Acid resistant cement material

Abstract

To provide an acid-resistant cement material of a low environment load and excellent acid resistance.SOLUTION: The acid-resistant cement material includes Portland cement, blast furnace slag fine powder, an admixture of a residual solid component of concrete sludge, and aggregate, in which 20 to 60% of the Portland cement is substituted with blast furnace slag fine powder and 10 to 60% of the admixture relative to the total amount of the Portland cement and the blast furnace slag fine powder is used.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an acid-resistant cementitious material. [Background technology]

[0002] Typical concrete is alkaline and exhibits good resistance in many neutral or slightly acidic environments. However, in sewage treatment facilities such as sewage treatment plants, sludge treatment plants, and sewer pipes, sulfates and organic acids contained in wastewater are decomposed by sulfate-reducing bacteria, generating hydrogen sulfide. This hydrogen sulfide is then converted to sulfuric acid by the action of sulfur-oxidizing bacteria that live on the interior walls of concrete structures used in sewage treatment facilities. When sulfuric acid penetrates the concrete, it reacts with calcium ions in the concrete to produce compounds such as calcium sulfate. These compounds, such as calcium sulfate, are more soluble than the aggregates and cement of ordinary concrete, dissolving the components of the concrete. In particular, tricalcium aluminate and tetracalcium aluminate in cement react with sulfuric acid and dissolve.

[0003] As a countermeasure, for example, an acid-resistant cement material has been disclosed that contains Portland cement, ground granulated blast furnace slag, and metakaolin, with 50 to 200 parts by weight of ground granulated blast furnace slag per 100 parts by weight of Portland cement, and 2 to 10 parts by weight of metakaolin per 100 parts by weight of the total amount of Portland cement, ground granulated blast furnace slag, and metakaolin (see, for example, Patent Document 1).

[0004] However, acid-resistant concrete often uses mortar containing large amounts of blast furnace slag to improve its sulfuric acid resistance. However, sulfuric acid-resistant mortar is often used as a base for resin linings. When sulfuric acid-resistant mortar containing large amounts of blast furnace slag is used, the calcium content is high. If the resin lining on the surface develops a partial defect or pinhole, the reaction between sulfate ions and calcium on the mortar surface causes gypsum, ettringite, and monosulfate to precipitate, increasing the mortar's weight and reducing its adhesion to the resin lining. Furthermore, the use of pozzolana powders such as blast furnace slag and silica fume causes significant shrinkage of the mortar, which can lead to problems such as cracking after construction.

[0005] In addition, a solidified body obtained by mixing and reacting an alkali-active amorphous powder (active filler) such as fly ash (coal ash), metakaolin, or blast furnace slag with an alkaline solution (aqueous sodium silicate solution, aqueous potassium silicate solution, aqueous sodium hydroxide solution, or aqueous potassium hydroxide solution) that activates it has been disclosed (see, for example, Patent Document 2). This solidified body is acid-resistant because it does not use cement.

[0006] However, geopolymers have problems such as the fact that they begin to harden quickly, making them difficult to transport from the manufacturing plant to the construction site; they are highly viscous, making it difficult to clean the mixer; they must be hardened at high temperatures to achieve sufficient strength; they require special blending, which requires skilled techniques and experience during construction; and the materials are expensive, making them costly to manufacture. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-30968 [Patent Document 2] Japanese Patent Application Publication No. 2024-35202 Summary of the Invention [Problem to be solved by the invention]

[0008] The present invention has been made in view of the above points, and an object of the present invention is to provide an acid-resistant cement material which has a low environmental load and excellent acid resistance. [Means for solving the problem]

[0009] The acid-resistant cement material of the present invention comprises Portland cement, ground granulated blast furnace slag, an admixture made from the residual solid content of concrete sludge, and aggregate, in which the ground granulated blast furnace slag is substituted for 20% to 60% of the Portland cement, and the admixture is used in an amount of 10% to 60% of the total amount of the Portland cement and the ground granulated blast furnace slag. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide an acid-resistant cement material that has a low environmental impact and excellent acid resistance. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a table showing an example of a composition of a test specimen that is a comparative example according to an embodiment of the present invention. [Figure 2] 1 is a table showing examples of the composition of test specimens as comparative examples and test specimens as examples. [Figure 3] 2 is a graph showing test results of a compressive strength test on the test specimen shown in FIG. 1. [Figure 4] 3 is a graph showing test results of a compressive strength test on the test specimen shown in FIG. 2. [Figure 5] 1 is a table showing an example of standards for a sulfuric acid resistance test in this embodiment. [Figure 6] 10 is a graph showing an example of test results of a mass change rate test on a specimen as a comparative example. [Figure 7] 1 is a graph showing an example of test results of a mass change rate test for a test specimen that is a comparative example and a test specimen that is an example. [Figure 8]10 is a graph showing another example of test results of the mass change rate test for the specimens of the comparative example and the example. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the present invention will now be described.

