Method for producing accelerating admixtures from returned concrete

Abstract

The present invention concerns a method for producing an additive composition suitable as an accelerator and activator for promoting the hydration and the compressive strength development of cementitious compositions, said additive composition comprising waste sludge from concrete batching plants and a water-soluble comb polymer useful as superplasticizer.

Description

[0001] METHOD FOR PRODUCING ACCELERATING ADMIXTURES FROM RETURNED CONCRETE

[0002] The invention concerns a method for producing a composition suitable as an accelerator and activator composition for promoting the hydration and the compressive strength development of cementitious compositions.

[0003] BACKGROUND OF THE INVENTION

[0004] Returned concrete is the residual amount of fresh concrete which is not placed at the jobsite and it is brought back to the ready-mixed plant inside the truck mixer. Returned concrete can result from ordering an excess of concrete to avoid shortage of material at the jobsite. In this case, the volume of returned concrete is typically about 1-2 cubic meter. Another reason for returned concrete is the failure of the delivered concrete to meet project specifications, for example, excessive temperature or deviation from expected properties (consistency, air content and setting time). In this second case, the entire load is rejected and returned to the mixing plant. Once back at the mixing plant, returned concrete can be managed in different ways. First, it can be resold as lower-grade concrete based on demand, or it can be used to produce block elements for different applications, such as provisional barriers and separating walls. In the absence of prompt reuse, returned concrete is often dumped to landfill, with obvious economic burden, waste of resources and negative impact on the environment.

[0005] It is estimated that the amount of returned concrete could range from 3 to 5 per cent of the concrete production. Considering that the actual global production of concrete exceeds 30 Gton / year, the global annual estimated amount of returned concrete ranges from 0.9 to 1.5 Gton. To avoid the disposal of such large amount of returned concrete, different methods for its recovery have been proposed.

[0006] CN 116283101 A describes a method for treating and reusing returned concrete, by extending its plasticity and construction fluidity time, the hydration heat release rate of cementitious materials, without reducing the strength after hardening. The method comprises mixing additives and water with residual concrete, such additives comprising retarders, water reducers, water retaining agents, early strength agents and foam stabilizers. The residual concrete, after stabilization, can be stored at the fresh state for longer time and reused on demand at proper time.JP 2000 / 327449A describes a method for treating returned concrete, which comprises adding and mixing an ultra-retarder that prevents the setting of returned concrete for at least 36 hours and, if necessary, a superplasticizer for increasing the fluidity of returned concrete and sieving using a sieve with opening of 5 to 10 mm. The aggregates separated by sieving can be effectively utilized as coarse aggregates and the fraction passing the sieve consists of a mortar containing fine aggregates that is suitable as pre-delivery mortar in the concrete pumping method.

[0007] Nowadays, the most widespread technology to treat returned concrete is the wet reclaiming method. In such plants, returned concrete is diluted with at least an equivalent volume of water and mixed in a separating Archimede’s screw, where coarse and fine aggregates are separated by gravity from the fine cementitious material and water. The same plants can be conveniently used to treat the washings of the dirty drums of the truck mixers after their use, which still contain concrete residues. Both coarse and fine washed aggregates can be recovered and recycled in the batching plant for new concrete production and the wash water portion containing the fine cementitious can be treated in sedimentation ponds or treated by filter-press or by micro-filtration equipment. The resulting clarified water can be reused to produce new concrete, and the separated solid fractions are accumulated in dedicated areas inside the concrete mixing plant before disposal. Such residues consist of mixtures of hydrated cement and the finer fractions of the original aggregates, typically calcareous or clayey materials. According to the process of separation from the washing water, they can contain variable numbers of larger fractions. The residues originating from the filter-press or from microfiltration processes are the finest, with a typical particle size distribution in the range from 1 pm to 1000 pm, while those separated by sedimentation in ponds may contain fractions of coarser aggregates, variable from 10 to 50 % compared to the finer fractions. The solid content of these residues ranges from 20 to 80 %, with a pH values exceeding pH = 12. Due to the stiff and muddy consistency, these residues are commonly indicated as “concrete sludge”. The schematic description of the overall wet reclaiming process is shown in Figure 1.

