BIOCHAR OBTAINED BY PYROLYSIS OF A CONSTRUCTION MATERIAL COMPRISING A VEGETABLE GRANULATE

Abstract

BIOCHAR OBTAINED BY PYROLYSIS OF A BUILDING MATERIAL COMPRISING A VEGETABLE AGGREGATE Biochar of 1% to 25% total organic carbon (TOC), 1% to 30% total inorganic carbon (ITC), and 0.5% to 40% total C2S; process of preparing said biochar obtained by pyrolysis of a building material comprising a vegetable aggregate; and use thereof as a substitute for clinker or aggregate.

Description

Title of the invention: BIOCHAR OBTAINED BY PYROLYSIS OF A BUILDING MATERIAL COMPRISING A VEGETABLE GRANULATE

[0001] The present invention relates to the valorization of a construction material containing a plant aggregate at the end of its life, and more particularly a biochar prepared by pyrolysis of such a construction material and its use as a substitute clinker.

[0002] The manufacture of binders, particularly hydraulic binders, and especially cements, essentially consists of calcining a mixture of carefully selected and measured raw materials, also known as "raw material." Firing this raw material produces an intermediate product, clinker, which, when ground with calcium sulfate and possibly added minerals, yields cement. The type of cement produced depends on the nature and proportions of the raw materials as well as the firing process. Several types of cement are distinguished: Portland cements (which represent the vast majority of cements produced worldwide), aluminous cements (or calcium aluminate cements), natural quick-setting cements, sulfoaluminate cements, sulfobelic cements, and other intermediate varieties.

[0003] The most common cements are Portland cements. Portland cements are obtained from Portland clinker, which is produced by clinkerizing a raw material rich in calcium carbonate in a kiln at a temperature of around 1450°C. The production of one tonne of Portland clinker is accompanied by the emission of significant quantities of CO2 (approximately 0.8 to 0.9 tonnes of CO2 per tonne of cement in the case of clinker).

[0004] In 2014, the quantity of cement sold worldwide was approximately 4.2 billion tons (source: French Cement Industry Association - SFIC). This figure, which is constantly increasing, has more than doubled in 15 years. The cement industry is therefore currently seeking a viable alternative to Portland cement, that is, cements with at least the same strength and quality characteristics as Portland cements, but which release less CO2 during their production.

[0005] During the production of clinker, the main constituent of Portland cement, CO2 emissions are linked to:

[0006] - up to 40% for heating the cement kiln, grinding and transport;

[0007] - up to 60% of so-called chemical CO2, or decarbonation.

[0008] Decarbonation is a chemical reaction that takes place when limestone, the main raw material for the manufacture of Portland cement, is heated to high temperature. The limestone then transforms into quicklime and CO2 according to the following chemical reaction:

[0009] [Chem.l] CaCCh -> CaO + CO2

[0010] The natural carbonation of cement-based materials, particularly concrete, is a potential means of reducing the carbon footprint associated with the manufacturing process and the use of cement. However, although concrete made from these cements naturally recarbonates during the service life of the structures, accounting for 15% to 20% of the CO2 emitted during manufacturing, the overall carbon balance associated with Portland cement production remains positive. Therefore, it remains necessary to reduce CO2 emissions during Portland cement production and / or improve the processes for recycling end-of-life concrete.

[0011] To reduce CO2 emissions related to the production of Portland cement, several approaches have been considered so far:

[0012] - the adaptation or modernization of cement processes in order to maximize the efficiency of heat exchange;

[0013] - the development of new "low carbon" binders such as sulfo- cements aluminous products prepared from raw materials less rich in limestone and at a lower cooking temperature, which allows a reduction in CO2 emissions of approximately 35%;

[0014] - the recycling of end-of-life construction materials which can have an impact important on the carbon footprint associated with their production;

[0015] - or even the (partial) substitution of clinker in cements by materials allowing to limit CO2 emissions.

[0016] Among the above approaches, that of (partial) substitution of clinker in cements has been the subject of numerous developments.

[0017] Among the substitute materials used, one can cite in particular blast furnace slag and fly ash from coal-fired power plants. However, the closure of coal-fired power plants is causing a shortage of good quality fly ash. Furthermore, the substitution of clinker with limestone filler (i.e., an inactive material) essentially has a dilution effect and is accompanied by a significant decrease in strength, which is problematic.

