Projects / 5/13/2025

Dealing with difficult aggregate in concrete production

Mapei admixtures contribute to produce sustainable concrete with new demanding raw materials

Author

Sven-Henrik Norman

Corporate Product Manager – RE-CON line

As high-quality natural aggregates become scarcer, concrete producers are having to adapt to alternative raw materials, such as new aggregates with higher water requirements, impacting on sustainability. Mapei Re-Con AGG admixtures help concrete mitigate the higher water demand from a number of new demanding raw materials.

This is the first in a series of articles that will focus on how to meet the challenges of challenging new raw materials in concrete production, particularly coarse and fine aggregates from different sources. Mapei concrete admixtures have a decades long track record of helping our customers produce concrete with the required workability and slump retention without having to add water and cement. Keeping the cement dosage as low as possible is the best way to reduce the carbon footprint of concrete. When raw materials change towards having a higher water demand, it has a negative effect on the sustainability aspect of concrete.

The latest range of Re-Con AGG admixtures offers great help to mitigate the higher water demand from a number of new demanding raw materials. In combination with other Mapei concrete admixtures, like for example Mapecube strength enhancers, the challenges of today can be met in a cost effective way. Let’s take a look at how the aggregate landscape has changed in the last decade and how to continue on the road of sustainable concrete production, even if the raw material situation becomes challenging.

In many areas high-quality sands are no longer locally available and concrete producers are increasingly being forced to turn to natural sands with clay intrusions, manufactured sands, and recycled aggregates.

From depletion to innovation: the shift in aggregate sources

Global demand for concrete continues to rise, driven by urbanization, infrastructure development, and population growth. This has led to the progressive depletion of high-quality natural sand and gravel resources, many of which were traditionally sourced from glaciofluvial deposits with naturally rounded, clean particles and favorable grading.

Modern concrete mix design has been largely developed and optimized using clean, well-shaped, and consistently graded natural aggregates. Historically, industry standards for concrete aggregate and cement have reflect this legacy: mainly based on idealized, pure natural sands with low fines content and minimal variability. Cement and water dosages, workability assumptions, and durability targets have all been defined with these reference materials in mind.

However, the reality of today's material supply chain is changing. In many regions, these ideal sands are no longer locally available, or their extraction is restricted due to environmental regulations. As a result, industry standards and traditional mix designs are challenged as concrete producers are increasingly being forced to turn to:

Natural sands with clay intrusions, which were previously rejected but are now used out of necessity. Clay of certain types-swelling clays like can have an extremely high water demand.

Manufactured sands, produced by blasting and crushing rock, are often angular and high in water demanding fines. Granite based manufactured sands are common in Northern Europe and in America. This rock type can contain high levels of free mica. Mica is a mineral group with a flaky crystalline structure, which gives it a very high specific surface, resulting in high water demand. Another potential cause of increased water demand with manufactured sand is high so called Zeta potential. If the manufactured sand is used in concrete too soon after production, negative ionic surface charges (Zeta potential) can cause rapid slump loss in concrete.

Recycled aggregates, derived from construction and demolition waste, with varied composition and porosity. With Mapei’s Re-Con Zero and Re-Con Dry Washing solutions, a recycled aggregate based on returned concrete and transformed concrete truck residue is available to the concrete producer. The increased porosity of recycled aggregates is a well-known challenge from the point of view of water demand.

These alternative materials can exhibit higher fines content, irregular shapes, and unpredictable surface chemistry—factors that directly affect workability and water demand.

At the same time, the industry is under growing pressure to reduce the carbon footprint of concrete, especially by lowering cement and water content in concrete mixes. This creates a clear conflict: we are asking concrete to do more, with less, while working with lower-quality raw materials than ever before.

Understanding and adapting to this shift is essential—not only for maintaining performance but also for enabling more sustainable and resource-efficient construction practices. This is where admixture technology plays a critical role, and this series of articles will look at some basic aggregate properties that influence the water and cement paste demand in concrete.

In Part 1, we will focus on the influence of particle shape, particle size gradation, and particle surface texture on the need for total cement paste volume in a concrete mix design.

Part 2 will focus on manufactured sand, and the challenges it may give to sustainable concrete production.

In Part 3, the negative effects of porous materials like clay minerals or RCA will be further explained.

And finally, Part 4 will focus on how Mapei’s total admixture program can help concrete producers overcome the challenges described in Part 1-3 to achieve concrete with the required fresh properties without having to use higher levels of water and cement.

Part 1. The Influence of particle shape, gradation and surface texture on the need for cement paste in a concrete mix design.

