Phosphogypsum-based multi-solid-waste recycled aggregate as well as preparation method and application thereof

Abstract

The invention relates to the technical field of industrial solid waste recycling, and provides ardealite-based multi-solid-waste recycled aggregate as well as a preparation method and application thereof. The ardealite is used as a main raw material, and the fly ash, the steel slag and the fibers are used as auxiliary materials to perform a synergistic effect, so that the multi-solid-waste recycled aggregate is obtained. The recycled aggregate provided by the invention has a good filling effect on pores, has relatively small density and higher strength, can improve the compressive and flexural strength and smooth degree of a product, has a high replacement rate on natural gravel, and is excellent in performance, environment-friendly, simple in preparation method and low in cost. According to the method, high-valued utilization of various solid wastes can be realized, the utilization rate of the solid wastes is increased, and the concept of sustainable development is met.

Description

Technical Field

[0001] The present invention relates to the technical field of industrial solid waste resource utilization, and in particular to a phosphogypsum-based multi-solid waste recycled aggregate, and a preparation method and application thereof. Background Art

[0002] The development and utilization of mineral resources have accelerated social development, but at the same time, a large amount of industrial solid waste has also accumulated. The accumulation of solid waste has changed the natural landscape, affected the surrounding ecological balance and natural environment, and also had an impact on groundwater resources. In the process of human production and life, a large amount of solid waste with residual utilization value but difficult to directly use in the production and life process is generated. For example, phosphogypsum is an industrial solid waste generated in the process of wet phosphoric acid; fly ash is a solid waste captured in the flue gas of thermal power plants; steel slag is a by-product of the steelmaking process; waste chemical fiber is an important component of domestic waste. At present, the comprehensive utilization rate of the above-mentioned industrial solid wastes is relatively low. Therefore, it is very important to utilize industrial solid waste as a resource.

[0003] A Chinese invention patent with publication number CN116283118A discloses the preparation and application of a pavement base material based on the synergistic stabilization of phosphogypsum and recycled aggregate by an organic-inorganic cementitious material. The pavement base material is prepared with cement-based cementitious material, calcined phosphogypsum, natural fine aggregate, recycled coarse aggregate, sodium silicate, calcium stearate emulsion, polymer emulsion and water as main raw materials. While efficiently utilizing solid waste as a resource, it effectively ensures the obtained pavement base material has excellent mechanical strength and long-term stability.

[0004] The Chinese invention patent with publication number CN1149566288A discloses a high-strength phosphogypsum-based recycled aggregate, the preparation method of which is: adding the mixed material formed by adding water after mixing the materials into a ring die granulator, using a ring die and a pressure roller to press and shape it, and then performing carbonization curing and natural curing. It can replace natural crushed stone and is beneficial to energy saving and environmental protection.

[0005] The Chinese invention patent with publication number CN117964271A discloses a carbon dioxide-cured phosphogypsum-based cementitious material and a preparation method thereof, which can efficiently and energy-savingly cure solid waste cementitious materials, and the obtained cured cementitious materials have good mechanical properties, which can provide technical support for the recycling and reuse of solid waste materials.

[0006] Although the above methods all have certain utilization of solid waste, there are problems such as low solid waste replacement rate, complex process maintenance, and poor practical application effect. In addition, some methods require calcination or granulation to improve performance, which has high material costs and is not conducive to large-scale utilization of solid waste. Summary of the invention

[0007] In view of this, the present invention provides a phosphogypsum-based multi-solid waste recycled aggregate and its preparation method and application. The present invention uses phosphogypsum as the main raw material and uses multiple solid wastes to synergistically prepare recycled aggregate. The obtained recycled aggregate has a high replacement rate for natural sand and gravel, and the prepared road base material has high strength and low ecological risk. The preparation method is simple, which makes up for the performance defects of a single solid waste as recycled aggregate, and can realize large-scale resource utilization of solid waste.

[0008] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0009] A phosphogypsum-based multi-solid waste recycled aggregate comprises a dry mix and fibers; the dry mix comprises phosphogypsum, fly ash and steel slag; the mass ratio of the phosphogypsum, fly ash and steel slag is 1-2:1-2:1-2; the mass of the fibers is 0.5-3% of the mass of the dry mix.

