Preparation of Heavyweight Ultra-high Performance Concrete Using Barite Sand and Titanium-rich Heavy Slag Sand

DING Qingjun, DENG Chao, YANG Jun, ZHANG Gaozhan, HOU Dongshuai

(1. School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China; 2. Advanced Building Materials Key Laboratory of Anhui Province, School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei 230601, China; 3. School of Materials Science and Engineering, Department of Civil Engineering, Qingdao University of Technology, Qingdao 266033, China)

Abstract: The heavyweight ultra-high performance concrete (HUHPC) was prepared with barite sand partially replaced by titanium-rich heavy slag sand (THS) at replacement proportion of 0%, 30%, 50%, 70% and 100% in this work. The results show that THS incorporation can effectively improve the mechanical properties and reduce the volume shrinkage of HUHPC. The HUHPC with 50% THS replacement reaches an apparent density of 2 890 kg/m3 (for fresh HUHPC), 28 d compressive strength of 129 MPa, 28 d flexural strength of 23 MPa, 28 d flexural toughness of 28.4, 56 d volume shrinkage of 359×10-4 and, as expected, excellent durability.Microstructural investigation demonstrates that the internal curing of pre-wetted THS promotes the hydration of the surrounding cement paste thereby strengthening the interfacial transition zone, resulting in the “hard shell” formation around aggregate to “protect” the aggregate. Additionally, the “pin structure” significantly improves the cement paste-aggregate interfacial connection. The combination of “hard shell protection” and “pin structure” remarkably improve the mechanical properties of HUHPC produced with porous THS aggregate.

Key words: heavyweight ultra-high performance concrete, titanium-rich heavy slag sand, mechanical properties, durability, internal curing

1 Introduction

As the main structural material of nuclear engineering buildings, shielding concrete not only undertakes the task of shielding rays, but also an important safety guarantee for nuclear facilities. During its service, it bears various direct and indirect loads, so its mechanical properties are related to the safety of the whole nuclear engineering. However, the current design strength of existing shielding concrete is generally not high, most of which do not exceed C60[1-3], and their resistance to various loads during the service period is weak, leaving hidden dangers for nuclear engineering. Ultra high performance concrete (UHPC) has the characteristics of high density, ultra-high strength and high toughness, and is an ideal material for building ultra-high strength radiation shielding buildings.However, the radiation shielding ability of ordinary aggregates UHPC is still lower than that of shielding concrete prepared with heavy aggregates, so it is necessary to add heavy aggregates[4]. But the high amount of UHPC cementing material results in excessive early shrinkage, which requires steam curing to improve the volume stability and crack resistance of the components[5]. Therefore, there is an urgent need to reduce the early shrinkage and cracking of UHPC and enhance its ray shielding ability to prepare low-shrinkage and shielding UHPC.

The commonly used radiation-shielding aggregates in shielding concrete mainly includes heavy aggregates such as barite, iron ore, scrap iron, and substances with low atomic numbers that can slow down neutrons. For example, Ozenet al[6]studied the methods of using various high-density natural ores for shielding concrete, and formulated different shieldingγ-rays concrete with apparent density above 3 000 kg/m3. Akkurtet al[7]thought that with the increase of barite concentration, theγ-rays linear attenuation coefficient of concrete increased, while the neutron removal cross-section decreased, and found that the optimal ratio of barite to shield both neutrons andγ-rays at the same time is about 53.8%, and the optimal concrete density value is about 3 020 kg/m3. Gokceet al[8]found that the mixture with barite aggregate accounting for 40% of the total aggregate volume is the best reactive powder concrete (RPC) mixture for shielding neutrons andγ-rays at the same time, but due to the inclusion of barite in addition, the mechanical properties of RPC are significantly reduced, and the replacement of quartz by barite aggregates has a greater impact on the flexural tensile strength than the compressive strength. Sharmaet al[9]believe that lead fiber and lead alloy fiber can significantly improve theγ-rays absorption capacity of concrete compared with steel fiber. In the work of Gonzalez-Ortegaet al[10], by studying the influence of barite aggregates on the performance of concrete during the mixing process, the mechanism for reducing the mechanical properties of concrete is given—that is, due to the high density of barite aggregate, excessive vibrating will easily cause it to sink, and due to its fragility, the grading curve will be affected during the mixing process, and the fine dust formed will affect the performance of the concrete interface transition zone,thereby reducing the mechanical properties of barite concrete. Zhenget al[11]proposed that the mechanical properties of ilmenite concrete are higher than that of barite concrete, and its high temperature resistance is better, so it is more suitable to be used as the aggregate of shielding concrete. Professor Pan’s team[12-14]proposed the use of discarded cathode ray tube (CRT)glass instead of heavy aggregates to prepare radiation shielding concrete, and conducted a systematic study of concrete related properties. The above studies have shown that mixing heavyweight aggregates or fibers into UHPC can increase itsγ-rays absorption capacity.However, the commonly used natural minerals have high density and low strength[15]. They are easy to settle and segregate when used in the preparation of concrete,causing poor concrete homogeneity, resulting in uneven shrinkage and cracking during concrete hardening,which affects its mechanical properties and radiation shielding performance.

