It is common to use natural fine aggregates (natural sand) in concrete. However, the environmental impacts caused by the extraction of this natural resource and its scarcity in certain regions means that crushed sand is increasingly sought after for the production of concrete.
Due to the growing demand for artificial fine aggregates (crushed sand), the concrete technologist needs to act to prevent the granulometry and the shape of the grain from interfering negatively in the performance of the concrete. One way is to use additives to the mix to mitigate the negative effects of artificial sand. Therefore, air entraining agents allow concrete with little or no natural sand to be produced with economic and technical success.
The Use of Natural Sand vs. Crushed Sand in Concrete Mixtures
About 35 % volume of concrete is comprised of sand. Generally, cement and coarse aggregates are factory made products and their quality and standards can be easily controlled and maintained. The sand is usually excavated from riverbeds or riverbank by digging.
The artificial fine aggregate is a well-known and widely used material. However, the obvious and acceptable preference is for the natural aggregate.
While comparing the characteristics of both grain shapes, natural sand has a round shape and a smooth surface. Therefore, these characteristics contribute to the reduction of friction between the particles of the concrete mixture. In contrast, artificial sand usually has the opposite characteristic, such as lamellarity and high surface roughness. This explains the preference for natural sand.
However, economic and environmental drivers have increased the demand for artificial sand. Since natural sand is extracted from riverbeds and sedimentary deposits, the sand mining process negatively affects the local environment. Parallelly, due to the over-extraction of this natural resource, river sand has become increasingly expensive in certain regions. Thus, the use of an artificial fine aggregate has become necessary.
As natural sand is partially or completely replaced by artificial sand, the rheological behavior of the mixture is profoundly changed. On these occasions, the granulometry and the grain shape can negatively interfere in the performance of both the fresh and hardened state of concrete.
The Impact of Crushed Sand on Fresh Concrete
In its fresh state, the biggest impact of using artificial sand in the concrete mix design is on its rheology. Even when obtaining a good particle size distribution, the crushed sand’s grain shape and the interaction between particles make some types of concrete almost impractical. For example, fluid concrete and self-compacting concrete demand great fluidity with rheological stability. However, it is possible to replace natural sand with artificial sand in the composition of self-compacting concrete.
The Impact of Crushed Sand on Dry Concrete
In the hardened state, there are a few more points to note. Mix designs with artificial sand can suffer from having a higher demand for water per cubic meter of concrete. This occurs due to the larger surface area of the aggregate, especially when crushed sand has high levels of materials finer than 75-μm.
Higher water consumption is not the only change. Increasing the water also increases the addition of cement. Thus, concrete with artificial sand consumes more cement per cubic meter compared to a mixture with natural sand at the same water/cement ratio.
In addition to the above-stated economic factors, durability may be impaired. Concrete mix design with crushed sand will have a lower resistance to abrasion due to the excess of super fines. At the same time, there will be a greater probability of contact between grains of sand without the presence of paste, which may cause the microstructure to be more permeable.
However, there is a way to reduce the negative effects of artificial sand and make its use economically viable. Especially in concretes without natural sand, one solution is to use concrete additives that reduce the friction between the particles. Two examples are air entraining agents and water reducers.
Additives for a Concrete Mix Design with Crushed Sand
Concrete additives for mixtures with artificial sand, whether in combination with natural sand or not, need to incorporate air in controlled quantities to minimize the negative effects of the crushed sand. One solution is to add a water-reducer to the concrete mix. This type of additive disperses cement grains and small particles by electrostatic repulsion and reduction of surface tension. In this way, the water reducer can generate entrained air, which reduces the friction between the sand grains.
Another option is to use an air-entraining agent independently from a water-reducing additive. By handling these additives separately, it is possible to control their dosages and effects with efficiency. Hence, it is possible to avoid adverse effect of the water reducer to the concrete, such as excessive setting time delay, segregation, or bleeding.
