Multiple composite excitation material and its application in marine silt solidification

Abstract

The application belongs to the technical field of engineering materials, and particularly discloses a multiple composite excitation material and application thereof in marine silt solidification, wherein the excitation material is composed of the following components in percentage by weight: modified composite solid activator: 38%-52%; modified light-burned MgO component: 25%-40%; and modified nano-SiO2 composite component: 12%-22%. The application realizes stable input of solidification excitation, condensation and early strength regulation of active MgO, and dispersion promotion of nano-SiO2 for densification, and uses pretreatment to cooperatively reduce salt-induced volume instability, thereby solving the problems of unstable solidification effect, insufficient densification and high long-term failure risk of the prior art under the background of coastal high-salt marine black and odorous silt.

Description

Technical Field

[0001] This invention belongs to the field of engineering materials technology and relates to the resource utilization process of solidified silt as roadbed filler. Specifically, it relates to a multi-composite activating material and its application in the solidification of marine silt. Background Technology

[0002] By adding special solidification materials to silt, mixing and curing them, a series of chemical reactions occur between the silt and the solidification materials to form solidified soil with good engineering properties, which can be used in land reclamation, road construction, filling, and embankment protection projects.

[0003] Marine black and odorous sludge is generated from dredging or cleaning of nearshore aquaculture areas. This type of sludge waste typically exhibits a high water content and a predominantly fine-particle, viscous / silty system; the pore water contains high salinity and ionic strength; and due to organic matter accumulation and an anaerobic reducing environment, it possesses distinct black and odorous characteristics and a reducing background. These characteristics make it difficult to achieve a comprehensive performance that balances engineering application window and long-term stability when using existing solidification methods. Existing solidification agent formulations and solidification processes for sludge solidification are mostly based on relatively low-salinity or freshwater boundary conditions, making them difficult to adapt to high-salinity marine systems, as evidenced by: On the one hand, a high-salt environment can disrupt the effective reaction environment on which alkali activation or gelation reaction depends, causing fluctuations in the time window for coagulation and early gelation structure formation, thereby affecting the operability of direct landfill molding and roadbed filling and compaction.

[0004] On the other hand, high ionic strength and complex ionic interactions can alter colloidal stability and the nucleation, growth, and filling processes of reaction products, thereby weakening densification efficiency and increasing the risk of pore connectivity, ultimately affecting strength development and permeability barrier capacity. In addition, during the curing stage, high-salt components may induce salting out, ion migration, and secondary structural damage, amplify shrinkage stress, and increase the probability of microcrack initiation and propagation, thereby reducing volume stability and durability. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a multi-composite activating material. Applying the activating material of the present invention to marine sludge can solve the problems of unstable solidification effect, insufficient densification, and high risk of long-term failure in the application of existing solidification technologies to high-salinity marine black and odorous sludge.

[0006] This invention is achieved through the following technical solution: A multi-component composite excitation material, comprising the following components by weight percentage: Modified composite solid activator: 38%~52%; Modified lightly calcined MgO component: 25%~40%; Modified nano-SiO2 composite components: 12%~22%.

[0007] A further improvement to the present invention is as follows: The modified composite solid activator is prepared by the following steps: waste glass powder is mixed with caustic soda and subjected to an alkaline melting reaction to obtain a molten product; the molten product is cooled and ground to obtain a powder; after sieving, the powder is surface-coated with polyethylene glycol PEG-400 and sodium stearate; after modification, it is dried and sieved to obtain the modified composite solid activator.

[0008] Furthermore, the mass ratio of the waste glass powder to caustic soda is 100:30-50; Furthermore, the temperature of the alkali melting reaction is 800-900℃, and the time is 1-3 hours; Furthermore, the powder is ground to a D50 of 20-30 μm; Furthermore, the powder sieving retains powder with a particle size of 0.5~60μm; Furthermore, the mass ratio of PEG-400 to sodium stearate is 2:1, and the total mass of PEG-400 and sodium stearate is 1-2 wt% of the mass of the powder after sieving.

[0009] Furthermore, the modified light-burned MgO component is obtained through the following steps: light-burned MgO is spray-coated with a silica sol-polyvinyl alcohol composite coating solution, modified, dried, and sieved to obtain the modified light-burned MgO component.

