RU2769164C1 - Expansion admixture for cement containing steel slag - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention relates to an expanding admixture for tensioning and expanding cements. The expanding additive includes blast-furnace granulated slag, gypsum stone and alumina slag, and additionally contains iron-containing pulverized waste with a particle size of 1-200 microns. To prepare the mixture, components with a moisture content of 1–5% are used.

EFFECT: invention improves the strength characteristics of cements.

2 cl, 2 tbl

Description

The invention relates to the technology of mixed binders and can be used in the manufacture of expanding cements.

Expandable cement is a product obtained by thoroughly mixing cement and expanding additives. A distinctive property of expanding cements is their ability to expand during setting and hardening. The expansion of cement stone is based on the growth of crystals of calcium hydrosulfoaluminate formed during their hardening.

The introduction of an expanding additive during the preparation of a cement mixture regulates the expansion energy of the binder, which makes it possible to obtain cements with both compensated shrinkage and straining cements with different self-stress energies.

Expanding additives exclude shrinkage of the cement stone during its hardening, provide linear expansion of the cement stone.

Expanding cement is in demand due to the following operational advantages: increased adhesive properties and uniform expansion, contributing to the tight fit of the mortar to the base and the gradual filling of voids and cracks.

An expanding binder for cement is known from the prior art, containing dihydrate gypsum, blast-furnace granulated slag, cupola slag, iron-containing waste from sheet-rolling production, liquid glass, filter-press lime-containing waste from sugar production, quicklime, alumina-containing waste from an electroplating shop of aluminum production and cement clinker [Author's certificate No. 1413072, IPC С04B7/14, 1988].

The disadvantage of this invention is the complex composition of the expanding additive and the instability of its expanding properties, due to the unstable activity of lime, which quickly carbonizes in air and loses its activity, which can adversely affect the cement expansion process.

The closest in technical essence to the proposed technical solution is an expanding additive to cement, selected as the closest prototype analogue, including, wt.%: alumina slag 4 - 6, gypsum dihydrate 22 - 24, granulated blast furnace slag 60 - 69, production waste wet enrichment of iron ore based on SiO ₂ 5 - 10 [Patent RU No. 2049081, IPC C04B28/04, C04B28/04, C04B22/06, C04B7/00, 1995].

The disadvantage of this invention is that high physical and mechanical properties of density, compressive strength and tensile strength in bending, as well as self-stress in cement based on such an additive are achieved only at high temperatures up to 90°C. However, at room temperature of 20°C, the physical and mechanical properties are much lower.

The technical result of the claimed invention is to obtain an expanding additive used for the production of stressing and expanding cements, which improves the physical and mechanical characteristics of cement containing such an additive, compared with the prototype at room temperature 20 ° C, as well as the possibility of recycling unclaimed waste from steelmaking.

The specified technical result is achieved by the fact that the expansion additive to cement, including blast-furnace granulated slag, gypsum stone and alumina slag, and according to the invention, additionally contains steelmaking slag stabilized to prevent silicate decomposition with granular dust from gas cleaning of electric arc steelmaking furnaces, containing iron oxides Fe2O3 not less than 45%, in the amount of 2-5% by weight of the processed slag, and microsilica in the following ratios, wt. %:

blast furnace granulated slag 52.0–75.0 gypsum stone 18.0–22.0 aluminous slag 3.0–10.0 steel slag, stabilized to prevent silicate decay granulated gas cleaning dust electric arc steel furnaces 3.5–12.0 microsilica 0.5-4.0

The essence of the invention.

The essence of obtaining an expanding cement stone is the manifestation and regulation of its own stresses. In order for self-stresses to lead to a significant expansion without destroying the bonds between the elements of the porous structure, an expanding additive is introduced into the cement composition.

The proposed composition of the expanding additive differs from the known one in that it additionally contains steelmaking slag, stabilized to prevent silicate decomposition with granular dust from the gas cleaning of electric arc steelmaking furnaces with an iron oxide Fe ₂ O ₃ content of at least 45%, in an amount of 2-5% by weight of the processed slag .

The use of the aforementioned steelmaking slag as part of an expansion additive for cement helps to increase the density and strength of the cement.

As is known, blast-furnace metallurgical slags have long been used in the production of building materials as a component of Portland slag cement, while the use of steel-smelting slags is much smaller, which is primarily due to the heterogeneity of the composition of steel-smelting slags, the variability of their physical and mechanical properties and the tendency to silicate decomposition.

At the same time, steelmaking slags are highly active, which makes them an excellent raw material for the production of cement binders. The ability to expand and self-compact allows the use of steel slags to obtain expanding cement mixtures.

It is known that the iron content in the converter and open-hearth slags of steelmaking is very high (45-65 wt.%). At the same time, the chemical composition of slags from gas cleaning of electric furnaces is not constant, since it depends on the grade of steel being smelted and is characterized by a lower iron content, as well as the presence of non-ferrous metal impurities.

