The use of recycled concrete sand (RCS) in 3D-printed mortar mixtures offers sustainability benefits, but this material introduces challenges due to its elevated porosity and tendency to release water. These factors typically result in reduced mechanical strength. However, utilizing vacuum mixing techniques successfully enhances the material's workability and reduces unwanted air content, indicating that RCS holds significant sustainable promise when moisture content is carefully managed.
3D-printed construction is no longer a futuristic dream — it’s happening right now, shaping houses, bridges, and infrastructures through automated layering of cement-based materials. But as this technology grows, so does a massive sustainability question:
Can we replace natural sand with recycled concrete sand—without sacrificing printability or strength?
A team of researchers decided to investigate this challenge using a clever twist: vacuum mixing. Their study explores how recycled sand behaves in 3D-printable mortars, how mixing pressure affects properties, and what this means for future greener construction.
Let’s break it all down in a smooth, easy-to-read EngiSphere style. 🌍✨
Concrete production is notoriously resource-hungry. Sand — the backbone of mortar — is the third most used natural resource on Earth after air and water. Construction-grade sand is becoming scarce globally, pressuring the industry to seek alternatives.
🔁 Enter recycled concrete sand (RS).
This sand is created from crushed construction and demolition waste. Using it closes a material loop, reduces environmental footprint, and supports a circular economy.
But there’s a catch…
Recycled sand tends to be:
All of these affect printability, consistency, and strength — especially for 3D printing, where precision is everything.
So the research team asked:
👉 Can recycled concrete sand be used effectively in 3D-printable mortars?
👉 And does vacuum mixing improve performance?
The researchers created four mortars:
| Code | Sand Type | Mixing Type |
|---|---|---|
| NMO | Natural sand | Ordinary mixing |
| NMV | Natural sand | Vacuum mixing |
| RMO | Recycled concrete sand | Ordinary mixing |
| RMV | Recycled concrete sand | Vacuum mixing |
Each mix kept the same cement content, water-to-cement ratio, and additives. The only differences were:
Samples were tested both in cast form and 3D-printed layers. The researchers measured:
💧 Fresh properties: slump flow, air content, printability
📦 Hardened properties: porosity, pore distribution, compressive strength
Let’s explore what they found!
One surprising result?
Mortars containing recycled concrete sand were more fluid than those with natural sand — even though their compositions were nearly identical.
Why? Two main reasons:
RS absorbs far more water (8.3% vs. 0.7% for natural sand).
Even after 24-hour saturation, some absorbed water may be released during mixing, increasing effective water in the mortar.
This leads to higher slump — meaning the mix spreads more easily.
Rougher, more porous grains influence the rheology.
Result:
🟢 RMO (recycled + ordinary mixing) = high slump
🔵 RMV (recycled + vacuum mixing) = too fluid to print beyond 3 layers
Vacuum mixing increased fluidity even more, making RMV unprintable in standard layer heights.
Vacuum mixing reduced entrapped air significantly:
Less air =
✔ stronger, more durable hardened mortar
✔ more consistent printing
This shows that vacuum mixing can help stabilize mixtures, especially recycled ones.
Using a lab-scale 2D concrete printer:
This means vacuum mixing + recycled sand needs further water absorption control before it becomes reliable for printing.
Porosity is a big deal in concrete durability. The researchers measured:
Key findings:
This matches the higher intrinsic porosity of recycled concrete sand.
Surprisingly:
Print layering did not change porosity very much.
Both cast and printed samples had similar pore volumes.
These larger pores can weaken strength.
Now the million-dollar question:
How strong were the mortars?
Here’s the breakdown:
A 5–10% increase in strength occurred when using vacuum mixing for natural sand.
But recycled sand didn’t benefit much — likely because vacuum conditions caused some water to escape from saturated aggregates, slightly raising effective water content.
Printed samples always had lower strength because:
But the reduction varied:
Natural sand
Recycled sand
🧩 Unexpectedly, recycled mortars showed slightly more stable strength between cast and printed shapes.
Layer consistency
Comparing strength from layer 1 to layer 15:
Let’s simplify the key takeaways:
✔ Recycled sand is totally usable in 3D-printed mortars
…but it brings more porosity and lower strength.
