When constructing new buildings or demolishing old buildings, a large amount of waste concrete will be formed. In the past, the main treatment method is to transport the waste concrete to the suburbs for stacking or burial, but this will not only occupy land, but also cause environmental pollution. In fact, these waste concrete can be turned into treasure and recycled through the preparation of recycled concrete, thus effectively reducing carbon dioxide emissions and improving the environmental pollution caused by waste concrete treatment. This is a powerful measure for the construction industry to respond to the call of the state under the background of carbon peak and carbon neutralization. The connotation
of
carbon peak and carbon neutralization is that human production and life will certainly produce carbon emissions, so it is necessary to control the source of carbon emissions, but not every source can be controlled, which requires carbon neutralization, for the carbon emissions that can not be controlled, the method of absorption and neutralization is adopted, when the emissions are equal to the absorption. The equilibrium state of carbon neutrality is achieved, which is equal to relative zero emissions. The reason why carbon peak and carbon neutralization often occur together is that carbon peak needs to reach the turning point of decline, so that carbon neutralization can achieve the basis of relatively zero emissions. At the same time, the work of carbon reduction is not only to control the production of carbon dioxide, but also to take the way of carbon absorption at the same time, in order to achieve the best effect of carbon reduction. The goal of
carbon peak and carbon neutrality is now very clear, but some people may have a misunderstanding that the first goal of "double carbon" is to achieve carbon peak in 2030, and unlimited emissions can be achieved before 2030. This view is obviously wrong. Carbon dioxide has a certain life span, and carbon reduction is not achieved overnight. If it is not controlled now, the carbon peak in 2030 and the carbon neutral target after 2030 will not be achieved as scheduled. Therefore, all industries should take certain measures to reduce carbon from now on.
2. The significance
of concrete recycling of construction waste to carbon peak and carbon neutralization The production of building materials needs to consume a lot of energy, and a large amount of carbon dioxide will be produced in the exploitation of various raw materials and the consumption of coal, oil and natural gas. If construction waste concrete is recycled, carbon emissions will be greatly reduced. In addition, the traditional treatment methods of stacking and burying such waste concrete can cause environmental pollution. These waste concrete will release a large number of calcium silicate, sulfate, calcium hydroxide and sulfate in the natural environment, which will pollute the water environment, soil environment and atmospheric environment, lead to the death of a large number of plants, and affect the absorption of carbon dioxide by natural plants. As a result, the regional ecological environment is damaged, and it is very difficult to control. The recycling of building waste concrete to produce recycled aggregate, recycled cement and other recycled concrete will not only reduce the production of some new building materials, avoid carbon dioxide produced in some links, but also be conducive to environmental protection, maintain ecological balance, better play the role of natural environment in purifying carbon dioxide. It plays a positive role in promoting the realization of carbon peak and carbon neutralization.
With the accelerating pace of urban construction in China, various types of large, medium and small buildings have been built all over the country, and the demand for concrete materials has been increasing, which has led to the mining of raw materials and the preparation of building materials throughout the year. These activities are accompanied by a large amount of carbon dioxide emissions. In addition, construction projects and the demolition or renovation of old buildings, which are common everywhere every year, will produce a large amount of waste concrete. Except for a small part of these materials used for site backfilling, most of them are piled up as garbage, buried or even put into the water system, causing damage to the natural environment. Many green plants die because of the pollution caused by waste concrete. Based on this, since the end of the last century, European countries, Japan and other developed countries have carried out research on the recycling of waste concrete to solve the problem of insufficient raw materials, while achieving the purpose of protecting the environment, saving energy and reducing carbon. Since the beginning of this century, China has gradually realized the importance of green energy-saving for the sustainable development of the construction industry, and has begun to study the recycling of waste concrete. Although China started late in this respect compared with developed countries, it has developed rapidly in recent years under the high attention of the state, and the gap between China and developed countries is also rapidly narrowing.