Air Entraining Agents in Concrete
Air entrainers have hydrophilic and hydrophobic molecules, remarkably similar to soap and other surfactant formulations. They have the ability to decrease the surface tension of water and form a film around the air bubbles created during the mixing process. Thus, air entraining agents stabilize air bubbles until the concrete begins to harden and prevents air from escaping the mixture.
Types of Air-Entraining Agents
The 4 most common types of raw materials for air-entraining agents in the global market are:
- Wood resins,
- Sulphonated hydrocarbons,
- Fatty and resinous acids, and
- Synthetic materials.
How to Identify a High-Quality Air-Entraining Agent?
The size of the air bubbles embedded in concrete are essential for a quality performance. Ideally, air bubbles are less than 1 mm in diameter and are on average between 0.2 and 0.3 mm. Therefore, this type of bubble contributes to the mobility of crushed sand particles within the mix without impairing compressive strength. It is worth mentioning that this argument is only true if the volume of entrained air is less than 5%.
In order to guarantee the average diameter and the correct distribution of air bubbles, it is important to use raw materials in the manufacture of additives that promote the proper incorporation of air, does not affect the mechanical resistances, and helps in the rheology of the concrete.
Among the most common raw materials, wood resins have an advantage over other types of air entraining agents. This plant-based type increases the air content in a more controlled way in relation to the dosage and without a significant enlargement in bubble diameter for concretes with less than 5% of entrained air. In other words, air-entraining agents derived from wood resins elevate the air content to a certain extent, even if the mixing process continues. In comparison, other raw materials entrain air continuously while the mixture is being stirred. Thus, synthetic resins and sulfonated hydrocarbons are interesting for cellular concrete, for example.
What is the purpose of adding an air entraining agent to the concrete mix with crushed sand?
With the correct air entrainment, concrete containing artificial sand will have a substantial gain. The additive enables the use of crushed sand in the cementitious mix with up to 100% replacement of natural sand.
If applied correctly, even sands from construction demolition are suitable substitutes for river sand. Even with a rough particle size and high superfine content, this type of artificial material has potential. A high-quality concrete can be produced as long as an air entraining agent is added into the mix.
Furthermore, it is important to note that the addition of an air-entraining agent usually does not affect the setting times. This is true because normally these additives do not influence the hydration process of Portland cement.
However, the air content can be affected by the type of cement. Finer cements with higher levels of mineral addition tend to hinder the entrainment of air due to their particle packing and the interaction between particles in an aqueous system.
Below is a comparative image between 2 mixtures of concrete with artificial sand. Only the slump test on the right side contains an air entrainer in the mix.
Air-entraining agents play a fundamental role in the feasibility of producing concrete with crushed (artificial) sand. Especially in the absence of natural sands, an air entrainer allows for a flexible choice of materials in the production of concrete. This occurs because the additive creates thousands of microscopic air bubbles that are well distributed within the mixture. Thus, these air bubbles act as a lubricant between the rough particles of crushed rock sand. Consequently, places without available river sand can produce concrete with the required characteristics and without technical or economic disadvantage.
About the Author
Mr. Santos is a civil engineer with a degree from the Federal University of Pernambuco (UFPE), with a postgraduate degree in tunnel engineering and in general and industrial chemistry. He worked for decade as a product engineer and technical manager within multinational companies in the field of construction chemicals, mainly in concrete and mortar additives. In 2016, he founded GMG Engenharia e Consultoria and works as its technical director. With vast expertise, Geniclésio Santos and his team provide consultancy for companies in Brazil and abroad for infrastructure projects. Also, the company has experience working unconventional construction materials, such as adhesives, polymeric coatings, waterproofing, etc… Lastly, Geniclesio Santos participates in the review and development committees for ABNT standards. Recently he was appointed secretary in the board that studied the new additive standard: ABNT NBR 11.768 Concrete Additives for Portland Cement.
Check out GMG Engenharia e Consultoria LinkedIn’s page for more information.
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