[0018] As alternative substitute materials, the use of biochar, carbonated or not, is also envisaged. However, this patent application does not describe the use of biochar as a cementitious additive.

[0019] In the publication “The use of Biochar to reduce the carbon footprint of cement-based materials”, Procedia Structural Integrity, 26 (2020), 199-210, the authors Suarez-Riera et al. describe the use of biochar as a cementitious additive (or filler) to reduce the carbon footprint of both cement production and the building material made from it. International patent application WO-A-2023 / 281220 describes the use of carbonated biochar as a clinker substitute.

[0020] The term “biochar” is an abbreviation of “bio-charcoal,” in which the prefix “bio” designates biological origin and “charcoal” corresponds to the English term for charcoal. Thus, “biochar” refers to any product obtained by the pyrolysis of organic biomass from various sources such as plants, including wood, straw, and agricultural or green space residues, or organic compounds such as sewage sludge, poultry manure, or cattle manure.

[0021] Biochar differs from charcoal by its use as a fertilizer rather than as a fuel and by its environmental impact, since it acts as a carbon sink rather than releasing CO2 into the atmosphere when burned.

[0022] Biochar is therefore interesting for two reasons: - it concentrates a large amount of biogenic carbon (between 40 and over 80%); and - its structure develops a large specific surface area capable of adsorbing large quantities of CO2.

[0023] Biochar is thus classically used in agriculture to increase soil quality, and therefore its productivity. However, although carbon sequestration in the soil by burying biochar has been practiced for many years to combat soil acidification and increase fertility, losses and emissions of carbon in the form of CO2 have been identified when the chemical balances of the soil (pH, leaching, burial depth, etc.) vary.

[0024] The main difficulty associated with the use of biochar as a substitute clinker lies in the fact that biochar production devices use plant resources from by-products of the wood or agricultural sector, which are also used in many other applications (soil amendment, heating pellets, particle boards etc...) which leads to competition for use.

[0025] Other approaches, such as carbon capture and storage, have also been developed to limit CO2 emissions from cement plants or coal-fired power plants. For example, international patent application WO-A-2019 / 115722 describes a process for cleaning exhaust gases containing CO2 and the production of additional cementitious material. The described process involves using recycled concrete fines from stockpiles or a silo as a starting material, rinsing the starting material to provide a carbonaceous material, removing the carbonaceous material and the cleaned exhaust gas, and deagglomerating the carbonaceous material to form the additional cementitious material. It also involves using stockpiles or a silo containing recycled concrete fines as a starting material for cleaning CO2-containing exhaust gas and simultaneously producing additional cementitious material. However, this process requires drying the carbonate product before it can be used.

[0026] As of the date of the present invention, it therefore remains necessary to identify alternative substitute materials that can significantly reduce CO2 emissions during the production of Portland cement, but also to improve the recycling processes of used concrete, which can have a significant impact on the carbon balance associated with the production of construction materials.

[0027] However, it has now been found quite surprisingly that the pyrolysis of construction material containing a plant aggregate (or of a mixture comprising a construction material and a plant aggregate) makes it possible to obtain a biochar with particular characteristics allowing its use as a substitute for clinker or as aggregate, and thus to significantly lower the carbon footprint of both the construction material finally prepared while maintaining mechanical properties, and in particular medium and long term compressive strengths compatible with the intended uses, and of the construction material from which it is obtained.

[0028] Thus, the present invention relates to a biochar comprising: • 1% to 25% total organic carbon (TOC); • 1% to 30% total inorganic carbon (TIC); and • from 0.5% to 40% of C2Stotai.

[0029] The biochar according to the present invention offers a double benefit in terms of carbon footprint. First, it can be used as a substitute clinker or as a substitute aggregate, which significantly reduces the carbon footprint of the final construction material while maintaining mechanical properties, and in particular compressive strength in the medium and long term, compatible with the intended uses. Furthermore, since the biochar according to the present invention is obtained from a construction material, in particular concrete, at the end of its life, it also makes it possible to significantly lower the carbon footprint associated with the production of said construction material.

