Basil Ash Boosts Ultra-High-Performance Concrete

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 3 2024

A recent article published in Case Studies in Construction Materials investigated the effect of partially substituting cement with agricultural waste in ultra-high-performance concrete (UHPC). Specifically, 5-25 wt.% of ordinary Portland cement (OPC) was substituted with basil plant ash (BPA).

Background

The demand for UHPC has increased rapidly during the last three decades to build special structures subject to high stresses. According to international standards, UHPC's compressive strength should be over 120 MPa and tensile strength should be over 12 MPa at the 28-day test period.

To achieve these properties and reduce cement consumption, the application of supplementary materials for cement has expanded to include industrial and agricultural waste. Incorporating such waste materials into UHPC can further enhance its durability and mechanical characteristics.

Furthermore, utilizing agricultural and other organic waste in UHPC helps address environmental concerns related to the disposal of this waste in landfills and carbon emissions from cement production. Thus, although the use of BPA in UHPC is currently unexplored, this research investigated its potential as a cement substitute in UHPC.

Methods

Locally sourced Beni Suef's plant waste of basil, OPC, silica fume (SF), pure river sand, and a third-generation superplasticizer (SP) were used as raw materials for concrete production. BPA was prepared from the plant waste by air-drying at 25 °C and subsequently subjecting to heat treatment at different temperatures of 300 °C, 500 °C, 700 °C, and 900 °C. 

The microstructure, elemental composition, and spatial distribution of the resulting BPA samples were examined using X-ray fluorescence (XRF), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX).

Subsequently, homogeneous concrete mixes were prepared including a reference mix (without BPA) and twenty mixes containing different OPC replacement ratios of BPA (5, 10, 15, 20, and 25 wt.%) treated at distinct temperatures.

The concrete’s compressive strength was analyzed using cubic samples after 7, 28, and 90 days of curing on a hydraulic testing machine. Alternatively, the tensile splitting strength, modulus of elasticity, flexural strength, and water permeability were determined using cylindrical specimens after 28 days of curing.

A sorptivity test was conducted on all mixes for durability assessment while their microstructures were analyzed using SEM. Moreover, thermogravimetric analysis (TGA) was performed to estimate the mass loss due to high-temperature (up to 900 °C) treatment of BPA and to evaluate hydration products' decomposition phases. Finally, X-ray diffraction (XRD) analysis was used to investigate the concrete samples that achieved optimum compressive strength after 28 days of curing.

Results and Discussion

The XRF, EDX, and SEM results revealed that thermal treatment and subsequent grinding significantly varied the chemical and physical properties of BPA, including particle shapes and surface characteristics. 

The BPA-containing concrete exhibited increased compressive strength, splitting tensile strength, flexural strength, and elastic modulus compared to the reference concrete. This increase was observed for all OPC replacement ratios of BPA heat-treated up to 700 °C. Overall, the best results were achieved for 20 wt.% OPC replacement with BPA treated at 700 °C.

The enhanced mechanical strength of BPA-concrete was attributed to the pozzolanic reaction of BPA with Ca(OH)₂, which is a by-product of OPC hydration, and the creation of extra hydration gel (calcium-silicate/aluminum silicate-hydrates). These conclusions were derived from SEM micrographs and TGA results.

Additionally, the finer BPA particles resulting from the heat treatment and grinding process increased packing efficiency in the concrete mix. These factors also reduced the water permeability of UHPC containing heat-treated BPA. However, the concrete containing 25 wt.% BPA exhibited a slight decrease in mechanical strength for all treatment temperatures. This was attributed to the diluting impact caused by the decreased quantity of OPC in UHPC. 

Additionally, BPA treated at 900 °C harmed the transport properties of the concrete, increasing water permeability. This was explained by the XRD results, which demonstrated that increasing temperature has a harmful effect on the crystalline arrangement of SiO₂, reducing BPA’s effectiveness in reacting with Ca(OH)₂.

Conclusion

Overall, the concrete fabricated using BPA treated at 700 °C as a partial substitution for 20 wt.% of OPC exhibited superior mechanical properties compared to the reference concrete without BPA. For instance, the compressive strength and splitting tensile strength were enhanced by 15.07 % and 20.39 %, respectively, compared with the control mix at 28 days.

The TGA, XRD, and SEM analyses were consistent with the determined mechanical and durability characteristics. Thus, using BPA for partial substitution of OPC at 20 wt.% can result in UHPC at low costs. Additionally, heat treatment of BPA at high temperatures up to 700 °C can raise the replacement rate to 25 wt.% while achieving excellent transport properties.

The results of this study can help accelerate the utilization of agricultural wastes in the construction sector to produce sustainable building materials.

Journal Reference

Zeyad, A. M., Agwa, I. S., Abd-Elrahman, M. H., & Mostafa, S. A. (2024). Engineering characteristics of ultra-high performance concrete containing basil plant ash. Case Studies in Construction Materials, e03422. DOI: 10.1016/j.cscm.2024.e03422, https://www.sciencedirect.com/science/article/pii/S2214509524005734

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