[0010] Furthermore, the silica sol-polyvinyl alcohol composite coating solution is composed of silica sol and polyvinyl alcohol solution mixed at a mass ratio of 3-5:1, wherein the solid content of the silica sol is 10-30 wt%, and the concentration of the polyvinyl alcohol solution is 1-5 wt%. Furthermore, the amount of the silica sol-polyvinyl alcohol composite coating solution is 1-5 wt% of the mass of lightly calcined MgO.

[0011] Furthermore, the modified nano-SiO2 composite component is prepared by the following steps: surface modification of nano-SiO2 is performed using γ-aminopropyltriethoxysilane, and after separation, washing, and drying, surface-modified nano-SiO2 powder is obtained. The surface-modified nano-SiO2 powder is compounded with polycarboxylate superplasticizer and then dried at low temperature to obtain the modified nano-SiO2 composite component.

[0012] Furthermore, the particle size of the nano-SiO2 is 10~50 nm; Furthermore, the amount of γ-aminopropyltriethoxysilane used is 1-5 wt% of the mass of nano-SiO2; Furthermore, the mass ratio of the surface-modified nano-SiO2 powder to the polycarboxylate superplasticizer is 100:5-10.

[0013] Furthermore, the compounding process employs a combination of mechanical stirring and ultrasonic dispersion.

[0014] Preferably, the multiple composite excitation material is composed of the following components by weight percentage: Modified composite solid activator: 45%; Modified lightly calcined MgO component: 32%; Modified nano-SiO2 composite component: 18%.

[0015] A further improvement of the present invention is as follows: The application of the above-mentioned multi-composite activating materials in the solidification of marine silt, wherein the marine silt is black and odorous marine silt generated from dredging or desilting of nearshore aquaculture areas, the specific process is as follows: (1) The three components of the excitation material are premixed and compounded; (2) The marine black and odorous sludge is subjected to desanding and salt conditioning pretreatment; (3) Add 60-80% of the total mass of the activating material to the sludge slurry after pretreatment in step (2), and coarsely mix at 300-400 rpm for 5-10 min; then add the remaining activating material and finely mix at 500-700 rpm for 10-20 min. (4) The slurry mixed in step (3) is directly filled and molded or the roadbed is filled and compacted. After molding, it is demolded and cured.

[0016] Furthermore, the total amount of the activating material is 6% to 12% of the dry solids mass of the sludge obtained after pretreatment; Furthermore, the stirring rate of the coarse mixture is 300-400 rpm, and the stirring rate of the fine mixture is 500-700 rpm. Furthermore, the curing process involves wet curing at 15~30℃ for 7-28 days.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the multi-composite excitation material provided by the present invention, the modified composite solid activator is used to provide a stable excitation phase and buffer the fluctuation of the reaction window. The lightly calcined MgO is surface-modified to regulate the early coagulation and gelation process under high salt conditions. The nano-SiO2 is surface-modified and meets the requirements of nanoscale dispersion morphology (particle size 10~50 nm), which can improve the effective nucleation sites and pore filling efficiency, thereby promoting the transformation of pore structure towards densification and reducing the risk of pore connectivity.

[0018] 2. During the curing process, this invention employs staged stirring to ensure that the curing agent maintains stable slurry homogeneity and reaction coupling consistency even under high salt and black odor conditions. The staged stirring includes coarse mixing and fine mixing. By extending the fine mixing stage, the three components of the curing agent complete the transformation from initial agglomeration and local enrichment to a continuous and uniformly distributed slurry phase, thereby reducing the dispersion of setting time and local over-reaction caused by the dominant ionic strength of NaCl. At the same time, under the black odor reduction background, the stable supply of the solid activated phase and the synergistic enhancement of the nano-SiO2 nucleation and filling pathway suppress the unfavorable pore structure construction caused by the damage to colloidal stability. Detailed Implementation

[0019] This invention provides a multi-composite activating material and its application in the solidification of marine silt. In the specific embodiments, unless otherwise specified, the experimental methods used are conventional methods, and the materials and reagents used are commercially available. The invention will now be described in detail with reference to specific embodiments.

