A fully recyclable concrete and a method for its production and recycling

Abstract

The application discloses a fully recyclable concrete and a preparation method and a recycling method thereof, which are characterized by fully mixing silicate cement, limestone coarse aggregate, river sand, limestone machine-made sand, copper residue, metakaolin and a water reducing agent, adding water and stirring uniformly to obtain the fully recyclable concrete, wherein the fully recyclable concrete can be directly regenerated into high belite cement clinker and recycled after being crushed and calcined after service, and the cement clinker obtained has good hardening performance and low calcination temperature.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to a fully recyclable concrete and its preparation and recycling methods. Background Technology

[0002] Cement concrete is the most widely used and consumed building material. It not only consumes large amounts of high-quality sand and gravel resources, but also presents significant challenges in disposal and utilization due to the massive quantities of cement concrete generated after its disposal. Furthermore, high-quality natural raw materials for cement production, such as calcium carbonate, are becoming increasingly scarce. Since the main oxide composition of cement concrete is similar to that of cement clinker, consisting mainly of CaO and SiO2, it has the potential to replace cement raw materials. Moreover, the use of hardened cement stone to replace calcium carbonate can reduce the CO2 emissions from calcium carbonate decomposition.

[0003] Existing technologies for preparing recycled cement from waste concrete mainly utilize the coarse and fine or hardened cement stone separated from crushed waste concrete to replace raw cement meal. Some researchers have even replaced all raw cement meal with waste concrete, but this still requires a significant amount of corrective materials, and the hydration and hardening performance of the recycled cement clinker still lags behind that of ordinary Portland cement. Chinese patent (application number 201010569489.6) discloses a method for preparing recyclable concrete. This method uses the same raw materials as ordinary Portland cement clinker to prepare coarse and fine aggregates. Cement, coarse and fine aggregates, and water are then mixed in a specific ratio to produce cement concrete. The waste cement concrete is then crushed, ground, and calcined at 1200-1450℃ to obtain cement clinker. However, this method requires the manual preparation of coarse and fine aggregates, making the process relatively complex.

[0004] Therefore, there is a need to provide a recyclable concrete that can be directly used as cement raw material after being discarded, and can be calcined at high temperature to obtain cement clinker again with good hardening performance. Summary of the Invention

[0005] In view of this, this application provides a fully recyclable concrete, its preparation method and application, which can be directly recycled into high-belite cement clinker after being discarded and reused, and the resulting cement clinker has good hardening properties.

[0006] To achieve the above technical objectives, this application adopts the following technical solution:

[0007] In a first aspect, this application provides a method for preparing fully recyclable concrete, characterized by the following steps: Silicate cement, limestone coarse aggregate, river sand, limestone manufactured sand, copper slag, metakaolin, and a water-reducing agent are thoroughly mixed evenly; water is added and stirred evenly to obtain fully recyclable concrete; after the fully recyclable concrete has been in service, it is crushed, ground, and then calcined, yielding high-belite cement clinker without the need for or with minimal correction of the raw materials; during the preparation of the recyclable concrete, the direct calcination after disposal is considered to prepare recycled cement, wherein river sand mainly provides SiO2, copper slag mainly provides Fe2O3, metakaolin mainly provides Al2O3, and limestone coarse aggregate and limestone manufactured sand mainly provide CaO; and the raw materials contain various impurity ions, including Na... + K + Mg 2+ Al 3+ Fe 3+ P 5+ S 6+ .

[0008] Preferably, before crushing the recyclable concrete, a curing step is also included. After the recyclable concrete is cured, it is fed into a jaw crusher for crushing, then into a ball mill for grinding, and sieved through a 0.074mm square-hole sieve until the residue is less than 10% of the material to be sieved, to obtain recycled cement raw meal. A small amount or no corrective material is added to the recycled cement raw meal, and after being mixed evenly, it is calcined and then rapidly cooled to obtain high-belite cement clinker. Gypsum is then added to obtain recyclable concrete.

[0009] Preferably, the mass ratio of silicate cement, limestone coarse aggregate, river sand, limestone manufactured sand, copper slag, metakaolin, and water is 1:(2.82-4.21):(0.84-1.20):(0.59-0.97):(0.19-0.72):(0.01-0.07):(0.54-0.80).

