Shell sand concrete and preparation method thereof

Abstract

The invention discloses shell sand concrete and a preparation method thereof, and the shell sand concrete is prepared from the following components in parts by weight: 550 to 800 parts of shell sand, 650 to 850 parts of coarse aggregate, 250 to 450 parts of cement, 250 to 450 parts of auxiliary cementing material, 200 to 300 parts of water and 0 to 10 parts of water reducing agent, wherein the sand ratio ranges from 0.40 to 0.50. According to the method, key concrete mix proportion design parameters such as the optimal sand ratio range and the use amount of the cementing material are considered, resource recycling of the waste shells can be achieved, and the method is economical and environmentally friendly. On one hand, waste can be turned into wealth, the ecological environment problem caused by stacking of waste shells and over-mining of river sand and the current situation of river sand resource shortage are relieved, and the concept of green and environment-friendly sustainable development is met; on the other hand, local materials can be used, and the offshore engineering cost is reduced to a certain extent. In addition, the shell sand is subjected to water pre-absorption treatment, so that the defect of high water absorption of the shell sand material is overcome, and the working performance of the concrete mixture is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical fields of solid waste recycling and cement-based building materials, and particularly to a shell sand concrete and a preparation method thereof. Background Art

[0002] With the new stage of construction of offshore projects such as cross-sea bridges, undersea tunnels, breakwaters, and deep-water ports, the demand for reinforced concrete has increased sharply. River sand is one of the most commonly used fine aggregates for concrete, but over-exploitation will have an adverse impact on the human ecological environment. The chemical composition of waste shells is stable and does not participate in the hydration reaction process of cementitious materials, and this characteristic is similar to that of other concrete aggregates. If local materials can be used and shell sand is used to replace ordinary river sand to prepare a shell sand concrete, it is of great significance for reducing the engineering cost of the offshore area and alleviating the problems caused by the stacking of waste shells and over-exploitation of river sand.

[0003] How to obtain high-performance concrete based on shell sand is one of the effective ways to realize the resource utilization of waste shells and is also the main technical difficulty that needs to be overcome currently. At present, the following problems mainly exist in the related technical research on crushed shell aggregate concrete:

[0004] (1) Existing research is mostly based on the mix ratio system of ordinary river sand concrete. By taking the method of partially replacing the fine aggregate of river sand at a certain mass ratio on the basis of the mix ratio of ordinary river sand concrete, the influence of the differences in the physical properties of aggregates on the key concrete mix design parameters such as the optimal sand ratio and the dosage of cementitious materials is not considered, resulting in the unclear mix design parameters of concrete applicable to crushed shell aggregates.

[0005] (2) Existing research shows that crushed shells can be used as aggregates to produce concrete. However, when its dosage is too high, its workability and mechanical properties are worse than those of ordinary river sand concrete. Therefore, the recommended optimal replacement ratio of crushed shells in existing research is relatively low, and the ecological benefit of "turning waste into treasure" of crushed shells cannot be fully exerted.

[0006] (3) Compared with traditional river sand, the fine aggregate of crushed shells has higher water absorption. When preparing concrete, the higher water absorption of the aggregate may cause it to absorb water from the concrete mixture, resulting in a decrease in the free water content in the concrete mixture, a reduction in the fluidity of the concrete, slump loss, and poor workability.

[0007] To solve the above technical problems, the present invention uses shell sand as all the fine aggregate to prepare a shell sand concrete with good workability and mechanical properties. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a shell sand concrete and a preparation method thereof in view of the deficiencies of the above-mentioned prior art. The shell sand concrete and the preparation method thereof take into account the key concrete mix design parameters such as the optimal sand ratio range and the amount of cementitious materials, and can realize the resource recycling of discarded shells, which is economical and environmentally friendly. On the one hand, it can turn waste into treasure, alleviate the ecological and environmental problems caused by the stacking of discarded shells and excessive exploitation of river sand, and the current situation of shortage of river sand resources, which is in line with the concept of green and environmentally friendly sustainable development; on the other hand, it can also realize the use of local materials, and reduce the cost of offshore projects to a certain extent.

[0009] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0010] A shell sand concrete comprises the following components, measured by weight: 550-800 parts of shell sand, 650-850 parts of coarse aggregate, 250-450 parts of cement, 250-450 parts of auxiliary cementitious materials, 200-300 parts of water, and 0-10 parts of water reducing agent; wherein the sand ratio ranges from 0.40 to 0.50.

[0011] The particle size of shell sand is less than 5mm.

[0012] The coarse aggregate is continuously graded crushed stone with a particle size of 5 to 25 mm.

[0013] The cement is one or a combination of silicate cement or ordinary silicate cement; the auxiliary cementitious material is fly ash.