Titanium-rich heavy slag (THS) is an inorganic material mainly composed of perovskite, diopside and diopside ferrian minerals[16]. As a by-product in the metallurgical process of vanadium-titanium magnetite enrichment, THS is formed during blast furnace smelting of vanadium-titanium magnetite. About ten million tons of THS are discharged every year in southwest China[17]. Now there are still 30 million tons of titanium slag that has not been effectively used[18,19]. The large accumulation of industrial waste slag has caused problems such as land occupation, environmental damage, and resource waste. Previous experiments have investigated the feasible usage of THS as aggregate in concrete production[20,21]. The experimental evidences show that the THS exhibits high chemical stability in concrete and can be used in preparing high strength concrete[16]. Recently, THS has been used to prepare C30 self-compacting concrete successfully and applied in Jinsha River Bridge of Lipan Highway in southwest China[22]. From another perspective, the THS contains more than 20% of TiO2in its chemical composition[23].Previous experimental study demonstrated that titanium oxides have a potential application in radiation shielding[24]. Like natural ores, THS has a higher atomic number and apparent density (2 970-3 300 kg/m3)[25],and can be used as a potential high-density material for making shielding concrete. Meanwhile, the surface of THS is rough, porous and rich in edges and corners.This renders it with an ability to play an internal curing role in the concrete material, which can significantly improve the volume stability of concrete. The characteristics of high apparent density, high strength and high porosity of THS[26]make it an good candidate material for preparing heavyweight ultra-high performance concrete (HUHPC), which can effectively improve theγ-rays shielding performance, mechanical properties and volume stability of the HUHPC.

The object of this research is to prepare heavyweight ultra-high performance concrete (HUHPC) with barite sand and titanium-rich heavy slag sand (THS).The influence of THS replacement on the mechanical properties and long-term durability of HUHPC were also studied. Finally, microstructure investigation was performed to elucidate the strengthening and shrinkage-reducing mechanism of THS in the HUHPC.

2 Experimental

2.1 Raw materials

The cement used in this work P·O52.5 cement is produced by Huaxin Co., Ltd., Hubei Province, China.Silica fume is produced by Langtian Wastes Recovery Co., Ltd. (Sichuan Province, China) and fly ash microbeads is produced by Zhucheng New Material Co., Ltd., Tianjing City, China. In order to reduce the shrinkage of HUHPC paste, an HME type of expansiveagent produced from Subote New Material, Co., Ltd.(Jiangsu Province, China) was also used in preparing the paste. The chemical compositions of these raw materials are shown in Table 1.

Table 1 Chemical composition of cementitious materials/wt%

Table 2 Chemical composition of aggregate/wt%

The fine aggregate used in current work is barite sand and THS. The porous THS is produced by the industrial metallurgy company of Panzhihua Iron and Steel Group Co., Ltd. The fineness modulus, apparent density and saturated water absorptivity of THS are 3.0, 3 100 kg/m3and 8%, respectively. The methylene blue (MB) value and crushing value of THS are 0.9 and 5.3%, respectively, according to standard JTG E42-2005[27]. Previous experiments have showed that THS has good chemical stability and radiation shielding performance[16][28]. The expansion value of THS is much lower than the standard limitation of GB/T 14684-2011[29]on the alkaline-silica reaction of aggregate[16].As forγ-ray of 662 keV energy, the linear attenuation coefficient (μ) of THS is 0.183 cm-1, which reaches 70% of that of barite (μ=0.261 cm-1) and is far higher than that of basalt (μ=0.108 cm-1)[28]. The barite sand in this work is of continuous grade with a fineness modulus of 2.7. Its apparent density and saturated water absorption are 4 300 kg/m3and 1.6%, respectively.The XRF analyses of THS and barite sand are specified in Table 2. The chemical composition of THS shows a titanium dioxide content of 22.3% and that of barite sand shows a BaSO4content of 96.8%. This indicates goodγray radiation shielding performance for both types of sand.

