Preparation and characterization of titanium gypsum arti ﬁ cial aggregate

: In this study, solid waste titanium gypsum (TG) was used as raw material to design the basic mix ratio of aggregate, and TG arti ﬁ cial aggregate (TGA) was prepared based on alkali-activated cold bonding technology. The e ﬀ ects of di ﬀ erent additives (slag, silica fume, and ﬂ y ash) on the properties of TGA were preliminarily investigated by using NaOH as activator in laboratory test, and the additives of TG aggregate were determined. Furthermore, the aggregate mix ratio was designed based on the additive, and the physical and mechanical properties, X-ray di ﬀ raction analysis, scanning electron microscopy analysis, and dry – wet cycle test of arti ﬁ cial aggregate were carried out. The results show that the arti ﬁ cial aggregate prepared by the same process and the aggregate prepared by silica fume as an additive has a high balling rate; it is technically feasible to use TG as the main raw material, silica fume as an additive, and NaOH as an activator to select a suitable mix ratio to prepare arti-ﬁ cial aggregates. The microscopic test results reveal the internal products and structural degradation process of TG aggregate.


Introduction
At present, due to the rapid urbanization and industrialization of the world, the accumulation of industrial waste has led to a series of problems, such as environmental pollution, land occupation, deforestation, and global warming.It is urgent to determine an effective and environmentally friendly method to solve this problem [1][2][3][4].In the past few years, the use of industrial waste to alleviate civil engineering problems has attracted great interest in sustainable development [5,6].This approach can provide cost-effective, environmentally friendly, and sustainable solutions for construction and industrial waste management [7,8].In the world, there are three main types of solid waste gypsum produced by the rapidly developing industrial production: titanium gypsum (TG), phosphor gypsum, and desulphurized gypsum.TG is a kind of weak alkaline waste residue, which is added with lime by sulfuric acid method to neutralize a large amount of acidic wastewater [9,10].The main component of TG is CaSO 4 •2H 2 O. Due to the high free water content, high impurity content, and poor mechanical properties, it is difficult to recycle.At present, the annual emissions of TG in China alone have reached 300,000 tons.These wastes are usually treated in the form of stacking, which not only takes up a large area of land resources but also causes groundwater and air pollution during weathering.
Guo et al. [11] used fly ash as the main raw material and separately mixed metallurgical sludge and municipal solid waste incineration fly ash to prepare heavy metalcontaining solid waste polymers with good acid and alkali erosion resistance.Xu et al. [12] studied the effect of silica fume on the macro and micro properties of cold-bonded unburned fly ash ceramsite and found that the increase of silica fume content improved the compressive strength and compactness of unburned ceramsite.Ceramsite is a kind of lightweight aggregate produced by using waste resource diameter technology.Gao et al. [13] studied the influence of solid-waste interaction on the strength of modified fly ash geopolymer by taking the content of carbide slag, desulfurization gypsum, and steel slag as the influencing factors and optimized the mix ratio design to improve the early strength of modified fly ash geopolymer.Shi [14] explored the process of wet extrusion granulation of dredged sediment to prepare cold-bonded unburned aggregate.The optimum process parameters were 35-40% water content of sediment, 3 mm extrusion diameter, and 30 min molding time.Hao et al. [15] prepared ceramsite by using steel slag and mineral powder as the main raw materials and adding water glass as an activator through the nonfired process, changing the proportion of steel slag and mineral powder, and designing multiple sets of experiments to study its effect on the performance of non-fired ceramsite.Xu et al. [16] proposed a simple physical modification to improve the performance of TG in the stability of expansive soil.The engineering properties and mechanism of stabilized soil are studied, and it is found that the stabilized soil with 25% TG admixture can meet the requirements of Chinese standard for subgrade below grade two.At present, in the field of building materials, it is mainly to modify TG or other solid wastes to prepare high-content TG cementing materials and explore the application [17][18][19][20].
Although the resource application of TG is varied, there is no research on the resource utilization of TG in the form of making aggregate [21], which may be due to the easy occurrence of alkali aggregate reaction in the process of making artificial aggregate by using solid waste, which is one of the important factors affecting the quality of aggregate [22][23][24].TG artificial aggregate (TGA) was prepared by alkali-activated cold bond granulation using TG as precursor and NaOH as activator.TGA test schemes with different additives (fly ash, slag powder, and silica fume) and different TG contents were designed.The physical properties, density, water absorption, and reaction products were characterized and compared to explore its applicability in the construction field, alleviate environmental pollution problems, and the situation of resource shortage in the construction industry.

