Recycled Aggregate: Why Does Strength Still Scatter Even When the Mix Design Is Right?
Using construction waste recycled aggregate to replace natural sand and gravel for brick making – getting the mix design to work in the laboratory is not difficult. What is difficult is that on the production line, with the same mix design sheet, the bricks produced today have sufficient strength, while tomorrow a batch may show visible porosity or cracks. The problem rarely lies in the "mix ratio itself," but in the three unstable variables that recycled aggregate brings into the production process.
1. Water Absorption: The "Water" in the Mix Design Is Wrong from the Start
The water absorption of natural sand and gravel is typically around 1%–2%, while recycled aggregate – especially crushed waste concrete and brick rubble – can have water absorption several times higher. The old mortar attached to the surface of recycled aggregate has high porosity. After mixing water is added, a portion of the water is "grabbed" by the aggregate, and the water that actually participates in cement hydration and provides workability is lower than the mix design set value. The result is: the mix looks "dry," the operator adds water based on experience, and the water-cement ratio spirals out of control; or the mix looks "wet," but the interior of the aggregate is actually dry, and after forming there is localized water deficiency, leading to insufficient cement hydration.
The first correction in recycled aggregate batching is not to change the mix ratio, but to compensate the mixing water according to the actual water absorption of the aggregate at the batching stage. Without this step, the reproducibility of the mix design is untenable from the source.
2. Micro-Fines Content: Not "Impurities," but They Eat Strength
Recycled aggregate inevitably contains a certain proportion of micro-fines – fine particles with a particle size of less than 0.15 mm, originating from brick powder, mortar powder, and soil generated during the crushing process. These micro-fines are not completely inactive; some brick powder has certain pozzolanic potential, but their large specific surface area and high water demand significantly change the rheological properties of the mix. More critically, if the micro-fines content fluctuates too much – 10% in this batch of aggregate, 20% in the next – the effective ratio of cement to aggregate in the mix design is actually changing every day. Under the premise of controlling the strength grade of recycled blocks, the micro-fines content needs to be incorporated into the mix design, rather than being ignored as "impurities."
3. Crushing Value and Gradation: Vibration Parameters Cannot Be "One Set for All"
The crushing value of recycled aggregate is typically higher than that of natural aggregate, and the strength of the particles themselves is lower, making them more likely to be crushed and refined during the forming vibration process. This means that the same set of vibration frequency, amplitude, and pressing parameters that produce just the right density on natural aggregate may cause over-vibration on recycled aggregate – aggregate crushing, increased fines, and localized segregation. The forming parameters of a recycled aggregate production line need to be adjusted on a recipe basis according to the crushing value and gradation of the aggregate, rather than following the "standard parameters" for natural aggregate.
The mix design for recycled aggregate is essentially "responding to change with change": the water absorption of the aggregate is changing, the micro-fines content is changing, and the crushing value is changing – so the mix ratio, mixing water, and forming parameters must change accordingly. Only by managing these three variables can the strength dispersion of recycled aggregate bricks be considered controllable.