[0010] Preferably, when the mass ratio of silicate cement, limestone coarse aggregate, river sand, limestone manufactured sand, copper slag, metakaolin, and water is 1:(2.82-3.21):(0.84-0.89):(0.59-0.77):(0.19-0.29):(0.001-0.07):(0.54-0.61), under this mix proportion, the recyclable concrete of this scheme, in addition to meeting the mix proportion design requirements and basic performance of ordinary concrete, will also generate waste materials after service life. After crushing, grinding, and sieving, ordinary high-belite cement clinker was obtained after calcination. This ordinary high-belite cement clinker contains SiO2: 25.45-26.99%, Al2O3: 4.07-5.25%, Fe2O3: 4.62-5.28%, CaO: 63.91-65.03%, and KH = (0.75-0.77), SM = (2.11-2.62), and IM = (0.88-1.02), meeting the characteristic requirements of high-belite cement clinker. The mineral phase composition of the high-belite cement clinker meeting the above conditions, calculated by the Bouger method, is: tricalcium silicate (C3S): 23-31%, dicalcium silicate (C2S): 52-57%, tricalcium aluminate (C3A): 3-5%, and tetracalcium aluminoferrite (C4AF): 14-16%.

[0011] Preferably, the mass ratio of silicate cement, limestone coarse aggregate, river sand, limestone manufactured sand, copper slag, metakaolin, and water is 1:(3.54-4.21):(1.14-1.20):(0.68-0.97):(0.47-0.72):(0.01-0.07):(0.68-0.80). Under this mix proportion, the recyclable concrete of this scheme, in addition to meeting the mix proportion design requirements and basic performance of ordinary concrete, after the end of its service life, the waste material is crushed, ground, sieved, and calcined to obtain high-ferrous phase high-belite cement clinker; in this high-ferrous phase high-belite cement clinker, SiO2: 25.75-27.14%, Al2O3: 4.62-5.46%, Fe2O3: 7.26-8.58%, CaO: 60.19-60.94%, KH = 0.67, SM = (1.83-2.28), IM = 0.64; the mineral phase composition of the high-ferrous phase high-belite cement clinker meeting the above conditions, calculated by the Borg method, is C2S: 74-78%, C4AF: 22-26%.

[0012] Preferably, the water-reducing agent has a mass of 0.15-1.0% of the silicate cement.

[0013] Preferably, the cement is PI cement, which contains 62-64% CaO by mass.

[0014] Preferably, the copper slag contains 50%-55% Fe2O3 by mass. The residue on a 0.074mm square-hole sieve is less than or equal to 10%.

[0015] Preferably, the metakaolin contains 40%-44% Al2O3 by mass. The metakaolin has a residue of less than or equal to 10% after being screened through a 0.074mm square-hole sieve.

[0016] Preferably, the river sand contains 65%-70% SiO2 and 10%-13.72% Al2O3 by mass.

[0017] Preferably, the limestone manufactured sand contains 50%-52% CaO by mass.

[0018] Preferably, the limestone coarse aggregate contains 50%-52.13% CaO by mass.

[0019] Preferably, the calcination temperature is 1250-1350℃.

[0020] Secondly, this application provides a recycled high-belite cement clinker.

[0021] Thirdly, this application provides a method for recycling fully recyclable concrete. After the fully recyclable concrete has been in service, it is crushed, ground, and then calcined to obtain high-belite cement clinker. After adding gypsum, the high-belite cement is obtained. Specifically, for high-belite cement clinker with a ratio of KH = (0.75-0.77), SM = (2.11-2.62), and IM = (0.88-1.02), 3-5% of the clinker mass of gypsum dihydrate is added; for high-belite cement clinker with a ratio of KH = 0.67, SM = (1.83-2.28), and IM = 0.64, 1-3% of the clinker mass of gypsum dihydrate is added.