[0014] The water reducer is a powdered naphthalene-based high-efficiency water reducer. When the weight of the water reducer is greater than 0, the proportion of the water reducer is 0.5% to 1% of the total weight of the cement and the auxiliary cementitious materials.

[0015] A method for preparing shell sand concrete comprises the following steps.

[0016] Step 1, preparing shell sand: washing the discarded shells to remove impurities, drying them naturally, crushing and sieving them to obtain shell sand;

[0017] Step 2, weighing raw materials: weigh the following raw materials in parts by weight: 550-800 parts of shell sand, 650-850 parts of coarse aggregate, 250-450 parts of cement, 250-450 parts of auxiliary cementitious materials, 200-300 parts of water and 0-10 parts of water reducing agent.

[0018] Step 3, pre-absorption of shell sand: the 200-300 parts of water weighed in step 2 are divided into pre-absorption water and mixing water; wherein the pre-absorption water is determined according to the shell sand content and the optimal pre-absorption rate; the shell sand is pre-absorbed with the pre-absorption water;

[0019] Step 4: Dry mixing: Put the cement, auxiliary cementitious materials and coarse aggregate weighed in step 2 into a mixer and stir and mix to obtain a dry mix.

[0020] Step 5, wet mixing: first mix the water reducing agent weighed in step 2 with the mixing water to form mixing water; then, mix the shell sand pre-absorbed in step 3, the dry mix obtained in step 4 and the mixing water to form a slurry.

[0021] Step 6: Shape and cure the slurry to obtain shell sand concrete.

[0022] In step 2, the ratio of shell sand to the sum of shell sand and coarse aggregate is formed as the sand ratio; the optimal value of the sand ratio and the optimal pre-water absorption rate in step 3 are both obtained through orthogonal experimental design, and slump and compressive strength tests are performed on each group of test samples.

[0023] The influencing factors in orthogonal experimental design include cement dosage, pre-water absorption rate, net w / b and sand ratio.

[0024] Increasing the sand ratio can increase the slump of shell sand concrete and reduce the compressive strength. The optimal range of sand ratio is 0.45-0.50.

[0025] The increase in pre-water absorption rate can reduce the compressive strength of shell sand concrete, but when the cement dosage is constant, the initial slump and time loss of shell sand concrete with a pre-water absorption rate of a% are relatively compromised; therefore, the optimal pre-water absorption rate is a%, where a% is half of the saturated water absorption rate of shell sand.

[0026] The present invention has the following beneficial effects:

[0027] 1. The shell sand concrete of the present invention can realize the resource recycling of discarded shells, which is economical and environmentally friendly. On the one hand, it can turn waste into treasure, alleviate the ecological and environmental problems caused by the stacking of discarded shells and excessive exploitation of river sand and the current situation of shortage of river sand resources, and conform to the concept of green and environmentally friendly sustainable development; on the other hand, it can also realize the use of local materials, and reduce the cost of offshore engineering to a certain extent.

[0028] 2. The present invention compensates for the defect of high water absorption of shell sand material itself by pre-absorbing water to the shell sand, and significantly improves the working performance of concrete mixture; the pre-absorbed shell sand can play the "internal curing" role of shell sand in the middle and late stages of hydration, and improve the mechanical properties of shell sand concrete; the porous physical properties of shell sand make it have a certain adsorption capacity for harmful ions, thereby improving the durability of shell sand concrete. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A comparison of the gradation curves of shell sand and river sand is shown.

[0030] Figure 2 The mix design flow chart for shell sand concrete is shown.

[0031] Figure 3 The comparison chart of slump loss over time of shell sand concrete of groups G1 to G9 is shown; among them, (a) corresponds to shell sand concrete of groups G1 to G3; (b) corresponds to shell sand concrete of groups G4 to G6; (c) corresponds to shell sand concrete of groups G71 to G9.

[0032] Figure 4 The figure shows the influence of cementitious material dosage, net w / b, sand ratio and pre-water absorption rate on the slump of shell sand concrete; among them, (a) is the cementitious material dosage; (b) is the net water-cement ratio; (c) is the sand ratio; and (d) is the pre-water absorption rate.

[0033] Figure 5 The influence of cementitious material dosage, net w / b, sand ratio and pre-water absorption rate on the compressive strength of shell sand concrete is shown; among them, (a) is the cementitious material dosage; (b) is the net water-cement ratio; (c) is the sand ratio; and (d) is the pre-water absorption rate.

[0034] Figure 6 The relationship between the compressive strength and the curing age of the three embodiments of the present invention and the comparative example 1 is shown.

[0035] Figure 7 The relationship between the splitting tensile strength and the curing age of each group of concrete prepared by the present invention is shown.

[0036] Figure 8 A comparison chart of the non-steady-state chloride ion migration coefficients of each group of concrete prepared in the present invention at 28 days and 90 days is shown. DETAILED DESCRIPTION

[0037] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific preferred embodiments.