[0010] Preferably, the CaSO in the phosphogypsum 4 ·2H 2 The mass percentage of O is ≥80%; the particle sizes of the phosphogypsum, fly ash and steel slag are independently less than 0.075 mm.

[0011] Preferably, the composition of the steel slag includes SiO 2 The slag has a bulk density of 2.0 to 3.5 tons / cubic meter, a moisture content of 3 to 8%, a porosity of 10% to 30%, and a specific surface area of ​​4000 to 6000 cm 2 / g.

[0012] Preferably, the SiO in the fly ash 2 The mass percentage is ≥50%; the fly ash has a bulk density of 0.5-1.0 kg / L, a porosity of 60-70%, and a specific surface area of ​​2000-4000 cm 2 / g.

[0013] Preferably, the fiber is a monofilament fiber obtained by crushing waste chemical fiber textiles; the length of the fiber is 5 to 20 mm.

[0014] The present invention also provides a method for preparing the phosphogypsum-based multi-solid waste recycled aggregate described in the above scheme, comprising the following steps:

[0015] dry-mixing phosphogypsum, fly ash and steel slag to obtain a dry mix;

[0016] The dry mixed material and fiber are mixed to obtain the phosphogypsum-based multi-solid waste recycled aggregate.

[0017] Preferably, the dry mixing time is 1 to 5 minutes.

[0018] Preferably, an activator is added during the dry mixing or when the dry mix and the fiber are mixed, and the activator is calcium hydroxide and sodium silicate; the mass ratio of calcium hydroxide and sodium silicate is 1-2:1-2; the mass of the activator is 1-5% of the total mass of the phosphogypsum, fly ash and steel slag.

[0019] The present invention also provides the use of the phosphogypsum-based multi-solid waste recycled aggregate described in the above scheme or the phosphogypsum-based multi-solid waste recycled aggregate prepared by the preparation method described in the above scheme in concrete; the replacement rate of the dry mix in the phosphogypsum-based multi-solid waste recycled aggregate for natural sand and gravel in concrete is 45 to 90%.

[0020] Preferably, in the phosphogypsum-based multi-solid waste recycled aggregate, the replacement rate of phosphogypsum to natural sand and gravel in concrete is 15-30%, the replacement rate of fly ash to natural sand and gravel in concrete is 15-30%, and the replacement rate of steel slag to natural sand and gravel in concrete is 15-30%.

[0021] The present invention provides a phosphogypsum-based multi-solid waste recycled aggregate, including a dry mix and fiber; the dry mix includes phosphogypsum, fly ash and steel slag; the mass ratio of the phosphogypsum, fly ash and steel slag is 1-2:1-2:1-2; the mass of the fiber is 0.5-3% of the mass of the dry mix. The present invention uses phosphogypsum as the main raw material, and simultaneously supplements fly ash, steel slag and fiber for synergistic effect to obtain the multi-solid waste recycled aggregate of the present invention; wherein the sulfur content in phosphogypsum is high, and it can replace the sulfur-containing components in natural aggregate as a high-sulfur material in the recycled aggregate; the aluminum content in fly ash is relatively high, and it has the characteristics of volcanic ash reaction, which can optimize the hydration process of cementitious materials prepared from recycled aggregates; steel slag is a high-silicon material, and its high strength can replace the high-strength component in natural aggregate. The present invention combines phosphogypsum, fly ash and steel slag. After self-excitation by an activator, various solid wastes can undergo hydration and gelation reactions under the action of the excitation to produce more CSH and trisulfur hydrated calcium sulfoaluminate (Aft) and other components that can effectively improve the strength of the material. The network structure formed by hydration can effectively wrap up harmful substances such as heavy metals, phosphorus and fluorine in the solid waste, greatly reducing the leaching toxicity of the material; at the same time, chemical fibers are added to form effective traction inside the material, greatly improving the efficiency and uniformity of internal force conduction. Therefore, the road base material prepared using the multi-component solid waste of the present invention has high strength and low ecological risk.

