Methods of pozzolanic activity of scoria
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Methods of Assessing Pozzolanic Activity of Scoria
Introduction to Pozzolanic Activity of Scoria
Scoria, a volcanic rock, is increasingly being used as a natural pozzolan in concrete due to its beneficial properties. Pozzolanic activity refers to the ability of a material to react with calcium hydroxide (portlandite) in the presence of water to form compounds possessing cementitious properties. Various methods are employed to assess the pozzolanic activity of scoria, each providing insights into its effectiveness as a supplementary cementitious material (SCM).
Chemical and Mineralogical Analysis
X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) XRD and XRF are commonly used to determine the mineralogical and chemical composition of scoria. These techniques help identify the presence of reactive minerals such as plagioclase feldspars and pyroxenes, which influence the pozzolanic reactivity under different curing conditions . The presence of amorphous sodium aluminosilicate phases, such as albite, is particularly significant as they react with portlandite to form cementitious compounds.
Steam Curing Techniques
Low-Pressure and High-Pressure Steam Curing Steam curing, both at low and high pressures, is employed to accelerate the pozzolanic reaction. High-pressure steam curing, or autoclaving, is particularly effective in stimulating the reactivity of inert siliceous and aluminosiliceous materials. Studies have shown that scoria containing plagioclase feldspars reacts better under high-pressure conditions, while those with pyroxene minerals are more reactive under low-pressure steam curing. The consumption of portlandite during these processes is a direct measure of pozzolanic activity, with scoria samples showing significant portlandite consumption under both curing conditions.
Mechanical Properties and Strength Activity Index (SAI)
Compressive Strength Tests The compressive strength of concrete mixtures containing scoria is a critical indicator of its pozzolanic activity. Scoria samples have demonstrated high compressive strengths, comparable to other SCMs like fly ash and rice husk ash, with strengths exceeding 50 MPa at 28 days for high-volume replacements . The Strength Activity Index (SAI) is another metric used, with scoria achieving SAIs of up to 99% under high-pressure steam curing.
Microstructural Analysis
Scanning Electron Microscopy (SEM) and Thermogravimetric Analysis (TGA) SEM and TGA are used to study the microstructural development and thermal properties of scoria-containing concrete. These analyses reveal the formation of tobermorite-like structures, which contribute to the strength and durability of the concrete. The development of these structures is dependent on the mineralogical composition of the scoria and the curing conditions.
Heat of Hydration
Isothermal Calorimetry Isothermal calorimetry measures the heat of hydration, which is crucial for mass concrete applications. Scoria has been shown to effectively reduce the heat of hydration and the associated temperature rise, making it suitable for use in mass concrete constructions. This reduction in heat is comparable to that achieved with fly ash, highlighting scoria's potential as a natural pozzolan.
Conclusion
The pozzolanic activity of scoria can be effectively assessed using a combination of chemical, mineralogical, mechanical, and microstructural analyses. Techniques such as XRD, XRF, steam curing, compressive strength tests, SEM, TGA, and isothermal calorimetry provide comprehensive insights into the reactivity and performance of scoria as a supplementary cementitious material. These methods confirm that scoria is a viable and effective natural pozzolan for enhancing the properties of concrete.
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