[0020] Example 1 1. Preparation and compounding of excitation materials (1) Preparation of modified composite solid activator Waste glass powder and solid sodium hydroxide were mixed at a mass ratio of 100:40, melted at 850℃ for 2 h, cooled to room temperature, and then ground to D50≈25μm. The powder was then separated by air classification and retained in the range of 0.5~60 μm.

[0021] First, polyethylene glycol PEG-400 was dissolved in an appropriate amount of deionized water to form a 10 wt% modified solution. Sodium stearate was then added and stirred for 20 min until uniformly dissolved. Subsequently, the modified solution was sprayed evenly onto the surface of the graded molten powder, and simultaneously mixed at 300 rpm for 15 min in a high-speed mixer to ensure uniform adsorption of the modifier onto the powder surface. After drying at 80℃ for 3 h, the powder was cooled and passed through a 200-mesh sieve to obtain a modified composite solid activator with stable activation supply characteristics. The mass ratio of PEG-400 to sodium stearate was 2:1, and the total amount of PEG-400 and sodium stearate modifiers was 1.5 wt% of the powder mass.

[0022] (2) Preparation of modified lightly calcined MgO component A 20 wt% silica sol and a 2 wt% polyvinyl alcohol (PVA-1788) solution were mixed at a mass ratio of 4:1 to form a spray coating solution. Lightly calcined MgO was premixed in a high-speed mixer for 5 min at 250 rpm. A 3 wt% coating solution of the lightly calcined MgO was then sprayed evenly into the mixer using an atomizing nozzle for 10 min, while simultaneously increasing the mixer speed to 500 rpm to ensure uniform adhesion of the coating solution to the MgO particle surface. After spraying, mixing continued for 15 min to form a uniform inert layer. The resulting material was then dried at 105℃ for 2 h, cooled, pulverized, and passed through a 200-mesh sieve to obtain the modified lightly calcined MgO component. The coating layer forms a thin, barrier interface on the surface of MgO particles, thereby slowing down their instantaneous reaction in high-salt pore water and reducing structural disturbances caused by localized exothermic reactions and expansion.

[0023] (4) Preparation of modified nano-SiO2 composite components Take nano-SiO with a particle size of 10~50 nm 2， Surface modification of nano-SiO2 was performed using γ-aminopropyltriethoxysilane (KH-550), with the amount of KH-550 added being 3 wt% of the mass of nano-SiO2. The specific steps were as follows: nano-SiO2 was first dispersed in an 80% (v / v) ethanol-water mixed solution to prepare a suspension with a solid content of 5 wt%; after mechanical stirring at room temperature for 20 min, the pH was adjusted to 4.5–5.5; then KH-550 was added, and the reaction was continued at 40 °C for 2 h; after the reaction was completed, the mixture was centrifuged, washed twice with ethanol, and then vacuum dried at 60 °C for 6 h to obtain surface-modified nano-SiO2 powder.

[0024] The surface-modified nano-SiO2 powder was compounded with polycarboxylate superplasticizer at a mass ratio of 100:8. The polycarboxylate superplasticizer was first added in the form of a 10 wt% aqueous solution, and then dispersed by mechanical stirring for 15 min and low-power ultrasonication at 30 kHz for 10 min. Subsequently, it was dried at low temperature at 50℃ for 12 h to obtain the modified nano-SiO2 composite component.

[0025] The modified nano-SiO2 composite component obtained was ultrasonically treated in an aqueous medium with a D90 particle size of no more than 200 nm. It showed no obvious sedimentation after 2 h of suspension and could maintain a uniform dispersion without obvious agglomeration in the sludge slurry.

[0026] (5) Take the above-prepared components and premix them according to the proportions in Table 1.

[0027] Table 1 Modified composite solid activator 45% Modified lightly calcined MgO component 32% <![CDATA[Modified nano-SiO2 composite component]]> 18% 2. Solidification of marine black and odorous sludge The black and odorous sludge from the coastal aquaculture area was tested and found to have a water content of 78.6%, a pore water NaCl content of 28.5 g / L, a pH of 7.4, a TOC of ≈2.1%, an Eh of -120 mV, and a fine particle content of >85%.