[0022] In addition, the recycled aggregate provided by the present invention has a high replacement rate for natural sand and gravel, good filling effect on pores, and low density. In practical applications, it can improve the compressive and flexural strength and smoothness of the product and reduce the transportation pressure of the material. In addition, the present invention makes full use of the characteristics of high tensile strength of waste fibers, solves the problem that waste fibers are of many types, mixed components, and difficult to separate and regenerate; at the same time, the present invention makes full use of favorable resources such as calcium, iron, aluminum, and silicon in industrial solid wastes such as phosphogypsum, steel slag, and fly ash, and can realize the high-value utilization of various solid wastes, improve the utilization rate of solid wastes, and conform to the concept of sustainable development.

[0023] The present invention also provides a method for preparing the phosphogypsum-based multi-solid waste recycled aggregate described in the above scheme. In the present invention, phosphogypsum, fly ash and steel slag are first dry-mixed, and the obtained dry-mixed material is then mixed with fiber to obtain the phosphogypsum-based multi-solid waste recycled aggregate. The preparation method of the phosphogypsum-based multi-solid waste recycled aggregate of the present invention is simple, does not require calcination and granulation, has low cost, and is conducive to the large-scale resource utilization of solid waste. DETAILED DESCRIPTION

[0024] The present invention provides a phosphogypsum-based multi-solid waste recycled aggregate, comprising a dry mix and fibers; the dry mix comprises phosphogypsum, fly ash and steel slag; the mass ratio of the phosphogypsum, fly ash and steel slag is 1-2:1-2:1-2; the mass of the fibers is 0.5-3% of the mass of the dry mix.

[0025] In the present invention, the mass ratio of the phosphogypsum, fly ash and steel slag can be specifically 1:1:1, 4:3:3 or 5:3:3.

[0026] In the present invention, the CaSO 4 ·2H 2 The mass percentage of O is preferably ≥80%, specifically 80%, 85% or 90%; the particle size of the phosphogypsum is preferably less than 0.075 mm; in a specific embodiment of the present invention, the phosphogypsum can be directly used after passing through a 200-mesh standard sieve.

[0027] In the present invention, the SiO 2 The mass percentage of is preferably ≥50%, specifically 50%, 55% or 65%; the bulk density of the fly ash is preferably 0.5-1.0 kg / L, specifically 0.5 kg / L, 0.8 kg / L, 1.0 kg / L; the porosity is preferably 60%-70%, specifically 60%, 65% or 70%; the specific surface area is preferably 2000-4000 cm 2 / g, specifically 2000cm 2 / g、2500cm 2 / g、3000cm2 / g or 4000cm 2 The present invention controls the bulk density, porosity and specific surface area of ​​fly ash within the above ranges, which can most effectively approach the strength and other performance indicators in the embodiments, and is conducive to controlling the performance error of the product in actual production.

[0028] In the present invention, the particle size of the fly ash is preferably less than 0.075 mm; in a specific embodiment of the present invention, the fly ash can be directly used after passing through a 200-mesh standard sieve.

[0029] In the present invention, the composition of the steel slag includes 10-25wt% SiO 2 and 40-60wt% CaO; the bulk density of the steel slag is preferably 2.0-3.5 tons / cubic meter, specifically 2.0 tons / cubic meter, 3 tons / cubic meter or 3.5 tons / cubic meter, the moisture content is preferably 3%-8%, specifically 3%, 5%, 6% or 8%, the porosity is preferably 10%-30%, specifically 10%, 15%, 20%, 25% or 30%, and the specific surface area is preferably 4000-6000cm 2 / g, specifically 4000cm 2 / g、5000cm 2 / g or 6000cm 2 The present invention controls the specific gravity, moisture content, porosity and specific surface area of ​​the steel slag within the above ranges, which can most effectively approach the strength and other performance indicators in the embodiments, and is conducive to controlling the performance error of the product in actual production.

[0030] In the present invention, the particle size of the steel slag is preferably less than 0.075 mm; in a specific embodiment of the present invention, the steel slag is preferably crushed and then passed through a 200-mesh standard sieve for use; the crushing is preferably carried out in a crusher, and the crushing time is preferably 90 seconds.

