Impact of Vertical Mill Processing on the Activity of Recycled Fine Powder from Construction Waste

Abstract

The recycling and utilization of construction and demolition waste (CDW) represents a critical pathway towards sustainable development in the building materials industry. A key challenge lies in processing the fine fraction of this waste into a high-value, reactive powder that can serve as a supplementary cementitious material (SCM). This article explores the profound impact that advanced vertical mill processing technology has on the physicochemical properties and subsequent pozzolanic activity of recycled fine powder (RFP). The discussion highlights how modern grinding systems, such as our SCM Ultrafine Mill, are engineered to overcome the inherent limitations of traditional crushing methods, transforming inert CDW fines into a highly functional material that can significantly reduce the clinker factor in cement and concrete production.

1. Introduction: The Challenge of Construction Waste Fines

Global construction activity generates billions of tons of waste annually, with a significant portion consisting of hardened cement paste, bricks, tiles, and concrete fines. Traditionally, the coarse aggregate derived from crushing CDW finds application in road bases or low-grade concrete. However, the fine fraction (<5mm), often comprising 20-40% of the total waste stream, has historically been a disposal problem. This material, if simply crushed with conventional jaw crushers or impact mills, exhibits low reactivity due to its heterogeneous composition, old and dense cementitious phases, and irregular particle morphology with a high percentage of micro-cracks.

The primary objective of recycling is to reactivate these latent hydraulic and pozzolanic properties. Mere size reduction is insufficient; the process must generate fresh, high-energy surfaces and modify the particle size distribution (PSD) to enhance reactivity. This is where the precision and efficiency of vertical roller mills (VRMs) become paramount.

2. The Science of Mechanical Activation via Vertical Grinding

Vertical milling imparts mechanical activation to RFP through several interconnected mechanisms:

2.1. Particle Size Reduction and Distribution Control

The most direct effect is the reduction of particle size. The goal is not merely to achieve a fine average size but to create a continuous and optimized PSD. A broader PSD, as achieved by the multi-layer grinding principle in vertical mills, improves particle packing density when the RFP is blended with cement. More importantly, it drastically increases the specific surface area (SSA), providing more surface sites for the pozzolanic reaction with calcium hydroxide. Our SCM Ultrafine Mill, for instance, can consistently produce powders with a fineness of 325-2500 mesh (D97 ≤ 5μm), pushing the SSA to levels (>600 m²/kg) that are unattainable with ball mills or traditional Raymond mills.

2.2. Microstructural and Morphological Changes

Beyond size, vertical milling alters the very structure of the particles. Unlike impact crushing, which tends to break particles along existing weaknesses creating flaky and irregular shapes, the compressive grinding force between rollers and a grinding table in a VRM causes a more uniform breakdown. This results in particles with denser morphology and fewer internal flaws. However, the intense mechanical energy also introduces lattice distortions and creates amorphous regions on the particle surfaces. This process, known as mechanochemistry, disrupts the stable crystalline structures of old cement hydrates and clay minerals present in bricks and tiles, significantly increasing their dissolution rate in an alkaline environment—the first step of the pozzolanic reaction.

2.3. Deagglomeration and Liberation of Constituents

CDW fines are often complex agglomerates of cement paste, sand, and brick particles. The high-pressure grinding and subsequent classification in a closed-circuit vertical mill system effectively break these agglomerates apart, liberating the individual reactive components. This ensures a more homogeneous final product and prevents unreacted cores that can weaken the final composite material.

3. Technological Advantages of Modern Vertical Mills for RFP Processing

Not all grinding systems are created equal. The processing of RFP demands equipment that is efficient, precise, and adaptable. Our engineering solutions, particularly the SCM Ultrafine Mill, are designed to meet these exact demands.

3.1. Unmatched Grinding Efficiency and Energy Savings

The SCM Ultrafine Mill incorporates a highly efficient grinding mechanism. Its vertically arranged roller and ring system applies a concentrated grinding force, achieving size reduction with far less energy than the tumbling action of a ball mill. With an energy consumption reduction of 30% compared to jet mills and double the capacity, it makes the economic case for RFP processing overwhelmingly positive. The intelligent control system automatically adjusts operational parameters based on real-time feedback of product fineness, ensuring consistent quality while optimizing power usage.