[0030] Within the scope of the present invention:

[0031] - "Biochar" means any material obtained by pyrolysis of biomass organic matter of various origins such as plants, including wood, straw and agricultural or green space residues, or organic compounds such as sewage treatment plant sludge known as "STEP sludge", poultry droppings or cattle manure;

[0032] - "C2Stotai" refers to the set of polymorphs constituting the belite phase, in in particular the C2Sp polymorph, the C2Sa polymorph and the C2SY polymorph;

[0033] - The term "mineral aggregate" means any mineral or bio-based filler, constituting principal aggregate of concrete by volume, such as a rough or rounded rock fragment or an artificial granular material with a median particle size greater than 2 mm. Examples of mineral aggregate include sands, gravels, or crushed stone. Preferably, "mineral aggregate" refers to sand;

[0034] - the term "vegetable aggregate" means any aggregate of essentially vegetable origin composed of cellulose, hemicellulose and / or lignins. Preferably, "plant aggregate" means a plant aggregate selected from: • the seminal hairs of seeds, particularly cotton, • bast fibers from plant stems such as hemp fiber, flax fiber, nettle fiber; • the woody part of the stem such as hemp shives, flax shives, wood chips or miscanthus; • cork such as cork beads; • fibers extracted from the leaves or trunk, particularly sisal; and • fruit shells such as nuts, and in particular coconut, or of cereals such as rice.

[0035] Preferably, "vegetable granule" means a vegetable granule selected from cotton, hemp fiber, hemp shives, flax shives, wood chips, cork balls, miscanthus, sisal, coconut husk, straw, rapeseed, rice hulls, sunflower, reed and / or typha;

[0036] - "construction material" means concrete or mortar, preferably a concrete ;

[0037] - the term "binder" means any hydraulic or aerial binder;

[0038] - the term "hydraulic binder" means any hydraulic binder free of aggregates classically used to prepare mortar or concrete. Examples of hydraulic binders include hydraulic lime (conforming to standard NF EN 459-1 in force at the date of the present invention) or a cement such as a cement aluminous, a sulfo-aluminous cement, a Portland cement or a natural rapid cement;

[0039] - the term "air binder" means any air binder free from conventionally aggregated materials used to prepare mortar or concrete. An example of an air-hardening binder is air lime (conforming to standard NF EN 459-1 in force at the date of the present invention). Among the air limes preferably used in the context of the present invention are: • a calcium hydrated lime (CL) containing calcium oxide (CaO) and / or calcium hydroxide (Ca(OH)2) in which the sum of CaO + MgO is at least 70% and the MgO content is < 5%; or • a dolomitic lime (DL) containing calcium magnesium oxide (CaO MgO) and / or calcium magnesium hydroxide (Ca(OH)2 Mg(OH)2) of which the sum CaO + MgO is at least 80%, and the MgO content varies from 5% to more than 30%;

[0040] - "aluminous cement" means any cement, amorphous or not, obtained by cooking of a mixture of limestone and bauxite and containing at least 5% monocalcium aluminate CA;

[0041] - "Natural quick-setting cement" means any hydraulic binder that sets and Rapid hardening in accordance with standard NF P 15-314: 1993 in force at the date of the present invention. Preferably, "natural rapid-setting cement" means a cement prepared from a clinker comprising: • from 0% to 20% of C3S; • 40% to 60% C2S; • 7% to 12% of C4AF; • 2% to 10% of C3A; • 10% to 15% CaCO3 (calcite); • 10% to 15% of Ca5(SiO4)2CO3 (spurrite); • 3% to 10% sulfate phases: yeelimite C4A3$, langbeinite (K2Mg2(SO4) 3, anhydrite (CaSO4); and • 10% to 20% of lime, periclase, quartz and / or one or more amorphous phases;

[0042] - "Portland cement" means any cement based on classified Portland clinker as EMC (I, II, III, IV, V or VI) according to standard NF EN 197-1, NF EN 197-5 or NF EN 197-6 in force at the date of the present invention;

[0043] - "sulfo-aluminate cement" means any cement prepared from clinker sulfo-aluminous containing 5% to 90% of 'yeelimite' C4A3$ phase, a sulfate source, and, optionally, a limestone addition;

[0044] - "Sand" means any sand that is suitable for use by a person skilled in the art for the preparation of construction materials;

[0045] - The term "clinker substitute" means any composition capable of being to partially substitute for clinker in the preparation of a cementitious composition while allowing an increase in the performance of the cementitious binder resulting from this combination;

[0046] - The term "aggregate substitute" means any composition capable of being to partially substitute for aggregates in the preparation of a building material; and

[0047] - the term "specific surface area" means the specific surface area measured by the method BET (Brunauer, Emett and Teller) which represents the ratio of the area of ​​the actual surface of a material to the mass of matter of that material.

