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Environmental Technology

ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20

Gypsum blocks produced from TiO2 production by-products

Yihe Zhang, Fan Wang, Hongwei Huang, Yuxi Guo, Baoying Li, Yangyang Liu& Paul K. Chu

To cite this article: Yihe Zhang, Fan Wang, Hongwei Huang, Yuxi Guo, Baoying Li, YangyangLiu & Paul K. Chu (2016) Gypsum blocks produced from TiO2 production by-products,Environmental Technology, 37:9, 1094-1100, DOI: 10.1080/09593330.2015.1102329

To link to this article: http://dx.doi.org/10.1080/09593330.2015.1102329

Accepted author version posted online: 23Oct 2015.Published online: 29 Oct 2015.

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Gypsum blocks produced from TiO2 production by-productsYihe Zhanga,b, Fan Wanga, Hongwei Huanga, Yuxi Guoa, Baoying Lia, Yangyang Liua and Paul K. Chub

aBeijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School ofMaterials Science and Technology, China University of Geosciences, Beijing 100083, People’s Republic of China; bDepartment of Physics &Materials Science, City University of Hong Kong, Kowloon, People’s Republic of China

ABSTRACTCalcined titanium gypsum was investigated by the ways of XRD (powder X-ray diffraction), XRF (X-ray fluorescence) and TG-DTA (thermogravimetric–differential thermal analyses). It was employedas raw material for making lightweight materials. The influence of cement, amount of water/solid(W/S) ratio, water-reducing agent, citric acid content and the hydration age on the gypsumblocks was investigated. The results showed that the optimum W/S ratio, cement content andwater-reducing agent are 0.9, 10% and 2 wt% for the calcined gypsum from titanium gypsum,respectively. The 5.96 MPa was attained after 7 days of ageing. It was also found that the citricacid is inappropriate to be used in the production of gypsum blocks from TiO2 production by-products.

ARTICLE HISTORYReceived 20 November 2014Accepted 27 September 2015

KEYWORDSTitanium gypsum; industrialwastes; recycling; buildingmaterials; gypsum blocks

1. Introduction

The use of waste materials and industrial by-productsin the manufacture of Portland and blended cements,and ordinary and lightweight blocks undoubtedly con-tributes to the saving of natural resources and protectionof environment.[1–10] Titanium gypsum is a by-productfrom the production of titanium dioxide (TiO2) pigment.Because of the red colour imparted by the iron fromilmenite (FeTiO3) ores, titanium gypsum is often calledred gypsum.[11] Titanium is extracted from ilmenite byways of concentrated sulphuric acid digestion; theexcess acid (H2SO4) and iron sulphate (FeSO4) are neu-tralized with lime or limestone, thus generating titaniumgypsum, usually a by-product mainly composed ofgypsum and iron hydroxide according to the followingreactions [12]:

Ca(OH)2 + H2SO4 � CaSO4 · 2H2O, (1)FeSO4+Ca(OH)2 � Fe(OH)2+CaSO4. (2)

The slurry is separated by filtration from water which ispartially recycled in the TiO2 process. Titanium gypsumis usually landfilled or left or stacked nearby the titaniumdioxide plant.

Similar to phosphogypsum,[13,14] flue gas desulphur-ization gypsum[15–17] and other solid wastes generatedby sulphuric acid neutralization, titanium gypsum con-sists of calcium sulphate and impurities including fluor-ides, silica, organic matters and alkalis. The presence ofimpurities restricts the use of this by-product in building

materials field. Flue gas desulphurization of gypsum andphosphogypsum products is currently used in thecement and gypsum industries as set retarders forcement for making gypsum plaster and bricks.[18–25]Titanium gypsum is seldom used in the cement industryas a set retarder. There are few studies reporting itsutilization for the production of gypsum bricks.

The waste disposal minimization could benefit bothhuman health and environment in several industrial pro-cesses. In addition to the generation of the main product,the appropriate treatment of a fraction of the waste gen-erated could obtain some co-products with economicvalue and broad application.[26,27] Obviously, theenvironmental and health impact of these co-productsshould comply with existing regulations at the nationaland international level.