[0038] A shell sand concrete comprises the following components, measured by weight: 550-800 parts of shell sand, 650-850 parts of coarse aggregate, 250-450 parts of cement, 250-450 parts of auxiliary cementitious materials, 200-300 parts of water, and 0-10 parts of water reducing agent.

[0039] Sand ratio = shell sand / (shell sand + coarse aggregate), then the sand ratio range is 0.40 to 0.50, and more preferably 0.45 to 0.50.

[0040] Furthermore, the particle size of the shell sand is preferably less than 5 mm, and the fineness modulus is preferably 3.0. The physical properties of the river sand to be replaced by the shell sand of the present invention are shown in Table 1, and the grading curve is shown in Figure 1 shown.

[0041] Table 1 Physical properties of river sand fine aggregate

[0042]

[0043] Furthermore, the coarse aggregate is preferably a continuously graded crushed stone with a particle size of 5 to 25 mm, and more preferably 20 mm. The crushed stone is sieved to remove impurities before use.

[0044] Furthermore, the cement is preferably one or a combination of silicate cement or ordinary silicate cement, etc.; further, Conch brand P·O 42.5 ordinary silicate cement is preferably used, and its performance meets the specification limit of "General Portland Cement" (GB 175-2020). The specific physical properties and chemical composition are shown in Table 2 and Table 3 respectively.

[0045] Table 2 Physical and mechanical properties of cement

[0046]

[0047] Table 3 Chemical composition of cement (%)

[0048] CaO <![CDATA[SiO 2 ]]> <![CDATA[Al 2 THE 3 ]]> <![CDATA[Fe 2 THE 3 ]]> <![CDATA[SO 3 ]]> <![CDATA[Na 2 The]]> <![CDATA[K 2 The]]> MgO 60.16 21.35 4.94 2.71 1.96 1.00 0.48 0.46

[0049] Furthermore, the auxiliary cementitious material is preferably fly ash, and the ratio of the amount of fly ash added to the total mass of the auxiliary cementitious material and cement is preferably 0.50. The auxiliary cementitious material can greatly reduce the amount of cement. Furthermore, it is preferred to use Class F Grade I fly ash produced by Henan Borun Refractory Materials Co., Ltd. The main characteristic parameters of the fly ash are shown in Table 4 below.

[0050] Table 4 Main characteristic parameters of fly ash (%)

[0051] Loss on ignition <![CDATA[SiO 2 ]]> <![CDATA[Al 2 THE 3 ]]> <![CDATA[Fe 2 THE 3 ]]> <![CDATA[SO 3 ]]> <![CDATA[TiO 2 ]]> <![CDATA[K 2 The]]> CaO 1.66 55.71 37.79 4.42 0.66 1.66 1.54 2.66

[0052] Furthermore, the water reducer is preferably a powdered naphthalene-based high-efficiency water reducer. When the weight of the water reducer is greater than 0, the water reducer dosage is preferably 0.5% to 1% of the total weight of cement and auxiliary cementitious materials. Furthermore, it is preferred to use the CQJ-NX01 naphthalene-based water reducer produced by Shanghai Chenqi Chemical, whose performance meets the requirements of "Concrete Admixtures" (GB 8076-2008). Among them, the solid content is greater than 96%, the sodium sulfate content is less than 20%, and the chloride ion content is less than 0.4%.

[0053] A method for preparing shell sand concrete comprises the following steps.

[0054] Step 1, prepare shell sand: wash the discarded shells to remove impurities, dry them naturally, and crush and sieve them to obtain shell sand.

[0055] Step 2, weighing raw materials: weigh the following raw materials in parts by weight: 550-800 parts of shell sand, 650-850 parts of coarse aggregate, 250-450 parts of cement, 250-450 parts of auxiliary cementitious materials, 200-300 parts of water and 0-10 parts of water reducing agent.

[0056] Step 3, pre-absorption of water by shell sand: divide the 200-300 parts of water weighed in step 2 into pre-absorption water and mixing water; wherein, the pre-absorption water is determined according to the shell sand content and the optimal pre-absorption rate; use the pre-absorption water to pre-absorb the shell sand, preferably stirring and mixing for 5 minutes.

[0057] The optimal value of the sand ratio in step 2 and the optimal pre-water absorption rate in step 3 were obtained through orthogonal experimental design, and the slump and compressive strength tests were performed on each group of test samples.

[0058] Shell sand concrete is a new type of concrete material, and its related research work is still in its early stages. The actual engineering application of crushed shell aggregate in the field of concrete is almost blank, so there is basically no engineering data for reference, and there is no theoretical formula and empirical formula for calculating the mix ratio of ordinary river sand concrete. In view of the objective problems existing in the mix ratio design of the above-mentioned crushed shell concrete, the present invention is based on the orthogonal design method, and designs the mix ratio of shell sand concrete through trial mixing, adjustment, calculation and other processes. The specific method flow is as follows: Figure 2 As shown, it preferably includes the following steps.