[0022] The beneficial effects of this application are as follows:

[0023] 1. This invention selects high-belite cement clinker as the target recycled cement clinker. High-belite cement clinker has a similar oxide composition to ordinary silicate cement clinker, but with dicalcium silicate (C2S) as the dominant mineral, it has a lower calcination temperature and lower CO2 emissions. However, due to insufficient early hydration capacity of dicalcium silicate, its early strength is lower. This application adds metakaolin and copper slag admixtures to obtain recyclable concrete. Compared with natural raw materials, the recyclable concrete of this application contains various impurity ions, including Na+. + K + Mg 2+ Al 3+ Fe 3+ P 5+ S6+ These impurity ions can stabilize the belite phase and prevent the transformation of the belite mineral phase to the allite mineral phase, which is an important reason affecting the hydration and hardening performance of recycled silicate cement clinker. At the same time, they can cause belite lattice distortion and improve the belite hydration capacity. This invention cleverly utilizes this characteristic to prepare high-belite cement clinker by calcining recyclable cement concrete ingredients. This can minimize the impact of impurity ions preventing the transformation of the belite mineral phase to the allite mineral phase on the hydration and hardening performance of clinker minerals, and can also improve the hydration activity of the belite mineral phase. This allows waste recyclable concrete to be directly recycled into high-belite cement clinker for recycling, and the resulting cement clinker has good hardening performance and a low calcination temperature.

[0024] 2. Compared to using natural raw materials as cement meal, this invention, when using recyclable concrete as cement meal, can reduce the calcination temperature. If the target clinker mineral phase is high-ferrous phase high-belite cement clinker, due to the high iron content, the calcination temperature can be further reduced to approximately 1250℃, which can reduce CO2 emissions and save energy. The reduced calcination temperature can avoid high-temperature decomposition of the iron phase mineral phase in the high-ferrous phase high-belite cement clinker, maximizing the hydration activity of the iron phase minerals and improving the early strength of the cement clinker. Simultaneously, impurities S present in recyclable concrete... 6+ Under low-temperature calcination, anhydrous calcium sulfoaluminate minerals can appear without high-temperature decomposition, further improving the early strength of high-iron phase high-belite cement clinker. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0026] The following specific embodiments further illustrate this solution.

[0027] Example 1

[0028] A method for preparing fully recyclable concrete includes the following steps: 46.96% by weight of limestone coarse aggregate (5-25mm continuous grade crushed stone), 13.67% river sand (medium sand with a fineness modulus of 2.8), and 10.50% limestone manufactured sand (fineness modulus 3.15, stone powder content 8%, CaCO3 content 93%) are mixed evenly. Then, 15.54% cement, 4.21% copper slag, and 0.16% metakaolin are added and mixed evenly. Finally, 8.96% water and 0.19% water-reducing agent are added until the mixture is homogeneous, yielding recyclable concrete. (Concrete 1)

[0029] Example 2

[0030] A method for preparing fully recyclable concrete includes the following steps: 46.96% by weight of limestone coarse aggregate (5-25mm continuous grade crushed stone), 13.98% river sand (medium sand with a fineness modulus of 2.8), and 10.19% limestone manufactured sand (fineness modulus 3.15, stone powder content 8%, CaCO3 content 93%) are mixed evenly. Then, 12.29% cement, 7.47% copper slag, and 0.16% metakaolin are added and mixed evenly. Finally, 8.96% water and 0.19% water-reducing agent are added until the mixture is homogeneous, yielding recyclable concrete. (Concrete 2)

[0031] Example 3

[0032] A method for recycling fully recyclable concrete involves first coarsely crushing the recyclable concrete obtained in Example 1 using a jaw crusher, and then grinding it in a ball mill to obtain recycled cement raw meal. The oxide composition of the recycled cement raw meal is shown in Table 2. 1.30% analytical grade correcting agents (0.25% SiO2, 0.39% Fe2O3, and 0.66% Al2O3) are added to the recycled cement raw meal, with adjustment values ​​of KH = 0.75, SM = 2.20, and IM = 0.92. The mixture is then calcined at 1300℃ for 30 min to obtain recycled cement clinker. This recycled cement clinker is ordinary high-belite cement clinker; adding 5% dihydrate gypsum yields recycled ordinary high-belite cement.

[0033] (Cement 1)

[0034] Example 4

[0035] A fully recyclable concrete recycling method was described in Example 2. The recyclable concrete obtained was first coarsely crushed using a jaw crusher, and then ground in a ball mill to obtain recycled cement raw meal. The oxide composition of the recycled cement raw meal was tested and is shown in Table 2. 2.04% corrective raw materials (1.27% CaCO3, 0.21% Fe2O3, and 0.56% Al2O3) were added to the recycled cement raw meal, with adjustment values ​​of KH = 0.67, SM = 2.04, and IM = 0.64. The recycled cement clinker, calcined at 1250℃ for 30 min, was a high-ferrous phase high-belite cement clinker. Adding 2% dihydrate gypsum yielded recycled high-ferrous phase high-belite cement. (Cement 2)

[0036] The recycled cement raw meal ratio values ​​of Examples 3 and 4 and the mineral composition of the obtained recycled high belite cement clinker are shown in Table 1.