[0031] In the present invention, the mass of the fiber is preferably 0.5% to 3% of the mass of the dry blend, specifically 0.5%, 1%, 1.5%, 2% or 3%; the fiber is preferably a monofilament fiber obtained by crushing waste chemical fiber textiles; the length of the fiber is preferably 5 to 20 mm, specifically 5 to 10 mm or 15 to 20 mm; the waste chemical fiber textiles can specifically be polyester, spandex, etc.; the crushing is preferably carried out in a fiber crusher.

[0032] The present invention also provides a method for preparing the phosphogypsum-based multi-solid waste recycled aggregate described in the above scheme, comprising the following steps:

[0033] dry-mixing phosphogypsum, fly ash and steel slag to obtain a dry mix;

[0034] The dry mixed material and fiber are mixed to obtain the phosphogypsum-based multi-solid waste recycled aggregate.

[0035] The present invention dry-mixes phosphogypsum, fly ash and steel slag to obtain a dry mix. In the present invention, the dry-mixing time is preferably 1 to 5 minutes, more preferably 2 to 3 minutes; the dry-mixing is preferably carried out in a mixer; the mixer is preferably a cement mortar mixer; the dry-mixing method is preferably low-speed stirring, and the rotation speed of the low-speed stirring is preferably 60 to 90 rpm.

[0036] In the present invention, the phosphogypsum-based multi-solid waste recycled aggregate is activated when used. Specifically, an activator can be added during dry mixing or when the dry mix is ​​mixed with fiber. The activator is preferably calcium hydroxide and sodium silicate; the mass ratio of calcium hydroxide and sodium silicate is preferably 1-2:1-2, more preferably 1:1; the purity of calcium hydroxide and sodium silicate is preferably analytically pure; the mass of the activator is preferably 1-5% of the total mass of the phosphogypsum, fly ash and steel slag, more preferably 2%.

[0037] After obtaining the dry mix, the present invention mixes the dry mix with fibers to obtain the phosphogypsum-based multi-solid waste recycled aggregate. The present invention has no special requirements on the mixing method of the dry mix and the fibers, as long as uniform mixing can be achieved.

[0038] The present invention provides the use of the phosphogypsum-based multi-solid waste recycled aggregate described in the above scheme or the phosphogypsum-based multi-solid waste recycled aggregate prepared by the preparation method described in the above scheme in pavement base materials; in the present invention, the phosphogypsum-based multi-solid waste recycled aggregate is specifically used to replace natural sand and gravel in pavement base materials, and the total replacement rate of the dry mixture in the phosphogypsum-based multi-solid waste recycled aggregate to the natural sand and gravel in the pavement base materials is 45-90%. Specifically, in the phosphogypsum-based multi-solid waste recycled aggregate, the replacement rate of phosphogypsum to the natural sand and gravel in the pavement base materials is preferably 15-30%, the replacement rate of fly ash to the natural sand and gravel in the pavement base materials is preferably 15-30%, and the replacement rate of steel slag to the natural sand and gravel in the pavement base materials is preferably 15-30%.

[0039] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] In the following examples, the fiber preparation method is as follows: chemical fiber textiles are put into a fiber crusher for crushing to obtain monofilament fibers with a length of 5 to 20 mm; CaSO 4 ·2H2 The mass percentage of O is 96%; the bulk density of steel slag is 2.8 tons / cubic meter, the moisture content is 4%, the porosity is 28%, and the specific surface area is 5120cm 2 / g; SiO in fly ash 2 The mass percentage is 77%, the bulk density is 0.8kg / L, the porosity is 62%, and the specific surface area is 2530cm 2 / g.

[0041] Example 1

[0042] (1) After the steel slag is crushed, it is passed through a 200-mesh standard sieve for later use.

[0043] (2) Pass fly ash and phosphogypsum through a 200-mesh standard sieve for later use.

[0044] (3) Based on the assumption that the replacement rate of phosphogypsum, steel slag and fly ash for natural sand and gravel is 15%, phosphogypsum, steel slag and fly ash are weighed in a mass ratio of 1:1:1 and added into a mixer in sequence, and dry-mixed for 3 minutes to obtain a dry mix.

[0045] (4) Adding 1.5% of fiber (length) of the total mass of the dry mix and 2% of an activator of the total mass of the dry mix into the dry mix, wherein the activators are calcium hydroxide and sodium silicate, and the mass ratio of calcium hydroxide to sodium silicate is 1:1, and mixing them well to obtain phosphogypsum-based multi-solid waste recycled aggregate.