[0008] A complete description of the wet reclaiming process and the characteristics of the resulting materials is given by A. Diotti, L. Cominoli, G. Plizzari and S. Sorlini in “Experimental Evaluation of Recycled Aggregates, Washing Water and Cement Sludge Recovered from Returned Concrete” published in Appl. Set. 2022, 12, 36. https: / / doi. org / 10.3390 / appl2010036. For both environmental and economic reasons, there is increasing interest in developingsolutions to recover and reuse the sludges from both ready-mixed and precast concrete batching plants.

[0009] Japanese Patent JP3154796B2 discloses a cement hardening accelerator composition obtained from concrete waste or sludge generated at concrete batching plant and a method of producing the same. The method consists in sieving the fine fraction from concrete demolition or sludge, molding this fine powder with water and cement, treating by autoclave curing and finally pulverizing the cured product. It is claimed that highly crystalline calcium silicate hydrates with very large specific area are formed, which act as crystal nuclei for newly formed cementitious hydrates and therefore accelerate the hydration and hardening of cementitious phases, mainly of alite. The main drawback of this accelerator composition is the time and energy consuming process for its production. Multiple steps of demolition and crushing, sieving, casting and thermal curing are necessary to obtain the final product active as accelerating additive. Therefore, due to the time and energy consuming process, this additive is unattractive from both the economic and sustainability point of view.

[0010] Japanese Patent JP7141867 claims an early strength enhancer for cementitious compositions consisting of a mixture of amorphous cement hydrates, magnesium hydroxide and / or calcium hydroxide and unreacted clinker mineral. The strength enhancer is produced by a mixing step where a cement composition is mixed with water to form a cement-based hydrate, followed by a curing step until the ratio of clinker mineral in the cement-based hydrate is in the range between 1 % to 50 % by mass. The above cement-based hydrate then wet-ground to a pulverization step such that the proportion of particles having diameter of 1 pm or less is 10% by volume or more. Sludge from ready-mixed concrete plants can be conveniently used as cement-based hydrate to produce the strength enhancer. The wet pulverization step can be carried out in a ball mill or the like using water as dispersing medium. Even if this method is simpler compared with the method described in JP3154796B2, as it doesn’t involve the modification of the crystalline habit of the cement hydrates by the autoclave curing step, it still requires the wet grinding step to reduce the particle size at the proper value, which implies additional mechanical processing of the cement-based hydrate and energy consumption, affecting both the economy and the sustainability of the whole process. Moreover, the final product remains in the form of a stiff slurry, which makes its use impractical as strength enhancer additive at the ready-mixed concrete batching plant.WO 2017 / 032719A1 discloses an accelerator composition derived from hydrating Portland cement under high shear conditions in the presence of a comb polycarboxylate ether dispersant. WO 2019 / 058313 Al discloses an accelerating composition based on Portland cement, obtained by hydration of Portland cement in aqueous suspension in the presence of carboxylic acid, the calcium salt thereof, polyethanolamines or mixtures thereof and, optionally, in the presence of polycarboxylate ether polymers.

[0011] US 2024 / 0174565A1 discloses a performance-enhancing particulate pre-mix to substitute for a portion of the cement and / or aggregate components normally used in concrete or other cementitious mixtures. Such performance-enhancing particulate pre-mix comprises mineral fines, including concrete washout fines, having D90 between about 30 pm and about 500 pm and / or Blaine fineness of less than about 375 m2 / kg and one or more performance-enhancing additives, including water reducers such as polycarboxylate ethers. The performance-enhancing particulate pre-mix is in powder form and can be produced by blending the mineral fines with performance-enhancing additives by intergrinding, planetary mixers, spraying methods. In the case of concrete washout fines, they are dried before use and the water reducer / superplasticizer is separately added to the cementitious mixtures, as described in the specific examples.