[0037] Table 1. Mineral composition and raw meal ratio of ordinary high-belite and high-ferro-phase high-belite cement clinker.

[0038]

[0039] This indicates that Examples 3 and 4 do indeed correspond to obtaining ordinary high-belite cement clinker and high-iron phase high-belite cement clinker.

[0040] Comparative Example 1

[0041] A method for preparing ordinary cement concrete includes the following steps: 46.96% by weight of limestone coarse aggregate (5-25mm continuous grade crushed stone) and 24.17% by weight of river sand (medium sand with a fineness modulus of 2.8) are mixed evenly; then 19.91% cement is added, followed by 8.96% water and 0.19% water-reducing agent, until the mixture is evenly mixed to obtain non-recycled concrete. (Concrete 3)

[0042] Comparative Example 2

[0043] A method for preparing ordinary cement concrete includes the following steps: 46.96% by weight of limestone coarse aggregate (5-25mm continuous grade crushed stone) and 24.17% by weight of limestone manufactured sand (medium sand with a fineness modulus of 2.8) are mixed evenly; then 19.92% cement is added, followed by 8.96% water and 0.19% water-reducing agent, until the mixture is evenly mixed to obtain non-recycled concrete. (Concrete 4)

[0044] Comparative Example 3

[0045] A method for preparing ordinary high-belite cement, maintaining the same yield value as the ordinary high-belite cement clinker in Example 3, uses analytical grade reagents, namely analytical grade SiO2, Al2O3, Fe2O3, and CaCO3. After uniform mixing, the raw materials are calcined at 1400℃ for 30 minutes to obtain ordinary high-belite cement clinker. Adding 5% gypsum dihydrate yields ordinary high-belite cement. Since Comparative Example 3 directly obtains cement clinker (Cement 3) by calcining analytical grade raw materials, Cement 3 is non-recycled cement. (Cement 3)

[0046] Comparative Example 4

[0047] A method for preparing high-ferrous phase high-belite cement, maintaining the same yield value as the high-ferrous phase high-belite cement clinker of Example 4, uses analytical grade reagents, namely analytical grade SiO2, Al2O3, Fe2O3, and CaCO3. The raw materials are mixed uniformly and calcined at 1350℃ to obtain high-ferrous phase high-belite cement clinker. Adding 2% gypsum dihydrate yields high-ferrous phase high-belite cement. Since Comparative Example 4 directly obtains cement clinker (Cement 4) by calcining analytical grade raw materials, Cement 4 is non-recycled cement. (Cement 4)

[0048] The range of oxide composition of raw materials used in the concrete preparation process of Examples 1-2 and Comparative Examples 1-2, as well as the concrete mix proportions, are shown in Table 2.

[0049] Table 2 Oxide Composition of Concrete Raw Materials

[0050]

[0051] The concrete mix proportions in Examples 1-2 and Comparative Example 1 are shown in Table 3.

[0052] Table 3. Mix proportions of cement concrete (%)

[0053]

[0054]

[0055] The oxide composition of the concrete obtained from Test Examples 1-2 and Comparative Examples 1-2 after crushing and grinding is shown in Table 4. As can be seen from the table, the CaO / SiO2 ratio in the concrete of Comparative Example 1 is lower than that in the concrete of the Test Examples, while the CaO / SiO2 ratio in the concrete of Comparative Example 2 is much higher than that in the concrete of the Test Examples. Furthermore, the iron content in the comparative examples is relatively low. This is because no mineral admixtures or limestone manufactured sand were added in Comparative Example 1, and no mineral admixtures or river sand were added in Comparative Example 2. Neither of these methods meets the requirements for preparing recycled cement by direct calcination without batching.