✔ Vacuum mixing reduces air content and improves workability
…but with recycled sand, it can accidentally increase free water.
✔ Printability is excellent for all except RMV
Recycled sand + vacuum = too fluid unless water is controlled perfectly.
✔ The structure of printed elements remains relatively uniform
No layer-by-layer weakening issues were found.
This study opens the door to greener 3D-printed construction — but several future improvements are needed.
The main challenge right now is uncertainty in how much water recycled concrete sand actually absorbs and later releases.
Automated or real-time moisture scanners for recycled concrete sand could solve this.
Vacuum could be applied:
Fine-tuning may drastically improve printability of recycled mixes.
The study used a very high cement content (~938 kg/m³) for printability.
Future sustainable recipes should reduce cement or include:
Additives or treatments that reduce harmful pore sizes could lift the performance of recycled mortars closer to natural ones.
While laboratory-scale printing serves as the initial phase, subsequent development will transition to implementing robotic arms, gantry systems, and validating full-scale structural walls:
This research demonstrates that recycled concrete sand is absolutely viable for use in 3D-printable mortars — provided that we carefully manage its water absorption and porosity challenges.
Here’s the bottom line:
🌱 Recycled sand makes mortar more sustainable
🧪 Vacuum mixing improves workability and reduces air
🧱 3D printing with recycled mortars is feasible and stable
⚠️ But porosity and strength need optimization for real-world adoption
With ongoing research, smarter moisture control, and better mix design, recycled concrete could become a mainstream ingredient in future 3D-printed buildings — making construction cleaner, greener, and more efficient. 🌍🏗️✨
🧱 Recycled Concrete Sand (RS) - Crushed concrete from old buildings that’s processed into fine sand, used as a greener alternative to natural sand in new mortars.
🏖️ Natural Sand (NS) - Sand taken from rivers or quarries; it has smooth grains and low water absorption, making it the traditional ingredient for mortar.
💧 Water Absorption - How much water a material can soak up — recycled sand absorbs a lot more than natural sand, affecting flow and strength.
⚙️ Vacuum Mixing - A mixing technique where air is removed during mixing, helping reduce trapped bubbles and create smoother, denser mortar.
🌬️ Entrapped Air - Tiny air pockets accidentally mixed into mortar; too much of it weakens the material, so reducing it improves strength.
🥣 Slump / Slump Flow - A quick test measuring how easily fresh mortar spreads — higher slump means more fluid, easier-to-print mix.
🖨️ 3D Concrete Printing - Layer-by-layer construction using a mortar-extruding machine, allowing rapid creation of walls and structures without traditional formwork. - More about this concept in the article "Transforming Cities with 3D Concrete Printing: Unlocking the Future of Sustainable Urban Development 🏙️".
🔬 Porosity - How much empty space exists inside the hardened mortar — higher porosity usually means lower strength and more water penetration. - More about this concept in the article "Water Crisis or Resource Management? The Future of Water in Minas Gerais, Brazil 💧🚰".
🧵 Pore Size Distribution - A breakdown of pore sizes inside mortar, important because larger or more irregular pores weaken the material.
🧪 Mercury Intrusion Porosimetry (MIP) - A lab test where mercury is forced into pores under pressure to measure their size and quantity.
🧊 Water-Accessible Porosity - The portion of pores that water can fill — useful for judging durability against moisture-related damage.
💪 Compressive Strength - How much force the material can withstand when squeezed; a key measure of how strong the mortar is. - More about this concept in the article "Engineered Bamboo in Building Materials 🎋 Stronger, Greener, Smarter".
🧱 Printed vs. Cast Samples
Source: Khoury, E.; El Cheikh, K.; De Schutter, G.; Cazacliu, B.; Rémond, S. Using Vacuum Mixing for 3D Printing of Mortars Made with Recycled Sand. Buildings 2025, 15, 4217. https://doi.org/10.3390/buildings15234217
From: Institut de Recherche de la Construction, ESTP; Institut Mines Télécom; Buildwise; Ghent University; Université Gustave Eiffel; University Orléans, University Tours.