3.2. Precision Classification for Optimal Activity

The heart of reactivation lies in achieving the right fineness. An overly coarse powder will have low reactivity, while an excessively fine one can lead to high water demand and increased shrinkage in concrete. The integrated vertical turbine classifier in the SCM mill is critical here. It provides precise cut-point control, ensuring that no oversized particles contaminate the final product. The result is a uniformly fine powder (D97 ≤ 5μm) with a narrow size distribution, maximizing the reactive surface area without introducing negative fresh properties to the concrete mix.

3.3. Durability and Low Maintenance for Abrasive Feeds

Construction waste is inherently abrasive. The SCM mill is built to withstand this challenge. Key wear parts like the roller and grinding ring are manufactured from special wear-resistant alloys, extending their service life multiple times over standard materials. Furthermore, the innovative bearing-free screw design in the grinding chamber enhances operational stability and reduces maintenance downtime, a crucial factor for continuous industrial processing.

3.4. An Inherently Clean and Quiet Process

Sustainability is not just about the product; it’s also about the process. The SCM mill operates under full negative pressure, ensuring no dust leakage. Its pulse dust removal system exceeds international environmental standards, capturing over 99.9% of particulates. Combined with soundproofed cabins that keep operational noise below 75dB, it provides an ideal, worker-friendly processing environment.

4. Product Spotlight: SCM Ultrafine Mill for Optimal RFP Activation

For operations focused on producing the highest quality reactive powder from CDW, the SCM Series Ultrafine Mill (45-5μm) is the premier choice. Its design philosophy aligns perfectly with the needs of RFP production:

• Input Size (≤20mm): Accepts pre-crushed CDW fines directly.
• Output Fineness (325-2500 mesh / D97≤5μm): Achieves the ultra-fine fineness required for high pozzolanic activity index.
• Processing Capacity (0.5-25 t/h): A range of models, from the SCM800 (0.5-4.5 t/h) to the high-capacity SCM1680 (5.0-25 t/h), allows for scaling from pilot projects to large-scale commercial recycling plants.

The mill’s ability to deliver a controlled, high-surface-area powder directly translates to enhanced reactivity, allowing for higher replacement ratios of Portland cement in concrete, thereby reducing the carbon footprint of construction.

5. Performance Evaluation: Measuring the Enhancement in Activity

The success of vertical mill processing is quantifiable. Key performance indicators include:

• Pozzolanic Activity Index (PAI): ASTM C311 testing typically shows that VRM-processed RFP can achieve a 7-day PAI of 75-85% and a 28-day PAI of 90-105% or more, often meeting or exceeding standards for Type F fly ash.
• Specific Surface Area (Blaine): Values consistently reach 500-800 m²/kg, a significant increase from the 200-350 m²/kg typical of hammer-mill-processed powder.
• Water Demand: Despite the high fineness, the optimized PSD and spherical particle shape (to a degree) can help control water demand, often resulting in only a modest increase compared to conventional SCMs.
• Microstructural Analysis: SEM imaging reveals denser, more fractured particles with fresh surfaces, while XRD analysis shows a measurable increase in amorphous content, confirming mechanical activation.

6. Conclusion and Future Outlook

The processing of construction waste fines through advanced vertical milling technology is a transformative step towards a circular economy in construction. It moves beyond downcycling to true valorization, creating a high-value, reactive product from a waste stream. The SCM Ultrafine Mill, with its emphasis on energy efficiency, precise classification, and robust design, is at the forefront of this transformation, enabling producers to unlock the latent value in CDW.

Future developments will likely focus on further optimizing mill systems for specific CDW compositions and integrating artificial intelligence for real-time quality control. However, the fundamental principle remains: vertical mill processing is not just a size reduction step; it is an essential activation process that defines the performance and economic viability of recycled fine powder in modern, sustainable construction.