[0059] Step 3-1: Orthogonal experimental design

[0060] Crushed shell fine aggregate has higher water absorption than traditional river sand. When preparing concrete, the higher water absorption rate of aggregate may cause it to absorb water from the concrete mixture, resulting in a decrease in the free water content in the concrete mixture, a decrease in the fluidity of the concrete, and a loss of slump. In order to prevent the shell sand from "stealing water" from the cement hydration process during the mixing process, affecting the working performance of the concrete mixture, and improving the working performance of the shell sand concrete mixture, the shell sand is pre-absorbed with water in the present invention.

[0061] The amount of cementitious materials, pre-water absorption rate, net water-cement ratio and sand ratio are determined to be the main influencing factors. The specific levels of each factor are shown in Table 3.3.1. It should be noted that the sand ratio adopts the mass sand ratio. The saturated water absorption rate of shell sand is a% = 8%, and the pre-water absorption rate factor level considers three situations: no pre-absorption (0%), unsaturated (4%) and saturated (8%) pre-absorption. Affected by pre-absorption, in order to control the total water-cement ratio (total w / b), the net water-cement ratio (net w / b) factor value should not be too large, which are 0.35, 0.40 and 0.45 respectively, so as to design the orthogonal table shown in Table 5.

[0062] Table 5 Orthogonal design factors and levels

[0063] Influencing factors Amount of cementitious material used: weight parts / g Pre-absorption rate Net w / b Sand rate Level 1 500 0％ 0.35 0.40 Level 2 550 4％ 0.40 0.45 Level 3 600 8％ 0.45 0.50

[0064] Step 3-2: Preliminary mix ratio

[0065] According to the orthogonal table designed in step 3-1, the preliminary mix design of shell sand concrete is carried out. A total of 9 groups of concrete need to be prepared, as shown in Table 6. Among them, cement and fly ash (auxiliary cementitious material) use the same components.

[0066] Table 6 Preliminary mix ratio

[0067]

[0068] Step 3-3: Influence of mix design parameters

[0069] Based on the preliminary mix proportions of shell sand concrete groups G1 to G9, slump and compressive strength tests were carried out. Based on the test results, the influence of cementitious material (cement + fly ash) dosage, pre-water absorption rate, net water-cement ratio and sand ratio on the working performance and mechanical properties of shell sand concrete were analyzed.

[0070] A. Amount of cementitious materials

[0071] Figure 3 is the change of slump loss over time of each group of shell sand concrete. Figure 3 (a) The amount of the gelling material in groups G1 to G3 is 500 parts by weight / g; Figure 3 (b) The amount of cementitious material used in groups G4 to G6 is 550 parts by weight / g; Figure 3 In (c), the amount of gelling material used in groups G7 to G9 is 600 parts by weight / g.

[0072] from Figure 3 It can be found that the amount of cementitious materials has a significant effect on the slump of shell sand concrete. With the increase of the amount of cementitious materials, the slump generally shows an upward trend. In addition to the amount of cementitious materials, the pre-water absorption rate, net w / b and sand ratio have a certain degree of influence on the slump of shell sand concrete. When the net w / b and sand ratio are small, the G1 group with 0% pre-water absorption rate (no pre-absorption) has the worst working performance and almost no initial slump. When the net w / b and sand ratio are large, the G9 group with 8% pre-water absorption rate has a large slump within 120 minutes. Under the random combination of net w / b and sand ratio, when the amount of cementitious materials is constant, the initial slump and time loss of shell sand concrete using the 4% pre-water absorption rate group (G2, G5, G8) are relatively compromised.

[0073] Figure 4The following is the influence of cementitious material dosage, net w / b, sand ratio and pre-absorption rate on the slump of shell sand concrete. It can be found that the slump of shell sand concrete increases with the increase of net w / b and sand ratio. Figure 4 (a) It can be seen that increasing the amount of cementitious materials can significantly increase the slump of shell concrete, which is consistent with Figure 3 The overall change patterns of the graphs (a) to (c) are consistent. Figure 4 (d) shows that the use of different pre-absorption rates has little effect on the initial slump (0min) of shell sand concrete, which is also consistent with Figure 3 The findings in Figures (a) to (c) are consistent.

[0074] In order to provide a direction for optimizing the mix design of shell sand concrete in the future, the influence of the key factors of mix design on the compressive strength of shell sand concrete is analyzed. cu ) data, plotted as Figure 5 The schematic diagram of each influencing factor is shown, and the influence of each factor on the compressive strength of shell sand concrete is qualitatively analyzed.

