METHOD FOR RECYCLING CONCRETE CONSTRUCTION AND/OR DEMOLITION WASTE

Abstract

A method for recycling concrete waste from construction and/or deconstruction, including at least one step of microwave processing of concrete blocks, at least one step of mechanical processing, and at least one step of carbonation of recycled concrete aggregates and/or recycled concrete fines; and a recycling installation for carrying out the recycling method and to recycled concrete aggregates obtained by the steps of microwave processing of concrete blocks and mechanical processing.

Claims

1-15. (canceled)

16. A method for recycling concrete from construction and/or demolition, comprising at least the following steps: i) one or more steps of microwave treatment of concrete blocks from construction and/or demolition with a size on the centimeter scale, to form microfractured concrete blocks, ii) one or more steps of mechanical treatment making it possible to extract, from said microfractured concrete blocks, recycled aggregates comprising bare natural aggregates and natural aggregates associated with an adherent residual cementitious matrix; and recycled fines, iii) one or more steps of carbonating the recycled aggregates, in the presence of carbon dioxide, to form carbonated recycled aggregates, and/or iv) one or more steps of carbonating the recycled fines, in the presence of carbon dioxide, to form carbonated recycled fines.

17. The method according to claim 16, wherein the concrete blocks from construction and/or demolition have a size of at most 10 cm.

18. The method according to claim 16, wherein the microwave treatment of step i) is carried out for a time ranging from 30 s to 10 min.

19. The method according to claim 16, wherein the microwave treatment i) is carried out at a frequency ranging from 915 MHz to 2450 MHz.

20. The method according to claim 16, wherein the bare natural aggregates are selected from gravel, sands with a particle size of 1 mm or more, and one of the mixtures thereof; and the natural aggregates associated with an adherent residual cementitious matrix are selected from gravel, sands with a particle size of 1 mm or more, and one of the mixtures thereof.

21. The method according to claim 16, wherein the adherent residual cementitious matrix associated with the natural aggregates comprises natural aggregates having a particle size of less than 1 mm.

22. The method according to claim 16, wherein step ii) comprises at least one step ii-1) of milling the microfractured concrete blocks.

23. The method according to claim 22, wherein step ii) further comprises at least one screening step ii-2).

24. The method according to claim 16, wherein the recycled aggregates have a maximum particle size G.sub.M of at most 45 mm, or at most 25 mm, or at most 11.2 mm, or at most 8 mm, or at most 4 mm.

25. The method according to claim 16, wherein the recycled aggregates have a particle size G.sub.m of at least 1 mm.

26. The method according to claim 16, further comprising at least one step a) of cleaning the recycled aggregates.

27. The method according to claim 26, further comprising at least one step b) of washing the recycled aggregates.

28. The method according to claim 16, wherein step iii) is carried out in the presence of a gas comprising at least 10 mol. % of CO.sub.2, with respect to the total number of moles of gas.

29. A recycling installation for carrying out a recycling method as defined in claim 16, comprising: at least one microwave tunnel for forming microfractured concrete blocks from concrete blocks from construction and/or demolition with a size on the centimeter scale, at least one roller mill for fragmenting the microfractured concrete blocks and releasing recycled aggregates comprising bare natural aggregates and natural aggregates associated with a cementitious matrix; and recycled fines, at least one vibrating, gyratory or rotary screen or sieve for separating the recycled aggregates from the recycled fines, optionally cleaning them, and optionally washing them, and at least one carbonation reactor for forming carbonated recycled aggregates and/or carbonated recycled fines.

30. A recycled aggregates (RCA) obtained according to a method as defined in claim 16, wherein the RCAs with a given particle size fraction, of 1 mm or more, comprise at least 50 wt. % of bare natural aggregates with said given particle size fraction, with respect to the total weight of the RCAs with said given particle size fraction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0222] The appended drawings illustrate the invention:

[0223] FIG. 1a shows the fracturing of concrete according to a method of the background art.

[0224] FIG. 1b shows the fracturing of concrete comparatively according to the method of the invention.

[0225] FIG. 2 depicts a schematic diagram of the various steps of the method of the invention.

[0226] FIG. 3 depicts the measurement of RCA cleanliness through the variation in residual CP (wt. %) depending on the size of the RCAs; and the measurement of RCA water absorption (as a %) depending on the size of the RCAs, according to the method of the invention, and comparatively according to a reference method.