[0013] The acid-resistant cement material of this embodiment contains ordinary Portland cement, ground granulated blast furnace slag, an admixture made from the remaining solid content of concrete sludge, and aggregate in a predetermined blend.

[0014] Ground granulated blast furnace slag has a Blaine value (Blaine specific surface area) of 4000 to 8000 cm 2 / g is preferable. The higher the Blaine value of ground blast furnace slag, the faster the hydration hardening reaction progresses, and the structure of the hardened body becomes denser early. This results in a hardened body with excellent early strength development, making it possible to shorten the construction period. In addition, because the structure of the hardened body becomes dense, acid penetration is prevented, providing excellent acid resistance, and also improving water impermeability and adhesiveness.

[0015] On the other hand, if the amount of ground granulated blast furnace slag mixed is too high, there is a concern that the mortar will shrink excessively, so 25 to 150 parts by mass, preferably 40 to 100 parts by mass, of ground granulated blast furnace slag is mixed with 100 parts by mass of Portland cement. Therefore, the amount of ground granulated blast furnace slag used is such that it replaces 20% to 60% of the Portland cement (the amount such that the slag replacement rate (= (ground granulated blast furnace slag) / (Portland cement + ground granulated blast furnace slag)) is 20% to 60%), preferably 30% to 50%.

[0016] The above admixtures have a Blaine value of 4000 to 11000 cm 2 / g, particle size is preferably 15 μm to 5 mm, and the Blaine value is 4000 to 8000 cm 2 / g and a particle size of 15 μm to 30 μm are more preferred. If the admixture is within the above range, the effect on strength is small and the acid-resistant cement material exhibits excellent acid resistance.

[0017] The admixture is a recycled material obtained by drying and crushing concrete sludge (sludge cake) discharged from concrete plants. Concrete sludge is the solids discharged during centrifugal compaction of secondary concrete products, as well as those contained in concrete mixer and agitator truck cleaning and return concrete. Both solids contain cement, but are characterized by the fact that they are pre-hardened cement components that have not yet hardened like conventional concrete. Because these solids are exposed to air before hardening, calcium in the cement reacts with carbon dioxide in the air to produce calcium carbonate. This calcium carbonate is primarily formed on the surface of the cement, presumably preventing hardening. In other words, the admixture is preferably a solid obtained by suspending pre-hardened concrete sludge in water and then separating it into solid and liquid using a filter press. More preferably, the admixture is the so-called slag obtained by centrifugal compaction of concrete sludge. This slag is less likely to contain fine aggregate powder in the concrete, so the cement content is higher and it has excellent weighing properties.

[0018] On the other hand, even if the above-mentioned admixture contains fine powders in concrete, this does not cause any problems in this embodiment. Because the fine powders are inert, they do not have any negative effect on the activity of the solids, including cement. To quantify the amount of fine powders, the crystalline SiO2 content contained as a crystalline substance, i.e., the component known as quartz, can be quantified using powder X-ray diffraction or other methods. The amount of quartz contained in the slag is generally around 1% to 10%. Even if this amount of quartz is contained, it does not have any effect on the admixture.

[0019] The amount of the admixture is 10% to 60%, preferably 20% to 50%, based on the total amount of Portland cement and ground granulated blast furnace slag.

[0020] The aggregate may be any general aggregate having various particle sizes that is used in mortar or concrete.

[0021] In addition, other additives such as cement admixtures, water reducing agents, coloring pigments, water-retaining materials, etc. may be further added to the acid-resistant cement material within a range that does not impair the effects of this embodiment.

