Global interest in ultra-high performance (UHP) concrete is growing. The best method for creating high-performance and super-high-performance concrete is using nanoparticles.
Silica fume (SF) is typically thought of as the most readily accessible ingredient for producing high-performance concrete, despite current efforts to incorporate nanomaterials, such as carbon nanotubes, nano-clay, and nano-SiO2, to increase the performance of concrete.
However, the silica fume also has similar health hazards associated with nanoparticles since silica fume has an average particle diameter of 150 nm, making it an ultrafine material. These ultrafine particles may get absorbed in the digestive tract, lungs, or skin, causing severe health and safety issues for living beings.
Motivation Behind the Study
This study is a continuation of a previous study conducted by the same researchers, which demonstrated that the physical morphology and the composition of silica fume could be recognized by the combination of Raman spectroscopy and a light optical microscope. Moreover, the researchers also identified silica fume agglomerates in raw silica fume slurry and hydrated SF–PC blends.
However, before this technology could be applied practically, further studies were required since, in real-life situations, concrete interacts with deleterious substances in the surroundings such as SO4 2-, Cl-, CO2 O2. These enter concrete causing chemical reactions that degrade some hydration products and modify the microstructures. These degradation processes may affect silica fume agglomerates' stability, imposing threats to living systems.
The Deterioration Mechanisms Used in the Study
The degradation of concrete is mainly caused by specific deterioration mechanisms, including sulfate attack, chloride attack, and carbonation.
Cement hydration products react with atmospheric carbon dioxide causing deterioration of concrete. Calcium-bearing hydration products, including calcium aluminates, calcium hydroxide (Ca(OH)2, CH), and calcium silicate hydrate (C–S–H), react with carbon dioxide producing hydrated aluminum, silica gel, and calcium carbonate (CaCO3). However, the most dominant reaction is between carbon dioxide and calcium hydroxide (CH), converting CH into calcium carbonate.
pH of the concrete pore solution is reduced, causing corrosion of reinforcing bars. Moreover, calcium carbonate densifies the microstructure of the cement matrix, and carbonation shrinkage can cause microcracks formation, potentially affecting silica fume agglomerates' stability.
Sulfate salts (SO42-) enter cement to react with hydrates such as ferrite phase, unreacted aluminate, monosulfoaluminate, tricalcium aluminate hydrates, calcium hydroxide forming ettringite (AFt) and gypsum.
This harms the hardened cementitious materials as ettringite causes spalling, cracking and destroying cement. Moreover, the cracks can lead to further interaction between the surrounding environment and cement matrix affecting SF agglomerates.
Hydrated aluminate phases react with chloride ions to yield Friedel's salt (3CaO Al2O3 CaCl2 10H2O) that occupies more space, causing pore refinement. Moreover, aluminate hydrates are consumed in this process, altering cement matrices' chemistry and the stability of the silica fume agglomerates.
Owing to these problems with concrete, this study intends to trace the status of silica fume in cementitious materials subjected to different deterioration mechanisms. It also shows the working capabilities of Raman microscopy to identify the silica fume in deteriorated cementitious materials.
How the Experiment was Carried Out
Several silica-fume-blended-PC-paste samples were prepared and matured at 20 °C for six months. Deterioration regimes were then applied to these samples.
Moreover, some silica fume blended PC pastes were also put through severe possible deterioration conditions. For this purpose, silica fume combined PC pastes were converted into powder form of 63 μm fineness after six months of a maturing period and were then exposed to severe deterioration regimes for three months before Raman microscopy analysis was conducted.
Results of Raman Microscopy Analysis
The study employed a Raman microscope to observe the stability of the unreacted silica fume within SF–PC blends subjected to sulfate attack, chloride ingress, and carbonation deterioration mechanisms.
In the SF–PC blends subjected to sulfate attack and carbonation, the Raman bands at 350–540 cm-1 identified amorphous silica. However, no silica phase was identified in SF–PC blends subjected to chloride attack due to continued hydration of silica fume due to enhanced cement hydration. These observations determine potential risks to living systems in long-term servicing structures subjected to such a deterioration environment.
Moreover, the study demonstrated the application of the Raman microscope for tracing silica fume in cement under deterioration mechanisms. This indicates Raman microscopy's effectiveness in monitoring the status of nanomaterials such as silica fume in concrete structures.
Yanfei Yue, Jingjing Wang and Yun Bai (2022) Tracing the Status of Silica Fume in Cementitious Materials Subjected to Deterioration Mechanisms with Raman Microscope. Materials. https://www.mdpi.com/1996-1944/15/15/5195/htm