[0227] FIG. 4a depicts the speciation analysis of the constituents of the initial concrete and RCAs at the end of the method of the invention.

[0228] FIG. 4b depicts comparatively the speciation analysis of the constituents of the initial concrete and RCAs of a reference method.

[0229] FIG. 5 shows the performance of the method of the invention with respect to a reference method, and with respect to the theoretical limit.

[0230] FIG. 6 shows the performance of the method of the invention with respect to a reference method, and with respect to the theoretical limit.

DETAILED DESCRIPTION

[0231] Further features and advantages of the present invention will become apparent from the description of non-limiting examples of the method according to the invention.

EXAMPLES

Example 1: Depictions of the Method According to the Invention and of a Method of the Background Art not According to the Invention

[0232] FIG. 1 comprises a depiction of concrete fracturing according to a method of the background art (FIG. 1a)) and a representation of concrete fracturing using the method of the invention (FIG. 1b)).

[0233] In FIG. 1a), the fracturing is random and non-selective between the NAs and the mortar. It produces RCAs with low NA content (<60 wt. % of NA with respect to the total weight of the RCAs) and with high CP content (>10 wt. % of CP with respect to the total weight of the RCAs), with a high potential for transgranular fracturing of the original natural aggregates NA. As a result, aggregates made entirely of cementitious matrix can be formed, while natural aggregates NA with their original structure preserved are released only to a limited extent. After fracturing, one type of recycled concrete aggregates (RCA) is mostly obtained: mixed aggregates composed of pieces of natural aggregate and a high proportion of cementitious matrix or mortar within the mixed aggregates. As a corollary, fracturing produces few recycled fines, which concentrate no more than 25% of the CP of the initial concrete to be recycled.

[0234] In FIG. 1b), the fracturing of a concrete according to the method of the invention is selective between the NAs and the mortar, thus avoiding fracturing of the original natural aggregates NA, and favoring the release of natural aggregates NA with their original structure preserved, as well as of the cementitious matrix. The cement paste in the microfractured cementitious matrix can lead to fines, which may or may not be carbonated, with a particle size that is easily separable from that of the aggregates. On the surface of the released natural aggregates, a small proportion of adherent residual cementitious matrix may be found, which may or may not be carbonated to obtain recycled aggregates RCA or carbonated recycled aggregates CRCA that perform as well as the original natural aggregates. In other words, the fracturing of a concrete according to the method of the invention favors both the concentration of the NAs in RCAs with a low content of adherent mortar without altering the particle size and mechanical properties of the NAs, and the recovery of the CP contained in the initial concrete to be recycled in a fines fraction. The microfracturing of the CP concentrates most of the CP from the initial concrete to be recycled in the fines, and promotes the carbonation of the CP in the fines and/or of the CP adhering in small amounts to the surface of the RCAs. The carbonation of the RCAs produces CRCAs with performance levels as good as those of the NAs.

[0235] FIG. 2 shows step i) of microwave treatment in a microwave tunnel of concrete blocks from construction and/or demolition in order to form microfractured concrete blocks. These are then milled in step ii-1) in a roller mill to form a mixture of recycled aggregates RCA with different particle sizes; and recycled fines. Milling ii-1) makes it possible to obtain a particle size of 8 mm or less, for example. A first screening or sieving step ii-2a) is then carried out using a rotary screen to extract recycled aggregates having a particle size fraction G.sub.1-G.sub.1 (e.g. particle size fraction of 4 mm to 8 mm); as well as a mixture of recycled fines and recycled aggregates having a particle size<G.sub.1 (particle size of less than 4 mm).

[0236] Clean recycled aggregates RCA having a particle size fraction G.sub.1-G.sub.1 are then carbonated in a step iii) in a carbonation reactor in order to form a first CRCA fraction. The recycled fines and the recycled aggregates having a particle size<G.sub.1 undergo a further screening or sieving step ii-2b) using a rotary screen to extract recycled aggregates RCA having a particle size fraction G.sub.2-G.sub.2 (e.g. particle size or particle size fraction of 1 mm to 4 mm); as well as recycled fines (particle size of less than 1 mm).