The main potential utilization of titanium gypsum isto replace natural gypsum (CaSO4·2H2O) for makingbuilding materials. Calcined gypsum can be successfullyproduced from titanium gypsum by dehydration at110–150°C. It is formed according to following equation(Equation (3)):

CaSO4 · 2H2O � CaSO4 · 1/2H2O+ 11/2H2O. (3)When the hydration reaction occurs, gypsum is formedagain.[1] This compound is relatively soluble at 20°C,about 0.256 g in 100 g of water. Therefore, it cannot beutilized directly on the exterior of buildings becauserain water could negatively affect its mechanical proper-ties.[28,29]

© 2015 Taylor & Francis

CONTACT Yihe Zhang zyh@cugb.edu.cn

ENVIRONMENTAL TECHNOLOGY, 2016VOL. 37, NO. 9, 1094–1100http://dx.doi.org/10.1080/09593330.2015.1102329

Gypsum blocks are environmentally friendly andenergy-saving materials due to their good properties,small dimensional change and easy application, andare very popular in buildings.[30,31] Natural gypsumhas long been used as raw materials for gypsumblocks. The development of copper smelting processesresulted in a growing number of titanium gypsum-based waste stackings. The manufacture of gypsumblocks from titanium gypsum can consume greatamounts of it. Not only can the stacking field of titaniumgypsum in the copper smelting plant be reduced andsecondary pollution avoided, but the exploitation ofnatural gypsum can also be decreased and the miningcost saved.

In this study, series of the gypsum blocks were pre-pared from calcined titanium gypsum. They were sub-jected to compressive strength and blending strengthtests. The obtained results provide some positive sugges-tions as well as an effective way to recycle the neutraldregs and alleviate their impact on the environmentand the economy.

2. Experiment

Titanium gypsum was supplied by the titanium dioxideplant Anhui Annada Titanium Industry Co., Ltd, in Ton-gling Anhui (south-eastern part of China). In order toobtain calcined gypsum, titanium gypsum was heatedin an electric oven at 170°C for 2 h. Afterwards, it wasstored in a desiccator at room temperature to avoidany contamination.

Commercial Portland cement (OPC), Zuanpai 32.5R(Hebei Yanxin Building Materials Co., Ltd.), naphtha-lene sulphonate-formaldehyde condensate (FDN),water-reducing agents (Muhu Concrete AdmixtureCo., Ltd) and Citric acid (Beijing Chemical Works)were used.

Powder X-ray diffraction (XRD) analysis was carriedout using a Philips PW 1050 powder diffractometer(Cu-Kα diffraction filtered by graphite and excited atintensity of 100 mA and tension of 40 kV), with the scan-ning rate 8°/min and the scanning range 5°–80° at roomtemperature.

The chemical composition of titanium gypsum wasinvestigated by ways of X-ray fluorescence (XRF) analysis

(Thermo electron corporation) using Au ARL ADVANT XP+ system equipped with Rh radiation operating at 50 kV.

Thermogravimetric–differential thermal analyses (TG–DTA) of the precursors were performed in air, from 20°Cto 600°C, at a heating rate of 10°C/min.

Titanium gypsum and admixture samples werepassed through a 4.75-mm sieve under dry conditionsand mixed with regular tap water. They were cast intothree-gang moulds and compacted by using a vibrationtable for 60 seconds. After demoulding, they were curedfor 7 days at a relative humidity of 50% and 20°C. Theapparent density of the materials was determined bymeasuring the weight and the dimensions of the pro-duced materials. Porosity was evaluated according toEquation (4), where P′ is the porosity (%), da is the appar-ent density and d is the bulk density measured by Hepycnometer.

P′(%) = 1− dad

( )× 100. (4)

The compressive strength of the gypsum blocks wasdetermined according to Chinese standards (GB/T9776–2008: calcined gypsum) on samples 40 × 40 ×40 mm3. Mechanical tests were employed at a loadingrate of 5 mm/min, and the results were the average ofthree specimens.

3. Results and discussions

The application of the calcined gypsum for buildingmaterials is limited by the presence of hemihydrategypsum and impurities which have a great effect onthe performance of the products. XRF and XRD analyseswere carried out in order to determine the chemical andmineralogical components of titanium gypsum (Table 1and Figure 1(a)). Table 1 shows that the chemical compo-sition of titanium gypsum detected by XRF analysis. Tita-nium gypsum is mainly SO3 (41.83%), followed by CaO(34.31%) and Fe2O3 (7.58%). As expected, CaSO4·2H2Ois the main phase of titanium gypsum, as shown inFigure 1(a), and Figure 2 shows TG-DTA of titaniumgypsum which loses weight at the range of 140–170°Cas highlighted by two endothermic peaks. They wereclose to each other, so it is difficult to discriminate thepoint where one reaction ends and the other starts on

Table 1. Chemical composition of titanium gypsum test, mass%.Compound SO3 CaO Fe2O3 SiO2 Al2O3 TiO2 MgO

m/m (%) 41.830 34.310 7.580 2.910 2.250 1.230 0.842Compound Na2O MnO V2O5 Cl K2O P2O5 SrOm/m (%) 0.434 0.251 0.081 0.060 0.053 0.044 0.032Compound ZnO Cr2O3 ZrO2 MoO3 Co3O4 NiO Othersm/m (%) 0.029 0.029 0.019 0.005 0.002 0.001 1.009