Experiment design 2.1 Raw material
The TG raw material used in the test is from a chemical plant in Henan Province, and the scanning electron microscope (SEM)/energy dispersive spectrometer (EDS) image is shown in Figure 1.S95 ore powder from Henan Yuanheng Environmental Protection Engineering Co., Ltd.; the grade I ash of Yulian Power Plant is selected; the high quality and high content silica fume from Henan Platinum New Material Co., Ltd. was used.The chemical composition of each raw material is shown in Table 1.Henan Tianrui P•O 42.5 ordinary Portland cement was used.
The activator uses industrial grade sodium hydroxide (NaOH), white flake, 99% content.The water reducer used is purchased from Shanxi Huangteng Chemical Co., Ltd., which is a HT-HPC polycarboxylate superplasticizer.When the dosage is 1%, the water reduction rate is 15%.

Aggregate mix ratio design and preparation method
To determine the basic composition of aggregate, reference is made to relevant literature [17,18].The ratio of fixed TG, cement, and mineral additive excitation is 7:3, that is, (TG + cement)/(mineral admixtures) = 7:3, and the ratio of TG to cement is fixed at 5:

TGA preparation method
Raw material pretreatment: Figure 3 shows the reddishbrown solid original TG of uneven size.The large block TG is broken into small and uniform pieces, which are placed in the oven set at 50℃, dried at low temperature for 24 h, and then placed in the ball mill for grinding.Mixing: The treated TG and cement, additives (silica fume, fly ash, S95 mineral powder), and admixtures (activators, water reducing agents) are mixed according to the set mix ratio and then aged for use.The aging time is 1 h.
Granulation: the dry material is mixed with a certain proportion of water to obtain the mix in a plastic state, and then, the mix is pressed into sheets through the cake press station of the granulation equipment, and the strip is made through the strip making station.Finally, the prepared strip is placed horizontally on the pill making station to obtain granular or short rod-like wool.
Ball forming: the wool is placed in the pelletizer into a ball cylinder, and under the action of centrifugation, spherical particles are formed, that is, artificial aggregate.
Maintenance: Natural maintenance method is adopted.The formed aggregate is put into a plastic container and sprayed with water for maintenance every 24 h until the corresponding age is reached.During the period, the surface of the aggregate should be kept wet, and there is no water in the plastic container, and the curing temperature is room temperature 25℃.

Experimental method
According to GB/T 174312-2010 'Lightweight aggregate and test method-Part 2', the industrial TGA cylinder compressive

Experimental results and discussion
According to the previous section, the aggregate properties of the prepared TGA were characterized and the basic components of the TGA were determined.According to the results obtained, the physical and mechanical properties and micro phase analysis of TGA with different TG content are recorded in the following chapters.

