A new paper in the journal Materials authored by scientists from Nanjing University of Aeronautics and Astronautics and Yangzhou Polytechnic University has investigated the properties of low-carbon concrete and its use in sustainable construction.
Study: Experimental Research on Mechanical and Shrinkage Properties of Alkali Activated Low-Carbon Green Concrete. Image Credit: Elnur/Shutterstock.com
Low-Carbon Concrete
The construction industry is a key driver of climate change. This is due to factors such as the energy and carbon-intensive manufacturing processes of conventional materials such as ordinary Portland cement. The environmental impact of construction has exponentially increased in recent decades due to rapid industrialization and urbanization in developed and developing nations.
Anthropogenic climate change is a critical existential threat to human society, and due to the scale of environmental damage caused by numerous industries, scientists have focused on innovative low-carbon strategies and materials. The role the construction industry plays in this key issue has crystallized research in the sector into sustainable, environmentally friendly materials.
Amongst the various sustainable cementitious materials investigated in recent decades, low-carbon alkali-activated cement has displayed significant environmental benefits compared to ordinary Portland cement.
Studies have revealed that this low-carbon cementitious material emits one-fifth of the amount of CO2 produced by ordinary Portland cement during manufacture. Additionally, this material utilizes aggregates from industrial waste streams, improving the circularity and sustainability of multiple industrial sectors.
Many studies have demonstrated that this sustainable cementitious material possesses comparable or better mechanical, physical, and physiochemical properties than conventional cementitious materials. These green building materials possess a wide range of application potentials.
The Influence of Additives
Low-carbon alkali-activated carbon includes a variety of waste additives which improves its sustainability. However, to compete commercially with ordinary Portland cement, it must possess equivalent or better properties. Due to this key engineering demand, research has focused on evaluating the influence of additives on the mechanical and physiochemical properties of this green building material.
Studies have revealed that the proportions of fly ash, alkali activators, slag, admixture, and sol ratio have an influence on the shrinkage rates of this material. For instance, an increased sol ratio leads to decreased drying shrinkage. However, the drying shrinkage rate of these sustainable cementitious materials is larger than ordinary Portland cement, which presents a challenge for researchers.
Recent research has highlighted the influence of limestone on shrinkage. At 30% inclusion of limestone in low-carbon concrete, which incorporates slag waste, shrinkage increases. But, if this proportion is increased to fifty percent, shrinkage decreases. Shrinkage usually occurs within seven days with concrete that incorporates fly ash and slag before slowing down, with shrinkage ultimately less than that in ordinary Portland cement.
Shrinkage within the first seven days occurs due to a decrease in relative internal humidity, which causes a decrease in surface tension. Expanders can be used to decrease drying shrinkage, but there are challenges with using them due to their negative influence on material strength. Designing durable low-carbon cement is a key research challenge currently.
The Study
To date, studies on alkali-activated low-carbon concrete have focused primarily on material characteristics such as acid resistance, freeze-thaw behavior, and carbonation. Knowledge of shrinkage properties in this green building material is currently limited. As shrinkage can lead to cracking and other factors which impact the performance of final products, this is a critical research gap.
In addition, few studies have investigated the effects of composite proportions on low-carbon concrete. The new paper in Materials has attempted to fill these research gaps by preparing low-carbon alkali-activated concrete using orthogonal experiments.
Using these experiments, the authors have evaluated the influence of waste additives on the strength and shrinkage properties of the material. Scanning electron microscopy, mercury intrusion porosimetry, and X-ray diffraction were employed to study microstructure and product compositions.
Study Conclusions
The study makes some important conclusions based on experimental study and theoretical analysis. Firstly, increased fly ash content causes a gradual decrease in compressive strength. At 28 days, the compressive strength decline is smaller than at seven days. An increase in sol ratio increases compressive strength, with a ratio of over 0.42 achieving maximum compressive strength at 28 days. Increased expander content decreases compressive strength.
Secondly, fly ash content has an influence on shrinkage strain over 28 days, with no discernible difference observed past 30% incorporation. This strain can be reduced by increasing the sol ratio. Increased expander content decreases shrinkage strain gradually at twenty-eight days.
Finally, the incorporation of small amounts of the expander and fly ash leads to decrease micro-cracks in the internal structure due to an increase in hydration products. Moreover, fly ash and expander reduce the porosity of the material, with an increase in small hole volume and a decrease in large hole volume. Crystallization decreases with increased proportions of fly ash.
Overall, the paper makes an important contribution to the field of sustainable construction, revealing the influence of sustainable waste additives on alkali-activated low-carbon concrete shrinkage and compressive strength properties. Thus, the paper provides future researchers with key information on this green building material.
Further Reading
Xu, S et al. (2022) Experimental Research on Mechanical and Shrinkage Properties of Alkali Activated Low-Carbon Green Concrete Materials 15(17) 5984 [online] mdpi.com. Available at:
https://www.mdpi.com/1996-1944/15/17/5984
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