[0022] Thus, the acid-resistant cement material of this embodiment uses the residual solids of concrete sludge as an admixture in concrete containing cement and ground granulated blast furnace slag, thereby minimizing mass change even in corrosive environments such as those caused by sulfuric acid. The admixture contains Portland cement, ground granulated blast furnace slag, and aggregates made from the residual solids of concrete sludge. The ground granulated blast furnace slag replaces 20% to 60% of the Portland cement, and the admixture accounts for 10% to 60% of the total amount of Portland cement and ground granulated blast furnace slag. The use of ground granulated blast furnace slag in this acid-resistant cement material forms a dense hardened structure, making it difficult for sulfuric acid to penetrate into the hardened structure and reducing deterioration. Although sulfate ions in sulfuric acid destroy the skeleton of calcium silicate (CSH) compounds, the formation of gypsum dihydrate on the surface, which reacts more slowly with sulfate ions than other substances, contributes to its strong acid resistance. In particular, the use of the residual solids of concrete sludge as an admixture enhances reactivity with sulfate ions, resulting in the formation of a thick gypsum dihydrate layer. As a result, it is possible to provide an excellent acid-resistant cement material, particularly a sulfuric acid-resistant cement composition specialized in sulfuric acid resistance, even with a formulation that uses a reduced amount of ground granulated blast furnace slag.Therefore, it is possible to provide an acid-resistant cement material that uses a reduced amount of ground granulated blast furnace slag and is inexpensive and has a low environmental impact, is made from industrial waste and by-products as raw materials, and is capable of forming an acid-resistant (sulfuric acid-resistant) hardened mortar (cement, mortar, concrete) that has a small mass change rate and excellent acid resistance even when exposed to high concentrations of sulfuric acid, etc.

[0023] The acid resistant cementitious material can also be used in the manufacture of civil engineering or building components used in acid resistant applications. [Example]

[0024] <Compression strength test> In order to confirm the compressive strength properties of cement materials using the residual solids of the sludge from the slag of centrifugal compacted concrete as an admixture, the compressive strength was measured by substituting the residual solids with the standard sand of JIS mortar and curing underwater.

[0025] The compressive strength of mortars in which 0, 50, 75, and 100% of the above admixtures were substituted for JIS standard sand by mass with ordinary Portland cement (specimens A0 to A3) was measured. Also, cement equivalent to blast furnace cement type B (hereafter abbreviated as BB) was prepared by mixing ordinary Portland cement and blast furnace slab powder in a 60:40 ratio, and the compressive strength of mortars in which 0, 10, 20, 30, 40, 50, 75, and 100% of the above admixtures were substituted for JIS standard sand by mass with BB (specimens B0 to B7) was measured.

[0026] The materials used were tap water (W), ordinary Portland cement (C, manufactured by Taiheiyo Cement Corporation), ground granulated blast furnace slag (GBFS, Esment®, gypsum-free), the above-mentioned admixture (R), and JIS standard sand (S). The water-reducing agent used was SSP-104 (SP, manufactured by Takemoto Yushi Co., Ltd.), added in an amount that would provide the desired pouring fluidity for each mix. Because a large amount of water-reducing agent was added, the amounts of fine aggregate and water were adjusted to account for the 30% solids content of the water-reducing agent. The mix proportions for the prepared mortars are shown in Figures 1 and 2.

[0027] The specimens were prepared in accordance with JIS R 5201. Mixing was performed using a JIS mortar mixer, and the formwork was a 40 x 40 x 160 mm rectangular column. After molding, the specimens were pre-cured for 24 hours in a constant temperature and humidity chamber at 20°C and 65% RH. They were then demolded and underwater cured at 20°C. However, because mixing mortar containing the above admixtures is extremely difficult, the mixing procedure and mixing time were devised differently from JIS R 5201. Specifically, cement and water were added first, followed by the admixture, ground granulated blast furnace slag, and JIS standard sand, in that order. Mixing was also performed for a long time to allow the water-reducing agent to act on the powder. Mixing times ranged from 4 to 12 minutes, depending on the amount of water-reducing agent added.

[0028] The compressive strength test was performed in accordance with JIS R 5201 Appendix C, with a loading rate of 2.4 kN / sec. Six specimens were measured, and the average was used as the measured value.

[0029] The test results for specimens A0 to A3 are shown in Figure 3. Figure 3 shows the relative strength (%) based on the compressive strength of JIS mortar containing no admixture.

[0030] The results showed that the relative strength decreased as the additive rate increased from 50 to 100%, which means that the additives did not have any effect on increasing compressive strength.

[0031] The test results for specimens B0 to B7 are shown in Figure 4. With an addition rate of 10 to 40%, it was possible to obtain a compressive strength that was approximately 90% of the compressive strength of JIS mortar without any additives. In other words, even if the amount of additive added was increased, it was possible to obtain a compressive strength that was roughly equivalent to that of JIS mortar without any additives.

