(257u) Raman Spectroscopic Characterization of the C-S-H and C-a-S-H Structures and Investigation of Their Behavior in Atmospheric CO2

Sinem Ortaboy^a,b, Jiaqi Li^c, Rupert J. Myers^c, Guoqing Geng^c, Paulo J. M. Monteiro^c, Roya^a Maboudian, Carlo Carraro^a 

^a Department of Chemical and Biomolecular Engineering, University of California at Berkeley, USA

^b Chemistry Department, Engineering Faculty, Istanbul University, Turkey

^c Department of Civil and Environmental Engineering, University of California at Berkeley, USA

Concrete is the most widely used construction material in the world. The primary constituent of this material is Portland cement (PC), which is mainly produced by mixing limestone (lime bearing) and clay (silica, alumina, and iron) and other ingredients such as alumina and iron oxide. The main reaction product of PC hydration is calcium silicate hydrate (referred to as C-S-H) gel, which has unique properties in the cement matrix as a solid binder phase¹. The structure and composition of C-S-H gel greatly influences the strength, durability, and other physical and chemical properties of hydrated PC. The incorporation of aluminum into the C-S-H structure (referred to as C-A-S-H) is typical in modern PC-based cements, which are commonly synthesized using relatively more sustainable and aluminum-bearing supplementary cementitious materials e.g. fly ash². Understanding the chemistry and structure of C-(A-)S-H is important in order to obtain concrete of desired quality and help in formulating processes to reduce the carbon footprint of the cement industry, which is responsible for 5-8% of global anthropogenic CO₂ emissions.

The uptake of CO₂ by C-S-H is generally regarded as a detrimental process because the carbonation process degrades the C-S-H gel structure³ and potentially leads to corrosion of reinforcing steel and deleterious cracking of the reinforced concrete. However, the carbonation process is also a possible means to achieve CO₂ sequestration^4-6. Regardless of the motivation, the ability to monitor CO₂ uptake in C-(A-)S-H can provide information about the atomic-scale and microstructural changes occurring in cement-based materials.

Raman spectroscopy is a powerful method to analyze functional groups through their molecular vibrations. This spectroscopic method provides detailed insight without significant damage to the sample and is sensitive to small changes in the chemical and structural composition of local atomic environments. In this paper, we report Raman spectroscopy results and use them to investigate the influence of temperature, Al content and atmospheric CO₂ uptake on the chemistries and structures of C-(A-)S-H gels prepared with different Ca/Si molar ratios.

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