Below is the average composition of the slag from gas cleaning of electric steel-smelting furnaces, wt.%:

Fe 30-55; SiO ₂ 2-12; Al ₂ O ₃ 0.3-10.0; CaO 1.5-17.0; MgO 5-27; MnO 1.5-5.5; Р2О5 0.02-0.25; Cr up to 10; Ni up to 8; Zn up to 2; Pb up to 1.

Granulation is mainly subjected to blast-furnace slags. The bulk of granulated blast-furnace slag is supplied to the production of Portland slag cement. They are also used to obtain clinker-free binders, slag-alkaline concretes, mineral wool, slag-ceramic products, as a filler in cement and asphalt concretes.

Steel-smelting slags of silicate decomposition are characterized by a high basicity modulus, therefore, when they are cooled, they almost completely crystallize and contain almost no glass. These slags are not granulated, but merge into dumps, where they slowly cool down.

Further, slags, being in dumps, break up into pieces due to calcareous decomposition into pieces of different sizes. Despite the fact that the mechanical properties of such slags are quite high, for example, the strength of open-hearth slags is 80–150 MPa, they withstand more than 200 frost resistance test cycles. Lump slags have a dense structure, the average density is 3100-3400 kg/m ³ . Compressive strength 60 - 130 MPa.

In this regard, at the moment, the disposal of unclaimed waste from steelmaking is relevant.

Steel slags are not resistant to silicate decomposition. Silicate decomposition in steelmaking slags is due to the fact that the mineral dicalcium silicate from an unstable form (γ2CaO SiO ₂ ) at a temperature below 525 ° C passes into a stable state (2CaO SiO ₂ ), which is accompanied by an increase in its volume by 10%, as a result rock cracks and breaks into pieces.

The effect on silicate decomposition is exerted by the cooling temperature of the melt. It is possible to prevent silicate decomposition by its rapid cooling and granulation. When using self-disintegrating slags in production as a filler, their stabilization is required, i.e., a change in the chemical and mineralogical compositions to obtain a stable crystalline structure.

In the claimed invention, as part of an expanding additive, steelmaking slag is used, stabilized to prevent silicate decomposition by granular dust from gas cleaning of electric arc steelmaking furnaces with an iron oxide content of Fe2O3 of at least 45%, in an amount of 2-5% by weight of the processed slag, as described in the RU patent 2752914 [IPC C04B5/06, priority dated 07/29/2020]

The chemical composition of blast-furnace slags is represented mainly by four oxides: CaO (29-30%), MgO (0-18%), Al ₂ 0 ₃ (5-23%) and Si0 ₂ (30-40%). In a small amount they contain oxides of iron (0.2-0.6%) and manganese (0.3-1%), as well as sulfur (0.5-3.1%).

Most blast-furnace slags are characterized by basicity (CaO/SiO ₂ ) in the range of 1.0-1.2 and content of Al ₂ 0 ₃ and MgO 3-20 and 5-15.

Depending on the ratio of CaO / Si0 ₂ and (CaO + MgO) / Si0 ₂ , slags are divided into basic and less basic.

At the Cherepovets Metallurgical Plant, when smelting low-sulfur iron, blast-furnace slag is less basic, containing: MgO is 10-12%, and the ratio of CaO / Si0 ₂ and (CaO + MgO) / Si0 ₂ is 1 and 1.3.

It follows from the above that the chemical composition of blast-furnace slag makes it possible to use it instead of clay and part of the carbonate components in the composition of the raw mixture of the expanding additive.

Gypsum stone is a rock of sedimentary origin with impurities of dolomite, anhydride, iron hydroxide, sulfur, quartzite, consisting mainly of the mineral CaSO ₄ ⋅ 2H ₂ O.

The main purpose of adding gypsum to cement binders is to slow down the hydration process of the cement after it has been mixed with water.

The process associated with cement hydration is as follows. Water is added to the cement, after which the cement begins to harden. The hardening process is very short, so the cement is constantly stirred.

When gypsum and water are added to cement, a reaction occurs with the cations of the particles of the aluminates included in the cement with semi-aqueous gypsum (gypsum stone or calcium hydroxide).

As a result, calcium hydrosulfoaluminate, or ettringite, is formed - a mineral with a whitish or yellowish color, a derivative of aluminum and calcium, sulfate.

Ettringite can initially be formed as very fine grained crystals that form a coating on the surface of the particles. These crystals are too small to bridge the gaps between cement particles. Therefore, the cement mixture remains plastic and suitable.

One of the main reasons for the decrease in the strength of cement is the late formation of ettringite under the influence of various environmental factors.

The primary formation of ettringite crystals in the initial phase of hydration has a constructive effect on the setting control effect, and later the formation of ettringite crystals causes destructive processes.

Introduction to the composition of the expanding additive microsilica - ultrafine material, consisting mainly of silicon dioxide SiO ₂ .

For the primary formation of ettringite crystals in the initial phase of hydration, microsilica is introduced into the composition of the expanding additive, according to the claimed invention - an ultrafine material consisting mainly of silicon dioxide SiO ₂ .