[0237] The clean recycled aggregates having a particle size fraction G.sub.2-G.sub.2 are then carbonated in a step iii) in a carbonation reactor in order to form a second CRCA fraction and/or the recycled fines are carbonated in a carbonation reactor in a step iv).

[0238] Optionally, the concrete blocks used in step i), having a size on the centimeter scale (e.g. 30 mm), can be obtained from the crushing of construction and/or demolition concrete, preferably using a gravel crusher or jaw crusher [step i.sub.0)].

[0239] During steps ii-2a) and ii-2b), steps of abrasion a) and washing b) (water spraying) are preferably carried out concomitantly with each of steps ii-2a) and ii-2b).

Example 2: Recycling of Concrete from Construction and/or Demolition Using a Method According to the Invention

2.1 Production of Concrete Samples and Characteristics

[0240] A concrete was formulated from cement and crushed siliceous natural aggregates (NA) with a particle size of less than 8 mm and a water/cement ratio of 0.6.

[0241] 2.1.1 The cement used is Lafarge CEM I from the Le Teil plant. Its strength class is 52.5R, that is, rapid-setting. It meets the European requirements of standard EN 197-1.

[0242] The physical and mechanical characteristics of the cement used are as follows:

[00001]$\mathrm{Blaine}\mathrm{specific}\mathrm{surface}\mathrm{area}=4160{\mathrm{cm}}^2\text{/g},\mathrm{and}$$\mathrm{Density}=3.15g/{\mathrm{cm}}^3.$

[0243] 2.1.2 The natural aggregates (NA) used are a mixture of sand(S) and gravel (G) from the Palvadeau sandpit (Saint-christophe-du-Ligneron) of the crushed siliceous type. The characteristics of these aggregates are as follows:

[00002]$\mathrm{Density}=2640\text{kg/}{m}^3,$$\mathrm{Absorption}\mathrm{coefficient}=0.7\%,$$\mathrm{Chlorides}<0.001\%,$$\text{Water-soluble}\mathrm{sulfates}<0.01\%,\mathrm{and}$$\mathrm{Active}\mathrm{alkali}\mathrm{content}=0.0022\%.$

[0244] Table 1 hereunder depicts the particle size distribution of the NAs used to formulate the concrete, and more particularly the wt. % (mass fractions) of five different particle size fractions, namely 4/8 mm, 2/4 mm, 1/2 mm, 0.25/1 mm and 0/0.25 mm, with respect to the total weight of the natural aggregates NA used to formulate the concrete.

TABLE-US-00001 TABLE 1 NA particle Mass fraction size fraction Nomenclature (wt. %) 4/8 mm 4/8 NA 51.2 2/4 mm 2/4 NA 12.9 mm NA 7.4 0.25/1 mm 0.25/1 NA 14.2 0/0.25 mm 0/0.25 NA 14.3 Total 100.0 0/1 mm <1 NA 28.5

[0245] These mass fractions are interpreted as average values of the actual NA composition of the concrete used hereinafter. The last line of table 1 defines the 0/1-mm NA particle size fraction which is referred to hereunder. This fraction combines the 0.25/1-mm and 0/0.25-mm NA.

2.1.3 Concrete Formulation

[0246] The proportions of binder (cement and superplasticizer), water and NA used to produce concrete are shown in table 2 hereunder:

TABLE-US-00002 TABLE 2 CEM I 280 kg/m.sup.3 Concrete Mass metering of concrete constituents fraction (%) Water/cement 0.6 CEM I 11.8 mass ratio Gravel/sand 1.33 Water 7.6 mass ratio Superplasticizer 0.3 wt. %, with respect NA 80.6 to the total weight of water + cement + NA + superplasticizer

[0247] A plurality of concrete samples are prepared by mixing the constituents in a mortar mixer for 60 seconds at low speed (62 rpm), 40 seconds at high speed (125 rpm), pausing for 90 seconds and then 60 seconds at high speed (125 rpm) to form a mixture; then pouring the mixture into gray polypropylene cylindrical molds with a diameter of 25 mm and a height of 30 mm. The molds are vibrated for 210 seconds on a vibrating table. This leads to the formation of a plurality of concrete cylinders of controlled composition and shape, with a size of about 30 mm in their largest dimension.