[0028] (1) Pretreatment (a) Desanding: The sampled marine black and odorous sludge was first screened through a 2 mm sieve to remove shells, plant debris, and other impurities. Then, 0.5 times the weight of the sludge was added to clean water and stirred for 5 min to fully separate coarse and fine particles. After standing for 30 s, the rapidly settling coarse sand particles at the bottom were discarded, and the upper sludge slurry was retained. If necessary, this process was repeated once to reduce the mass fraction of particles larger than 0.25 mm to below 5%. (b) Salt adjustment: Deionized water was added to the desanded sludge slurry and diluted at a mass ratio of sludge slurry to deionized water of 1:0.8. After mechanical stirring for 10 min, the slurry was allowed to stand for 2 h and the supernatant was extracted. The above dilution-settling-supernatant extraction operation was repeated twice. Finally, the settled solids were recovered and the salinity of the pore water was measured to adjust the NaCl content from 28.5 g / L to 10~15 g / L. (c) The pretreated sludge was used for subsequent solidification tests.

[0029] (2) Coarse Mix The amount of activating material was 10% of the dry solids mass of the sludge. 70 wt% of the activating material was added to the pretreated sludge slurry and coarsely mixed at 300 rpm for 8 min. During the coarse mixing process, the sludge and the previously added curing agent were initially mixed under low to medium shear conditions, so that the curing agent changed from a localized feeding state to a macroscopically uniform distribution state.

[0030] (3) Fine mixing After coarse mixing, the remaining 30 wt% of activating material was added, and fine mixing was carried out at 500 rpm for 15 min. During fine mixing, based on coarse mixing, local agglomerates were further broken up by higher shear force, promoting uniform dispersion of nano-SiO2 composite components and strengthening the contact between each component and sludge particles.

[0031] (4) Shaping and curing The roadbed was filled and compacted, and demolded within 24 hours after molding to avoid excessive water loss. It was then wet-cured under a film at 20℃, maintaining surface moisture and periodically replenishing water. Samples were prepared in parallel for 7 days and 28 days of curing, with three samples in each group. The sample dimensions were 50 mm in diameter and 100 mm in height.

[0032] Example 2 In this embodiment, the amount of activating material is 6% of the mass of the dry solids of the sludge, and other operations are the same as in Embodiment 1, which will not be repeated here.

[0033] Example 3 In this embodiment, the amount of activating material is 12% of the mass of the dry solids of the sludge, and other operations are the same as in Embodiment 1, which will not be repeated here.

[0034] Comparative Example 1 In this comparative example, the lightly calcined MgO was not coated or modified, and other operations were the same as in Example 1, which will not be repeated here.

[0035] Comparative Example 2 In this comparative example, the modified nano-SiO2 composite component was not used in the excitation material, and other operations were the same as in Example 1, which will not be repeated here.

[0036] Comparative Example 3 In this comparative example, the activating material only contains a modified composite solid activator, and other operations are the same as in Example 1, which will not be repeated here.

[0037] Test case The specimens prepared in the above embodiments and comparative examples were subjected to performance testing. The average value of three specimens in each group was calculated. Specifically: (1) The unconfined compressive strength test was carried out in accordance with GB / T 50123 "Standard for Geotechnical Testing Methods".

[0038] (2) Linear shrinkage rate after drying (%): The specimens that have been molded and cured for 28 days are placed in a 40℃ forced-air drying oven and dried to constant weight. The change in length of the specimens before and after drying is measured and calculated according to the following formula: Linear shrinkage rate (%) = (L0 - L1) / L0 × 100% Where L0 is the gauge length before drying and L1 is the gauge length after drying.

[0039] (3) Mass change rate (%): Record the mass m0 of the specimen before drying and the mass m1 after drying, and calculate according to the following formula: Mass change rate (%) = (m1 - m0) / m0 × 100% (4) Apparent crack rating (0-3): The apparent crack rating method is adopted, where 0 indicates no visible cracks; 1 indicates a small number of fine surface cracks but no through cracks; 2 indicates obvious cracks with local connection; and 3 indicates through cracks or obvious cracking and spalling. Each specimen is independently rated by 3 testers, and the average rating is taken as the final result.

[0040] (5) Salt-related phenomena were observed and recorded by observing the white crystallization, powdering and crack development on the surface of the specimen after natural drying.