Construction waste concrete blocks can replace the coarse aggregate required for concrete preparation after crushing, and can also be directly used for building foundation reinforcement after crushing, or for road engineering, airport runway cushion, indoor stadium floor cushion, etc. If further crushed, it can also replace fine aggregate to prepare bricklaying mortar or plastering mortar. In the past, building waste concrete recycling products were mainly used for building structures with low strength requirements. However, with the improvement of related technologies in recent years, by optimizing the mix proportion and adjusting the amount of recycled aggregate, it can also be used in construction projects with high strength requirements. At present, China has been able to obtain high-quality recycled aggregate through hammering, cutting, crushing, screening and other processes, which can be used to prepare C25 ~ C30 strength grade concrete. The main idea is to recycle the waste concrete first, through a series of processes such as hammering, removing debris, crushing and so on, to obtain recycled concrete, and to process the waste concrete into recycled concrete including recycled aggregate and recycled cement, which is the main direction of recycling construction waste concrete under the background of carbon neutralization.
4. The recycling method
of building waste concrete first collects and recycles the building waste concrete in a unified way, and then obtains the recycled concrete including recycled aggregate and recycled cement through the following steps.
(1) Hammer primary election. Use the hammering equipment to hammer the waste concrete, and cooperate with manual screening to remove the sundries of wood, steel bars, plastics and other materials mixed in the waste concrete.
(2) Crushing and separating. The waste concrete after primary selection is crushed by a jaw crusher, and the crushing is successively treated by washing, magnetic separation, drying, wind separation and other processes. Among them, washing can clean up the sticky marl impurities on the broken concrete, magnetic separation can clean up the metal impurities contained in the broken concrete, and finally drying and wind separation can clean up the light impurities such as sawdust and plastics.
(3) Crushing and screening. The crushed concrete treated by the above process is further screened by a double-layer mesh screening machine to obtain recycled aggregates with particle sizes ranging from 0 to 40 mm, and the aggregates with particle sizes exceeding 40 mm are continuously crushed to below 40 mm.
(4) calcine cement. Waste concrete will produce a lot of marl after hammering, crushing and other processes, which is actually hardened cement stone and can be used to prepare recycled cement. Therefore, washing will be added in the crushing and separation process to collect these marl components. After being dried, the washed and separated slurry can be mixed with ingredients and calcined to obtain regenerated cement.
5. Performance and improvement
of construction waste concrete recycling products 5. Whether this method is worth popularizing on a large scale should be evaluated. The measurement of concrete performance mainly includes workability, strength, deformation, durability, economy and other aspects, so the following is an analysis of the performance of construction waste concrete recycling products from these aspects.
(1) Commutation. Workability refers to the workability of the mixture of various materials, because the apparent density of the recycled aggregate obtained from the recycling of construction waste concrete is lower than that of the natural aggregate, and the porosity is relatively larger, resulting in higher water absorption. In the case of the same water-cement ratio, the fluidity, plasticity, compaction and stability of recycled concrete obtained by recycling of construction waste will be weaker than that of concrete prepared by natural aggregate, thus affecting its workability and performance. Because of this, the recycled products of construction waste concrete have more pores, smaller thermal conductivity, and can obtain higher thermal insulation effect, so they have certain merits. At present, green energy-saving buildings will use recycled concrete to improve the thermal insulation effect of buildings and reduce the energy consumption for refrigeration and heating, which is in line with the relevant concepts of green buildings.
(2) Strength. For construction waste concrete recycling products, the strength of construction waste concrete recycling products is affected because the recycled aggregate has large pores, large fissures, higher water absorption, and the impurity problem is difficult to completely deal with. For construction projects with medium and low concrete strength requirements, the use of construction waste concrete recycling products can meet the strength requirements, but for construction projects with high strength requirements, it needs to be cautious.
(3) Contraction deformation. In the case of the same amount of cement, because the construction waste concrete recycling products have large pores and strong water absorption, the shrinkage deformation range is relatively small after mixing and putting into use, so it is ideal in terms of shrinkage deformation.
(4) Durability. There is a close relationship between the durability of construction waste concrete recycling products and the original durability of waste concrete, because the performance of recycled aggregate and recycled cement is mainly inherited from the performance of waste concrete itself. After hammering, crushing and other treatments during recycling, the performance will be lower than original, and some impurities will be mixed more or less in the preparation process, which will also affect the durability. Therefore, on the whole, the durability of construction waste concrete recycling products is lower than that of conventional concrete materials.