[0248] These concrete cylinders are then stored under cellophane in a room tempered to 19 C. for 28 days to cure the concrete.

[0249] After curing for 28 days in a temperate room, the cylindrical concrete samples are oven-dried at 80 C. for 48 hours. This time was determined following a test on a batch of aggregates weighed at regular intervals until reaching a constant mass (m<0.1%). This drying method ensures reproducibility of the concrete samples produced, by giving them a controlled moisture content comparable to that of naturally aged concrete.

[0250] The composition of the concrete in the samples, after curing and storage, is measured by dissolving the CP in 19 wt. % hydrochloric acid. The values in table 3 hereunder show the range of composition values measured on a plurality of 30-mm concrete samples, each sample being dissolved individually. This makes it possible to assess the variability in the composition of the concrete samples used.

TABLE-US-00003 TABLE 3 Concrete Mass fraction Average mass constituents (%) fraction (%) 4/8 NA 41.7-50.9 44.9 2/4 NA 8.7-14.5 11.7 NA 5.1-6.6 6.0 0.25/1 NA 9.4-12.1 10.4 0/0.25 NA 8.8-13.1 11.0 CP (hardened 14.1-17.1 16.1 cement + water) Total 100 Mortar (CP + <1 NA) 32.3-42.3 37.5

2.2 Implementation of the Method of the Invention

2.2.1 Microwave Fracturing Step i)

[0251] The microwave bench used is a single-mode cavity waveguide manufactured by SAIREM with the following characteristics:

[00003]$\mathrm{Frequency}=245025\mathrm{MHz},$$\mathrm{Output}\mathrm{power}=2000<0.1\%W,$$\mathrm{Maxiumum}\mathrm{operating}\mathrm{power}=1700W,$$\mathrm{Waveguide}=\mathrm{WR}340\left(\mathrm{rectangular}\mathrm{section}8643\mathrm{mm}\right),$$\mathrm{Wave}\mathrm{ripple}<0.3\%\mathrm{RMS},\mathrm{and}$$\mathrm{Magnetron}\mathrm{cooling}\text{system:}\mathrm{water}.$

[0252] This microwave bench can treat one 30-mm concrete sample at a time, at a given power and for a given time. A cylindrical concrete sample is placed in a quartz tube, which acts as a sample holder, with the tube placed in the chimney of the microwave bench. Each sample is treated for 4 minutes with an incident microwave power of 1.5 KW. The temperatures reached on the concrete surface range from 450 C. to over 600 C. (limit measurable by the infrared thermometer used).

2.2.2 Mechanical Processing Step ii)

2.2.2.1 Non-Percussive Crushing (Step i-1)

[0253] Non-percussive crushing following microwave treatment is performed using a manual hydraulic press (Specac Atlas 15T). The concrete samples are individually compressed. The stroke of the hydraulic press stops at 8 mm from the surface on which the RCAs are placed, corresponding to the maximum size of the NAs of the concrete samples.

2.2.2.2 Screening (Step ii-2)

[0254] The crushed sample is then screened by a dry process on standard 8-mm, 4-mm, 2-mm, 1-mm and 0.25-mm square mesh sieves in order to produce the 4/8-mm, 2/4-mm, 1/2-mm, 0.250/1-mm and 0/0.25-mm particle size fractions.

2.2.2.3 Abrasion (step a))

[0255] The 1/4-mm and 4/8-mm fractions of recycled concrete aggregates (RCA) produced by crushing then screening are abraded for a short time of 2 minutes separately in a cylindrical metal jar with an internal diameter of 10 cm, rotating at a speed of 100 rpm. The abrasive charge consists of 336 g of 20-mm alumina beads and 200 ml of water. The products are then re-screened at 8 mm and 4 mm to separate the 8-mm and 4-mm RCAs respectively from the abrasive fines.

[0256] At the end of steps ii-1), ii-2) and a), a plurality of fractions of fines are recovered and collected.