[0075] Depend on Figure 5 (a) It can be found that, except for the relatively discrete compressive strength of the group with lower cementitious material dosage, the compressive strength of shell sand concrete generally increases with the increase of cementitious material dosage. Figure 5 (b) It can be found that the water-cement ratio is an important factor affecting the compressive strength of both ordinary river sand concrete and shell sand concrete. Consistent with the rule of ordinary river sand concrete, the compressive strength of shell sand concrete is also inversely proportional to the net w / b. In addition, Figure 5 (c) and (d) show that the 28d compressive strength of shell sand concrete decreases linearly with the increase of sand ratio and pre-water absorption rate. This is mainly because the larger the sand ratio, the smaller the proportion of harder coarse aggregate, resulting in a relatively lower compressive strength of concrete. Increasing the pre-water absorption rate will also increase the water consumption of shell sand concrete to a certain extent, increase w / b, and lead to a decrease in its compressive strength. In summary, if we want to further improve the strength of shell sand concrete, we can take measures such as reducing w / b, increasing cement dosage, and controlling sand ratio.

[0076] Step 3-4: Optimize mix ratio

[0077] On the basis of the above test analysis, the preferred sand ratio and pre-water absorption rate were selected to further adjust and optimize the mix ratio of shell sand concrete. Taking into account the influence of various factors on the working performance and compressive strength of shell sand concrete, the preferred sand ratio of 0.45 and the preferred pre-water absorption rate of 4% were used to re-optimize the mix ratio of 6 groups of shell sand concrete. In order to control the density of each group of concrete to be the same, when the amount of coarse aggregate is reduced, the amount of cement needs to be increased accordingly. The optimized mix ratio of shell sand concrete is shown in Table 7, in which cement and fly ash (auxiliary cementitious material) use the same components.

[0078] Table 7 Optimized mix ratio

[0079]

[0080] The slump and compressive strength test results of shell sand concrete of groups Y1 to Y6 in the optimized mix design are shown in Table 8.

[0081] Table 8 Slump and compressive strength of concrete of groups Y1 to Y6

[0082]

[0083] Step 3-5: Obtain the mix ratio of shell sand concrete

[0084] Based on the preliminary mix design and optimized mix test results, a shell sand concrete mix with good working performance that can meet the top requirements of different strengths was finally determined.

[0085] In summary, the increase of sand ratio can increase the slump of shell sand concrete and reduce the compressive strength. Therefore, the optimal range of sand ratio is 0.45-0.50. The increase of pre-water absorption rate can reduce the compressive strength of shell sand concrete, but when the cement dosage is constant, the initial slump and time loss of shell sand concrete with a% pre-water absorption rate are relatively compromised; therefore, the optimal pre-water absorption rate is a%, where a% is half of the saturated water absorption rate of shell sand.

[0086] Step 4, dry mixing: the cement, auxiliary cementitious material and coarse aggregate weighed in step 2 are placed in a stirrer and stirred and mixed for preferably 2 minutes to obtain a dry mix. Furthermore, at this time, the stirrer stirs and mixes at a speed of 40 to 60 r / min.

[0087] Step 5, wet mixing: first, mix the water reducing agent weighed in step 2 with the mixing water to form the mixing water; then, preferably mix the shell sand pre-absorbed in step 3, the dry mix obtained in step 4, and the mixing water for 2.5 minutes to form a slurry. Further, after adding the mixing water, stir at a speed of 80-90r / min, pause 1-2 times in the middle to clean the mixture adhering to the mixer blade with a shovel, and manually turn the mixture.

[0088] Step 6: The slurry is cast in a mold, and after standing for 24 hours, the slurry is demoulded and cured to a specified age at an ambient temperature of 20° C. and a relative humidity of 90 to 95%, thereby obtaining the shell sand concrete.

[0089] The following is a detailed description taking three embodiments of the present invention and a comparative example as examples.

[0090] Example 1

[0091] A method for preparing shell sand concrete comprises the following steps:

[0092] Weigh the following raw materials in parts by weight: 275 parts of cement, 275 parts of fly ash, 780 parts of shell sand, 790 parts of crushed stone, and 280 parts of water, wherein the ratio of fly ash to the total mass of fly ash and cement is 0.5.

[0093] The shell sand fine aggregate was pre-absorbed with a pre-absorption rate of 5%, and 39 parts of pre-absorption water were weighed from the total water and poured into the shell sand, and stirred for 5 minutes. The remaining 241 parts of water were used as mixing water.

[0094] Put cement, auxiliary cementitious materials and coarse aggregate into a mixer and stir for 2 minutes to obtain a dry mix; add pre-absorbed shell sand and mixing water to the dry mix and stir for 2.5 minutes to mix evenly to obtain a slurry. The slurry is formed and cured to obtain the slurry.