One of the key impacts of switching from natural to alternative aggregates is the change in how efficiently the particles pack together. The better a mix of coarse and fine aggregate will pack itself by free flow into a volume, the lower will be the volume of air between the particles. This empty air volume is described as voids content and if the aggregate has not been forced into the volume, it is called the uncompacted voids content. Why is this important and what parameters influence this formation of air pockets (voids) in an aggregate mix? Let’s take a closer look.

Particle shape

The first parameter is particle shape. Imagine that you have two glass jars of the same volume, and that you fill them with particles. One with a mix of very flaky and elongated shapes, and one with perfect spheres. The jar filled with perfect spheres will require much less of liquid to fill the air pockets that still remain in the jar compared to the other jar.

Pictures 1 and 2. The effect of particle shape on voids content, illustrated here by comparing bolts to marbles (AI-generated images).

Imagine two glass jars of the same volume that are filled with either bolts or glass marbles. The jar containing the glass marbles will require much less fluid in order to be filled completely. In a real concrete mix design situation-where the bolts represent crushed sand and aggregate and the marbles are the natural sand- this difference can be as much as 50 litres of cement paste needed- just to reach the same base level of a completely filled voids between all particles, ie. the “zero-slump” concrete. The particle shape will also affect how much cement paste is needed to then make the concrete easy to pump place and finish. More on this in the following articles of this series.

Gradation

The second parameter that influence the air voids content of an aggregate mix is the composition of the different size fractions. An aggregate mix with a variety of different sizes of particles will have a smaller voids content than a mix with more mono-sized particles. Natural sand and gravel from high quality deposits will have an even distribution of particle sizes. For every fraction forming voids, there will be particles from smaller fractions filling those voids. But as mentioned, the natural resources of this kind of high quality sand has been depleted. In many deposits, what is left is sand which is either finer or coarser than the ideal gradation- sometimes simply because it has been discarded while there was still well graded sand available. When replacing the fine aggregate in concrete with either a natural sand of poorer gradation or a manufactured (crushed) sand, the ability for the sand to pack itself in a natural and dense way is often lost. This in turn leads to increased voids and an increased need for cement paste to fill those voids. Thus, more cement paste is consumed, just to achieve a fully packed aggregate/paste system to form a solid concrete volume with only 1,5-2% air content. This starting point can be referred to as “zero slump” concrete.

An AI generated impression of a mono-sized, poorly graded aggregate on the right and a well graded material on the left. The voids content in the well-graded material are lower because there are particles of varying sizes to fill up any gaps.

Surface texture

The third parameter is the surface texture of each individual particle. High quality natural sand has been subjected to heavy polishing by natural forces as it has been transported in ice layers or by rivers. This gives a very smooth surface that improves the movement and packing. With manufactured sand, the production process is munch more violent and short. Blasting and crushing in 2-3 stages gives a much more rough surface and the locking of the particles gives less flowability and packing.

AI generated illustration of surface texture of manufactured aggregate versus natural aggregate.

If we summarize the effect of the three parameters above-Particle shape, Gradation and Surface texture, we can see a significant difference in the need for cement paste in a concrete mix design.

In mixes based on well-graded, rounded natural aggregates—such as river or glaciofluvial sands and gravels—the voids content typically falls around 38%. This allows for efficient packing and moderate cement paste demand. However, when the aggregate skeleton includes angular, elongated, or poorly graded materials, such as manufactured sand or crushed coarse aggregates, the voids content can rise significantly—up to 43% or more.

This 5% increase may appear small, but in practice, it has substantial implications. In a cubic meter of concrete, an increase from 38% to 43% means an additional 50 liters of voids that must be filled with paste to maintain similar workability and cohesion.

This drives up the demand for:

Water, to maintain slump or flow
Cement or supplementary cementitious materials (SCMs), to achieve a higher volume of cement paste
Admixtures, to optimize workability, retention, and dispersion under these new conditions

And critically, this comes at a time when producers are actively trying to reduce both cement and water content to meet sustainability and carbon footprint targets. The result is a growing gap between the raw materials available today and the assumptions underpinning traditional concrete mix designs.

Quantifying and responding to changes in voids content is therefore essential. It’s a key first step in adapting mix designs and selecting the right admixture solutions for modern concrete production.

In the following articles we will go deeper into the challenges and solutions on this theme.

Author

Sven-Henrik Norman

Corporate Product Manager – RE-CON line

Tag

#circulareconomy

Product Lines

Admixtures for concrete