2.2.3 Carbonation of Fines (Step iii))

[0257] The fines obtained are ground to 80 m using an automatic mortar and pestle, with an average size d.sub.50=around 10 m. This fineness is justified for the use of carbonated fines as a mineral additive. The step of carbonating the milled fines is carried out in suspension in a stirred reactor with a useful volume of 300 ml. The suspension used consists of a mixture of 15 g of recycled concrete fines and 150 ml of distilled water. The mixing is ensured by a mobile agitator rotating at 800 rpm. The carbonation reactor operates under a partial pressure of CO.sub.2 of 5 bar with a controlled temperature of 60 C. A carbonation test lasts 20 hours, during which the consumption of CO.sub.2 by the fines is continuously measured. The final carbonation rate of the carbonated fines is obtained by thermogravimetric analysis (TGA) or by total carbon measurement (CHNS analysis).

Comparative Example 3: The Recycling of Concrete from Construction and/or Demolition Using a Method not According to the Invention

[0258] The method of the invention was compared to a non-percussive crushing method, which represents the most efficient current technology for the production of recycled concrete aggregates. This comparative method, referred to hereinafter as the reference method, was carried out using the same concrete samples as those described in point 2.1. The reference method consists of a mechanical treatment via non-percussive crushing and screening steps under the same conditions as those described in point 2.2.

[0259] A step of carbonating the fines is carried out as described in example 2, part 2.2.3.

Example 4: Performance of the Method of the Invention

4.1 Performance Evaluation Criteria

[0260] The performance criteria used to characterize the performance of the method relate to the quality and use properties of the recycled concrete aggregates (RCA) and the recycled concrete fines.

For the Recycled Concrete Aggregates (RCA):

Quality:

[0261] Content (wt. %) of CP in a given RCA particle size fraction, [0262] Content of clean RCAs (i.e. RCA having a CP content<5 wt. %) in a given RCA particle size fraction, and [0263] Content (wt. %) of natural aggregates (NA) with the same particle size fraction as the RCAs.

Use Properties:

[0264] Water absorption of the RCAs by the WA24 method. This test measures the water content of the RCAs after drying according to standard NF EN 1097-5. It is expressed as a weight percentage. [0265] Wear resistance of the RCAs measured by the Micro Deval (MDE) test. This test measures the resistance to wear caused by friction between the RCAs and an abrasive load of steel balls, according to standard NF EN 1097-1. The test result is expressed as a % and represents the weight percentage of particles<1.6 mm produced during the test.

For the Recycled Concrete Fines (<1 mm):

Quality:

[0266] Content (wt. %) of mortar that the method concentrates in the fines (<1 mm) produced [0267] Content (wt. %) of CP that the method concentrates in the fines (<1 mm) produced [0268] Rate (wt. % of CO.sub.2 captured) of carbonation of the fines.

4.2 Cleanliness of the Recycled Concrete Aggregates

[0269] Appended FIG. 3 shows the measurement of cleanliness of the RCAs in terms of the variation in the content of residual CP (wt. %) (left) depending on the size of the RCAs for the method of the invention (solid circles) and for the reference method (empty circles). This content is measured by dissolving the RCAs with 4/8-mm, 2/4-mm, 1/2-mm, 0.250/1-mm and 0/0.25-mm particle size fractions in a solution of 19 wt. % hydrochloric acid.

[0270] The absence of overlap between the 95% confidence intervals for the mass fraction of residual CP down to an RCA particle size of 0.5 mm demonstrates that the method of the invention produces RCAs that are significantly cleaner than the RCAs produced by the reference method down to an RCA size of 0.5 mm.

[0271] Appended FIG. 3 also shows the measurement of the water absorption of the RCA (as a %) (right) depending on the size of the RCAs for the method of the invention (long dotted curves) and for the reference method (short dotted curves). This content is measured by dissolving the RCAs with 4/8-mm, 2/4-mm, 1/2-mm, 0.250/1-mm and 0/0.25-mm particle size fractions in a solution of 19 wt. % hydrochloric acid.

4.3 Product Speciation During the Method of the Invention

[0272] The fine analysis of product quality during the method of the invention (vs. the reference method) consists in measuring the speciation of the initial constituents of the concrete (NA and CP, see example 2, part 2.1) before and after the method of the invention (vs. the reference method) (example 2, part 2.2 and example 3). Speciation is used to refer to the traceability of the initial constituents in the products of the method of the invention (vs the reference method).

[0273] As a reminder, the products used to characterize the performance of the concrete recycling methods are 4/8 RCA, 2/4 RCA, 1/2 RCA and fines (particles<1 mm). The constituents of interest in the speciation measurement are the natural aggregates 4/8 NA, 2/4 NA, 1/2 NA, and the fines represented by 0.25/1 NA, 0/0.25 NA and the CP.