[0032] From the above studies, it can be seen that when mixing the above-mentioned admixtures into mortar, using them together with ground granulated blast furnace slag rather than ordinary Portland cement is effective. This is thought to be because the ground granulated blast furnace slag activated the above-mentioned admixtures. Ground granulated blast furnace slag is known to increase early strength by activating alite in cement. The above-mentioned admixtures are derived from ready-mix concrete and contain amorphous, unhydrated clinker. Just as ground granulated blast furnace slag activates the reaction of alite in cement, it is presumed that ground granulated blast furnace slag activated the amorphous alite in the above-mentioned admixtures, forming a dense hardened structure and leading to increased compressive strength.

[0033] It is also known that mixing ground granulated blast furnace slag increases the amount of initial CSH (calcium silicate compound) production, and it is thought that the produced CSH acts as a nucleus that serves as the starting point for the reaction of the above-mentioned admixtures, thereby acting on the alite in the amorphous cement.

[0034] The above test results showed that when the above admixtures are mixed, the compressive strength does not decrease when added to cement mortar equivalent to blast furnace cement type B compared to ordinary cement mortar, and that the preferred addition rate at which the compressive strength does not decrease when the above admixtures are used as a wholesale replacement for JIS standard sand in BB is 10 to 40%.

[0035] <Sulfuric acid resistance test> Test specimens of the above admixtures, which were deemed to be effective based on the results of the compressive strength test, were subjected to an experiment in which they were immersed in an aqueous sulfuric acid solution to confirm whether they had acid resistance (sulfuric acid resistance).

[0036] For this sulfuric acid resistance test, we searched for a mix that would satisfy two standards: the "Concrete Repair Technology Manual - Sludge Treatment Facilities Edition" prescribed by the Tokyo Metropolitan Government Bureau of Sewerage, and the "Manual for Corrosion Inhibition and Prevention Technology for Sewerage Concrete Structures" prescribed by the Japan Sewage Works Agency. Each standard is shown in Figure 5. As shown in these standards, the tests conducted included a mass change rate test and a sulfuric acid penetration depth test.

[0037] (a) Mass change rate test To confirm sulfuric acid resistance, the mass change rate was measured by comparing the mass after immersion in sulfuric acid aqueous solution with the mass before immersion according to the number of days that had passed. The Tokyo Metropolitan Government Bureau of Sewerage's standard is that the mass change rate after immersion in 10% sulfuric acid aqueous solution for 56 days must be within ±5%, while the Japan Sewage Works Agency's standard is that the mass change rate after immersion in 5% sulfuric acid aqueous solution for 28 days must be within ±10%. Therefore, tests were first conducted in 10% sulfuric acid aqueous solution, followed by experiments in 5% sulfuric acid aqueous solution.

[0038] The specimens were prepared in the same manner as for the compressive strength test.

[0039] The sulfuric acid solution in which the specimens were immersed was replaced in its entirety every seven days. After immersion, the specimens were taken out of the sulfuric acid solution and washed evenly over their entire surfaces with tap water from a fully open tap for one minute, after which their masses were measured.

[0040] The mass change rates of specimens A0 to A3 are shown in Figure 6. After 56 days of immersion, specimen A0 showed a mass change rate of 45.9%, while specimen A1 showed a mass change rate of -36.8%, specimen A2 -30.9%, and specimen A3 -28.7%. The mass change rate decreased as the addition rate of the above admixtures increased.

[0041] Figure 7 shows the mass change rate when mortars (specimens C0 to C2) in which JIS standard sand was substituted with the above admixtures at 0, 30, and 50% of BB were immersed in a 10% aqueous sulfuric acid solution. Note that these addition rates are based on the compressive strength test results, which showed that the optimum addition rate for the above admixtures is around 40%.

[0042] After 56 days of immersion, specimen C0 showed a mass change of -44.2%, while specimen C1 showed a mass change of -29.1% and specimen C2 showed a mass change of -23.3%, decreasing as the additive rate of the admixture increased. As a result, it was found that high sulfuric acid resistance could be achieved with a small additive rate compared to when ordinary cement mortar is mixed with admixtures.

[0043] FIG. 8 shows the mass change rate of specimens C0 to C2 when they were immersed in a 5% aqueous sulfuric acid solution.

[0044] After 28 days of immersion, specimen C0 showed a mass change rate of -22.3%, while specimen C1 showed a mass change rate of +6.27% and specimen C2 showed a mass change rate of +8.28%, confirming that the mass increased as the addition rate of the above admixtures increased.

