Nano-Alumina Enhances Ultra-High-Performance Cement

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Aug 26 2024

A recent article published in Materials investigated the influence of nano-silica (NS), nano-alumina (NA), and nano-calcium oxide (NC) particles on the properties of fresh cement pastes and their compressive strength variation over a year. Among these, NA was most suitable for ultra-high-performance cement (UHPC)-based systems.

Background

The superior compressive strength of UHPC compared to regular concrete makes it ideal for various critical applications, such as bridges, ultra-high-rise buildings, and areas prone to seismic activity. Recent advancements in UHPC development have introduced additives that enhance and maintain concrete strength while also reducing environmental impacts and costs.

Incorporating nanoparticles into UHPC reduces its microscopic pores, thereby improving its density. This filling effect and packing improvement further expand the range of applications for UHPC systems. Moreover, nanomaterial manufacturing contributes very little to carbon emissions, making it a sustainable choice for additives.

While most literature on UHPC modification with nanoparticles focuses on the use of nano silica (NS) and nanocalcium carbonate (NC), this study evaluated the performance of cement pastes modified with NS, nano-alumina (NA), and NC. The most suitable nanoparticles among these were then incorporated into the UHPC mix.

Methods

The study was conducted in two stages. In the first stage, cement pastes were prepared with 1.5 wt.% of different nanoparticles. These pastes were cast into 25×25×50 mm³ specimens, de-molded after one day, and then immersed in water until testing.

The hydration heat of the freshly prepared cement pastes was recorded immediately after determining their consistency using an eight-channel isothermal calorimeter. Additionally, the mineralogical composition of the hydration products in the pastes was analyzed using X-ray diffraction (XRD).

The compressive strength of the synthesized pastes was measured at 7, 28, 90, 180, and 365 days of aging using a computer-controlled universal testing machine. Furthermore, the open porosity of the pastes was determined in water under vacuum conditions at the same testing intervals.

Based on the results from this stage, nano-alumina (NA) was selected for incorporation into UHPC in the second stage. Two different proportions of NA (1.5 and 3.0 wt.%) were added to the UHPC to evaluate their impact on mechanical properties, flowability, and carbonation.

The compressive strength and open porosity of the UHPC specimens in water were measured at 7 and 28 days. Additionally, carbonation was assessed both with and without NA under two different curing regimes: one at a temperature of 22 ±2 °C with 55 % humidity and another at 24 ±2 °C in a 3 % CO₂ atmosphere. Finally, the calcium carbonate content was quantified at 7, 28, and 90 days through thermogravimetric analysis in an N₂ atmosphere, ranging from 50 to 1000 °C.

Results and Discussion

The nano-additives NA, NS, and NC each distinctly influenced the properties of cement due to their unique characteristics. NA, for instance, accelerated alite hydration by providing nucleation sites, which resulted in a more pronounced early peak in the calorimetric analysis. Additionally, NA promoted the formation of calcium-aluminum compounds, as evidenced by XRD spectra, further accelerating hydration, enhancing compressive strength, and modifying the porosity of the cement paste.

In contrast, NS enhanced early hydration rates through pozzolanic reactions with calcium hydroxide, leading to the formation of additional hydrates and increasing the density of the cement. This contributed to improved early strength but could potentially slow down hydration in later stages. Notably, while NS increased the hydration heat, it did not significantly alter the compressive strength of the cement.

NC, on the other hand, significantly accelerated early hydration kinetics and the initial formation of the microstructure, thereby enhancing early strength development. However, this rapid initial activity led to an early onset of slower diffusion-controlled hydration phases, which limited its long-term impact on compressive strength.

Among these additives, NA demonstrated the most consistent improvement in the mechanical properties of cement over time, enhancing compressive strength by 15 % at 7 days and by 1-6 % at later stages. Furthermore, NA reduced open porosity, resulting in denser and more durable cement pastes. Consequently, NA was selected for incorporation into UHPC samples rather than NS and NC.

NS-containing pastes required higher water content compared to other systems, while NC exhibited the lowest trend in compressive strength, making them less suitable for UHPC applications.

The optimal NA content for incorporation into UHPC was determined to be 1.5 wt.%, as it did not negatively impact the workability or consistency of the cement. NA-modified pastes maintained their initial water-to-binder ratio while achieving minimal porosity and maximum compressive strength. Moreover, NA did not interfere with the carbonation resistance of the UHPC.

Conclusion

Overall, the researchers compared the effects of incorporating NS, NA, and NC into cement pastes, finding that each of these nanomaterials accelerated the hydration process through distinct mechanisms.

NA enhanced the formation of hydration products at early stages and increased compressive strength by 10 % at later ages. In contrast, NC reduced the porosity of cement pastes by 54 % at 28 days, while NS failed to significantly improve cement strength. As a result, NA was identified as the most suitable nanomaterial for use in UHPC systems.

Incorporating NA preserved the ultra-high-performance characteristics of the cement while also increasing its compressive strength in a CO₂ atmosphere. This makes NA a promising advancement for improving the mechanical properties and durability of UHPC systems under various environmental conditions.

Journal Reference

Tsardaka, E.-C., Tsampali, E., & Stefanidou, M. (2024). The Contribution of Nano-Alumina to Ultra-High-Performance Cement-Based Systems. Materials, 17(16), 4120. DOI: 10.3390/ma17164120, https://www.mdpi.com/1996-1944/17/16/4120

Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.