[0046] The prepared phosphogypsum-based multi-solid waste recycled aggregate is made into cement mortar specimens. The preparation of the cement mortar specimens is carried out according to the method in GB / T 17671-2021, and the phosphogypsum-based multi-solid waste recycled aggregate is used to replace part of the standard sand. The replacement rate of phosphogypsum, steel slag and fly ash in the phosphogypsum-based multi-solid waste recycled aggregate for natural sand and gravel is 15%, and the total replacement rate is 45%.

[0047] The strength of cement mortar specimens was tested according to the method in GB / T 17671-2021. The cement model used was P·S·A32.5. The results are shown in Table 1. The bulk density of cement mortar specimens was tested. The results are shown in Table 1. The toxicity of cement mortar specimens was tested according to the method in HJ557-2010. The results are shown in Table 2.

[0048] Example 2

[0049] (1) After the steel slag is crushed, it is passed through a 200-mesh standard sieve for later use.

[0050] (2) Pass fly ash and phosphogypsum through a 200-mesh standard sieve for later use.

[0051] (3) According to the substitution rate of phosphogypsum for natural sand and gravel of 20%, the substitution rate of steel slag for natural sand and gravel of 15%, and the substitution rate of fly ash for natural sand and gravel of 15%, phosphogypsum, steel slag and fly ash were weighed in a mass ratio of 4:3:3 and added into a mixer in sequence, and dry-mixed for 3 minutes to obtain a dry mixture.

[0052] (4) Adding 1.5% of fiber (length) of the total mass of the dry mix and 2% of an activator of the total mass of the dry mix into the dry mix, wherein the activators are calcium hydroxide and sodium silicate, and the mass ratio of calcium hydroxide to sodium silicate is 1:1, and mixing them well to obtain phosphogypsum-based multi-solid waste recycled aggregate.

[0053] The obtained recycled aggregate is made into cement mortar specimens and the performance is tested. The preparation method of the cement mortar specimens is the same as that in Example 1. The replacement rates of phosphogypsum, steel slag and fly ash in the phosphogypsum-based multi-solid waste recycled aggregates for natural sand and gravel are 20%, 15% and 15%, and the total replacement rate is 50%.

[0054] The bulk density and strength test results of the cement mortar specimens are shown in Table 1, and the toxicity test results are shown in Table 2.

[0055] Example 3

[0056] (1) After the steel slag is crushed, it is passed through a 200-mesh standard sieve for later use.

[0057] (2) Pass fly ash and phosphogypsum through a 200-mesh standard sieve for later use.

[0058] (3) According to the calculation that the replacement rate of phosphogypsum for natural sand and gravel is 25%, the replacement rate of steel slag for natural sand and gravel is 15%, and the replacement rate of fly ash for natural sand and gravel is 15%, phosphogypsum, steel slag and fly ash are weighed in a mass ratio of 5:3:3 and added into a mixer in sequence, and dry-mixed for 3 minutes to obtain a dry mixture.

[0059] (4) Adding 1.5% of fiber (length) of the total mass of the dry mix and 2% of an activator of the total mass of the dry mix into the dry mix, wherein the activators are calcium hydroxide and sodium silicate, and the mass ratio of calcium hydroxide to sodium silicate is 1:1, and mixing them well to obtain phosphogypsum-based multi-solid waste recycled aggregate.

[0060] The prepared phosphogypsum-based multi-solid waste recycled aggregate was made into cement mortar specimens and the performance was tested. The preparation method of the cement mortar specimens was the same as that in Example 1. The replacement rates of phosphogypsum, steel slag and fly ash in the phosphogypsum-based multi-solid waste recycled aggregate for natural sand and gravel were 25%, 15% and 15% respectively, and the total replacement rate was 55%.

[0061] At the same time, natural sand and gravel were used as aggregates, and cement mortar specimens were prepared according to the method in GB / T 17671-2021 as a comparative experiment.

[0062] The bulk density and strength test results of the cement mortar specimens are shown in Table 1, and the toxicity test results are shown in Table 2.