[0045] (b) Sulfuric acid penetration depth test As shown in Figure 5, the Tokyo Metropolitan Government Bureau of Sewerage and the Japan Sewage Works Agency both require a phenolphthalein-free depth of 3.0 mm or less after 28 days of immersion in a 5% sulfuric acid solution. Based on the results of the compressive strength test, experiments were conducted on specimens C0–C2. The sulfuric acid solution in which the specimens were immersed was replaced every seven days. In accordance with the "Manual for Corrosion Inhibition and Prevention Technology for Sewerage Concrete Structures," the specimens were split perpendicular to the sulfuric acid-impregnated surface, a 1% phenolphthalein solution was sprayed on the cut surface, and the vertical length of the red-colored portion of the specimen was measured with calipers. The average of these measurements was subtracted from the initial specimen width of 20 mm, and half of this value was calculated to determine the sulfuric acid penetration depth. The sulfuric acid penetration depth was measured on the side, which did not take into account the surface in contact with the immersion container.

[0046] After 28 days of immersion, the sulfuric acid penetration depth of the mortar was 2.9 mm for specimen C0, 2.7 mm for specimen C1, and 2.25 mm for specimen C2, confirming that the penetration depth decreased as the addition rate of the above admixtures increased. All addition rates met the standards of the Tokyo Metropolitan Government Bureau of Sewerage and the Japan Sewage Works Agency, confirming that increasing the addition rate had the effect of reducing penetration depth.

[0047] The cross-section of the immersed mortar confirmed that when the addition rate of the above admixture was high, many white layers, thought to be gypsum layers, were formed. A comparison of the time periods after immersion in sulfuric acid solution, 7 and 28 days, confirmed that immersion in 10% sulfuric acid solution caused significant surface corrosion compared to immersion in 5% sulfuric acid solution.

[0048] From the results of (a) and (b) above, it is thought that one of the reasons for the difference between cases where ground granulated blast furnace slag was added and cases where it was not added is that, as shown in the compressive strength test, the addition of ground granulated blast furnace slag formed a dense hardened structure, making it difficult for sulfuric acid to penetrate into the hardened structure and making deterioration less likely to occur.

[0049] Furthermore, the reason for the decrease in mass change rate as the amount of the admixture increased is thought to be related to the formation of a substance that appears to be a gypsum dihydrate layer. It is known that sulfate ions in sulfuric acid destroy the CSH framework, creating a brittle gypsum dihydrate layer. Further deterioration leads to the formation of ettringite inside the gypsum dihydrate layer, resulting in cracks. The fractured cross-sections confirmed that a thicker gypsum dihydrate layer formed when the admixture was added than when the admixture was not added. Based on these findings, it can be said that the admixture is highly reactive with sulfate ions in sulfuric acid, making it more likely to form a gypsum dihydrate layer. This is thought to be because the components of cement are crystalline, while the calcium component in the admixture becomes amorphous after undergoing a hydration reaction.

[0050] The above experiment was carried out using 5% sulfuric acid aqueous solution and 10% sulfuric acid aqueous solution, but considering that the pH of 5% sulfuric acid aqueous solution is 0.275 and that of 10% sulfuric acid aqueous solution is 0.04, it can be seen that the pH differs by about 7 times. This shows that although the concentration of 10% sulfuric acid aqueous solution is twice that of 5% sulfuric acid aqueous solution, it has a greater effect on the test specimen.

[0051] In this way, by adding the above-mentioned admixtures, it is possible to create acid-resistant cement materials with excellent sulfuric acid resistance, and just like with compressive strength, adding them to BB has a high effect. It has also been shown that mortars in which JIS standard sand is substituted with admixtures that become the remaining solids of concrete sludge at 0, 30, and 50% of the BB meet the sulfuric acid resistance standards set by the Japan Sewage Works Agency.

Claims

1. The mixture contains Portland cement, ground granulated blast furnace slag, an admixture made of residual solids from concrete sludge, and aggregate, The Portland cement is substituted with 20% to 60% of the ground granulated blast furnace slag, The admixture is used in an amount of 10% to 60% of the total amount of the Portland cement and the ground granulated blast furnace slag. An acid-resistant cement material characterized by:

2. The Blaine value of the admixture made from the residual solid content of concrete sludge is 4000 to 11000 cm ２ / g 2. The acid-resistant cement material according to claim 1.

3. The particle size of the admixture made from the remaining solid content of concrete sludge is 15 μm to 5 mm.

3. The acid-resistant cement material according to claim 1 or 2.

4. The remaining solids of concrete sludge are obtained from the slag produced by centrifugal molding of secondary concrete products.

3. The acid-resistant cement material according to claim 1 or 2.

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