Hydration activity , crystal structural , and electronic properties studies of Ba-doped dicalcium silicate

High belite cement has a wide application potential due to its low energy consumption, low CO2 emission, and excellent durability performance. Due to the low hydration rate and strength development at an early age, the activation of beta-dicalcium silicate (β-C2S) crystallographic structure is essential to improve the early strength of high belite cement. In this study, the β-C2S phase is activated by dissolving Ba2+ ions into the crystal lattice to improve the hydration rate. Unlike the traditional analysis methods of thermodynamics and dynamics theory, the first principle and density functional theory were applied to study the effect of Ba2+ ions on the activation of β-C2S, especially on the crystallographic structure, lattice parameters, and electronic structure change. The crystallographic structure of β-C2S can be activated by doping Ba atom and the crystal formation energy increases and the bandgap between VBM and CBM become narrow in the activated βC2S crystallographic structure. Comparing the Ca 2+ substitution in [CaO6] or [CaO8], the lattice deformation and hydraulic reactivity is more significant in Ba2-C2S and Ba22C2S. The first principle and density functional theory explains the change of the electronic structure of the activated crystallographic structure and provides a theoretical basis for the purposeful design of material structures.

Abstract: High belite cement has a wide application potential due to its low energy consumption, low CO 2 emission, and excellent durability performance. Due to the low hydration rate and strength development at an early age, the activation of beta-dicalcium silicate (β-C 2 S) crystallographic structure is essential to improve the early strength of high belite cement. In this study, the β-C 2 S phase is activated by dissolving Ba 2+ ions into the crystal lattice to improve the hydration rate. Unlike the traditional analysis methods of thermodynamics and dynamics theory, the first principle and density functional theory were applied to study the effect of Ba 2+ ions on the activation of β-C 2 S, especially on the crystallographic structure, lattice parameters, and electronic structure change. The crystallographic structure of β-C 2 S can be activated by doping Ba atom and the crystal formation energy increases and the bandgap between VBM and CBM become narrow in the activated β-C 2 S crystallographic structure. Comparing the Ca 2+ substitution in [CaO 6 ] or [CaO 8 ], the lattice deformation and hydraulic reactivity is more significant in Ba 2 -C 2 S and Ba 22 -C 2 S. The first principle and density functional theory explains the change of the electronic structure of the activated crystallographic structure and provides a theoretical basis for the purposeful design of material structures.

Introduction
The traditional cement manufacture has been lasting for less than 200 years. Due to the consumption of highgrade limestones, the high-grade limestone resource in our country is not enough for the next 40 years [1,2]. Therefore, low-grade ore should be used in cement manufacture in point of view of sustainable development. High belite cement has a wide application potential due to its low energy consumption, low CO 2 emission, and excellent durability performance [3][4][5][6][7][8]. Compared with Ordinary Portland cement, high belite cement consists of more than 40% dicalcium silicate (C 2 S) [9,10]. Due to its low CaO consumption, the cement calcining process becomes more energy-saving and a large sum of lowgrade ore can be recycled [11][12][13].
β-C 2 S belongs to an island structure, due to the lack of coplanar structure in the [CaO x ] 2x−2 polyhedron, the length of the Ca 2+ ion migration path is quite long, thus inhibits the hydraulic reaction of β-C 2 S during the hydration process [14,15]. Due to the low hydration rate and strength development in an early age, the application of high-belite cement is limited. The activation of the C 2 S crystallographic structure is essential to improve the early strength of high belite cement. By calcining at high temperature, dopant ions are incorporated in the crystal lattice of β-C 2 S in the form of the lattice vacancy or site substitution [16,17]. With the reduction of the crystal symmetry and the formation of crystal lattice distortion, the microstress in the lattice increases and the lattice activation of β-C 2 S is achieved. Activators such as As 2 O 5 , V 2 O 5 , Cr 2 O 3 , MgO, BaO, CrO, P 2 O 5 , R 2 O, etc. are commonly used in cement industry production [18,19]. At the same time, the addition of activators can effectively reduce the formation energy in chemical reaction thermodynamics [3].
Many researches have studied the activation mechanism of doped ions on the crystal structure transformation of β-C 2 S [16,[20][21][22][23]. Cuesta et al. [24] studied the activation mechanism of doped B 3+ ions on belite cement and found that Ca 2+ ions and Si 4+ ions in the [SiO 4 ] tetrahedron have been replaced, which contributes to the hydraulic reaction during cement hydration. Wang et al. [25] studied the activation effect of BaSO 4 on β-C 2 S in belite cement clinker during calcination, and the results showed that BaSO 4 could dissolve into the crystal structure of β-C 2 S and improve the early compressive strength.
The first principle and density functional theory can explain the change of the electronic structure of the activated crystal structure and also provide a theoretical basis for the purposeful design of the material structure [26][27][28]. Based on the traditional thermodynamics and kinetics theory, first-principles quantum mechanics calculations have been applied to study the relationship between electronic structure and reaction activity of C 2 S polymorphs [29,30]. In this study, the β-C 2 S phase is activated by dissolving ions into the crystal lattice to improve the hydration rate. The first principle and density functional theory were applied to study the effect of different ions on the activation of C 2 S, especially on the crystallographic structure and lattice parameters and electronic structure change.