ENVIRONMENTAL TECHNOLOGY 1095

the TG curve. In the DTA curves, the main reaction isshown as translating to a hemihydrate compound.According to the TG curves, the first weight loss corre-sponds to the removal of 3/2 water moles, and thesecond to that of 1/2 water mole. Therefore, gypsumdihydrate transformed into β-calcined gypsum. The tita-nium gypsum was crushed and dried at 170°C. Becausethe titanium gypsum waste contains some impuritiesapart from titanium gypsum, the impurities are alsoinvolved in the reaction when being heated. The exactcomponents of the impurities are unknown, and theamount of impurities is small in the waste. Therefore,we cannot explain the specific reasons for the increaseat low temperature (temperature lower than 50°C) orthe decrease at high temperature (temperature higherthan 400°C) in weight. However, the presence of theseimpurities does not affect the use of titanium gypsumwaste as raw materials to prepare gypsum blocks. From

the XRD results (Figure 1(b)), after drying and dehy-dration, the main phases change to β-calcined gypsumand this calcined gypsum will be used after experiments.

Calcined gypsum firstly reacted with water to formgypsum dihydrate again (Equation (5)); at the sametime, the crystal of gypsum grows fast in C-axis directionand easily into needle-like crystals. The intercross pilingof the crystals enhanced the strength of the gypsumblocks. According to Equation (2), water demand forthe hydration of gypsum is 19% by mass. However, thisvalue is higher due to both water adsorbed by gypsumand that used for the mixing process.

CaSO4·1/2H2O+ 11/2H2O � CaSO4·2H2O. (5)

As shown in Figure 3, when no cement was used, thecompressive strength was 2.88 MPa. It was less thanthe minimum compressive strength values establishedby the Chinese National Standards, for the presence ofimpurities do not allow the gypsum blocks to reach theminimum compressive strength values. It is thus requiredto add certain amount of cement in order to improve themechanical strength of gypsum blocks. The trend ofcompressive strength and density values with cementcontent is shown in Figure 3. By varying the amount ofcement from 0% to 20%, the compression strength ofgypsum blocks increased significantly (from 2.88 to5.23 MPa). Also, the density values increased from 986to 1237 kg/m3. As the Chinese National Standardsrequired density value of a gypsum block is below1000 kg/m3, cement content equal to 10% was estab-lished as the optimum additive amount; thus, compres-sive strength equal to 4.32 MPa and density valueequal to 953 kg/m3 were obtained.

The crystalline phases formed in the samples withdifferent cement content are illustrated in Figure 4. Ithas been found that all the samples consisted of a dihy-drate calcium sulphate characteristic peak. However, the

Figure 3. Influence of cement content on both compressivestrength and density of gypsum blocks.

Figure 1. XRD patterns of (a) titanium gypsum; (b) calcinedgypsum; (c) CaSO4·2H2O in gypsum blocks.

Figure 2. DTA-TG of titanium gypsum.

1096 Y. ZHANG ET AL.

relative intensity gradually decreases with the increase incement content, indicating the presence of more cementcontent in the samples. The cement pushed CaSO4·2H2Ocrystals to form a dense structure, hence increasing thecompressive strength.

Moreover, water content has a great influence on thestrength of gypsum blocks, and the water/solid (W/S)ratio is very important to study the preparation ofgypsum blocks. The compressive strength and densityof the gypsum element with the W/S ratio are shownin Figure 5. It can be observed that the density of thegypsum blocks decreases with the increase in the W/Sratio, but the compressive strength would descendwith more water. This is mainly due to the formation ofa porous structure resulting from water evaporation.The porosity of the samples is shown in Figure 6.These results show that an increase in the W/S ratioincreased the porosity. The excess moisture willevaporate after moulding, thereby creating small holes;as a result the porosity of the samples increases. On

the other hand, if the W/S ratio is too low, the rawmaterials exhibit higher viscosity value and hencereduced strength. Therefore, the best results in termsof compressive strength were obtained from a W/Sratio equal to 0.9.

The calcined titanium gypsum shows higher waterabsorption than calcined natural gypsum, thus requiringhigher W/S ratio when gypsum blocks are prepared. Thisaspect showed compressive strength values whichwould decrease with more water. Thus, it is necessaryto add a water-reducing agent in order to reduce thewater demand and increase the compressive strengthof gypsum blocks. FDN is a type of β-naphthalene sul-phonic acid-based superplasticizer usually used as awater-reducing agent in gypsum, cement and otherbinding materials. The change in the W/S ratio and com-pressive strength with the FDN content is displayed inFigure 7. When FDN is added in amounts lower than 2wt%, its effect is more evident and the density of theblocks significantly increased (from 953 to 1102 kg/m3).When the FDN addition is higher than 9%, the effect is

Figure 5. Influence W/S ratio on both compressive strength anddensity of gypsum blocks.