[0012] DESCRIPTION OF THE INVENTION

[0013] The present invention relates to a method for producing an accelerator / activator additive of the hydration of cementitious compositions from the residues of the sedimentation, filter-press and micro-filtration processes of the wet reclaiming method of returned concrete. With the method of the present invention, “concrete sludges” are no longer a waste to be disposed but they become a valuable ingredient for concrete production, as schematically shown in Figure 2. The accelerator / activator additive obtained according to the present invention promotes hydration and the compressive strength development of cementitious compositions and overcomes the drawbacks of the prior art. The additive of the present invention is particularly useful to produce high-performance sustainable concrete, particularly compositions characterized by a low clinker content and high fractions of supplementary cementitious materials (SCMs), such as those containing slag (GGBS), fly ash, calcined clays, limestone powder and their mixtures.

[0014] The present invention comprises the combination of “concrete sludge” produced at batching concrete plants by the wet reclaiming process with a water-soluble comb-polymersuperplasticizer to produce an accelerating / activating additive for cementitious compositions. Their combination exhibits strong accelerating effect at early age of hydration and enhancement of compressive strength at long curing times. Moreover, the additive of the present invention is capable to activate low clinker and even clinker-free cementitious compositions, like those based on GGBS, to produce geopolymer concrete.

[0015] The “concrete sludges” useful for the present invention are the residues from the concrete trucks and concrete mixers washings and from the treatment of returned concrete, or mixtures thereof, originated by the wet reclaiming process. Sludges have variable composition according to the type of original concrete, the type of process they originated from (sedimentation in ponds, filter-press, micro-filtration, centrifugation) and aging. In general, they consist of a mixture of hydrated cementitious phases and other fine inert minerals. Among anhydrous cementitious phases, if present, tricalcium silicate C3S, dicalcium silicate C2S, tricalcium aluminate CsAand tetracalcium aluminoferrite C4AF are the most representative. Anhydrous phases are higher in fresh sludge and quickly decrease by aging, because of their hydration reaction with the water of the sludge. Among hydrated cementitious phases, calcium silicate hydrate C-S-H, calcium hydroxide Ca(OH)2, ettringite AFt and other aluminate phases AFm are generally present. Calcium carbonate CaCCh may be present both as limestone filler and as the result of the carbonation reaction of calcium hydroxide by atmospheric CO2. Water in the sludge may vary over a wide range, typically from 20% to 80% by weight of sludge, according to the maturity of the sludge and the type of process adopted to remove the excess water. pH value of concrete sludge is generally higher than pH = 12.

[0016] Water-soluble comb polymers useful as superplasticizers are the latest generation concrete superplasticizers, which have supplanted the previous generation of naphthalene sulfonate, melamine sulfonate and lignosulfonate superplasticizers, thanks to their superior performance and cost effectiveness. They consist of an anionic charged polymer backbone with pendant polymer side chains. Their molecular structure can be varied over a wide range, by changing the molecular weight and the type of monomers of the backbone, which may be acrylic, methacrylic, vinyl, maleic, phenolic or mixture thereof. The ionic functional group may be carboxylate, phosphate, sulfonate as anionic groups and alkylammonium as cationic groups or mixtures thereof. The pendant side chains are based on polyoxyethylene moieties, with terminal alkoxy, hydroxy, phosphonate radicals or mixture thereof. The multiple variables available for the synthesis of comb polymers allow the production of a full range of products with specific functionalities. A comprehensive review of comb polymers superplasticizers, with particularemphasis to the most popular polyether-polycarboxylates PCEs, is given by L. Lei, T. Hirata and J. Plank in “40 years of PCE superplasticizers - History, current state-of-the-art and an outlook” published in Cement and Concrete Research, 157, (2022), 160826.

[0017] Typically, water-soluble comb polymers useful for the present invention are selected from a polyether-polycarboxylate, a polyether-polyphosphonate copolymer or mixtures thereof. The accelerator / activator of the present invention can be easily produced by adding the comb-polymer superplasticizer to the concrete sludge under mixing. The addition of the superplasticizers to the concrete sludge, regardless it is in the state of settled mud or in the form of panel cakes from filter-press, causes its instantaneous liquefaction, with the transformation from a solid material or a puddy and stiff sludge into a low-viscosity, well dispersed aqueous suspension, even in the presence of little water and with low shear mixing. This process can be easily accomplished in stirred tanks of adequate volume or in rotary mixers. The usual rotarycup concrete mixers can be conveniently used.