Synthesis of Ba-doped C 2 S
Analytical reagents and deionized water are applied during the solution preparation. β-C 2 S is synthesized according to ref. [16]. Analytical SiO 2 and CaO with a stoichiometric proportion of Ca:Si = 2:1, 4% BaSO 4 with total mass were mixed and ground uniformly by a small ball mill for 20 min until the sieve residue is less than 5%. After mixing anhydrous ethanol, the samples were made into a pill with a diameter of 15 mm and dried in a vacuum drying oven at 105℃. The samples were calcined in the furnace with the calcination temperature 1250°C, the heating rate 10°C/min, and heat preservation for 3 h. The samples were rapid cooling in the case of crystal transformation. The clinker was crushed and ground into a fine powder, and the chemical composition of β-C 2 S was analyzed by X-ray fluorescence spectrometry (ThermoFisher, ESCALAB 250Xi).

Characterization method
The structural variation of β-C 2 S doping Ba 2+ ions was further analyzed by Nuclear Magnetic Resonance ( 29 Si MAS-NMR) and Fourier transform infrared (FT-IR) spectroscopy. 29 Si MAS-NMR was measured by a Bruker Avance II 400 MHz spectrometer with a field strength of 9.4 T, operating at 99.2 MHz. The chemical shifts were referenced to tetramethylsilane (TMS). FT-IR patterns were obtained by a Fourier transform infrared spectrometer (Nicolet is5003) with a wave length range from 250 to 4,000 cm −1 .

Molecular dynamics simulation
In this paper, the first principle calculation is based on the density functional theory (DFT) and the plane-wave pseudopotential method (PWP) [31], and the CASTEP module of molecular structure and mechanics simulation software Materials Studio@ (Accelrys 6.2) is employed to calculate the electronic structure and cohesive energy β-C 2 S with/without doping Ba ions.
The state of the electronic structure is described by density functional and the generalized gradient approximation (GGA) [32]. The exchange-correlation functionals were calculated using Generalized Gradient Approximation (GGA) with the Perdew-Burke-Ernzerhof functional (PBE functional) [33]. The Brillouin zone was sampled with 4 × 4 × 4 k-points in the primitive cell. The values of kinetic energy cutoff E c and the k-points number are increased until the calculated energy converges within the required tolerance, where E c determines the number of plane waves and k points does the sampling of the irreducible wedge of the Brillouin zone [34]. The cutoff energy of plane-wave (PW) was 380 eV, the energy tolerance was 5 × 10 −7 eV/atom, the force tolerance was 0.03 eV/Å, the stress tolerance was 0.05 GPa, and the displacement tolerance was 0.001 Å.
3 Results and analysis 3.1 Activation of Ba-doped β-C 2 S The chemical composition of the calcined Ba-doped C 2 S is shown in Table 1, which shows that 2.55% Ba 2+ is stabilized in the system. The NMR spectrum can reflect the atomic coordination and adjacent atomic effect. The crystallographic structure variation of C 2 S reported can be described by the shift of 29 Si NMR spectrum [35,36]. Figure 1 shows 29 Si spectrum peak shifts and the appearance of the secondary peak right beside the main peak in the NMR spectrum. It infers to the atom subordination change and octahedral transformation in the Ba-doped C 2 S.
FI-IR analysis was further carried out to verify the lattice variation in Ba-doped C 2 S. FT-IR patterns of hydrated β-C 2 S and Ba-doped β-C 2 S at different curing age are shown in Figure 2. New peaks at 900 and 845 cm −1 in Ba-doped β-C 2 S are ascribed to the asymmetric stretching vibration of the [SiO 4 ] tetrahedron, which confirmed that Ba 2+ enters the crystal lattice after calcination and is consisted with the observation in NMR analysis [37]. The [SiO 4 ] tetrahedral stretching vibration at 3,440, 1,420, and 1,020 cm −1 can be attributed to the formation of C-S-H gel [38]. The appearance of a stronger absorption at 1420 cm −1 is attributed to the faster β-C 2 S hydration rate after the Ba doping. It can be concluded that Ba 2+ doping can significantly promote the β-C 2 S hydration at an early age.
When compared with general β-C 2 S, the addition of Ba leads to the lattice deformation and the reduction of formation energy in Ba-doped C 2 S. The spacing of each crystal plane is increased, and the inner holes are enlarged accordingly. Higher degrees of Ba 2+ substitution leads to higher variants of crystalline structure. Comparing the Ca 2+ substitution in [CaO 6 ] or [CaO 8 ], the lattice deformation and hydraulic reactivity are more significant in Ba 2 -C 2 S and Ba 22 -C 2 S.     The low hydraulic reactivity of β-C 2 S is ascribed to two aspects, one is the H 2 O molecules cannot enter the cavity in the β-C 2 S island structure, the other is that Ca attached to the tetrahedron [SiO 4 ] is not easily dissolved out [28,42]. Pritts et al. [43] have found that Ba, Fe, Al, and Pb doped in β-C 2 S clinkers can reduce crystalline symmetry and increase the hydraulic reactivity [40][41][42][43][44]. Due to the limitations of microscopic characterization methods, the correlation between the mechanism of hydration kinetics and the crystallographic structure of Ba-doped C 2 S remains to be further studied; therefore, the electronic structure Badoped C 2 S is further studied in the following.