[0063] Table 1 Bulk density and strength test results

[0064]

[0065] Table 2 Toxicity test results

[0066]

[0067] According to the data in Table 1, it can be seen that the early strength of the cement mortar specimens prepared by using the phosphogypsum-based multi-solid waste recycled aggregate instead of natural sand and gravel is equivalent to that of the comparative experiment, and the strength at 70 days exceeds that of the comparative experiment, indicating that the cement mortar specimens prepared by using the phosphogypsum-based multi-solid waste recycled aggregate of the present invention have excellent strength. According to Table 2, it can be seen that the leaching toxicity of phosphorus, cadmium, nickel and chromium in the cement mortar specimens prepared by the present application is significantly lower than that of the original phosphogypsum, indicating that the cement mortar specimens prepared by using the phosphogypsum-based multi-solid waste recycled aggregate of the present invention have low ecological risks.

[0068] In summary, the phosphogypsum-based multi-solid waste recycled aggregate provided by the present invention has a high replacement rate for natural sand and gravel. When applied to pavement base materials, the obtained material has high strength and low ecological risk, and can achieve efficient utilization of solid waste.

[0069] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A phosphogypsum-based multi-solid waste recycled aggregate, characterized in that: It comprises a dry mix and fibers; the dry mix comprises phosphogypsum, fly ash and steel slag; the mass ratio of the phosphogypsum, fly ash and steel slag is 1-2:1-2:1-2; the mass of the fibers is 0.5-3% of the mass of the dry mix.

2. The phosphogypsum-based multi-solid waste recycled aggregate according to claim 1, characterized in that: The mass percentage of CaSO4·2H2O in the phosphogypsum is ≥80%; and the particle sizes of the phosphogypsum, fly ash and steel slag are independently less than 0.075 mm.

3. The phosphogypsum-based multi-solid waste recycled aggregate according to claim 1, characterized in that: The steel slag comprises SiO2 and CaO; the bulk density of the steel slag is 2.0 to 3.5 tons / cubic meter, the moisture content is 3 to 8%, the porosity is 10 to 30%, and the specific surface area is 4000 to 6000 cm 2 / g.

4. The phosphogypsum-based multi-solid waste recycled aggregate according to claim 1, characterized in that: The mass percentage of SiO2 in the fly ash is ≥50%; the fly ash has a bulk density of 0.5-1.0 kg / L, a porosity of 60%-70%, and a specific surface area of ​​2000-4000 cm 2 / g.

5. The phosphogypsum-based multi-solid waste recycled aggregate according to claim 1, characterized in that: The fiber is a monofilament fiber obtained by crushing waste chemical fiber textiles; the length of the fiber is 5 to 20 mm.

6. The method for preparing phosphogypsum-based multi-solid waste recycled aggregate according to any one of claims 1 to 5, characterized in that: The following steps are involved: dry-mixing phosphogypsum, fly ash and steel slag to obtain a dry mix; The dry mixed material and fiber are mixed to obtain the phosphogypsum-based multi-solid waste recycled aggregate.

7. The preparation method according to claim 6, characterized in that: The dry mixing time is 1 to 5 minutes.

8. The preparation method according to claim 6, characterized in that: During the dry mixing or when the dry mixed material and the fiber are mixed, an activator is also added, and the activator is calcium hydroxide and sodium silicate; the mass ratio of the calcium hydroxide and the sodium silicate is 1-2:1-2; the mass of the activator is 1-5% of the total mass of the phosphogypsum, fly ash and steel slag.

9. Use of the phosphogypsum-based multi-solid waste recycled aggregate described in any one of claims 1 to 5 or the phosphogypsum-based multi-solid waste recycled aggregate prepared by the preparation method described in any one of claims 6 to 8 in pavement base materials; the total replacement rate of the dry mix in the phosphogypsum-based multi-solid waste recycled aggregate for natural sand and gravel in pavement base materials is 45 to 90%.

10. The use according to claim 9, characterized in that: In the phosphogypsum-based multi-solid waste recycled aggregate, the replacement rate of phosphogypsum to natural sand and gravel in road base materials is 15-30%, the replacement rate of fly ash to natural sand and gravel in road base materials is 15-30%, and the replacement rate of steel slag to natural sand and gravel in road base materials is 15-30%.

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