[0095] Example 2

[0096] A method for preparing shell sand concrete comprises the following steps:

[0097] Weigh the following raw materials in parts by weight: 350 parts of cement, 350 parts of fly ash, 660 parts of shell sand, 805 parts of crushed stone, 235 parts of water, and 3.5 parts of water reducing agent, wherein the ratio of fly ash to the total mass of fly ash and cement is 0.5.

[0098] The shell sand fine aggregate was pre-absorbed with a pre-absorption rate of 5%, and 33 parts of pre-absorption water were weighed from the total water and poured into the shell sand, and stirred for 5 minutes. The water reducer was poured into the remaining 202 parts of water, and mixed thoroughly to prepare the mixing water.

[0099] Put cement, auxiliary cementitious materials and coarse aggregate into a mixer and stir for 2 minutes to obtain a dry mix; add pre-absorbed shell sand and mixing water mixed with a water reducer into the dry mix, stir for 2.5 minutes, mix evenly, and obtain a slurry. The slurry is formed and cured to obtain the slurry.

[0100] Example 3

[0101] A method for preparing shell sand concrete comprises the following steps:

[0102] Weigh the following raw materials in parts by weight: 425 parts of cement, 425 parts of fly ash, 575 parts of shell sand, 700 parts of crushed stone, 280 parts of water, and 8.5 parts of water reducing agent, wherein the ratio of fly ash to the total mass of fly ash and cement is 0.5.

[0103] The shell sand fine aggregate was pre-absorbed with a pre-absorption rate of 5%, and 28.8 parts of pre-absorption water were weighed from the total water and poured into the shell sand, stirring and mixing for 5 minutes. The water reducer was poured into the remaining 251.2 parts of water, mixed thoroughly, and the mixing water was prepared.

[0104] Put cement, auxiliary cementitious materials and coarse aggregate into a mixer and stir for 2 minutes to obtain a dry mix; add pre-absorbed shell sand and mixing water mixed with a water reducer into the dry mix, stir for 2.5 minutes, mix evenly, and obtain a slurry. The slurry is formed and cured to obtain the slurry.

[0105] Comparative Example 1

[0106] The method for preparing river sand concrete with the same mix ratio as that in Example 1 comprises the following steps:

[0107] Weigh the following raw materials in parts by weight: 275 parts of cement, 275 parts of fly ash, 780 parts of river sand, 790 parts of crushed stone, and 280 parts of water, wherein the ratio of fly ash to the total mass of fly ash and cement is 0.5.

[0108] Put cement, auxiliary cementitious materials and coarse aggregate into a mixer and stir for 2 minutes to obtain a dry mix; add pre-absorbed shell sand and mixing water to the dry mix and stir for 2.5 minutes to mix evenly to obtain a slurry. The slurry is formed and cured to obtain the slurry.

[0109] The raw material compositions of the above-mentioned Examples 1, 2, and 3 are compared with those of Comparative Example 1, as shown in Table 9 below.

[0110] Table 9 Raw material composition of Examples 1 to 3 of the present invention and Comparative Example 1, unit: weight parts / g

[0111] serial number cement Fly ash Shell sand river sand gravel Water reducing agent water Example 1 275 275 780 0 790 0 280 Example 2 350 350 660 0 805 3.5 235 Example 3 425 425 575 0 700 8.5 280 Comparative Example 1 275 275 0 780 790 0 280

[0112] The above-mentioned Example 1 is a shell sand concrete with a strength of C30, Example 2 is a shell sand concrete with a strength of C40, and Example 3 is a shell sand concrete with a strength of C50. In order to compare with ordinary river sand concrete, the mix ratio of the C30 group shell sand concrete (Example 1) is used as a benchmark, and the river sand group concrete of Comparative Example 1 is designed as a control.

[0113] The raw material composition information of Examples 1 to 3 and Comparative Example 1 is shown in Table 9. Referring to the relevant specifications, the slump, cube compressive strength, splitting tensile strength and chloride ion penetration resistance of the above groups of concrete were tested. Among them, the slump was measured immediately after mixing and discharging, and the cube compressive strength, splitting tensile strength and chloride ion penetration resistance were measured after the concrete was hardened and demolded.

[0114] The specimen size of the concrete cube compressive strength test is 100mm×100mm×100mm. The HG-WF1000 microcomputer electro-hydraulic servo universal machine is used for force-controlled loading. The loading rate refers to the "Test Procedure for Hydraulic Concrete" (SL 352-2020). The test ages are 3d, 7d, 28d and 90d. Three specimens are tested in each group, and the results are obtained by taking the average value.

