Raman spectroscopy as a tool to understand the mechanism of concrete durability—A review
*Chenxi Tang as first author
Journal: Construction and Building Materials (CBM)
Since concrete durability issues due to deterioration involve physical and chemical reactions, it is important to understanding the fundamental knowledge of the chemistry changes over time. While a number of analytical tools are available for this purpose, there are certain drawbacks in terms of cost, time consumed and technical limitations. On the other hand, the development of Raman spectroscopy has enabled the determination of symmetric chemical bond stretching and bending vibration change of both amorphous and crystalline phases in cement-based materials.
Raman spectroscopy is a vibrational spectrum technology which can be used for studying the vibration states of excited molecules in a material. This particular technique is often used as a complementary tool for infrared spectroscopy as it is equipped with a wider incident wavelength. When the incident photon impinges on materials, scattering would occur and it is sensitive to the molecular structure. The extent of the Raman shift is related to the vibration of molecular bonds in the impinged materials, and hence could provide information on the structure such as bond strength, chemical states of the molecular and some local interactions between molecular, for instance hydrogen bonding. In cement-based materials, there are various silicate and sulfate phases which have O-M-O (SiO42-, SO42-) chemical bond structures. Molecules containing O-M-O structure have four vibration modes, namely V1 = symmetric stretching of O-M bonds, V2 = in-plane bending vibration of O-M-O bonds, V3 = asymmetric stretching of O-M bonds and V4 = out-of-plane bending vibration of O-M-O bonds. Previous studies found that these molecules (SiO42-, CO32-, SO42-, etc.) which have O-M-O structures were mostly Raman active, and can be used for explaining the reaction mechanisms of cement by accurately detecting the chemical environmental evolution of M atoms.
The first study of using Raman spectroscopy for cement materials was conducted by John Bensted in 1976. In this study, white Portland cement clinker minerals, calcium carbonate and calcium sulfate were tested. The results clearly demonstrate that Raman spectral can suitably characterize the reaction mechanisms of cement. Subsequently, Bensted continued the study on understanding the carbonation phenomena of white cement using Raman spectroscopy. However, these early studies were constrained by limited technology with a relatively low spatial and spectral resolution. With the development of Fourier transform, charge coupled device (CCD) and other modern technologies in the past four decades, the main mineral phases in cement (Alite/C3S and Belite/C2S) can be efficiently and accurately detected by Raman spectroscopy. Moreover, Raman spectroscopy has the potential for characterizing amorphous phases in cementitious materials. Raman spectroscopy is sensitive to the local structure of molecule, typically its nearest and next nearest atomic neighbors (MM and MMM). Hence, there is no need for the presence of long-range order structures in tested samples as required by other methods such as X-ray diffraction (XRD). Thus, Raman spectroscopy is mostly useful for characterizing materials which lack long-range order structures, such as C-S-H gels and alkali-silica reaction (ASR) gels.
Raman spectroscopy has opened up new perspectives that differ from previous analytical methods to elucidate the whole chemical reaction process. In this review, previous researches which utilized Raman spectroscopy for characterizing concrete durability problems (Carbonation, sulfate attack and alkali-silicate reaction) are summarized and these are compared with results from other tests such as XRD, Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), etc. It is the objective of the authors to demonstrate the potential of Raman spectroscopy in explaining deterioration mechanisms in relation to common concrete durability issues, as well as to instigate wider use of Raman spectroscopy in the area of cement chemistry.
混凝土耐久性问题涉及复杂的物理和化学反应，了解这些不同时期下这些化学变化的基本知识是非常重要的。虽然现在有许多的分析方法可直接或者间接的测定这些化学变化，但在成本、时间消耗和技术限制方面都或多或少存在一些限制。拉曼光谱法是一种振动光谱测试方法，可用于研究材料中激发态分子的振动状态。由于它相对于红外光谱来说具有更宽的波长，因此常被用作红外光谱的补充手段。当光子与待测材料相遇时，会发生散射现象，其中有约1%的散射光会发生频率变化，这一部分的散射称作拉曼散射。拉曼散射过程中这一频率的变化是固定的，它的大小与被测试材料中分子键的振动有关，因此可以提供材料的结构信息，如键强度，分子的化学状态以及分子之间的局部相互作用，如氢键。在水泥基材料中，有大量的硅酸盐和硫酸盐，它们具有类似O-M-O结构(SiO42-， SO42-)的化学键。通常来说，含有O-M-O结构的分子有四种振动模式，即：V1 = O-M键的对称伸缩，V2 = O-M-O键的平面内弯曲振动，V3 =O-M键的不对称伸缩，V4 = O-M-O键的平面外弯曲振动。这些O-M-O结构的分子(SiO42-、CO32-、SO42-等)大多是具有拉曼活性的，可以通过准确检测M原子的化学环境演化来解释水泥耐久性破坏过程的反应机理。
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