In recent years, with the continuous expansion of industrial production and engineering construction, the amount of solid waste generated, such as fly ash, slag, steel slag, coal gangue, tailings, and construction waste has also been increasing. If these wastes are stored for a long period, they will not only occupy a large amount of land resources but may also cause environmental problems such as dust, water pollution, and soil pollution, placing significant pressure on ecological protection and resource utilization. Against the backdrop of the ongoing promotion of "dual-carbon" goals and the concept of green building materials development, how to achieve the resource utilization of industrial solid waste has become an important issue for the building materials industry.
Concrete, as the most widely used building material in engineering construction, is an important carrier for disposing of industrial solid waste and improving resource recycling. Traditional concrete generally consists of cement, sand, aggregate, water, and admixtures. However, in actual production, many materials with potential activity or physical filling effects can be added to the concrete system as aggregate. The rational use of Mineral Admixtures can not only partially replace cement, reducing cement usage and project costs, but also improve the workability, mechanical properties, and durability of concrete.The following are some new types of materials that can be used as concrete aggregates.
First, there's fly ash. Fly ash is the fine ash collected from flue gas during coal combustion in coal-fired power plants, and it's one of the largest industrial solid wastes emitted in my country. Fly ash particles are relatively fine, possessing certain pozzolanic activity and micro-aggregate effects. When added to concrete, it can improve the workability of the mixture, reduce the heat of hydration, and enhance the later-stage strength and durability of the concrete. Extensive engineering practice shows that adding an appropriate amount of fly ash to structural concrete helps improve its impermeability, frost resistance, carbonation resistance, and sulfate attack resistance. Generally, when the fly ash content does not exceed 20%, the impact on the early-stage performance of concrete is relatively small; as the content increases further, its effect on improving durability and reducing the heat of hydration becomes more pronounced. However, fly ash quality varies considerably, and when using it, key indicators such as fineness, loss on ignition, water requirement ratio, and activity index should be carefully considered.
Secondly, there is silica fume. Also known as microsilica, silica fume is ultrafine dust collected in ferroalloy plants during the smelting of ferrosilicon or industrial silicon . Due to its extremely fine particles, large specific surface area, and high silica content, silica fume has a strong filling effect and pozzolanic reactivity, and is often used in the preparation of high-strength concrete, high-performance concrete, and self-compacting concrete. Silica fume can significantly improve the internal pore structure of concrete, making the paste denser, thereby improving compressive strength, impermeability, and resistance to chloride ion attack. However, the addition of silica fume significantly increases the water demand of concrete and may lead to increased self-shrinkage; therefore, it usually needs to be used in conjunction with a high-efficiency water-reducing agent. In practical engineering, the silica fume dosage is generally best controlled between 5% and 10%, which can exert a reinforcing and densifying effect while avoiding a decline in workability due to excessive dosage.
Thirdly, there is slag powder. Slag is a byproduct of blast furnace ironmaking, which is granulated into blast furnace slag powder after water quenching, drying, and grinding. Slag powder is a high-quality mineral admixture with relatively stable performance and mature applications. Adding slag powder to concrete can improve concrete fluidity, reduce slump loss, and enhance later-stage strength and durability. Slag powder is particularly suitable for large-volume concrete, underground engineering, marine concrete , and structural engineering with high durability requirements. It should be noted that the finer the slag powder , the higher its activity, but excessive fineness may lead to accelerated early hydration reactions, increased heat of hydration, and increased risk of autogenous shrinkage. Therefore, when using slag powder in low water-cement ratio concrete, the specific surface area and dosage should be reasonably controlled according to project requirements to avoid simply pursuing high activity while neglecting volume stability.
Fourth is steel slag. Steel slag is a major solid waste generated during steelmaking, with a relatively large output and certain cementitious activity and aggregate utilization potential. However, compared with fly ash and slag powder, the application risk of steel slag is higher, mainly because it may contain more free calcium oxide, free magnesium oxide, and other components. These substances will expand in volume during the later hydration process, and if not properly controlled, they can easily cause concrete cracking and damage, affecting structural safety. Therefore, when steel slag is used in cement concrete, especially structural concrete, strict stability testing and quality control are necessary. The source, aging degree, grinding method , and chemical composition of steel slag will all affect its performance. Overall, the resource utilization prospects of steel slag are promising, but it is not advisable to blindly use large quantities. It is more suitable as a road base material, non-load-bearing component material, or a fully treated auxiliary cementitious material.
Fifth is coal gangue. Coal gangue is a solid waste generated during coal mining and washing. Long-term stockpiling of coal gangue occupies land and causes environmental pollution. After crushing, screening, calcination, or activation treatment, coal gangue can be used as concrete aggregate or auxiliary cementitious material. Because coal gangue usually has a porous structure, low density, and relatively high water absorption, its strength and stability vary greatly depending on its source. Therefore, it is more suitable for medium- and low-strength concrete, lightweight aggregate concrete, or non-critical structural parts. It is generally recommended to partially replace coarse aggregate in C20 and below concrete, and the replacement ratio should not be too high. Before use, crushing index, water absorption, mud content, loss on ignition, and harmful components should be tested, and the optimal dosage should be determined through trial mixing.
Sixth is iron tailings. Iron tailings are waste products generated during iron ore beneficiation. After processing such as crushing, washing, and screening, they can be processed into fine or coarse aggregates that meet the requirements of concrete. Iron tailings sand and gravel are usually hard and have high strength, and can replace natural sand and gravel to a certain extent, reducing the pressure on the extraction of natural mineral resources. As natural sand and gravel resources become increasingly scarce, the application value of iron tailings in concrete is gradually gaining attention. In practical applications, iron tailings can be used in concrete of various strength grades, and in some projects, the replacement ratio can even reach more than half of the total aggregate. However, due to the high specific gravity and unstable particle size distribution of some iron tailings, problems such as segregation, bleeding, or decreased pumpability may occur when mixing concrete. Therefore, it is necessary to optimize the mix proportion based on the characteristics of raw materials and construction requirements.
Seventh is recycled concrete aggregate. With the expansion of urban renewal and the demolition of old buildings, a large amount of waste concrete is generated from construction waste. After crushing, screening, impurity removal, and grading, this waste concrete can be processed into recycled aggregate for reuse in concrete production. This method reduces construction waste landfill and conserves natural sand and gravel resources, demonstrating significant environmental benefits. Currently, recycled coarse aggregate is widely used in road base courses, subbases, non-load-bearing components, and some brick masonry materials. However, recycled aggregate often has old mortar adhering to its surface, resulting in higher water absorption. Its crushing index and quality stability are inferior to natural aggregate, which may affect the strength, durability, and shrinkage performance of structural concrete if used. Therefore, improving the level of construction waste sorting, refining recycled aggregate processing technology, and establishing unified quality standards are key to promoting the large-scale application of recycled aggregate.
Overall, all types of Mineral Admixtures have their unique value and corresponding application limitations. Fly ash, slag powder, and silica fume have relatively mature applications, mainly used to improve concrete performance; steel slag, coal gangue, iron tailings, and recycled aggregates demonstrate greater potential for solid waste resource utilization and the development of green building materials. It is important to emphasize that the utilization of admixtures should not solely focus on cost reduction, nor should it come at the expense of project quality. Different materials vary significantly in chemical composition, physical properties, activity levels, and stability; therefore, the appropriate dosage must be determined through testing, trial mixing, and engineering verification. Only by promoting solid waste utilization while ensuring quality and safety can a true balance of economic, engineering, and ecological benefits be achieved.