(5) Economy. Conventional concrete materials need to exploit raw materials, and need to consume a lot of oil, coal, natural gas and so on every year for the preparation of concrete, which will not only consume a lot of costs, but also increase carbon emissions. In addition, China's construction projects are very active, raw materials will inevitably decrease year by year, under the law of market supply and demand, the price of raw materials will continue to rise, and the cost will continue to rise. On the contrary, construction waste concrete recycling products do not need to be re-mined and refined, which saves the cost of these links and carbon emissions, and because construction waste concrete has been used as waste, the cost of obtaining such materials is very low, which greatly reduces the cost input.In recent years, the construction industry has attached great importance to the recycling of concrete, and the related technology has been developing continuously. The gap between concrete and conventional concrete can be shortened by adjusting parameters and adding reinforcement additives. Generally speaking, although some properties of construction waste concrete recycling products are weaker than conventional concrete, their economy is outstanding, and they are also conducive to carbon reduction and environmental protection, so they have a very ideal application prospect.
5. In this regard, in order to effectively implement the goal of carbon neutralization, it is necessary to improve the process of waste concrete recycling and further improve the performance of construction waste concrete recycling products, specifically from the following aspects.
(1) In the crushing and separation stage of cyclic regeneration, the crushed concrete after crushing, washing and magnetic separation treatment is subjected to heat treatment at 500 deg C. After heat treatment, the strength of recycled concrete will be further improved. This is because heat treatment can change the internal structure of the crushed material, so that the coarse and fine aggregates and mortar can be more fully separated, the impurities are less, the pores are smaller, and then the effect of improving its strength is achieved.
(2) In the process of crushing, separation and screening, the performance of recycled aggregate can be improved by optimizing the crushing process. Because the crushing size and surface structure of recycled aggregate have a direct impact on the performance of recycled concrete, through the vertical centrifugal crushing process and equipment, smaller and more uniform recycled aggregate can be obtained, thus improving the performance of recycled concrete.
And (3) when the cement is calcined, materials such as slag, furnace slag and the like can be added into the ingredients to serve as crystal seeds, and the treatment method can improve the performance of the cement base material and further improve the performance of the recycled cement, can treat industrial solid wastes such as the slag, the furnace slag and the like, avoids the pollution of the wastes to the environment, and is in line with the concept of environmental protection and low carbon.
(4) When using recycled aggregate and recycled cement to prepare recycled concrete, the recycled aggregate can be modified by water glass first, which is helpful to enhance the performance of recycled concrete, including early strength, acid corrosion resistance and frost resistance.
6 Specific recycling
of construction waste concrete 6.6
. Generally speaking, the concrete strength requirement of road engineering is significantly lower than that of high-rise building engineering, so recycled concrete is very suitable for road engineering, that is, to prepare construction waste concrete into recycled aggregate. It is used in road engineering. For the recycling products with low strength and more impurities, they can be used in road cushion construction. The performance of the material after screening treatment is enhanced to a certain extent, and the material can be applied to the construction of the road base after being mixed with some reinforcing agents, and the performance of the material can be further improved by adding some lime, cement, reinforcing additives and the like, so that the recycled aggregate with better performance is obtained, and the recycling technology of the building waste concrete is effectively applied to the pavement concrete.
To sum up, with the formulation of carbon peak and carbon neutral double carbon targets, higher carbon reduction and environmental protection requirements are put forward for the construction industry. Recycling the waste concrete produced by building construction and demolition, making recycled aggregate, recycled cement and other recycled concrete by hammering, crushing, screening and so on, can realize recycling, on the one hand, it can reduce the preparation of new concrete materials, avoid the carbon emissions produced by these preparation links, on the other hand, it can also protect the ecological environment. Avoid the destruction of waste concrete to nature, and play the role of natural environment in purifying carbon dioxide. It can be seen that under the background of carbon peak and carbon neutralization, the recycling of construction waste concrete is a powerful measure for the construction industry to respond to the call of the state, which is worth promoting.
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