The cumulative gradating curves of the fine aggregates are shown in Fig.1. It can be noticed that the grading curves of barite sand and THS are similar. Both of their gradations belong to grade II according to GB/T 14684-2011[29]. Fig.2 are the SEM images of these two kinds of aggregates. The THS has coarse surface and porous structure. On the contrary, barite sand exhibits relatively smooth surface, glass luster and dense structure. This explains the difference in the water adsorption capacity between these two types of fine aggregates and indicates that THS can play a role in internal curing the HUHPC paste.

Fig.1 Grading curve of two kinds of aggregate

Fig.2 SEM images of two types of aggregate

2.2 Sample preparation and tests

The HUHPC mixing proportion was designed by the modified Andreasen and Andersen model[30], which eliminates coarse aggregates and adds ultrafine mineral admixtures to optimize the particle gradation in the HUHPC. The mixing proportion of the raw materials was determined by fitting the accumulative grading curve with target curve. The optimization was performed using the least square method (LSM) and the minimizing deviation between mixing grading curve and target curve indicates the optimal mixing proportion[4]. In order to evaluate the effect of barite sand andTHS incorporation on the mechanical properties of concrete, HUHPC with different fine aggregate composition were prepared. The volume replacement rates of THS for barite were 0, 30%, 50%, 70% and 100% in these HUHPC samples, which are defined as T0, T30,T50, T70 and T100, respectively. The mixing ratios of these sample batches are listed in Table 3. Prior to the sample preparation, the THS was pre-wetted for 24 h to reach saturated state. Due to the fragility of barite, the mixing time of each stage should be strictly controlled,otherwise the aggregate grading would be seriously affected[10]. In this work, the mixing time does not exceed 435 s after barite is added. The apparent density of fresh HUHPC sample were also tested with the results given in Table 3. With increasing replacement of THS,the density of the HUHPC sample decreases. Nevertheless, all the heavyweight samples show apparent densities higher than 2 600 kg/m3.

Table 3 Mixing ratios of each HUHPC group and their apparent density/(kg/m3)

After mixing, the fresh HUHPC paste were cast into mold and cured at standard condition. Mechanical properties tests and microscale structure exploration were performed on the samples at specified ages.

3 Results and discussion

3.1 Compressive strength and flexural tensile strength

Fig.3 shows the compressive strength and flexural tensile strength of HUHPC samples at different ages.It can be observed that the compressive strength and flexural tensile strength of HUHPC gradually increase with increasing replacement of THS. This implies that the substitution of THS can reduce the negative effects of barite sand on the mechanical properties of HUHPC.Relevant research results show that the apparent density of heavy aggregate barite sand is quite different from the cement paste, which results in the aggregate segregation in fresh cement paste. Meanwhile, its breakage and smashing in the mixing procedure would affect the aggregate grading curve. This exerts negative impact on the mechanical properties of concrete produced by barite sand[4]. On the other hand, the apparent density of THS is smaller than that of barite sand, which would weaken the aggregate segregation and improve the homogeneity of concrete. Moreover, the mechanical strength of THS is relatively high. This makes it difficult to form fine powder in the mixing process of concrete, thus improving the strength of interface transition zone. Compared with barite sand, the adhesion between the aggregate and the cementitious material is enhanced, which improves the mechanical properties of concrete.

Fig.3 Compressive strength of HUHPC at different ages

It can also be noticed that when the replacement amount of THS is higher than 50%, the increases in compressive strength and flexural tensile strength become moderate with still higher THS incorporation.In fact, although the barite powder formed during the mixing process will affect the aggregate gradation curve, small amount of barite powder can fill the voids in the HUHPC framework and improve its compactness. Therefore, when the replacement of THS further increases, its strengthening effect on the mechanical properties of HUHPC becomes slight.

3.2 Flexural toughness

The flexural toughness of concrete was loaded with the MTS fatigue test system. Before the initial cracking of the concrete, the load was applied at a rate of 6 kN/min, and after the initial cracking, the load was applied with displacement until the specimen completely lost its bearing capacity at a rate of 0.1 mm/min.Fig. 4 shows the load-deflection curve of each HUHPC group mix ratio. It can be seen that the load-deflection curve of HUHPC specimens can be basically divided into three stages: the first stage is the elastic stage,which is mainly manifested as linearly rise in the load-deflection curve; the second stage is the stage of crack initiation and propagation, defined as the parabolic increase in the load-deflection curve; the third stage occurs when the crack penetrates the concrete, which indicates the fast decrease in the load-deflection curve and failure of the sample.