[0048] In the context of the present invention, the following notations are adopted to designate the mineralogical components of cement:

[0049] - C represents CaO;

[0050] - A represents A12O3;

[0051] - F represents Fe2O3;

[0052] - S represents SiO2; and

[0053] - $ represents SO3.

[0054] In the context of the present invention, the "organic carbon content" or TOC corresponds to the quantity (% w / w) of organic carbon contained in an entity (e.g. biochar) relative to the total weight of said entity (e.g. said biochar), and the "inorganic carbon content" or "ITC" corresponds to the quantity (% w / w) of inorganic carbon contained in an entity (e.g. biochar) relative to the total weight of said entity.

[0055] To determine these levels, approximately 180 mg of the product to be analyzed is placed in a nickel capsule. This capsule is then introduced into a quartz tubular furnace, allowing for a gradual temperature increase and temperature plateaus to separate the different carbonaceous species in a sample. This allows the following to be determined: - the "TOC", i.e., the quantity (% w / w) of total organic carbon of the entity determined by analyzing the signal obtained between 100°C and 500°C with a plateau at 450°C for 180 seconds; and - the "CIT", i.e. the quantity (% w / w) of total inorganic carbon of the entity determined by analysis of the signal obtained between 600°C and 1000°C with a plateau at 800°C for 180 seconds.

[0056] In the context of the present invention, the median diameter or dv50 corresponds to the diameter below which 50% of the total volume of the particles is found the sample in question. This can be determined by any method known to a person skilled in the art, in particular by dry laser granulometry.

[0057] Finally, within the framework of the present invention, the proportions expressed in % correspond to mass percentages relative to the total weight of the entity (e.g. biochar, construction material...) considered.

[0058] The present invention therefore relates to a biochar comprising 1% to 25% total organic carbon (TOC), 1% to 30% total inorganic carbon (TIC), and 0.5% to 40% C2Stotai. Preferably, the present invention relates to a biochar as defined above having the following characteristics, chosen alone or in combination: - biochar comprises 2% to 25% of TOC, preferably 3% to 25% of TOC, preferably 4% to 23% of TOC, most preferably 5% to 15% of TOC; - the biochar comprises from 1% to 15% of CIT, preferably from 1% to 6% of CIT, preferably from 1.5% to 5% of CIT, most preferably from 2% to 4.5% of CIT; - the biochar comprises from 1% to 35% C2Stotai, preferably from 1.5% to 30% C2Stotai, and most preferably from 2% to 30% C2Stotai; and / or - biochar has a specific surface area ranging from 3 m2 / g to 500 m2 / g, preferably from 5 m2 / g to 350 m2 / g, most preferably from 5 m2 / g to 100 m2 / g.

[0059] The biochar according to the present invention can therefore be obtained by pyrolysis of a construction material comprising a plant aggregate or of a mixture comprising a construction material and a plant aggregate. Thus, the present invention also relates to a process for preparing biochar as defined above, comprising the following steps: a. grinding of a construction material comprising a plant aggregate or of a mixture comprising a construction material and a plant aggregate; and b. pyrolysis under an inert atmosphere at a temperature ranging from 300°C to 1000°C.

[0060] Preferably, the process according to the present invention is carried out under the following conditions, taken alone or in combination: - the construction material or mixture comprising a construction material and a plant aggregate comprises 1% to 60% binder, 5% to 50% plant aggregate and 0% to 50% soil, preferably 10% to 45% binder and 15% to 40% plant aggregate and 0% to 40% soil; - the binder contained in the construction material is chosen to be hydraulic lime, aluminous cement, a sulfo-aluminous cement, Portland cement or a natural quick-setting cement and / or aerial lime; - the construction material or the mixture comprising a construction material and a plant aggregate is ground to obtain a particle size dv50 varying from 0.5 cm to 15 cm, preferably from 1 cm to 10 cm, most preferably from 4 cm to 7 cm; - pyrolysis is carried out under hydrogen, CO2, nitrogen or pyrolysis gas; preferably pyrolysis is carried out under nitrogen; - pyrolysis is carried out at a temperature ranging from 350°C to 900°C, preferably even at a temperature ranging from 400°C to 800°C; - pyrolysis is carried out for 5 to 120 minutes, preferably for 10 to 60 minutes, most preferably for 15 to 45 minutes; - the process includes an additional cooling step down to ambient temperature; - the process includes an additional grinding step to obtain a particle size compatible with its use as a clinker substitute or as an aggregate substitute; and / or - the process includes an activation step at approximately 800°C under a CO2 / water vapor atmosphere.