Figure 4. XRD patterns for gypsum block with different amountsof cement.

Figure 6. Influence W/S ratio on porosity of gypsum blocks.

Figure 7. Influence of FDN content on compressive strength anddensity of gypsum blocks.

ENVIRONMENTAL TECHNOLOGY 1097

not negligible, and the density change is also very small.Moreover, with the addition of FDN, the compressivestrength of gypsum samples will increase. The highestcompressive strength (5.96 MPa) is reached when theFDN content is equal to 2 wt% by mass. Further increasein FDN would reduce the compressive strength ofgypsum blocks. So the optimum FDN content is 2 wt%,when the compressive strength value is equal to 5.96MPa and the density is 981 kg/m3.

Figure 8 shows the SEM images for the gypsum blockmicro-structure from the sample without any admixtureaddition and the sample containing 10% cement and2% FDN by mass. From the figures we can see that thesemi-water gypsum transforms into crystals of dihydrouscalcium sulphate through hydration reaction. A densestructure is formed as the crystals ‘pack’ together,which is the source of strength of the blocks. By compar-ing the two figures, we can see that in the former case,the structure is relatively loose, while in the latter case,the cement and FDN formed gelatinous and rod-likematerials. These materials formed besides dihydrouscalcium sulphate in the hydration process and they

intersect and overlap each other, thus increasing thecompressive strength.

Sometimes it is necessary to add a retarder in theprocess of preparing gypsum blocks to prolong the soli-dification time of calcined gypsum and make the processeasier. Citric acid is a common retarder for gypsum. Theeffects of the citric acid content on the compressivestrength and density of the gypsum blocks are illustratedin Figure 9. The results indicated that even the additionof a small amount of citric acid will have a significantimpact on the compressive strength of gypsum blocks.When 0.5% by mass of citric acid is added, the compres-sive strength of gypsum elements is only 0.51 MPa. Citricacid can be a proper retarder if only hemihydrate wasused to prepare gypsum blocks. The experiment resultsshowed that citric acid is inappropriate to be used toprepare gypsum blocks by using TiO2 production by-pro-ducts. Maybe it is the impurities in the by-products thathave an influence on the use of the retarder. We didnot also find another alternative as a retarder to

Figure 8. SEM image of (a) gypsum blocks without cement and FDN, (b) gypsum blocks containing 10% cement and 2% FDN.

Figure 9. Influence of citric acid content on compressivestrength and density of gypsum blocks.

Figure 10. Compressive strength values of gypsum blocks withcuring periods.

1098 Y. ZHANG ET AL.

improve the mechanical properties of the preparedsamples. The experiment of adding a retarder demon-strated that it is unnecessary to use a retarder forthe preparation of titanium gypsum when using TiO2

production by-products.Compressive strength measured in the previous

experiments was the strength after 7 days of formingblocks. But the strength of gypsum blocks will developin the hydration process of calcined gypsum. Compres-sive strength of gypsum blocks at different ages ofcuring is depicted in Figure 10. The results show thatthe compressive strength at 2 h and 1 day is very low(only 0.7 MPa and 1.18 MPa, respectively). After 3 daysof curing, the compressive strength rises to 2.48 MPa,and while at 7 days it rises up to 5.96 MPa. After 7days, the increase of compressive strength is almostnegligible.

4. Conclusion

Gypsum blocks, lightweight gypsum-based buildingmaterials, in this study were prepared by using calcinedgypsum from Neutralization Sludge of Copper Smelting.The influence of cement amount, W/S ratio, water-redu-cing agent, citric acid content and hydration age on thegypsum blocks was evaluated. The results showed thatthe best W/S ratio, cement content and water-reducingagent are 0.9, 10% and 2 wt%, respectively. Themaximum compressive strength of gypsum blocks was5.96 MPa after 7 days of curing. It has also been foundthat citric acid is inappropriate to be used for theproduction of gypsum blocks. The outcomes of ourexperiments are positive, suggesting an effective wayto recycle neutral dregs and find extensive applicationsin manufacturing building components and materials.The utilization of titanium gypsum in the constructionmaterials industry could not only provide a low-costmaterial, but also contribute to lessening the environ-mental impact.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This research was jointly supported by the project of ChinaGeological Survey [No. 1212011120309], National High Tech-nology Research and Development Programme [863 Pro-gramme 2012AA06A109] of China, the Fundamental ResearchFunds for the Central Universities [2010ZD08], the special co-construction project of Beijing city education committee andDoctoral Program Foundation of Institution of higher educationof China [2-2-08-07].

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