Exploring Nano-Calcined Excavation Soil as a Green Cement Alternative

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.May 20 2024

A recent article published in Nanomaterials proposed the application of nano-sized calcined excavation soil (NCES) prepared from waste soil in cementitious materials. It provides a sustainable alternative use of excavated soil waste and an efficient method to enhance the durability of concrete structures.

Background

Cementitious materials are abundantly used in modern infrastructure for their strength and durability. Nano-material admixtures can significantly enhance the mechanical properties and durability of concrete structures prone to gradual decay over time. Thus, various nano-admixtures have been reported, including silica fume, ground granulated blast furnace slag, fly ash, and nano metakaolin (NMK). Among these, NMK is preferred for its excellent performance.

Rapid urbanization produces massive amounts of problematic excavation soil, leading to significant land use and severe environmental concerns due to untreated disposal. Notably, soil mostly has a composition of kaolinite, which can be transformed into metakaolin through calcination. However, this potential for waste soil has barely been researched. Thus, this study proposed a sustainable solution to transform soil waste into high-quality NCES using ball milling as a substitute for cement in construction.

Methods

Firstly, the raw material for NCES, excavation soil, was obtained from a subway construction project in China. It was dried, crushed, and sieved to obtain kaolinite-rich soil, which was then analyzed by X-Ray diffraction (XRD), thermogravimetric analysis (TGA), X-Ray fluorescence (XRF), and scanning electron microscopy (SEM).

Ball-milling was employed for NCES preparation as it can effectively break NCES particles into much finer ones. The influence of milling parameters, including rotational speed, duration, ball diameter, and strategy, was investigated to produce NCES with various specific surface areas.

Finally, a three-round milling strategy with optimized parameters was adopted to prepare NCES with the finest particle size and maximum specific surface area. The surface area of the prepared NCES was characterized by Brunauer-Emmett-Teller (BET) analysis while its surface morphology was examined through SEM.

Subsequently, a Control cement mix (without NCES) was prepared using ordinary Portland cement (OPC). Alternatively, for the NCES-added cement mix, OPC was replaced by NCES (15 wt.%) as a supplementary cementitious material (SCM).

The influence of the NCES particle size on the performance of cementitious materials was examined by preparing four cement mixes (through zero, one, two, and three rounds of grinding of calcined soil, respectively) apart from the Control mix. These cement pastes were characterized using XRD and nuclear magnetic resonance (NMR) spectroscopy.

The effect of NCES substitution in cementitious materials was then examined for mechanical performance (compressive strength test electronic universal testing machine), hydration dynamics, hydration products, microstructure, and heat-releasing properties. Furthermore, the underlying mechanisms were revealed through Fourier-transform infrared (FTIR) spectroscopy, SEM, and mercury intrusion porosimetry (MIP).

Results and Discussion

The cement mix with very fine NCES (specific surface area of 108.76 m²/g) exhibited a 29.7 % enhancement in mechanical strength and refined pore structure. In contrast, a cement mix with un-grounded calcined soil suffered a mechanical loss compared to the Control mix. Additionally, delayed and reduced heat release at an early age was observed in a cement paste mixed with NCES.

The calcined excavation soil replaced 15 wt.% of OPC before being ground to nano-sized particles. This reduced the compressive strength of NCES-blended specimens tested at different ages (3 to 28 days). However, the compressive strength of the blended specimen increased when the calcined soil was ground into nano-size, and the increment highly depended on the NCES particle size. The specimen containing the finest NCES exhibited a compressive strength greater than the Control mix.

This exceptional mechanical behavior of the NCES-blended specimens was attributed to two aspects. First is the pore structure control, as the higher surface areas of nano-size calcined soil offered more reactive sites for either OPC hydration or the pozzolanic reactions between the NCES and hydration product portlandite. This accelerated the formation of calcium silicate hydrate (C-S-H) and/or calcium aluminosilicate hydrate (C-A-S-H) gels filling the pores. In addition, the nano-sized NCES particles could efficiently fill the capillary pores.

The second aspect is the variation of hydration products due to the refinement of NCES, transforming partial C-S-H to a denser C-A-S-H and elongating the C-A-S-H chain. All these factors densified the matrix of NCES-containing cement mixes and improved their mechanical property (durability).

Conclusion

Overall, this study successfully demonstrated the application of excavation soil waste by producing NCES as SCM to enhance the 28-day compressive strength and microstructure of cementitious materials. The ball-milled calcined soil proved to be significantly beneficial in improving the mechanical properties of NCES-blended cement through pore structure and composition modification. The fineness of NCES was crucial in determining the mechanical and heat-releasing properties of the proposed cement mixes.

The researchers propose investigating the property change of the impurities in calcined soil during intermittent milling and their influence on cement product performance. Furthermore, the preparation conditions of NCES need to be optimized for economic and environmental benefits. The results of this work may contribute to the sustainable application of excavation soil waste.

Journal Reference

Ling, L., Yang, J., Yao, W., Xing, F., Sun, H., & Li, Y. (2024). Preparation and Application of Nano-Calcined Excavation Soil as Substitute for Cement. Nanomaterials, 14(10), 850. https://doi.org/10.3390/nano14100850, https://www.mdpi.com/2079-4991/14/10/850

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