Electronic structure of Ba-doped C 2 S
The total and partial density of states (TDOS and PDOS) for β-C 2 S, Ba 1(2) -C 2 S and Ba 11 (12,22) -C 2 S calculated are shown in Figure 4. The main contribution to valence band in β-C 2 S arises from the O-2p located from −4 to 0 eV below the Fermi energy, the main contribution to the conduction band in β-C 2 S arises from the Ca-3d located from 5 to 7 eV above the Fermi energy. The band gap between VBM and CBM is 4.887 eV. In the case of Ba 1(2) -C 2 S, the original bond state in the crystal structure has changed with Ba-adopted. Partial contribution to the conduction band comes from the Ba-3d located from 5 to 7 eV. The band gaps in the Ba 1 -C 2 S and Ba 2 -C 2 S are 4.762 and 4.441 eV correspondingly. In the case of Ba 11 (12,22) [37,45]. The formation of coplanar structures of [Ca(Ba)O x ] 2x−2 polyhedra leads to the structures of the coplanar polyhedron change from spiral chains to three-dimensional network structures [46]. It is suggested that the hydraulic activity of Ba-doped β-C 2 S is greatly improved with the formation of Ba-O-Si chains instead of Ca-O-Si chains. When β-C 2 S contact with water, the Ca 2+/ [CaO x ] 2x−2 polyhedra is dissolved, where OH − and Si-OH are formed accordingly [40]. Due to the internal forces in the [CaO x ] 2x−2 are unbalanced, a component force points to the location with Ca 2+ extracted, which accelerates the continuous dissolution of Ca 2+ ions.
In conclusion, the electron structure variation is related to the variation of charge exchange and atomic hybridization orbital in Ba-doped β-C 2 S with different doping positions.

Conclusions
The following conclusions can be drawn based on the laboratory investigations: 1. Doping Ba 2+ in β-C 2 S can significantly promote the early hydration. According to the electronic structure and hydraulic reactivity results, the electron structure variation is related to the variation of charge exchange and atomic hybridization orbital in Ba-doped β-C 2 S with different doping positions. 2. The hydration kinetics and electronic structure of β-C 2 S can be established systematically and intuitively by the first principle calculation. Comparing the Ca 2+ substitution in [CaO 6 ] or [CaO 8 ], the lattice deformation and hydraulic reactivity is more significant in Ba 2 -C 2 S and Ba 22 -C 2 S. 3. The crystallographic structure of β-C 2 S can be activated by doping Ba atom and the crystal formation energy increases and the band gap between VBM and CBM become narrow in the activated β-C 2 S crystallographic structure.