[0061] The biochar described above can therefore be used as a clinker substitute. Thus, the present invention also relates to the use of a biochar as defined above as a clinker substitute.

[0062] Finally, the biochar described above can also be used as an aggregate substitute. Thus, the present invention also relates to the use of a biochar as defined above as an aggregate substitute.

[0063] The present invention can be illustrated in a non-limiting way by the following examples. Example 1 - Biochar according to the invention 1.1 - Construction material subjected to pyrolysis

[0064] Construction materials comprising a plant aggregate whose composition is reported in the following Table 1 are used to prepare biochars according to the invention.

[0065] [Tables 1 Building material 1 2 3 4 5 6 7 8 9 Binder (% w / w) CEMI 0 59 0 0 0 0 0 0 0 CEMII 0 59 0 0 0 0 0 0 CEMIV 0 0 0 59 0 0 0 0 Carat 0 0 0 0 59 0 0 0 Binder 1 0 0 0 0 0 59 0 0 Binder 2 0 0 0 0 0 0 59 0 0 BioCNP 59 0 0 0 0 0 0 59 59 Vegetable Granule (% w / w) Hemp Shave 41 41 41 41 41 41 41 0 0 Miscanthus 0 0 0 0 0 0 0 41 0 Flax 0 0 0 0 0 0 0 0 41

[0066] Table 1 - Construction material 1.2 - Preparation process

[0067] The previous construction material 1 is ground to obtain a particle size dv5o of approximately 20 mm and then placed in a furnace to be pyrolyzed under nitrogen at a temperature of 600°C or 800°C for 30 minutes.

[0068] After returning to room temperature in the oven, the biochars obtained are ground until a particle size of approximately 10 pm is obtained.

[0069] Biochars IA (pyrolysis at 600°C) and IB (pyrolysis at 800°C) are thus obtained.

[0070] Similarly, biochars 2A / 2B, 3A / 3B, 4A / 4B, 5A / 5B, 6A / 6B, 7A / 7B, 8A / 8B and 9A / 9B are obtained from (respectively) construction materials 2 to 9 pyrolyzed at 600°C or 800°C. 1.3 - Composition of biochars according to the invention

[0071] The composition of the biochars thus obtained is reported in the following Tables 2 and 3.

[0072] [Tables2] Biochar (% w / w) AI 2A 3A 4A 5A 6A 7A 8A 9A TOC (% w / w) 9.6 8.6 8.8 9.2 21.2 3.3 5.6 14.5 5.1 TIC (% w / w) 3.3 3.8 4.1 2.2 3.0 4.3 4.2 3.4 4.7 Total surface area (% w / w) 15.9 4.1 5.8 2.0 2.6 2.2 3.4 9.6 23.5 Specific surface area (m² / g) 20.0 8.0 10.4 16.1 34.2 27.2 30.9 20.0 18.1

[0073] Table 2 - Biochar IA at 9A (pyrolysis at 600°C)

[0074] [Tables3] Biochar (% w / w) IB 2B 3B 4B 5B 6B 7B 8B 9B TOC (% w / w) 2.1 N / DN / DN / DN / D 1.2 3.6 N / DN / D CIT (% w / w) 3.0 N / DN / DN / DN / D 2.0 1.8 N / DN / DC^Stotal (% w / w) 29.7 N / DN / DN / DN / D 6.2 8.1 23.7 27.4 Specific surface area (m2 / g) 23.4 N / DN / DN / DN / D 33.6 39.7 26.5 16.6

[0075] Table 3 - Biochar IB to 9B (pyrolysis at 800°C) 1.4 - SEM Analysis

[0076] Biochars IA and IB as well as the reference SOLER biochar were metallized (LE1CA EM ACE200) and then observed by scanning electron microscopy (EE1 QUANTA 200F) using a "high vacuum" mode and an ETD detector - Magnification used: x3469 (biochar IA), x2837 (biochar IB) and x289 (biochar SOLER).