Microsilica is a substance consisting of colorless crystals, which has high strength, hardness and refractoriness, is resistant to acids and does not interact with water.

Amorphous microsilica is capable of reacting with calcium hydroxide at normal temperature to form tobermorite-like calcium hydrosilicates, which ensures the strength and durability of the cement stone.

The alumina slag used in the claimed composition is a high alumina blast furnace slag containing, wt. %: SiO ₂ 7-12; Al ₂ O ₃ 44-50; CaO 40-43; S 0.7-1.0, MgO 1-3, FeO 0.2-0.5.

The use of aluminous slag in the composition contributes to the achievement of the normative strength of cement within a short time after mixing with water.

In the closest analogue, selected as a prototype, to obtain good physical and mechanical properties of cement samples, including an expanding additive, the samples are heated to 90°C. This effect is due to the fact that ettringite is unstable at elevated temperatures and at temperatures above 80-90°C it turns into the monosulfate form 3CaO⋅Al ₂ O ₃ ⋅CaSO ₄ ⋅12H ₂ O (С3А^Н12), which excludes later formation of ettringite crystals , which promotes the formation of cracks in hardened cement and an increase in strength characteristics.

In the claimed invention, the synergy lies in the fact that the addition of steelmaking slag stabilized to prevent silicate decomposition by granular dust from gas cleaning of electric arc steelmaking furnaces, with an iron oxide Fe ₂ O ₃ content of at least 45%, in an amount of 2-5% of the mass of the processed slag, and microsilica, without heating the finished cement samples to 90°C, contributed to the improvement of physical and mechanical properties compared to the prototype.

An example of the implementation of the claimed invention.

Granulated blast-furnace slag, gypsum stone, alumina slag, steel-smelting slag stabilized to prevent silicate decomposition, granulated dust from gas cleaning of electric arc steel-smelting furnaces, microsilica were dried to a residual moisture content of 1-5%, with a dosage in the above ratios, crushed in a ball mill to a specific surface 3500±200 cm ² /g.

The composition of the expanding additive is presented in table 1.

Table 2 presents the physical and technical parameters of cement samples after 30 days of hardening, obtained using the claimed composition of the expanding additive.

To obtain samples, an expanding additive in an amount of 20% was mixed with Portland cement M400 and the self-stress of the obtained samples was determined.

To determine the ultimate strength in bending, the bending tests of specimens were carried out on the corresponding test equipment on an MII-100 machine.

The ultimate strength in bending (MPa) was recorded by the counter of the device. The test result was calculated as the arithmetic mean of the two largest test results of the three samples.

The halves of the samples obtained after bending tests were immediately tested for compressive strength. The tests were carried out on a hydraulic press MS-100.

The arithmetic mean of the test results of six halves of the samples was taken as the compressive strength.

As can be seen from table 2, the introduction of the claimed expanding additives allows you to obtain samples of expanding cements, with increased physical and technical parameters at room temperature of 20°C relative to the same indicators of the prototype: density - 2620 kg/m ² ; compressive strength - up to 48.4 MPa; tensile strength in bending - up to 7.10 MPa; self-stress - up to 4.6 MPa.

Table 1

Compound The content of components, wt.% one 2 3 4 Blast furnace granulated slag 52.0 69.0 63.0 75.0 gypsum stone 22.0 19.0 20.0 18.0 aluminous slag 10.0 5.0 7.0 3.0 steelmaking slag stabilized to prevent silicate decomposition by granular gas cleaning dust of electric arc steelmaking furnaces 12.0 5.5 8.0 3.5 microsilica 4.0 1.5 2 0.5

Expandable Additive Compositions

table 2

Comparison of physical and technical strength indicators

Physical and technical indicators at a temperature of 20 ° C Lineups one 2 3 4 Prototype Density kg / m ² 2550 2500 2450 2620 2400 Compressive strength, MPa 46.70 48.10 48.40 47.60 41.25 Tensile strength in bending, MPa 6.9 6.85 6.90 7.10 6.75 Self-stress, MPa 4.1 2.9 4.0 4.6 2.3

Claims (3)

1. Expanding additive for cement, including granulated blast-furnace slag, gypsum stone and alumina slag, characterized in that it additionally contains steelmaking slag stabilized to prevent silicate decomposition with granular dust from gas cleaning of electric arc steelmaking furnaces, with an iron oxide content of Fe ₂ O ₃ not less than 45%, in the amount of 2-5% by weight of the processed slag, and microsilica at the following ratios of components, wt. %: blast furnace granulated slag 52.0–75.0 gypsum stone 18.0–22.0 aluminous slag 3.0–10.0 steel slag, stabilized to prevent silicate decay granulated gas cleaning dust electric arc steel furnaces 3.5–12.0 microsilica 0.5–4.0 2. Expanding additive to cement according to claim 1, characterized in that all components have a moisture content of 1–5%.