801 and BLR-699 in the paint were only 5%, the contrast ratios of the obtained paints respectively reached 0.94 and 0.95, and the two values were close. Moreover, the abovementioned contrast ratios of the paint were higher than the contrast ratio (0.93) of superior products required in China National Standard of Exterior Paints (GB/T 9756-2009) [37]), indicating that R-801 and BLR-699 exhibited excellent pigment properties when they were applied in the paint. When the addition proportions of R-801 and BLR-699 in the paint increased to 10% or more, the contrast ratio of the paint was increased slightly. The contrast ratio of the paint containing R-801 was always equivalent to that of the paint containing BLR-699, indicating that the application of CaCO3-TiO2 composite particles as the pigment in the paint for exterior wall could achieve the equivalent covering effect to the paint containing the same proportion of TiO2. When the addition proportion of pigments in the paint is 5% or 10%, the whiteness of the paint containing R-801 was always lower than that of the paint containing the same proportion of BLR-699 ( Figure 7b). However, when the addition proportion of pigments increased to 15% and 20%, the whiteness of the paint containing R-801 was close to that of the paint containing BLR-699 and the color difference value (ΔE) was reduced to about 0.7. As the addition proportion of pigment in the paint for exterior wall is generally high, the comprehensive performance of the paint containing R-801 is equivalent to that of the paint containing BLR-699. As shown in the SEM images of the dry paints respectively containing R-801 and BLR-699 (Figure 8), the two samples all present a continuous form composed of solid particles, such as the polymer formed after emulsion evaporation, binder, pigments, and fillers, indicating that both the R-801and BLR-699 have good compatibility with emulsion.

Conclusions
CaCO3-TiO2 composite particles were prepared with CaCO3 and TiO2 in stirred mill according to the wet grinding method. The composite particles are characterized by the uniform TiO2 coating  Figure 7 shows the property of the exterior paint containing CaCO3-TiO2 composite particles (R-801, the mass ratio of TiO2 is 70%) and rutile TiO2 (BLR-699). As shown in Figure 7a, when the addition proportions of R-801 and BLR-699 in the paint were only 5%, the contrast ratios of the obtained paints respectively reached 0.94 and 0.95, and the two values were close. Moreover, the abovementioned contrast ratios of the paint were higher than the contrast ratio (0.93) of superior products required in China National Standard of Exterior Paints (GB/T 9756-2009) [37]), indicating that R-801 and BLR-699 exhibited excellent pigment properties when they were applied in the paint. When the addition proportions of R-801 and BLR-699 in the paint increased to 10% or more, the contrast ratio of the paint was increased slightly. The contrast ratio of the paint containing R-801 was always equivalent to that of the paint containing BLR-699, indicating that the application of CaCO3-TiO2 composite particles as the pigment in the paint for exterior wall could achieve the equivalent covering effect to the paint containing the same proportion of TiO2. When the addition proportion of pigments in the paint is 5% or 10%, the whiteness of the paint containing R-801 was always lower than that of the paint containing the same proportion of BLR-699 ( Figure 7b). However, when the addition proportion of pigments increased to 15% and 20%, the whiteness of the paint containing R-801 was close to that of the paint containing BLR-699 and the color difference value (ΔE) was reduced to about 0.7. As the addition proportion of pigment in the paint for exterior wall is generally high, the comprehensive performance of the paint containing R-801 is equivalent to that of the paint containing BLR-699. As shown in the SEM images of the dry paints respectively containing R-801 and BLR-699 (Figure 8), the two samples all present a continuous form composed of solid particles, such as the polymer formed after emulsion evaporation, binder, pigments, and fillers, indicating that both the R-801and BLR-699 have good compatibility with emulsion.

Conclusions
CaCO3-TiO2 composite particles were prepared with CaCO3 and TiO2 in stirred mill according to the wet grinding method. The composite particles are characterized by the uniform TiO2 coating

Conclusions
CaCO 3 -TiO 2 composite particles were prepared with CaCO 3 and TiO 2 in stirred mill according to the wet grinding method. The composite particles are characterized by the uniform TiO 2 coating on the surface of CaCO 3 . The CaCO 3 and TiO 2 particles were firmly combined together through the dehydration reaction of hydroxyl groups on their surfaces. CaCO 3 -TiO 2 composite particles exhibited similar pigment properties and light absorption property to TiO 2 . Moreover, the CaCO 3 -TiO 2 composite particles also showed excellent application performance. The contrast ratio of the exterior construction paint containing CaCO 3 -TiO 2 composite particles as the pigment was similar to that of paint containing the same proportion of TiO 2 , and was higher than the contrast ratio (0.93) of superior products required in the China National Standard of Exterior Paints (GB/T 9756-2009 [36]).