Determination of basic components of TGA
Figure 4 is the compressive strength change curve of TGbased paste samples cured for 7 and 28 days.When silica fume (L3) is used as the admixture of TG-based composite cementitious material, the compressive strength of the sample at 7 and 28 days ages is the largest.As an admixture, fly ash (L2) has the lowest compressive strength at each age, because the hydration activity of fly ash is low, and the hydration products of cementitious materials are less, which affects the compactness of the specimen, resulting in low strength.When S95 ore powder (L1) is used as admixture, the strength is slightly lower than that of silica fume, but the strength growth rate from 7 to 28 days is higher than that of silica fume, which is about 43.6%.
To finally determine the admixture of TGA, it is also necessary to observe the aggregate formation.The admixture is prepared according to the ratio of Table 3, and the aggregate morphology is shown in Figure 5.The aggregate prepared by S95 ore powder (L1) as admixture can be balled, but the particles are soft; the aggregate prepared by fly ash (L2) as an admixture is not easy to form a ball; the aggregate prepared by silica fume (L3) as admixture has high balling rate and regular shape.Therefore, silica fume was selected as an admixture to prepare TGA.
According to relevant research, the amount of solid waste is generally more than 50%.To increase the utilization of TG resources, it is gradually increased on the basis of 50% TG content.At the same time, considering the high cost of cement, the amount of cement is reduced while increasing the TG and the total amount is kept unchanged, and silica fume is added.The design of artificial aggregate test with such as Table 3, the preparation of aggregate and aggregate related tests, to find the best performance of TGA.

Packing density and water absorption
Section 3.1 determines that the additive of TGA is silica fume.The statistical results of TGA particle shape coefficient and bulk density prepared according to Table 3 are  shown in Table 4.It can be seen that the TGA average particle size coefficient is 1.0-1.2,which meets the requirements of GB/T 174312-2010 in Light Aggregates and their Test methods-Part 1.The aggregate after molding is approximately spherical, and the bulk density is between 700 and 800 kg/m 3 , which belongs to the 800-grade aggregate.Figure 6 is the relationship between the water absorption of TGA 1 h and the cementitious material.In each group, three equal amounts of aggregate were placed in the container, and the average value was taken as the final water absorption rate of the aggregate.With the decrease of the total amount of cementitious materials, the water absorption of aggregate showed an upward trend.When L3 group (cement:silica fume = 20%:30%), the water absorption of aggregate was the lowest at 1 h, which was 24.40%.The maximum water absorption rate of aggregate in L10 group is 31.65%.The high water absorption rate of aggregate may be due to the high water absorption rate of TG itself, which needs to be further studied and solved.When the total amount of cementitious materials is 50%, that is, L3 group (cement:silica fume = 20%:30%), the aggregate cylinder compressive strength is the highest, which is 11.7 MPa.According to the tests of L4, L5, L6 and L7, L8, L9 groups, when the total amount of cementitious materials is constant, the compressive strength of aggregate cylinder decreases with the decrease of cement content, indicating that the decisive factor affecting the compressive strength of aggregate cylinder is cement.In addition, when the total amount of cementitious material is more than 20%, the compressive strength of the aggregate tube is more than 6 MPa, which belongs to high-strength aggregate according to the provisions of GB/T 174312-2010 'Lightweight aggregate and its test methods-Part 1'.