[0041] (6) Pore structure parameters were determined by mercury intrusion porosimetry (MIP): Before testing, internal samples of specimens cured for 28 days were taken and placed in anhydrous ethanol to terminate hydration for 24 h, and then vacuum dried at 50℃ to constant weight; subsequently, total porosity, average pore size and micropore (pore size <50 nm) volume fraction were tested using an automatic mercury intrusion porosimeter.

[0042] (7) The proportion of interconnected holes was semi-quantitatively determined by mercury injection curve and scanning electron microscope (SEM) image: when the mercury hysteresis phenomenon weakened and the continuity of the channel was observed to be enhanced by SEM, it was determined to be "increased"; otherwise, it was determined to be "decreased".

[0043] The test results are shown in Tables 2-4.

[0044] Table 2 Unconfined compressive strength test results Example 1 10% 4.3 7.6 Example 2 6% 3.6 6.8 Example 3 12% 4.9 8.2 Comparative Example 1 10% 3.4 6.2 Comparative Example 2 10% 3.8 6.0 Comparative Example 3 10% 2.9 4.8 Table 3 Volume stability Example 1 0.22 -1.8 0~1 Slight surface powdering, no through cracks Comparative Example 1 0.58 -3.1 2 Significant localized salting out and numerous microcracks Comparative Example 2 0.46 -2.6 2 The pore structure is not dense enough, resulting in rapid efflux / salt precipitation. Comparative Example 3 0.91 -4.2 3 Significant risk of instability and cracking Table 4 Hole Structure Parameters Example 1 32.5 18.6 62 decline Comparative Example 1 38.2 24.1 50 rise Comparative Example 2 36.7 22.8 48 rise Analysis of the test results shows that, compared with Example 1, the unconfined compressive strength of lightly calcined MgO without coating modification decreased from 4.3 MPa and 7.6 MPa at 7 days and 6.2 MPa at 28 days, respectively. Simultaneously, the linear shrinkage after drying significantly increased from 0.22% to 0.58%, the apparent crack score increased from 0-1 to 2, and obvious localized salting out and microcracks appeared. This indicates that silica sol-polyvinyl alcohol composite coating modification of lightly calcined MgO can effectively delay its early transient hydration reaction in a high-salt environment, avoiding structural damage caused by localized exothermics and expansion, thereby significantly improving the volume stability and salting-out resistance of the cured body while increasing strength.

[0045] Compared to Example 1, Comparative Example 2, which did not use the modified nano-SiO₂ composite component in the excitation material, showed a decrease in 28-day strength from 7.6 MPa to 6.0 MPa, an increase in total porosity from 32.5% to 36.7%, an increase in average pore size from 18.6 nm to 22.8 nm, a decrease in micropore (<50 nm) volume fraction from 62% to 48%, and an increase in the proportion of interconnected pores. This demonstrates that nano-SiO₂, surface-modified with KH-550 and compounded with polycarboxylate superplasticizer, can achieve uniform nanoscale dispersion in the curing system, providing numerous effective nucleation sites and filling micropores. This promotes the evolution of the pore structure towards densification with low porosity, small pore size, high micropore ratio, and low connectivity, which is key to improving strength and impermeability.

[0046] Compared to Example 1, Comparative Example 3's activating material only contained the modified composite solid activator, lacking the synergistic effect of MgO and nano-SiO₂ components. Its 28-day strength was only 4.8 MPa, far lower than the 7.6 MPa of Example 1. Simultaneously, it exhibited the worst volume stability, with a linear shrinkage rate of 0.91%, a crack score of the highest level (Level 3), and a significant risk of instability and cracking. This fully demonstrates the significant synergistic effect among the three components of this invention: the modified composite solid activator provides a stable basic activating environment, the modified lightly calcined MgO component regulates solidification and contributes to early strength, and the modified nano-SiO₂ composite component promotes later densification. All three are indispensable, jointly solving the problems of insufficient strength development, difficulty in densification, and poor durability in the solidification of high-salinity marine silt.

[0047] The above description of the embodiments is only for illustrating the technical concept and features of the present invention. Its purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly. Those skilled in the art can obviously easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative effort. Therefore, the above embodiments should not be used to limit the scope of protection of the present invention. All improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A multi-composite excitation material, characterized in that, It consists of the following components by weight percentage: Modified composite solid activator: 38%~52%; Modified lightly calcined MgO component: 25%~40%; Modified nano-SiO2 composite components: 12%~22%.

