Effect of Recycled Fine Aggregate Replacement Ratio on the Fresh State, Mechanical, and Water Absorption Properties of Self- Compacting Mortar
This study demonstrates that while increasing the replacement of natural fine aggregates with recycled aggregates in self-compacting mortar progressively reduces flowability, compressive strength, and durability due to the recycled material's higher porosity and rougher texture, moderate replacement levels between 25% and 50% offer a sustainable balance of workability and mechanical performance.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Imagine the construction industry as a giant, hungry beast that eats up mountains of sand and rocks to build our cities, while simultaneously spitting out massive piles of broken concrete from old buildings. This creates a double trouble: we are running out of natural sand, and we are drowning in waste. Enter the hero of our story: Self-Compacting Mortar. Think of this not as ordinary concrete, but as a super-smooth, thick soup that flows like honey. You can pour it into a mold, and it slumps and spreads all by itself, filling every nook and cranny without needing anyone to vibrate or shake it. It's the efficient but perfect builder's dream.
Now, imagine trying to make this perfect soup using ingredients from the trash can instead of the quarry. That's where Recycled Aggregates come in. These are tiny bits of crushed-up old concrete. The big question scientists have been asking is: "If we swap out the fresh, smooth river sand for these rough, recycled bits, does our super-soup still flow? Does it stay strong? Or does it turn into a lumpy, weak mess?" This paper dives right into that messy kitchen experiment to see if we can turn construction waste into a high-performance building material without ruining the recipe.
The Great Sand Swap Experiment
In this study, two researchers from Iran decided to play the role of mad scientists in a concrete lab. They wanted to see what happens when you replace the "good" sand in self-compacting mortar with "recycled" sand (crushed old concrete) in different amounts. They didn't just guess; they made five different batches of mortar soup:
- The Control: 0% recycled sand (100% fresh, natural sand).
- The Light Swap: 25% recycled sand.
- The Halfway House: 50% recycled sand.
- The Heavy Swap: 75% recycled sand.
- The Full Switch: 100% recycled sand.
They kept everything else exactly the same—the amount of cement, the water, and the special "flow-boosting" chemicals—so the only thing changing was the sand. Then, they put these batches through a series of tests to see how they behaved when wet (fresh state) and how they held up when dry and hard (mechanical properties).
The Flow Test: The "Slump" and the "Funnel"
First, they looked at how the mortar moved. Imagine pouring a thick milkshake. If it's too thick, it won't pour; if it's too thin, it splatters.
- The Mini Slump Flow: They dropped a cone of mortar and watched how far it spread. The "perfect" fresh mortar (0% recycled) spread out to 33.5 cm. But as they added more recycled sand, the spread shrank. At 25% recycled, it was 28.75 cm. At 50%, it dropped to 23.0 cm. By the time they hit 100% recycled sand, the mortar barely spread at all, only reaching 14.75 cm. That's a huge drop of about 56%!
- The V-Funnel: They also timed how long it took the mortar to drain through a funnel. The fresh mix drained in 7 seconds. The 100% recycled mix took 16 seconds.
Why did this happen? The authors explain that recycled sand is like a sponge with a rough, bumpy surface. It's covered in old, dried-out cement mortar that makes it thirsty and sticky. When you mix it, these rough bits grab onto each other and the water, creating friction. It's like trying to slide across a floor covered in sandpaper instead of ice. The more recycled sand you add, the "thicker" and slower the soup becomes.
The Strength Test: The Squeeze
Next, they waited 28 days for the mortar to harden and then squeezed it until it broke to measure its strength.
- The control mix (0% recycled) was strong, holding up at 38.9 MPa.
- The 25% mix was 38.2 MPa.
- The 50% mix was 37.6 MPa.
- The 75% mix dropped to 36.1 MPa.
- The 100% mix fell to 35.1 MPa.
Here is the good news: even with 100% recycled sand, the mortar was still pretty strong (above 35 MPa). The drop wasn't a disaster; it was a slow, gentle slide. The authors suggest that the recycled sand has tiny cracks and holes inside it (from being crushed), which makes the final block slightly weaker, but not weak enough to be useless. Up to 50% replacement, the strength barely changed (less than a 4% drop).
The Water Test: The Sponge Effect
Finally, they checked how much water the hardened mortar could soak up.
- The control mix soaked up 3.24% water.
- The 100% recycled mix soaked up 5.02% water.
This increase makes sense because the recycled sand is basically a sponge. It has old mortar stuck to it that is full of tiny holes. When you build a wall with this sand, those holes connect up, creating a highway for water to travel through. The more recycled sand you use, the more "sponge-like" the wall becomes.
The Verdict: Finding the Sweet Spot
So, what's the takeaway? The paper suggests that you can't just throw 100% recycled sand into your mortar and expect it to work exactly like the original. The flow gets too slow, and the water absorption gets too high.
However, the study found a "Goldilocks zone." If you swap out 25% to 50% of the natural sand for recycled sand, you get the best of both worlds. In this range:
- The mortar still flows well enough to be useful (though not as perfectly as the 0% version).
- The strength stays almost exactly the same (a tiny drop of less than 4%).
- The water absorption goes up, but not dangerously so.
The authors conclude that using recycled sand is a great way to save natural resources and clean up construction waste, but you have to be careful with the recipe. If you go too high (75% or 100%), you need to tweak the mix (add more flow-boosters or change the water) to keep it working. But for a moderate swap, it's a win-win for the planet and the builder.
What This Paper Didn't Do (The "Fine Print")
It's important to know what this study didn't do, so we don't get our hopes up too high. The researchers didn't test how long these walls would last in a hurricane, or if they would crack in freezing weather, or if salt water would eat them away. They only looked at the first 28 days. They also didn't use fancy microscopes to look at the tiny cracks inside the sand; they just guessed based on the results they saw. So, while the results are promising, they are a starting point, not the final word on every possible situation.
In short, this paper tells us that we can recycle our concrete waste into new building materials, but we have to mix it carefully. Swap a quarter or half of your sand, and you get a strong, eco-friendly mortar. Swap it all, and you might need to do some extra work to make it flow.
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