Phase analysis
Samples from group L3 to L10 at the age of 14 days were selected to analyze the composition of hydration products by X-ray diffractometer (XRD).From the analysis of the  characteristic diffraction peaks in Figure 7, it can be seen that the characteristic peaks of ettringite (AFt), CaSO  (1) (2) (3) The content of TG increased with the decrease of cementitious materials.From L3 sample to L10 sample, the diffraction peak of CaSO 4 •2H 2 O increased, and the content of CaSO 4 •2H 2 O which did not participate in the reaction increased.When the total amount of cementitious materials is constant, the amount of cement is reduced, the amount of silica fume is the diffraction characteristic peak of the hydration product AFt is weakened, the diffraction peak of C-S-H is weakened, and the diffraction characteristic peak of the alkali-silica gel is enhanced.Ca(OH) 2 can stimulate the activity of TG, and a small amount of active SiO 2 and Al 2 O 3 in TG react with Ca(OH) 2 to form C-(A)-S-H, which is similar to the C-(A)-S-H produced by cement hydration.These hydration products increase the strength of aggregates as part of cementitious materials.In addition, the amorphous SiO 2 pozzolanic activity in silica fume is higher, which can react with the cement hydration product Ca(OH) 2 to form C-S-H gel [19,20].When the TG content is large, the dissolved CaSO 4 •2H 2 O will continue to react with C-S-H to form AFt. With the decrease of cement content, the hydration products decrease, and too much active silicon reacts with NaOH under alkaline conditions to form alkali-silica gel, so the characteristic peaks are gradually enhanced in the XRD pattern.
Figure 8 is the SEM image of L3-L10 groups of samples at 14 days age, and the causes of TGA performance change are analyzed by analyzing the generation of hydration products and the microstructure of aggregates.With the increase of TG content, no matter how to change the content of cement and silica fume, the main component of TG, CaSO 4 •2H 2 O, has not been fully utilized.Combined with the relevant alkali-silica gel literature [22], the L3 group can be observed that the alkali-silica gel and ettringite intersect and closely connect with each other.The gelling system is excited by NaOH and CaSO 4 •2H 2 O dissolves to provide Ca 2+ , Na + , etc., which react with active silicon under alkaline conditions to produce alkali-silica gel.The structure of L4-L6 is loose and gel is produced.It can be observed that there is still unhydrated CaSO 4 •2H 2 O in the aggregate.At the same time, with the decrease of cement content, less and less ettringite is observed.Ettringite plays a supporting role inside the aggregate and ettringite decreases, so the cylinder compressive strength of the aggregate will gradually decrease, cracks and pores are observed in L6.It shows that the hydration products of L6 group are not much, which is one of the reasons for its low strength.Alkali-silica gel, Aft, and some dihydrate gypsum can be observed in L7 group.The structure is better than that of L8-L10 group and the products are also rich.This is because the cement is fully hydrated and the small amount of silica fume is fully utilized to produce more ettringite and alkali silica gel.Through the SEM images of L8-L10 groups of samples, it was also observed that there were a large number of columnar, plate-like and clastic unhydrated dihydrate gypsum in the internal structure of the aggregate, and only a small amount of gel was bonded to the unhydrated gypsum on the surface, with loose structure and a large number of pores.
As a whole, the needle-like ettringite crystal and fibrous calcium silicate hydrate are the main hydration products of the cement material and are accompanied by the formation of alkali-silicon gel.The ettringite crystal and the remaining unreacted gypsum dihydrate crystal are interspersed and dispersed in the structure and are cross-bonded with the calcium silicate hydrate and alkali-silicon gel.Spicular ettringite crystal plays the role of skeleton support in the microstructure, fibrous hydrated calcium silicate and alkali silicon gel can play the role of intertwining  Preparation and characterization of TG artificial aggregate  7 bond, and their mutual matching can improve the compactness and strength of the aggregate structure.After phase analysis and comparison, it is concluded that the aggregate structure and performance of L7 group are better, and the product is more abundant.This ratio can be used as the optimal ratio in this study.