[0018] The amount of the water-soluble comb polymer to be added to the concrete sludge to transform it into a liquefied slurry is in the range from 0.05 to 15 %, preferably from 0.1 to 10 %, more preferably from 0.3 to 5 % by weight of the solid fraction of the sludge.

[0019] If the concrete sludge contains coarse aggregates, possibly present in larger fractions in the sludge produced by the less selective sedimentation ponds method, the addition of the superplasticizer and the consequent liquefaction of the sludge causes the instantaneous sedimentation of the coarse aggregates at the bottom of the mixer and their easy separation from the slurry. By this way, it is possible to recover an additional fraction of clean coarse aggregate to produce new concrete.

[0020] The liquefied, low-viscosity slurry can be stored in tanks, and it is stable for days or weeks and can be added to concrete mixtures using metering pumps, in the same way as conventional concrete admixtures, with enormous advantage compared with the other methods. Typical viscosity values range from 100 cP to 100000 cP, preferably from 300 cP to 50000 cP, more preferably from 500 cP to 10000 cP. If the viscosity of the liquefied slurry increases on standing for longer periods, it is possible to easily restore the desired viscosity by adding supplemental amount of superplasticizer.

[0021] The addition of the superplasticizers to the sludge, with the deagglomeration and dispersion of the solid particles, causes a sharp reduction of the particle size distribution. This result isunexpected as water-soluble comb polymers useful as superplasticizers have been invented and developed to work as dispersants on anhydrous cement particles and not on the hydrated phases, which represent most of the sludge composition. Indeed, no reference exists in the scientific and patent literature disclosing the use of superplasticizers to disperse the hydrated cementitious phases. Furthermore, despite the use of the superplasticizers severely retards the hydration of cementitious phases, as reported by Zang, L. et al. in “Retardation effect of PCE superplasticizers with different architectures and their impacts on early strength of cement mortar”, Cement and Concrete Composites, 2019, 104, 103369, the additive of the present invention, comprising the combination of “concrete sludge” with PCE superplasticizers, exhibits high accelerating effect and promote the hydration and the early compressive strength development of cementitious compositions.

[0022] Further ingredients can be added to the accelerator / activator of the present invention to impart additional desired properties, such as workability retention and enhancement of final strength. Suitable retardants comprise carbohydrates such as sucrose, glucose, gluconate, glucoheptonate, glucose syrup, phosphate and phosphonates, borates or mixtures thereof, whose dosages are in the range from 0 to 10 %, preferably from 0.01 to 5 %, more preferably from 0.1 to 3 % by weight referred to the weight of the solid fraction of the sludge.

[0023] Suitable accelerators comprise alkanolamines, like triethanolamine (TEA), monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), triisopronanolamine (TIPA), ethanoldiisopropanolamine (EDIPA), N,N,N’ ,N’ -tetra(hydroxyethyl)ethylenediamine (THEED), ethylenediisopropanolamine (EDIPA), calcium nitrate, sodium thiocyanate, calcium formate, sodium sulphate, sodium thiosulphate, nanocomposites of synthetic metal silicate hydrates and mixtures thereof in percentages are ranging from 0 to 20 %, preferably from 0.01 to 10 %, more preferably from 0.1 to 5 % by weight of the solid fraction of the sludge.

[0024] Suitable alkaline activators comprise alkaline hydroxides like sodium hydroxide and potassium hydroxide, sodium carbonate, potassium carbonate, sodium sulphate or mixture thereof, in percentages ranging from 0 to 30 %, preferably from 0.01 to 20 %, more preferably from 0.1 to 10 % by weight of the solid fraction of the sludge.