2. The multi-composite excitation material according to claim 1, characterized in that: The modified composite solid activator is prepared by the following steps: waste glass powder is mixed with caustic soda and subjected to an alkaline melting reaction to obtain a molten product; the molten product is cooled and ground to obtain a powder; after sieving, the powder is surface-coated with polyethylene glycol PEG-400 and sodium stearate; after modification, it is dried and sieved to obtain the modified composite solid activator.

3. The multi-composite excitation material according to claim 2, characterized in that: The mass ratio of the waste glass powder to caustic soda is 100:30-50; And / or, the temperature of the alkali melting reaction is 800-900℃, and the time is 1-3h; And / or, the grinding is carried out to a D50 of 20-30 μm; And / or, the powder sieving retains powder with a particle size of 0.5~60μm; And / or, the mass ratio of PEG-400 to sodium stearate is 2:1, and the total mass of PEG-400 and sodium stearate is 1-2 wt% of the mass of the powder after sieving.

4. The multi-composite excitation material according to claim 1, characterized in that: The modified light-burned MgO component is prepared by the following steps: light-burned MgO is spray-coated with a silica sol-polyvinyl alcohol composite coating solution, modified, dried, and sieved to obtain the modified light-burned MgO component.

5. The multi-composite excitation material according to claim 4, characterized in that: The silica sol-polyvinyl alcohol composite coating solution is prepared by mixing silica sol and polyvinyl alcohol solution at a mass ratio of 3-5:1, wherein the solid content of the silica sol is 10-30 wt% and the concentration of the polyvinyl alcohol solution is 1-5 wt%. And / or, the amount of the silica sol-polyvinyl alcohol composite coating solution is 1-5 wt% of the mass of lightly calcined MgO.

6. The multi-composite excitation material according to claim 1, characterized in that: The modified nano-SiO2 composite component is prepared by the following steps: γ-aminopropyltriethoxysilane is used to modify the surface of nano-SiO2, and after separation, washing and drying, the surface-modified nano-SiO2 powder is obtained. The surface-modified nano-SiO2 powder is compounded with polycarboxylate superplasticizer and then dried at low temperature to obtain the modified nano-SiO2 composite component.

7. The multi-composite excitation material according to claim 6, characterized in that: The particle size of the nano-SiO2 is 10~50 nm; And / or, the amount of γ-aminopropyltriethoxysilane used is 1-5 wt% of the mass of nano-SiO2; And / or, the mass ratio of the surface-modified nano-SiO2 powder to the polycarboxylate superplasticizer is 100:5-10; And / or, the compounding process employs a combination of mechanical stirring and low-power ultrasonic dispersion.

8. The multiple composite excitation material according to claim 1, characterized in that: The multiple composite excitation material is composed of the following components by weight percentage: Modified composite solid activator: 45%; Modified lightly calcined MgO component: 32%; Modified nano-SiO2 composite component: 18%.

9. A method for solidifying marine silt, characterized in that, The marine silt is solidified using the multi-composite activating material as described in any one of claims 1-8, wherein the marine silt is black and odorous marine silt generated from dredging or desilting of nearshore aquaculture areas, and the method includes the following steps: (1) The three components of the excitation material are premixed and compounded; (2) The marine black and odorous sludge is subjected to desanding and salt conditioning pretreatment; (3) Add 60-80% of the total mass of the activating material to the sludge slurry after pretreatment in step (2), and coarsely mix at 300-400 rpm for 5-10 min; then add the remaining activating material and finely mix at 500-700 rpm for 10-20 min. (4) The slurry mixed in step (3) is directly filled and molded or the roadbed is filled and compacted. After molding, it is demolded and cured.

10. The method for solidifying marine silt according to claim 9, characterized in that: The total amount of the activating material used is 6% to 12% of the dry solid mass of the sludge obtained after the pretreatment in step (2); And / or, the stirring rate of the coarse mixture is 300-400 rpm, and the stirring rate of the fine mixture is 500-700 rpm; And / or, the curing is wet curing at 15~30℃ for 7-28 days.