[0115] The specimen size of concrete splitting tensile strength is 100mm×100mm×100mm, and the test age is 3d, 7d, 28d and 90d. Before the test, the contact surface of the specimen was cleaned with sandpaper, and a wooden pad with a size of 200mm×20mm×5mm was pasted along the splitting surface. The SHT4305 microcomputer-controlled electro-hydraulic servo universal testing machine was used with a special steel arc pad (base) for splitting tensile strength for force-controlled loading. The loading rate was based on the "Test Procedure for Hydraulic Concrete" (SL 352-2020). Three specimens were tested in each group, and the results were averaged.

[0116] Referring to the Standard for Test Methods of Long-term Performance and Durability of Ordinary Concrete (GB / T 50082-2009), the rapid chloride ion migration coefficient method (RCM method) was used to determine the chloride ion penetration resistance of each group of concrete. At the test age of 28d and 90d, the test was carried out using the Beijing Nairde RCM-NTB chloride ion diffusion coefficient tester, and the test used a cylindrical specimen with a size of φ100mm×50mm. Before the test began, the thickness of the specimen was measured and recorded with a vernier caliper. After the test, the specimen was split with a press, and 0.1mol / L silver nitrate solution was sprayed on the fracture surface. After standing for 15min for color development, the non-steady-state chloride ion migration coefficient was calculated. Three specimens were tested in each group, and the results were averaged.

[0117] It can be seen from Table 10 that the shell sand concrete (Examples 1 to 3) prepared by the present invention has a slump between 50 and 100 mm and good working performance compared with the conventional river sand concrete (Comparative Example 1). By pre-absorbing the shell sand, the "water grabbing" of the shell sand from the cement paste can be effectively reduced, and the working performance of the mixture can be improved.

[0118] Table 10 Slump test results of Examples 1 to 3 of the present invention and Comparative Example 1

[0119]

[0120] Figure 6 The relationship between the compressive strength and curing age of each group of concrete prepared by the present invention is given. Figure 6 It can be found that the change pattern of the compressive strength of the shell sand concrete in Examples 1 to 3 with age is consistent with that of the ordinary river sand concrete in Comparative Example 1, and the compressive strength of each group of concrete increases with age. This is mainly due to the fact that with the development of the cement hydration process, the hydration products continue to increase, and the interior of each group of concrete becomes more compact. In addition, the growth rate law of the compressive strength of the shell sand concrete in different embodiments is also similar to that of ordinary river sand concrete. The growth rate of the compressive strength of each group of concrete generally shows the characteristics of a faster growth rate in the early stage and a slower growth rate in the later stage. This shows that, like ordinary river sand concrete, the hydration reaction of shell sand concrete is most intense in the early stage and gradually slows down in the later stage.

[0121] By observing the shell sand concrete (Example 1) and ordinary river sand concrete (Comparative Example 1) with the same mix ratio, it can be found that the compressive strength of the two is not much different in the first 28 days. Since the shell sand aggregate can play an "internal curing" role, the compressive strength of the Example 1 group is significantly higher than that of the Comparative Example 1 group at 90 days. Under the same mix ratio, the shell sand concrete prepared by the present invention and the ordinary river sand concrete have the same parameters such as the amount of cementitious materials and the water-cement ratio. However, since the surface of the shell sand aggregate is rougher than that of ordinary river sand, it makes it more tightly connected with cement slurry, hydration products, etc., and the compressive strength is better than that of the river sand concrete with the same mix ratio.

[0122] Figure 7 The relationship between the splitting tensile strength and curing age of each group of concrete prepared by the present invention is given. It can be found that the change trend of the splitting tensile strength of each group of concrete with the curing age is generally consistent with the compressive strength. In addition, the same as the rule of ordinary river sand concrete, the splitting tensile strength of the shell sand concrete prepared by the present invention is smaller than its compressive strength. For shell sand concrete (Example 1) and ordinary river sand concrete (Comparative Example 1) with the same mix ratio, the splitting tensile strength of shell sand concrete is significantly higher than that of ordinary river sand concrete in the initial stage of hydration, and the difference between the two is not much in the later stage. This finding is slightly different from the aforementioned compressive strength change rule. At 3d and 7d, the splitting tensile strength of the shell sand concrete of Example 1 group is better than that of the ordinary river sand concrete of Comparative Example 1 group. At 28d, the splitting tensile strengths of the concrete of Example 1 group and Comparative Example 1 group are 2.83MPa and 2.88MPa, respectively. As the hydration process progresses, at 90 days, the splitting tensile strengths of the concrete of Example 1 and Comparative Example 1 are 3.04 MPa and 3.02 MPa, respectively.

