Recycled Concrete Aggregate Beneficiation by Cement Paste Removal
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Solution Overview
Problem
The utilization of recycled concrete aggregates (RCA) in new concrete is limited due to the negative effects of the porous and cracked residual hardened cement paste (HCP) attached to the RCA, which affects compressive strength, water absorption, drying shrinkage, creep, tensile strength, modulus of elasticity, frost resistance, bulk density, specific gravity, abrasion loss, and crushability, and requires costly chemical additives to achieve suitable workability.
Innovation Solution
A beneficiating process involving crushing and processing waste concrete to abrade off the hardened cement paste using a rotary drum and liberate unreacted portland cement and supplementary cementitious materials (SCMs) through fine grinding, producing processed recycled concrete aggregate and paste suitable for new concrete products.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If recycled concrete aggregate (RCA) is used directly in new concrete, then the utilization of recycled materials increases, but the quality of concrete deteriorates due to residual hardened cement paste causing increased water absorption, decreased compressive strength, and other negative properties
Solution Approach 1:
The patent extracts and removes the residual hardened cement paste from the surface of recycled concrete aggregates through mechanical means (crushing, screening, and washing processes). This extraction eliminates the harmful paste layer that causes increased water absorption and decreased compressive strength, while retaining and reusing the valuable aggregate material itself.
Solution Approach 2:
The patent changes the physical and chemical parameters of the recycled concrete aggregate by controlling the crushing intensity, screening mesh sizes, and washing conditions. These parameter changes transform the RCA from a low-quality material with attached paste to a high-quality aggregate with clean surfaces, improving compressive strength and other concrete properties.
2Ease of operation
If RCA is pretreated by prewetting to saturate residual cement paste, then workability improves, but the amount of water and chemical additives required increases
Solution Approach 1:
Instead of adding water and chemicals to mask the problems of residual paste, the patent extracts and removes the paste entirely through mechanical processing. This eliminates the root cause of workability issues rather than treating the symptoms, reducing the need for additional water and chemical additives.
Solution Approach 2:
The patent replaces the chemical treatment approach (using water reducers and admixtures) with a mechanical treatment approach (crushing, screening, and washing). This mechanical substitution achieves better workability through physical means, eliminating or reducing the need for costly chemical additives.
3Strength
If multiple treatment methods (coating, polymer treatment, silica fume impregnation) are applied to RCA, then certain properties improve, but the process complexity and cost increase
Solution Approach 1:
The patent segments the treatment process into distinct mechanical stages: crushing to break apart the concrete, screening to separate aggregates by size, and washing to remove residual paste. This segmentation creates a systematic, multi-step mechanical process that achieves comprehensive treatment without requiring complex chemical treatments or multiple specialized equipment.
Solution Approach 2:
The patent achieves improved compressive strength by changing the physical parameters of the RCA through mechanical processing - controlling particle size distribution through screening, removing surface contaminants through washing, and fracturing the paste-aggregate bond through crushing. These parameter changes provide consistent quality improvement without the complexity of chemical treatments.
4Adaptability or versatility
If RCA from multiple sources is processed uniformly, then the product becomes available as a bulk material without knowing its origin, but the variability in residual paste content and type affects concrete quality
Solution Approach 1:
The patent creates a universal processing system that can handle RCA from multiple sources and concrete types through a single integrated mechanical treatment process. The combination of crushing, screening, and washing functions universally on different RCA inputs, producing a consistent quality output that can be used as a bulk material without needing to know the specific origin or type of the recycled concrete.
Solution Approach 2:
The patent uses parameter changes in the mechanical processing - standardized crushing intensity, screening mesh sizes, and washing conditions - to transform variable input materials into a consistent quality product. These controlled parameter changes ensure that regardless of the source or type of RCA, the output meets uniform quality specifications for compressive strength and other concrete properties.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The process effectively exposes the unreacted cement and SCMs, restoring their reactivity, thereby improving the quality of recycled aggregates and reducing the need for chemical additives, and enhancing the mechanical properties of new concrete.
Implementation Method 1
processing the crushed waste concrete in a rotary drum to abrade the hardened cement paste (HCP) off the RCA
Implementation Method 2
processing the HCP in a grinding device to liberate the unreacted portland cement and supplementary cementitious materials (SCMs) contained in the HCP
Data Source
AI summary
Methods and systems are provided for processing post-consumer waste concrete into a usable recycled aggregate and supplementary cementitious material. The concrete is crushed and tumbled under heat and optional watering to liberate unreacted hardened cement paste. The aggregate is then screened for size for application, and the hardened cement paste is milled into a supplementary cementitious material, which can be used alone or blended with other supplementary cementitious materials such as ground glass pozzolan.