[0056] Table 4 Oxide composition of cement concrete after crushing and grinding

[0057]

[0058] The proportions of the cement raw meal in each test group are shown in Table 5. It can be seen that the recyclable concrete from Examples 3 and 4 accounts for approximately 98-99% of the cement raw meal. To achieve the designed yield, only a very small amount of corrective material is needed for direct calcination to obtain recycled cement. Within the same raw meal yield range, the calcination temperature of Examples 3 and 4 is approximately 100°C lower than that of Comparative Examples 3 and 4. This is mainly because the clinker mineral phase is predominantly belite, which has a relatively low formation temperature; and the impurity ions in the recyclable concrete also promote the formation of the liquid phase, lowering the reaction temperature.

[0059] Table 5. Mix proportions and calcination temperatures of cement raw materials for each group.

[0060]

[0061]

[0062] Testing and Evaluation

[0063] The strength and impermeability of the above four types of concrete were tested, and the results are shown in Table 6 below.

[0064] Table 6 Comparison of Concrete Strength and Impermeability

[0065] project Concrete 1 Concrete 2 Concrete 3 Concrete 4 3D strength / MPa 20 17 27 25 28d strength / MPa 37 35 42 38 90d strength / MPa 44 43 48 45 <![CDATA[28d chloride ion diffusion coefficient 10 -12 / ㎡ / s]]> 15 17 12 14 <![CDATA[56d Chloride ion diffusion coefficient 10 -12 / ㎡ / s]]> 12 14 10 12

[0066] As shown in Table 6, although concretes 1 and 2 had lower early-stage strength compared to concretes 3 and 4, their later-stage strength increased significantly. This is because concretes 1 and 2 contained mineral admixtures, copper slag and metakaolin. Copper slag and metakaolin have relatively low early-stage hydration activity, initially playing a dominant role in filling. However, as cement further hydrates, the content of the silicate mineral hydration product Ca(OH)2 gradually increases, and the alkaline environment in the system gradually strengthens. The active components of the mineral admixtures (SiO2, Fe2O3) are activated in this alkaline environment, reacting with cement hydration products to form volcanic ash reactions, generating strong cementitious substances such as hydrated calcium silicate gel, which improves the bond strength of the concrete interface zone. Furthermore, the later-stage durability of concretes 1 and 2 was also improved. After 56 days of curing, compared to 28 days, the chloride ion permeability coefficients of all groups decreased. The chloride ion permeability coefficient of Example 1 decreased by 33.3% compared to Comparative Example 1, and the chloride ion permeability coefficient of Example 2 decreased by 33.6% compared to Comparative Example 2.

[0067] The above four types of cement were tested for cement paste strength, and the results are shown in Table 7 below.

[0068] Table 7 Strength results of each group of recycled cement

[0069]

[0070]

[0071] As shown in Table 7, the 3-day strength of the cements in Examples 3 and 4 is higher than that of the cements in Comparative Examples 3 and 4, respectively, indicating higher early hydration and hardening capacity, and also higher 28-day and 90-day strengths. The superior performance of the obtained recycled cement is due, in part, to the activation effect of impurity ions on the belite phase, thus improving its hydration and hardening capacity; simultaneously, the lower calcination temperature is beneficial for increasing the activity of the iron phase, which in turn helps improve the hydration and hardening performance of the cement clinker.

[0072] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for recycling fully recyclable concrete, characterized in that, The process includes the following steps: Silicate cement, limestone coarse aggregate, river sand, limestone manufactured sand, copper slag, metakaolin, and water-reducing agent are thoroughly mixed evenly, water is added and stirred evenly to obtain fully recyclable concrete. After the fully recyclable concrete has been used for a period of time, it is crushed and calcined at 1250-1300℃ to obtain high belite cement clinker. 1-3% dihydrate gypsum by weight of clinker is added to obtain high belite cement. The mass ratio of silicate cement, limestone coarse aggregate, river sand, limestone manufactured sand, copper slag, metakaolin, and water is 1:(3.54-4.21):(1.14-1.20):(0.68-0.97):(0.47-0.72):(0.01-0.07):(0.68-0.80); the resulting high-belite cement clinker has a yield value of KH=0.67, SM=(1.83-2.28), and IM=0.

64. The water-reducing agent has a mass of 0.15-1.0% of the silicate cement. The copper slag contains 50%-55% Fe2O3 by mass. The metakaolin contains 40%-44% Al2O3 by mass.

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