[0077] The images obtained are presented in [Fig.1].

[0078] On biochars IA and IB, we observe the presence of a mineral deposit on the pyrolyzed plant particles which is not present on the reference biochar.

[0079] Example 2 - Use of biochar as a clinker substitute

[0080] Each biochar used as a clinker substitute is ground until a particle size of Dv50 = 10 pm is obtained.

[0081] The biochars IA and IB thus ground are used to prepare respectively the mortar compositions M1 and M2 reported in the following Table 4.

[0082] The reference mortar (Ref.M) is prepared without the addition of biochar.

[0083] [Tables4] Mortars Mortar M1 Mortar M2 Ref. Mortar (M Ref.) Biochar IA (kg / m3) 105 - - Biochar IB (kg / m3) - 105 - Portland Clinker (kg / m3) 453 453 552 Gypsum (kg / m3) 27 27 33 Sand (kg / m3) 1758 1758 1758 Water (kg / m3) 293 293 293

[0084] Table 4 - Composition of mortars M1, M2 and M ref.

[0085] In mortars M1 and M2, the mass substitution rate of clinker is 18%. Example 3 - Mechanical properties of mortars 3.1 - Spreading

[0086] The spreading properties of the previously obtained mortars were measured according to the protocol of standard NF EN 1015-3.

[0087] The results of the spreading measurements are reported in the following Table 5.

[0088] [Tables5] Mortar M1 Mortar M2 Mortar M ref. Spread (in mm) 151 134 219

[0089] Table 5 - Spreading of mortars M1 & M2

[0090] The results show spread values ​​for the Ml and M2 mortars lower than that of the reference mortar Mref., these values ​​nevertheless remaining compatible with conventional implementation. 3.2 - Compressive strength

[0091] The compressive and flexural strengths of the mortars obtained previously were measured at 1, 2, 7 and 28 days according to the protocol of standard NF EN 196-1.

[0092] The results of the resistance measurements are reported in the following Table 6.

[0093] [Tableauxô] Mortar M1 Mortar M2 Mortar M Ref. Rc (MPa) 1 day 21 23.6 23.0 2 days 33.8 34.5 35.0 7 days 42.6 44.3 48.0 28 days 52.0 52.6 58.0 RFI (MPa) 1 day 3.5 3.9 4.0 2 days 5.5 5.4 6.5 7 days 6.4 6.7 7.7 28 days 6.9 7 8.1

[0094] The results show that the mechanical strengths are comparable between M1, M2, and Mref at 1 and 2 days. Substituting clinker with biochar does not affect early-age strengths. Furthermore, the results show very good strengths, particularly in compression, for the M1 and M2 mortars, which exhibit 28-day strengths compatible with their use as construction materials.

Claims

Demands

1. Biochar comprising: • 1% to 25% total organic carbon (TOC); • 1% to 30% total inorganic carbon (ITC); and • 0.5% to 40% C2Stotai.

2. Biochar according to claim 1, characterized in that it comprises from 3% to 25% of TOC.

3. Biochar according to claim 1 or 2, characterized in that it comprises from 1% to 6% of CIT.

4. Biochar according to any one of claims 1 to 3, characterized in that it comprises from 1% to 35% of C2Stotai.

5. Biochar according to any one of claims 1 to 4, characterized in that it has a specific surface area ranging from 3 m2 / g to 500 m2 / g.

6. A process for preparing a biochar according to any one of claims 1 to 5 comprising the following steps: a) grinding a construction material comprising a plant aggregate or a mixture comprising a construction material and a plant aggregate; and b) pyrolysis under an inert atmosphere at a temperature ranging from 300°C to 1000°C.

7. A process according to claim 6, characterized in that the pyrolysis of step b) is carried out at a temperature ranging from 350°C to 900°C.

8. A process according to claim 6 or 7, characterized in that the building material or mixture comprising a building material and a plant aggregate comprises 10% to 60% binder and 16% to 50% plant aggregate.

9. Use of a biochar according to any one of claims 1 to 5 as a clinker substitute.

10. Use of a biochar according to any one of claims 1 to 5 as a substitute for aggregate.

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