[0274] FIGS. 4a) and 4b) show this speciation in detail for the reference method and for the method of the invention, respectively. In these figures: [0275] Xi and Xi depict the initial composition of the concrete samples, before carrying out the methods. This composition is the same for both the reference method and for the method of the invention. [0276] Columns A and A show the mass distribution of the products (4/8 RCA, 2/4 RCA, 1/2 RCA and the fines) after carrying out the reference method and the method according to the invention, respectively. The products are separated by screening after crushing for the reference method (example 3), and after steps ii) and a) for the method of the invention (example 2, part 2.2); and their mass fractions are obtained by weighing. [0277] Columns B and B show the speciation (in the form of mass percentage) of each of the products, in other words, of each of the fractions in columns A and A: B1 (or B1) corresponds to the speciation of 4/8 RCA of A1 (or A1), B2 (or B2) corresponds to the speciation of 2/4 RCA of A2 (or A2), etc. Speciation measurement involves selective dissolution of the CP in each product in 23 wt. % hydrochloric acid, followed by screening (and optionally abrasion), weighing and drying of the particle size fractions of NA found after dissolution. Mass loss after dissolution gives a direct measurement of the mass of CP found in the products. [0278] Columns C and C show the observed amount of clean NAs (or bare or clean RCAs) and NAs associated with an adherent residual mortar (or unclean or unclean RCAs), as well as the amount of CP in the selected fraction: C1 (or C1) corresponds to the observations and measurements for the 4/8 NA fraction of B1 (or B1), C2 (or C2) for the 2/4 NA fraction of B2 (or B2) and C3 (or C3) for the 1/2 NA fraction of B3 (or B3). [0279] Xr and Xr represent the recalculated composition of the concrete samples, after carrying out the reference method and the method according to the invention, respectively. It is established on the basis of the values in columns B and B, which give the speciation of the initial constituents of the concrete in the products.

[0280] As the fraction of the fines is defined as all particles smaller than 1 mm (0.25/1 RCA+0/0.25 RCA), the speciation analysis of the constituents shows that the method of the invention concentrates 31% of the mass of the concrete in this fines fraction (column A in FIG. 4b): 16% of A4+15% of A5), compared with only 12% for the reference method (column A in FIG. 4a): 8% A4+4% A5). The method of the invention therefore makes it possible to concentrate almost three times more fines than the reference method, fines which can be recycled as cement raw meal if they are not carbonated, or as mineral additives if they are.

[0281] The mass fraction of 4/8, 2/4 and 1/2 RCA in the method of the invention is very close to the mass fraction of the 4/8, 2/4 and 1/2 NA in the initial concrete, which can be explained by a mass fraction for recovery of NAs of more than 75%, or even 80%, in the RCAs. In other words, the RCAs produced by the method of the invention contain less than 20 wt. % of residual mortar, i.e. a CP mass fraction of 5 wt. % to 8 wt. % (see B1, B2 and B3 in FIG. 4b).

[0282] By comparison, the RCAs of the reference method have an NA recovery rate of 40 wt. % to 60 wt. %, and therefore a mass fraction of residual mortar of 60 wt. % to 40 wt. % in the RCAs, i.e. a CP mass fraction of 11% to 21% (see B1, B2 and B3 in FIG. 4a).

[0283] The recalculation of the compositions Xr and Xr of the concrete samples indicates that the method of the invention preserves the size integrity of the initial NAs better than the reference method. In fact, the 4/8 NA content in the recalculated composition Xr of the reference method is significantly lower than that obtained for the method of the invention, which de facto leads to a greater increase in the 2/4 NA content with the reference method. This is due to the microfracturing of the CP induced by the microwave step, which avoids excessive NA fragmentation during the subsequent crushing step.

