Terahertz Time-Domain Spectroscopy of Calcium Silicate Hydrates with Varying Calcium-to-Silicon Ratios

doi:10.16552/j.cnki.issn1001-1625.2026.0083

Abstract

Abstract:

Calcium silicate hydrate （C-S-H） is the main binding phase of hydrated Portland cement and largely determines strength and durability of concrete. It is nanocrystalline and mostly amorphous， with short-range order but no long-range periodicity. The medium-range order （MRO） that connects these two scales is thought to control the mechanical and transport behaviour of concrete， yet it is hard to measure， and the link between the nanostructure of C-S-H and its properties is still poorly understood. The boson peak， an excess of low-frequency vibrational states common to disordered solids， reflects both the MRO and the nanoscale fluctuation of elastic constants. For C-S-H it has been predicted by molecular-dynamics simulations but not yet measured. Terahertz （THz） radiation （0.1 THz to 10 THz） lies in the frequency range of these collective vibrations， so terahertz time-domain spectroscopy （THz-TDS） can be used to probe the boson peak directly.

C-S-H gels with nominal calcium-to-silicon molar ratios （Ca/Si） of 0.5， 0.8， 1.0， 1.4， and 1.7 （actual values 0.54 to 1.74 after impurity correction） were synthesized hydrothermally under nitrogen. They were characterized by X-ray diffraction （XRD）， Fourier-transform infrared spectroscopy （FTIR）， thermogravimetric analysis （TGA）， X-ray fluorescence， laser particle sizing and helium pycnometry， which revealed a poorly crystalline tobermorite-like gel with turbostratic stacking， and provided the contents of water and impurities （portlandite and calcite）. THz-TDS spectra were recorded in transmission on HDPE-diluted pellets over 0.4 THz to 3.0 THz， and the complex refractive index， absorption coefficient and dielectric function were obtained. To isolate the response of the gel itself， the dielectric contributions of the crystalline impurities were removed with an effective-medium （Bruggeman-Hanai-Sen） model， using volume fractions taken from XRD and TGA. For the measured particle sizes， the 1.0 THz to 1.5 THz range falls within the Rayleigh （weak-scattering） regime， so the spectral features come from intrinsic lattice dynamics rather than from particle scattering.

The intrinsic refractive index （1.58 to 1.85） and the real part of the permittivity show little dispersion， indicating that the water in C-S-H is nano-confined or chemically bound； FTIR showed the Si—O band shifting to lower wavenumber and broadening as Ca/Si increased， consistent with depolymerization of the silicate chains. After the dielectric loss was normalized， every sample showed a broad boson peak near 1 THz （0.82 THz to 1.02 THz）， close to values reported for silicate glasses and to the molecular-dynamics prediction for C-S-H. Its intensity varied non-monotonically with composition， peaking at Ca/Si=1.0 and decreasing on both sides of this ratio. This same composition also corresponds to an inflection in the structural-water loss （200 ℃ to 350 ℃） and the narrowest XRD diffuse peak， marking a structural crossover at Ca/Si≈1.0 from a regime governed by silicate-chain connectivity to one governed by interlayer calcium. The boson-peak frequency corresponds to a medium-range dynamical correlation length of about 1.0 nm to 1.5 nm.

These measurements provide experimental evidence of a boson peak in C-S-H and link its strength to the Ca/Si-dependent medium-range order and elastic heterogeneity of the gel. The terahertz boson peak therefore offers a way to probe the low-frequency dynamics of C-S-H that complements the usual static structural methods.

Key words: calcium silicate hydrate, terahertz time-domain spectroscopy, calcium-to-silicon ratio, dielectric property, boson peak, medium-range order

CLC Number:

TQ172

LI Zhanguo, CHEN Ying, LI Yue, LI Xiangyu. Terahertz Time-Domain Spectroscopy of Calcium Silicate Hydrates with Varying Calcium-to-Silicon Ratios[J]. BULLETIN OF THE CHINESE CERAMIC SOCIETY, 2026, 45(8): 2637-2649.

Figures/Tables 12

Fig.1 Schematic diagram of experimental optical path

Fig.2 XRD patterns of C-S-H samples

Fig.3 FTIR spectra of C-S-H samples

Fig.4 TG and DTG curves of C-S-H samples

Table 1 Calculation results of component content in C-S-H

Ca/Si	Mass fraction/%
Ca/Si	Free water	Interlayer water	Structural water	CH	CC
0.5	6.45	6.59	2.76	7.05	1.31
0.8	7.30	7.65	2.72	4.33	0.04
1.0	7.83	6.36	3.44	5.11	1.70
1.4	6.51	6.40	3.43	6.84	1.81
1.7	5.45	5.67	4.10	9.17	1.72

Table 2 XRF results of C-S-H samples

Ca/Si	Mass fraction/%
Ca/Si	SiO₂	CaO	MgO	Al₂O₃	Cl	Fe₂O₃	Cr₂O₃	SrO	NiO	SO₃
0.5	64.695	34.430	0.502	0.308	0.035	0.030	—	—	—	—
0.8	52.392	46.065	0.264	0.042	1.208	—	0.029	—	—	—
1.0	48.249	51.501	0.204	—	0.040	—	—	0.006	—	—
1.4	42.698	56.821	0.232	0.091	—	0.095	0.050	—	0.013	—
1.7	37.863	61.367	0.331	—	0.076	0.151	0.064	—	—	0.076

Fig.5 Particle size distribution of C-S-H samples

Fig.6 Time-domain signal and power spectra of C-S-H

Fig.7 Dielectric properties of CHand CC

Fig.8 Intrinsic refractive index， permittivity， absorption coefficient， and dielectric loss of C-S-H

Fig.9 Normalized dielectric loss spectra of C-S-H with different calcium-to-silicon ratios （Terahertz boson peaks）

Fig.10 Errors of terahertz boson peak for C-S-H with different calcium-to-silicon ratios

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