[0025] Other ingredients, like defoamers, air entraining agents (AEA) and viscosity modifying agents (VMA) can be added to impart desired properties.In another embodiment, the invention provides a process for the preparation of the additives of the invention, comprising the mixing of the sludge from concrete batching plants with a water-soluble comb polymer in mechanical mixers (mortar and concrete mixers, cowles mixers, paddle mixers or static mixers) to form a low viscosity suspension. Said suspension is added to fresh cementitious compositions, comprising inorganic binder, aggregates and water. Said cementitious compositions, which are another object of the invention, contain the additive in percentages ranging from 0.01 to 40 per cent, preferably from 0.1 to 30 per cent, more preferably from 1 to 25 per cent by weight of the binder of the cementitious composition. The inorganic binder is Portland cement, alumina cement, ground granulate blast furnace slag, sulphoaluminate cement or their mixtures.

[0026] Optionally, the cementitious composition may also comprise supplementary cementitious materials such as fly ash, natural pozzolana, limestone, metakaolin, calcined clay, recycled concrete fines, microsilica or mixtures thereof.

[0027] The aggregates comprise coarse and fine aggregates, natural and / or recycled aggregates or their mixtures.

[0028] The invention is described in further detail in the following examples.

[0029] Example 1

[0030] 1000 grams of a filter-press cake were collected from the wet reclaiming treatment plant of a precast concrete batching plant and introduced in a 12 cm diameter cylindrical plastic vessel with a mechanical stirrer consisting of a vertical rod equipped with 3 radial paddles, 3 cm long. The consistency of the filter press cake was a crumbly wet solid characterized by a total solid content of 54 % by weight and composition reported in Table 1, as determined by XRPD (Rietveld refinement) and thermogravimetric analyses.

[0031] Table 1. Composition of the filter press cake, expressed as per cent by weight of the solid fraction.

[0032] <

[0033]

[0034] 1 gram of AR23 comb polycarboxylate ether superplasticizer produced by Mapei S.p. A. was added dropwise to the filter-press cake in the plastic vessel under stirring at 100 rpm. AR23 is a 50 % solution of a polyether poly-acrylic / methacrylic acid comb polymer with polyoxyethylene side chains linked to the methacrylic backbone by ester linkages and terminated with methoxy radicals. Dynamon AR23 is produced by esterification at 160 °C of a linear poly-acrylic / methacrylic acid copolymer with methoxypolyethyleneglycol of molecular weight 3000 Dalton under vacuum. The reaction product was then neutralized with NaOH and the final concentration adjusted to 50 % solids with water. The molecular weight of the final product, as determined by Gel Permeation Chromatography, was Mw = 62000 Dalton and the charge density is 0.32 meq / g. Just after few drops of addition of Dynamon AR23, the consistency of the filter-press cake turned from a brittle material to a homogeneous paste with gradually less viscosity. Stirring was then continued at 100 rpm for 1 minute after the end of the addition. The final slurry SL1, additive of the invention, had the characteristics shown in Table 2.

[0035] Table 2. Characteristics of SL1 slurry obtained by treating the filter-press cake of Table 1 with AR23 superplasticizer.

[0036]

[0037] The comparison between the particle size distribution of the filter-press cake before the addition of AR23 and the additive of the present invention slurry SL1 is shown in Figure 3 showing the particle side distribution of the original filter-press cake (dotted line) and the slurry SL1 of the invention, where the original filter-press cake has been treated with AR23 superplasticizer (solid line).

[0038] It is evident from Figure 3 that the addition of AR23 superplasticizer favored the dispersion of the filter-press cake and shifted the particle size distribution toward lower values. Particularly, it is noteworthy that the additive SL 1 of the present invention was characterized by the presence of particles smaller than 1 pm, which are claimed to be highly effective as secondary nucleating agents to accelerate the early hydration of cementitious compositions (US8653186, EP 3080052) and promote the early strength development of cementitious compositions.SL1 slurry was checked for stability on standing. After few days from the preparation, a thin layer of supernatant water appeared, indicating that the dispersion was not perfectly stable. Nevertheless, a gentle mixing was sufficient to restore the homogeneity of the suspension and the original viscosity.

[0039] Example 2

[0040] 1000 grams of sludge from the wastewater treatment plant of a ready-mixed concrete batching plant were collected from the bottom of the sedimentation pond. The sludge had a total solid content of 40 % by weight with viscosity of 30000 cP and composition reported in Table 3. Table 3. Composition of the sludge from the sedimentation pond, expressed as per cent by weight of the solid fraction.