[0284] The degree of release (or degree of cleanliness) of the NAs in the RCAs was measured by visual sorting, with verification of the CP content of the separated fractions (columns C and C). This measurement demonstrates the selectivity of the method of the invention with regard to NA recovery, this selectivity being provided by the microfracturing of the concrete induced by the microwave treatment step. This selectivity results in the heterogeneous distribution of the residual CP, which is concentrated in a small fraction of the NAs, while the majority of the RCAs have a particularly low CP content. These are referred to here as clean (or bare) NAs. The very high proportion of clean NAs in the RCAs is a specific feature of the method of the invention. The measurement of release by visual sorting was carried out on the 4/8-mm (B1 and B1), 2/4-mm (B2 and B2) and 1/2-mm (B3 and B3) RCA fractions. On the right-hand side of FIG. 4b, illustrated by column C, it can be seen that over 60-70% of the RCAs produced by the method of the invention are clean (i.e. RCAs containing 1.6% to 3.2% of CP), with the remaining 30-40% concentrating the CP with a CP content of 3.9% to 10.7%. Illustrated by column C in FIG. 4a), the degree of release (or degree of cleanliness) of the RCAs of the reference method only ranges from 16% to 42%. In summary, for the three RCA particle size classes analyzed, the degree of release (or degree of cleanliness) of the NAs is two times higher for the method of the invention than for the reference method.

[0285] The performance of the method of the invention was compared with that of the reference method and the theoretical limit. By way of example, the theoretical limit of cleanliness for an RCA in terms of residual CP content is 0%, while the maximum % of 4/8-mm NAs in the 4/8-mm RCAs is 100%. All percentages are given as wt. %.

[0286] FIG. 5 shows the performance of the method of the invention with respect to the reference method, and with respect to the theoretical limit.

4.4 Characterization of Carbonation

[0287] At best, the aqueous carbonation methods can achieve about 80% of the theoretical carbonation limit of solids, calculated on the basis of their chemical composition.

[0288] The theoretical carbonation limit of the fines is directly related to the amount of calcium (Ca) they contain. Per mass unit of fines, this theoretical limit is similar for the fines of the method of the invention and of the reference method, since the measurements show that they have the same chemical composition. This theoretical limit is between 170 kg and 200 kg of CO.sub.2/metric ton of fines. Conversely, the method of the invention makes it possible to produce 2.6 times more fines than the reference method, so that the method of the invention can at most store 2.6 times more CO.sub.2 per metric ton of recycled concrete than the reference method. This represents a CO.sub.2 storage potential for the method of the invention of 60 kg of CO.sub.2/metric ton of recycled concrete, compared with 20 kg of CO.sub.2/metric ton of recycled concrete for the reference method, bearing in mind that the maximum potential for concrete carbonation is estimated on the same basis at 70 kg of CO.sub.2/metric ton of recycled concrete. Thus, in the fines, with respect to one metric ton of recycled concrete, the method of the invention makes it possible to store up to three times more CO.sub.2 than the reference method (table 4 hereunder).

TABLE-US-00004 TABLE 4 Fines of the Fines of the reference method of the method invention % CP 33% 37% % NA 67% 63% Theoretical limit of carbonation 170 200 (kg of CO.sub.2/metric ton of fines) Theoretical limit of carbonation 20 60 for fines, with respect to one metric ton of concrete (kg of CO.sub.2/metric ton of concrete)

[0289] The final carbonation rate of the carbonated fines is obtained by measuring the total carbon (CHNS analysis).

[0290] Table 5 hereunder lists the results for the fines obtained according to the reference method and according to the method of the invention.

TABLE-US-00005 TABLE 5 % C kg of (CHNS CO.sub.2/metric Carbonation measurement) ton of fines rate Reference method 2.69 116 68% Method of the 3.10 150 78% invention

[0291] The experimental measurement confirms that the carbonation potential of the fines is similar for the method of the invention and the reference method, which results from their similar chemical compositions. However, the results are substantially favorable for the fines produced by the method of the invention, with a carbonation increase of +1.3 with respect to the fines produced by the reference method. This result, plus the observation during carbonation testing of fines that the fines of the method according to the invention carbonate with better kinetics than those of the reference method, is probably the result of the microfracturing of the CP induced by the microwave treatment step, which facilitates the transfer of carbon dioxide (CO.sub.2) to the elements that can be carbonated (calcium) found in the CP.

[0292] FIG. 6 shows the performance of the method of the invention with respect to the reference method, and with respect to the theoretical limit.

[0293] Across all the performance criteria measured, the method of the invention is on average 2.3 times more efficient than the reference method.