Durability
To investigate the durability and stability of aggregates, dry and wet cycles are carried out on the dried aggregates.The damage situation of aggregates is shown in Figure 9, the quantitative calculation results are shown in Table 5, and the statistical chart of cracking rate is shown in Figure 10.
As can be seen from Figure 9, some aggregates crack a few minutes after the aggregate is injected into the water.The reason for this phenomenon [23,24] is that the alkalisilicon reaction (ASR) generates an expansive alkali-silicon gel (Na 2 O•nSiO 2 ).TG is used to prepare artificial aggregate.After mixing the cement material with water, highly active minerals such as dicalcium silicate, tricalcium silicate and tricalcium aluminate in the clinker will quickly dissolve in water and undergo chemical reactions.Hydrated calcium silicate, hydrated calcium aluminate, Ca(OH) 2 , and other hydrated products are generated.Ca(OH) 2 and NaOH dissolve and precipitate strongly alkaline OH − , which will react strongly with active minerals such as silica ash.Products such as calcium silicate hydrate gel, calcium aluminate hydrate and alkali silicon gel with gelling ability are generated.Calcium aluminate hydrate reacts with gypsum dihydrate dissolved in TG to produce ettringite crystals and other gelling substance.Ettringite and hydrated calcium silicate are the main basis of aggregate strength, which is also verified by XRD analysis of the aggregate.Alkalisilicon gel has expansibility and strong water absorption.After its expansion of water absorption, expansion stress is generated in the aggregate, resulting in the collapse of the aggregate structure.
As can be seen from Figure 10, the greater the dosage of silica fume, the greater the cracking rate of aggregate.The cracking rate of L6 group is the largest 35.97%, and the cracking rate of L7 group is the smallest 2.13%.Therefore, controlling the molar ratio of SiO 2 /Na 2 O has a good effect on inhibiting the cracking of aggregate.According to the calculated values of SiO 2 /Na 2 O molar ratio of each group in Table 5, the maximum SiO 2 /Na 2 O molar ratio of group L6 is  16.485, and the minimum SiO 2 /Na 2 O molar ratio of group L7 is 6.826, which is also consistent with the changing trend of silica fume content and cracking rate.This indicates that the content of silica fume changes the molar ratio of SiO 2 / Na 2 O.The greater the molar ratio of SiO 2 /Na 2 O, the more active silicon in the aggregate system, the more alkalisilicon gel is produced, and the greater the cracking rate of the aggregate.There are different cracking rates for the same dosage of silica fume, which is related to the amount of TG and cement in the aggregate system.Based on the above research results, the compressive strength, water absorption, and cracking rate of the aggregate cylinder are compared and analyzed.It is concluded that the aggregate of L7 group has the best overall performance and is the optimal mix ratio in this paper.

Conclusion
According to the research and analysis of the test results, the following conclusions are obtained: 1) The artificial aggregate was prepared by cold-bonded granulation technology using mineral powder, fly ash, and silica fume as additive mixed with TG and NaOH as activator.The results showed that the TGA prepared by silica fume as additive had the best performance.
2) The results show that the shaped TGA is approximately spherical, the average grain shape coefficient of TGA is 1.0-1.2, and the bulk density is 700-800 kg/m 3 .The higher the TG content, the higher the water absorption of TGA.In addition, the cylinder compressive strength of TGA is in the range of 6-12 MPa.
3) By XRD and SEM analyses, the main products of TGA system are C-S-H, alkalosilica-gel, and a small amount of AFt, and the internal structure of aggregate gradually deteriorates as the specific gravity of cementing material decreases.The experimental results show that group L7 (TG:cement:silica ash = 7:2:1, 1% NaOH and 1% waterreducing agent) TGA has the best comprehensive performance in this study.4) The cracking after TGA dry and wet cycle is due to the ASR of the gelling system.The selection of 10% silica fume as an additive can improve the dry and wet resistance of the aggregate and reduce the effect of ASR on the performance of TG artificial aggregate.

Figure 4 :
Figure 4: Compress strength of different admixtures at different ages.

Figure 6
Figure 6 is the change rule of the compressive strength of TGA cylinder under different mix proportions.With the

Figure 9 :
Figure 9: Cracking diagram of aggregate after dry and wet cycling.

Table 1 :
Chemical composition of raw materials

Table 2 :
Mixing ratio of artificial aggregate clean pulp test Figure 2: TGA preparation process.Figure 3: Raw TG.

Table 3 :
Mix ratio of artificial aggregate granulation test

Table 4 :
Particle form factor and bulk density of aggregate L10 1.22 1.02 1.12 766.70 Figure 6: 1 h water absorption and cylinder compressive strength of aggregate.
4 •2H 2 O and calcium silicate hydrate (C-S-H) in the XRD patterns of hydration products of different test groups at 14 days age are strong.The specific reaction of cement and TG is as follows:

Table 5 :
Statistical table of silica fume content, cracking rate, and SiO 2 /Na 2 O molar ratio of each group