[0041] < <

[0042]

[0043] 5 grams of SPH1 comb polymer poly carboxylate ether superplasticizer produced by Mapei S.p.A. were added dropwise to the filter-press cake in the plastic vessel under stirring at 100 rpm. SHP1 is a 50 % solution of a comb polymer with polyoxyethylene side chains linked to the vinyl / acrylic acid backbone by ether linkages and terminated with hydroxy radicals. SPH1 is produced by radical copolymerization of vinyl-based ethoxylated macromonomer with acrylic acid initiated by redox catalyst. The reaction product is then neutralized with NaOH and the final concentration adjusted at 50 % with water. The molecular weight of the final product, as determined by Gel Permeation Chromatography, is Mw = 77000 Dalton and the charge density is 0.26 meq / g. Stirring was continued at 100 rpm for 1 minute after the end of the addition. The final slurry SL2 had the characteristics shown in Table 2.

[0044] Table 4. Characteristics of SL2 slurry obtained by treating the sludge of Table 3 with Dynamon SPH1 superplasticizer.

[0045]

[0046] Examples 1 and 2 clearly indicate that water soluble comb polymers useful as superplasticizers can be conveniently used to disperse sludge from concrete batching plants to produce low viscosity slurries which can be easily stored in tanks and dosed by pumps, like any other commercial additives and admixtures for concrete and cementitious compositions.

[0047] Example 3

[0048] The accelerating effect of the additive of the invention on the hydration of cementitious compositions was evaluated by isothermal calorimeter test (TAM Air instrument by Waters Inc.) comparing the heat development of a cement paste made by mixing 5 % of the additive SL1 with a Portland cement CEMI 52.5R (EN 197-1 norm) with that of an equivalent cement paste without the additive, at the same water-to-cement ratio (W / C = 0.40). Data were collected during the first 48 hours of hydration and the results are shown in Figure 4. The results indicate that the additive SL1 of the invention strongly accelerated the hydration of cement paste. This is clearly demonstrated by the anticipation of the heat development rate curve of the paste containing the additive of the invention (dotted line), which showed the maximum heat development rate after about 9 hours from mixing, compared with the reference whose maximum was after about 13 hours (solid line). The cumulative heat flow curves indicate that, after 48 hours from mixing, the total heat developed by the 2 cement pastes was the same (300 J / g), indicating that the additive of the invention is capable to promote the hydration of cementitious compositions in the early period of hydration.

[0049] Example 4

[0050] The potential of the additive of the invention to develop early compressive strength of cementitious composition is shown in Table 5. Mortars were prepared according to EN:196-1 norm by using Portland cement CEMI 52.5 R (EN: 197-1 norm), standard normalized sand and tap water in the mass proportions 1 : 3 : 0.48, except for MIX 5 (see footnote 1 in Table 5). The additives SL1 and SL2 of the present invention were added to the mixing water in the mixing bowl at selected dosages, before the addition of cement and sand. Specimens 40 x 40 x 160 mm were casted in plastic molds and cured at 20 °C and 95% R.H. Compressive strength was measured after 6, 7 and 8 hours of curing.

[0051] The results of Table 5 indicate that both the additives of the invention are effective in developing the early compressive strength of cementitious compositions, as clearly evidenced by the comparison of the compressive strength of Mix 3 and Mix 4 with the mortar without additives(REFI). Moreover, the accelerating effect on the development of early strength is proportional to the dosage of the additive of the invention, as it results from the comparison between Mix 2 and Mix 3, where dosage of the additive SL1 has been varied from 3 to 5 % by weight of cement, respectively.

[0052] Table 5. Early age compressive strength (6, 7, 8 hours) of mortars with the additive of the invention in comparison with reference mortars.

[0053]

[0054] #1 : In this mortar mix, the dosage of cement was reduced by the quantity corresponding to the amount of the filter-press cake introduced with the additive SL1 of the present invention, expressed as solids.