Fig.4 Load-deflection curve of HUHPC

As shown in Fig.4, the load-deflection curve of each HUHPC group is different in the first stage. Generally, concrete failure is usually caused by microcracks caused by internal stress concentration and subsequent expansion. The flexural tensile properties of HUHPC specimens before initial cracking mainly depend on the cementation properties of hydration products of cementitious paste and the compactness of matrix materials.However, the fragility and powder behavior of barite reduce the density and connection of HUHPC, and aggravate the generation and development of internal microcracks when concrete is loaded, reducing the initial crack strength, initial crack deflection, peak strength and peak deflection of HUHPC, which also reduces the HUHPC flexural toughnessI20, which is defined as the integral area of the load-deflection curve, which reduces the failure energy and changes its failure form.

3.3 Long-term durability

3.3.1 Shrinkage properties

Due to the high amount of cementitious material, the influence of autogenous shrinkage on the performance of UHPC is dominant[31]. Fig.5 shows the shrinkage evolution over time of HUHPC samples with different THS content. As shown in the figure,the shrinkage of UHPC significantly reduced with increasing amount of THS addition. At the age of 56 d,the total shrinkage of HUHPC decreases from 7.5×10-4to 2.3×10-4, as the THS content increasing to 0% to 100%. This is because the surface water around concrete sample is drying and volatilizing as the cementitious materials hydrating, and the bleeding under gravity driving causes the internal humidity of matrix to be lower than the aggregate capillary pores. When the HUHPC is produced with pre-wetted THS, the internal moisture of the pre-wetted aggregate would gradually diffuse into the hydrated cement paste under the humidity gradient. This alleviates the self-drying phenomenon of micropores in the cement paste, thereby reducing the shrinkage ratio of HUHPC over the hydration process. Meanwhile, the internal moisture of the pre-wetted THS can also promote the hydration of expansion agent, which can compensate the shrinkage of the HUHPC paste during its hydration.

Fig.5 The evolution of HUHPC shrinkage with time for each mix proportion

It should also note that the shrinkage-reducing effect of THS becomes weak with the further increase of THS replacement. With the substituting content of THS increases from 0% to 50%, the shrinkage ratio of HUHPC decreases by 52.26%, but this value only decreases by 37.88% as the THS content increases from 50% to 100%. This can be attributed to two aspects: (1)When the content of barite sand is low, the amount of barite powder formed in mixing process is reduced, and the smaller barite powder has the effect of reducing the drying shrinkage of concrete, which is consistent with the study of Saidaniet al[32]. (2) The internal curing material added with additional water will increase the drying shrinkage, but will reduce the total shrinkage[33].Therefore, as the amount of THS increases, the reducing effect on the drying shrinkage of HUHPC becomes weaker.

3.3.2 Chloride penetration resistance

Rapid chloride ions migration coefficient (RCM)method and electric flux method are the main methods to test the chloride penetration resistance of concrete.As steel fiber is a good conductive material, which would affect the test results. The chloride penetration resistance tests were implemented with HUHPC samples without steel fiber. The test results are specified in Table 4.

As given in Table 4, the chloride ion migration coefficient and electric flux of HUHPC increases gradually with the increase of barite sand content. Nevertheless, the chloride ion migration coefficients for all the samples are ranging from (0.01 to 0.05×10-12m2/s, which are much less than the lower limit (1.5×10-12)of the best level of RCM rating. The electric fluxes of these samples range from 75 to 118 C, which are far below the lower limit (500 C) of Qs-V level for cement samples. This demonstrate the excellent impermeability of HUHPC to chloride ions.

3.3.3 Water penetration and carbonation resistances

The water penetration resistance and carbonation resistance tests were performed according to the GB/T 50082-2009 standard[34], with the results listed in Table 5.

As shown in Table 6, all the HUHPC samples show high impermeability grade and lower carbonation depth, regardless of the replacement content of THS.Generally, the ordinary UHPC prepared with quartz sand as the aggregate usually exhibits high impermeability and excellent carbonation resistance. Therefore,the testing results indicate that the replacement of quartz sand with barite sand and THS do not degrades its long-term durability.

Table 4 Test results of chloride penetration resistance

Table 5 Water penetration and carbonation resistance results

3.4 Hydration degree and microstructure of concrete

3.4.1 Hydration degree

In order to investigate the internal curing effect of pre-wetted THS on the HUHPC paste, the hydration degree of T0 and T100 HUHPC mortar sample was prepared and studied. Note that steel fiber is removed in preparing mortar samples to facilitate hydration degree test. The chemical bound water content of each mortar sample at hydration age of 3, 7, 14, 28 d was determined by ignition method to characterize its hydration degree evolution. During the test, the samples at specified age are placed in absolute ethanol to stop their hydration, and then put into the oven at 105 ℃along with crucibles until to constant weights. This process can remove the free water content in the sample.Subsequently, the sample and crucibles are kept in oven at 950 ℃ for 3 h. Three groups of tests are repeated foreach sample of different mix proportion and age. The calculation formula of the chemically bound water is shown in Eq.(1)[35].

where,Wnis the chemically bound water content of sample at hydration age ofndays(%),W2andW1are the mass of drying hardening system before and after ignition (g), respectively. LOI denotes loss on ignition.