[0123] Figure 8 The non-steady-state chloride ion migration coefficient D of each group of concrete prepared in the present invention at 28d and 90d RCM .Depend on Figure 8 It can be seen that at the age of 28 days, the chloride ion migration coefficient of the shell sand concrete of Example 1 is significantly lower than that of the ordinary river sand concrete of Comparative Example 1 with the same mix ratio. This may be because the porous physical properties of shell sand make it have a certain adsorption capacity for harmful ions, which is relatively beneficial to reinforced concrete structures in complex marine environments. As the hydration process advances, the hydration products continue to fill internal pores, cracks and other defects, resulting in an increase in the density of the concrete and an increase in the ability to resist external chloride ion erosion. Therefore, compared to the age of 28 days, the chloride ion migration coefficients of each group of concrete at 90 days are greatly reduced. The chloride ion migration coefficient of Comparative Example 1 group 90 days decreased by 62.8% compared to 28 days, and the groups of Example 1, Example 2 and Example 3 also decreased by 53.2%, 50.0% and 49.3% respectively. At the age of 90 days, the chloride ion migration coefficients of the concrete of Comparative Example 1 group and Example 1 group are not much different. The chloride ion migration coefficient of the shell sand concrete prepared by the present invention decreases with the increase of age, that is, the longer the age, the better the chloride ion penetration resistance of the shell sand concrete. In addition, whether it is 28d or 90d, the higher the strength of the shell sand concrete, the smaller its chloride ion migration coefficient. This is similar to the law of ordinary river sand concrete. According to the analysis of mechanical property related results, the lower the strength, the looser and more porous the shell sand concrete is, thereby providing more penetration channels for chloride ions, making the concrete's chloride ion penetration resistance worse.

[0124] The preferred embodiments of the present invention are described in detail above; however, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all belong to the protection scope of the present invention.

Claims

1. A shell sand concrete, characterized in that: By weight, it includes the following components: 550-800 parts of shell sand 650-850 parts of coarse aggregate 250-450 parts of cement Auxiliary cementitious materials 250-450 parts 200-300 parts water Water reducing agent 0~10 parts Among them, the sand ratio range is 0.40~0.

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2. The shell sand concrete according to claim 1, characterized in that: The particle size of shell sand is less than 5mm.

3. The shell sand concrete according to claim 1, characterized in that: The coarse aggregate is crushed stone with continuous grading of particle size from 5 to 25 mm.

4. The shell sand concrete according to claim 1, characterized in that: The cement is one or a combination of silicate cement or ordinary silicate cement; the auxiliary cementitious material is fly ash.

5. The shell sand concrete according to claim 1, characterized in that: The water reducer is a powdered naphthalene-based high-efficiency water reducer. When the weight of the water reducer is greater than 0, the proportion of the water reducer is 0.5% to 1% of the total weight of the cement and the auxiliary cementitious materials.

6. A method for preparing shell sand concrete, characterized in that: The steps include: Step 1, preparing shell sand: washing the discarded shells to remove impurities, drying them naturally, crushing and sieving them to obtain shell sand; Step 2, weighing raw materials: weigh the following raw materials in parts by weight: 550-800 parts of shell sand, 650-850 parts of coarse aggregate, 250-450 parts of cement, 250-450 parts of auxiliary cementitious material, 200-300 parts of water and 0-10 parts of water reducing agent; Step 3, pre-absorption of shell sand: the 200-300 parts of water weighed in step 2 are divided into pre-absorption water and mixing water; wherein the pre-absorption water is determined according to the shell sand content and the optimal pre-absorption rate; the shell sand is pre-absorbed with the pre-absorption water; Step 4, dry mixing: put the cement, auxiliary cementitious material and coarse aggregate weighed in step 2 into a mixer and stir and mix to obtain a dry mix; Step 5, wet mixing: first, mix the water reducing agent weighed in step 2 with the mixing water to form mixing water; then, mix the shell sand pre-absorbed in step 3, the dry mix obtained in step 4 and the mixing water to form a slurry; Step 6: Shape and cure the slurry to obtain shell sand concrete.

7. The method for preparing shell sand concrete according to claim 6, characterized in that: In step 2, the ratio of shell sand to the sum of shell sand and coarse aggregate is formed as the sand ratio; the optimal value of the sand ratio and the optimal pre-water absorption rate in step 3 are both obtained through orthogonal experimental design, and slump and compressive strength tests are performed on each group of test samples.

8. The method for preparing shell sand concrete according to claim 7, characterized in that: The influencing factors in orthogonal experimental design include cement dosage, pre-water absorption rate, net w / b and sand ratio.

9. The method for preparing shell sand concrete according to claim 7, characterized in that: Increasing the sand ratio can increase the slump of shell sand concrete and reduce the compressive strength. Therefore, the optimal value of the sand ratio is 0.45-0.

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10. The method for preparing shell sand concrete according to claim 7, characterized in that: The increase in pre-water absorption rate can reduce the compressive strength of shell sand concrete, but when the cement dosage is constant, the initial slump and time loss of shell sand concrete with a pre-water absorption rate of a% are relatively compromised; therefore, the optimal pre-water absorption rate is a%, where a% is half of the saturated water absorption rate of shell sand.

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