[0055] In mortar Mix 5 the dosage of cement was reduced by the quantity corresponding to the amount of the filter-press cake introduced with the additive SL1 of the invention, expressed as solids. By this way, the powder contents of both mortars Mix 5 and REFI were the same. The comparison of results of Mix 5 with the reference mortar REFI confirmed that the increase of compressive strength of mortar Mix 5 was due to the acceleration effect of the additive of the invention and not to the increase of the powder content. Moreover, these results confirmed that, by using the additive of the present invention, it is possible to reduce the cement dosage of cementitious compositions, still guaranteeing a high early strength development.

[0056] Example 5

[0057] The filter-press cake of Example 1 was formulated with diethanolamine (DEA), triisopropanolamine (TIPA), sodium thiocyanate (NaSCN), sodium gluconate (NaGlu) and Dynamon AR23 according to the proportions of Table 6.Table 6. Composition and characteristics of different formulations of the filter-press cake of Example 1 with different ingredients of the invention.

[0058]

[0059] All the resulting formulations were low viscosity aqueous suspensions and were used in mortar tests at the dosage of 5 % by weight of cement and compared with a reference mortar containing 5 % by weight of the original filter press cake at the same water-to-cement ratio (W / C = 0.47). The cement used was a limestone blended cement type CEMII / B-LL 42.5 (EN: 197-1 norm) with a clinker replacement by limestone powder in the range between 21 to 35 %. The results of the compressive strength of the mortars prepared with the additives of Table 6 are reported in Table 7.

[0060] Results of Table 7 indicate that the addition of the filter press cake alone (REF3) produced a remarkable decrease of the long-terms compressive strength compared to the plain mortar without any addition (REF2), confirming the decrease of the long-term strength already reported in the literature. On the other hand, all the mortars containing the additives of the present invention not only did they not exhibit a decrease, but rather showed an increase of compressive strength. These results confirmed that the additive of the invention promotes the hydration of cementitious materials not only at early age but also at long curing time.Table 7. Compressive strength of mortars with the additives of Table 4, at 1, 7 and 28 days of curing in comparison with the reference mortar.

[0061]

[0062] #2: In this mortar, 5 % by weight of cement of the original filterpress cake was used, without other additions.

[0063] Example 6

[0064] 500 grams of the additive SL2 of Example 2 of the invention were mixed, under mechanical stirring, with 20 grams of sodium carbonate Na2COs in powder form, 35.4 grams of AR23 and 75 grams water. The resulting suspension SL9 had the characteristics shown in Table 8.

[0065] Table 8. Characteristics of additive SL9 obtained treating sample SL2 with NaiCQj.

[0066]

[0067] Additive SL9 was used in mortar tests as alkaline activator for ground granulated blast furnace slag GGBFS, in comparison with a commercial new generation’s alkaline activator (Mapecube GEO produced by Mapei S.p.A.) produced according to US 2024 / 0174565 Al, and a reference mortar without additives (REF4). Additive SL2 was also tested as alkaline activator, without addition of sodium carbonate. Mortars were prepared by using 600 grams GGBFS supplied by ECOCEM, 1350 grams normalized standard sand and a water-to-slag ratio W / S = 0.30. Dynamon NRG 1012 superplasticizer produced by Mapei S.p.A. was added to the mortars to achieve similar flow for the different mortars. Mixing water was adjusted to balance the different amount of water introduced with the additives, characterized by different solidcontents. The mortar compositions and the compressive strength after 1, 2 and 3 days of curing at 23 °C and 95% R.H. are shown in Table 9.

[0068] Table 9. Compositions and 24 hours compressive strength for alkali activated slag mortars.

[0069]

[0070] Results of Table 9 indicate that the reference mortar did not show any appreciable compressive strength after 48 hours. On the other hand, the additive SL9 of the invention was effective in promoting the hydration of slag and in developing early compressive strength already after 24 hours and exhibited higher compressive strength development compared to commercial alkaline activators. Additive SL2, obtained as described in Example 2, was effective as activator for slag, confirming the possibility to produce alkali activated materials without Portland cement.