Fig.6 are the chemical bound water content at different ages of the two mix proportions. It can be seen that the content of bound water in T0 is higher than that in T100 over all the hydration ages. The water to binder (w/b) ratio of concrete plays an important role in its internal hydration reaction[36]. Although thew/bratio of the two groups of concrete is 0.18, the effectivew/bratio of T100 after removing its internal water content is only 0.08 due to the porous THS (Thew/bratio of HUHPC with THS may be higher, because there is some water on the surface of THS and the water absorbed inside will be released during the mixing process, but the actualw/bratio will be lower than that of ordinary aggregate). Since the water stored in the THS can react with the cementitious materials which are not involved in the reaction at the later stage of hydration,T100 should be more effective than T0 in promoting the hydration reaction. From the experimental results,the chemical bound water content of T100 increased by 19.65% from 7 to 28 days, which was higher than 10.36% of T0.

Fig.6 Chemical bound water content of HUHPC at different ages

3.4.2 Microstructure

In order to explore the strengthening mechanism of pre-wetted THS in HUHPC paste, SEM-EDS test was performed on the HUHPC mortar sample with 100% THS.

Fig. 7 shows the morphology of interface transition zone (ITZ) in T100 HUHPC paste. As shown in Fig.7(a), hydration products are closely stacked around the aggregate to form a dense ITZ structure, which demonstrates the tight connection in aggregate-cement paste interface. This can be ascribed to the internal curing effect of THS. Generally, the ultra-loww/bratio adopted in UHPC could not afford the sufficient hydration of its cementitious paste. This would lead to the remaining of certain amount of anhydrous cement particles around the aggregate. In current work, however,the pre-wetted aggregate can gradually release moisture into the cementitious paste after the hardening of the paste, which facilitates the further hydration of cement paste and forms more hydration products around the aggregates. This is also proved by the chemical bound water content experiment. Fig.7(b) describes the surface of a titanium-rich heavy slag particle. It should be noted that the hydration products are filled in the surface pore of the aggregate. On the one hand, these hydration products can fill the surface pore of the aggregate and improve its mechanical strength. On the other hand, these hydration products resemble a “pin”of cement paste that penetrates into the aggregate.

Fig.7 SEM images of the T100 HUHPC sample hydrated for 28 d

According to the microstructural observations,the interfacial connection behavior between cement paste and THS was summarized and described in Fig.8.As shown in the figure, the internal curing of THS can promote the hydration of surrounding cement paste to form dense interfacial transition zone, which show higher hydration degree and, consequently, higher mechanical strength compared to the paste matrix. The presence of these “hard shells” can effectively reduce the stress exerted on the aggregates when the HUHPC are under external loading. On the other hand, cement paste can penetrate into the surface pore of THS. This can densify the surface structure of the aggregate and improve its mechanical strength. Therefore, the negative effect of porous aggregate addition on the mechanical properties of HUHPC can be weakened to some extent. Furthermore, the “pin” around the aggregate can mechanically interlock the aggregate and cement past,which improves the aggregate-cement paste connection. Therefore, the replacement of THS can effectively improve the mechanical strength of HUHPC.

Fig.8 Schematic diagram of the cement paste-aggregate interface

4 Conclusions

a) Increasing the amount of THS replacement results in the gradual decrease of apparent density of HUHPC, but the increase of its compressive strength,flexural tensile strength and flexural toughness. HUHPC produced with 50% of THS replacement exhibits an apparent density of 2 890 kg/m3(for fresh HUHPC),28 d compressive strength of 129 MPa, 28 d flexural strength of 23 MPa, 28 d flexural toughness of 28.4 and excellent durability.

b) The incorporation of pre-wetted THS along with expansion agent addition can improve the volume stability of the HUHPC. On the one hand, the internal curing effect of THS can reduce the shrinkage of HUHPC. On the other hand, the moisture released by pre-wetted aggregate promotes the hydration of expansion agent and thereby compensating the shrinkage of HUHPC.

c) The internal curing effect THS can also facilitate the hydration of surrounding cement paste to improve their hydration degree. The “hydration shell”of dense interfacial transition zone formed around the aggregate would reduce the stress exertion on the aggregate when the HUHPC is under external loading.The hydration products can penetrate into the surface pores of the THS.