Claims

CLAIMS1. Method for producing an accelerator and activator composition for promoting the hydration and the compressive strength development of cementitious compositions comprising mixing the concrete sludge from the wet reclaiming process of returned concrete and the washings of concrete truck mixers with a water-soluble comb polymer as superplasticizer, wherein the concrete sludge is obtained by removal of coarse and fine aggregates and the drainage of excess water by sedimentation, centrifugation, filter-press or micro-filtration processes, said concrete sludge containing all the hydrated phases of Portland cement, including calcium silicate hydrate C-S-H, calcium hydroxide Ca(OH)2, ettringite AFt and other aluminate phases AFm.

2. Method according to claim 1 wherein the accelerator and activator composition is an aqueous dispersion with particle size distribution in the range from 0.1 pm to 1500 pm, preferably from 0.2 pm to 1000 pm, more preferably from 0.4 pm to 500 pm.

3. Method according to claim 1 and claim 2 wherein the accelerator and activator composition is an aqueous dispersion of viscosity in the range from 100 cP to 100000 cP, preferably from 300 cP to 50000 cP, more preferably from 500 cP to 10000 cP.

4. Method according to any of the previous claims wherein the accelerator and activator composition is an aqueous dispersion of total solid content in the range from 20 % to 80 %, preferably from 30 % to 70 %, more preferably from 40 % to 60 %.

5. Method according to any of the previous claims wherein the water-soluble comb polymer useful as superplasticizer is a polyether-poly carboxylate, a polyether-polyphosphonate copolymer or mixtures thereof.

6. Method according to any of the previous claims wherein the amount of water-soluble comb polymer useful as superplasticizer is in the range from 0.05 to 15 %, preferably from 0.1 to 10 %, more preferably from 0.3 to 5 % by weight referred to the weight of the solid fraction of the concrete sludge.

7. Method according to any of the previous claims, further comprising the addition to the accelerator and activator composition retardants, accelerators, alkaline activators, defoamers, air entraining agents, viscosity modifying agents or mixtures thereof.

8. Method according to claim 7 wherein the retardants comprise carbohydrates, phosphates, phosphonates, borates or mixtures thereof, whose dosages are in the range from 0 to 10 %, preferably from 0.01 to 5 %, more preferably from 0.1 to 3 % by weight referred to the weight of the solid fraction of the sludge.

9. Method according to claim 8 wherein the carbohydrates are selected from sucrose, glucose, gluconate, glucoheptonate, glucose syrup.

10. Method according to any of claims 7-9 wherein accelerators comprise alkanolamines, calcium nitrate, sodium thiocyanate, calcium formate, sodium sulphate, sodium thiosulphate, nanocomposites of synthetic metal silicate hydrates and mixtures thereof in percentages ranging from 0 to 20 %, preferably from 0.01 to 10 %, more preferably from 0.1 to 5 % by weight of the solid fraction of the sludge.

11. Method according to claim 10 wherein the alkanolamines are selected from triethanolamine (TEA), monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), triisopronanolamine (TIPA), ethanoldiisopropanolamine (EDIPA), N,N,N’,N’-tetra(hydroxyethyl)ethylenediamine (THEED), ethylenediisopropanolamine (EDIPA).

12. Method according to any of claims 7-11 wherein alkaline activators are selected from sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate or mixture thereof, in percentages ranging from 0 to 30 %, preferably from 0.01 to 20 %, more preferably from 0.1 to 10 % by weight of the solid fraction of the sludge.

13. Cementitious compositions comprising an inorganic binder and aggregates, said cementitious compositions containing the additive of claims 1-12 in percentages ranging from 0.01 to 40 per cent, preferably from 0.1 to 30 per cent, more preferably from 1 to 25 per cent by weight of the binder of the cementitious composition.

14. Cementitious compositions according to claim 13 wherein the inorganic binder is Portland cement, alumina cement, ground granulate blast furnace slag, sulphoaluminate cement or their mixtures.

15. Cementitious compositions according to claim 13 or 14 wherein the cementitious composition also comprises supplementary cementitious materials selected from fly ash, natural pozzolana, limestone, metakaolin, calcined clay, recycled concrete fines, microsilica or mixtures thereof.

16. Cementitious compositions according to any one of claims 13-15 wherein the aggregates comprise coarse and fine aggregates, natural and / or recycled aggregates or their mixtures.

Images