Claims:We Claim:
1. Processed non-metallic steel slag based particulate comprising abrasion based rounded/blunted shaped non-metallic steel slag particulates including selectively anyone or more of (i) sand particulates of substantially rounded /blunted particulates free of sharp edges as construction grade fine aggregates free of clay and silt (ii) Non-metallic steel slag larger particulates in the size range of > 4.75 mm suitable as grit for road making; (iii) Non-metallic steel slag finer particulates in the size range of <150µm suitable for cement manufacture.

2. Processed non-metallic steel slag based particulate as claimed in claim 1 wherein said construction grade fine aggregates have bulk density >1700 kg/m3preferably 1700-1900 Kg/m3,water absorption 1-3 % and specific gravity of 3.0 to 3.3 suitable as replacement for river sand.

3. Processed non-metallic steel slag based particulate as claimed in anyone of claims 1 or 2 wherein said construction grade fine aggregates have comparative strength with respect to conventional natural sand as hereunder:

Fine Aggregate 7th day Strength(Mpa) 28th day Strength, Mpa
100% Natural Sand 30-33 48-51
100% Processed steel slag sand 35-37 56-59

4. Processed non-metallic steel slag based particulate as claimed in anyone of claims 1 to 3 wherein said construction grade fine aggregates have compact structure and size distribution as per IS-383 zone –II suited for up to 100 % replacement of natural/river sand in mortar, concrete and road making.

5. Processed non-metallic steel slag based particulate as claimed in anyone of claims 1 to 4 wherein said construction grade fine aggregates provide for compressive strength of mortars of much higher in the range of 56 to 59 Mpavis a vis mortar with river sand.

6. Processed non-metallic steel slag based particulate as claimed in anyone of claims 1 to 5 wherein said non-metallic steel slag based particulate is obtained of selectively (a) BOF slag comprising of about 20-26 % FeO, 40–45% CaO, 12–15% SiO2, 6–10% MgO, 3–6% Al2O3, 1.0 – 6.0% MnO and 0.1–0.2% S by weight and (b) EAF slag comprising of 22-30 % FeO, 35–40% CaO, 15–18% SiO2, 8–9% MgO, 10–15% Al2O3, 1.5–2.0% MnO and 0.1–0.2% S by weight.

7. A process for manufacture of processed non-metallic steel slag based particulate as claimed in anyone of claims 1 to 6 comprising

the step of converting non-metallic steel making slag into fine aggregate involving subjecting the said non-metallic steel making slag to processing for changing shape and structure without breaking grains such as to generate selectively anyone or more of (i) sand particulates of substantially rounded/blunted particulates free of sharp edges as construction grade fine aggregates free of clay and silt (ii) Non-metallic steel slag larger particulates in the size range of > 4.75 mm suitable as grit for road making; (iii) Non-metallic steel slag finer particulates in the size range of < -150 µm suitable for cement manufacture.

8. A process as claimed in claim 7 comprising:

providing steel making slag and carrying out processing involving aged/unaged stable slag granules having compact structure and

subjecting the obtained slag granules to abrasion process for desired blunted/round shape and stable structure of granules avoiding elongated or flaky shape with sharp edges and obtaining therefrom the processed steel slag sand based dense and compacted constructional material like natural/river sand.

9. A process as claimed in anyone of claims 7 or 8 comprising:

subjecting slag granules to abrasion process with controlled parameters in vertical shaft impactor to modify the shape and structure of granules to have rounded shape without breaking grains, avoiding elongated, irregular or flaky shape with sharp edges;

subjecting the removal of coarser fractions (> 4.75 mm) using a vibratory screen to match the gradation.

subjected to removal of excessive fines (-150 µm) from steel slags using a dry air classifier.

10. A process as claimed in anyone of claims 7 to 9 wherein said abrasion process with controlled parameters in vertical shaft impactor comprises feed rate of 1.1 – 1.3 T/min and rotor speed of 2700-2900 rpm resulting in tip velocity in the range of 30-45 m/s whereby the slag particles do not break and only change to rounded shape.

11.A process as claimed in anyone of claims 7 to 10 wherein said angle of vibratory screen maintained between 12 - 20o.

12.A process as claimed in anyone of claims 7 to 11 wherein said processing step one of abrasion and screening, the bulk density of the slag is optimized at 1700 kg/m3 and water absorption at< 3% due to reduction in porosity.

13. A process as claimed in anyone of claims 7 to 12 wherein said non-metallic steel making slag used is selected from (a) BOF slag comprising of about 20-26 % FeO, 40–45% CaO, 12–15% SiO2, 6–10% MgO, 3–6% Al2O3, 1.0 – 6.0% MnO and 0.1–0.2% S by weight and (b) EAF slag comprising of 22-30 % FeO, 35–40% CaO, 15–18% SiO2, 8–9% MgO, 10–15% Al2O3, 1.5–2.0% MnO and 0.1–0.2% S by weight.

14. A system to carry out process for manufacture of processed non-metallic steel slag based particulate as claimed in anyone of claims 7 to 13 comprising
a vertical shaft impactor for subjecting the slag grains to abrasion process for modifying elongated, flaky shape of the particles to desired size and shape to uniform, cubical/rounded product;
a vibratory screen for screening out the larger (> 4.75 mm) sized unbroken slag particles;
an air classifier for removal of ultra-fines specially <150 µm, generating output after this processing as the processed slag sand having size distribution conforming to IS 383 - Zone II.

Dated this the 20th day of April, 2019
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FIELD OF INVENTION
The present invention relates to processed steel slag sand as fine aggregates and a three stage process for converting aged/unagedsteel slag (-10 mm) into fine aggregate to be used as 100% replacement of river sand for use in mortar, concrete, underground work, road making, meeting the standard specifications for construction purpose. More particularly, the present invention is directed to provide a multistage process with selective parameters for converting steel slag into fine aggregate by changing the structure and shape of the slag granules by subjecting to an abrasion process in a vertical shaft impactor, so that the angular, elongated, flaky shaped slag particles are converted into round or cubical of desired shape. Further separation of oversized material (> 4.75 mm) is carried out by using a vibratory screen. The under sized material or fines separation (-150 µm) is carried by subjecting it in a dry air classifier by controlling air velocities. The performance of prepared concrete samples, replacing natural sand by steel slag sand, indicates that it could be gainfully utilized up to 100 % as replacement of river sand as well as partial replacement with manufactured sand and river sand giving higher flexibility in civil engineering work and in road constructions.The developed process converts > 65 % of the feed to produce steel slag sand of desired fineness/size distribution having rounded shape without breaking grains meeting the specifications for use in road construction, non-critical applications and underground works as partial or total replacement of natural river sand.In addition to the sand, the under and oversized material can also be utilized. The finer fraction (<-150 µm)has application in cement making and coarser fraction (> 4.75 mm) has application as grit in road making.

BACKGROUND OF INVENTION

River sand is the most preferred fine aggregate for mortar and concrete. In the recent past, driven by the acute shortage of river sand, the other natural alternatives stone dust or manufactured sand are being used in the industry. To reduce the burden on the environment, alternative materials to be used as aggregates are being extensively investigated all over the world. In an integrated steel plant, slags are generated at two different stages of steel production, iron making and steel making known as BF (blast furnace) slag and steel slag respectively. Generally, BF (blast furnace slag) slags or iron making slags are granulated and used in cement making due to its high cementious properties and are also processed to convert into sand. Steel slags, both BOF (basic oxygen furnace) and EAF (electric arc furnace) slags were mostly dumped due to un-favorable physio-chemical conditions such as irregular shape, high density, presence of fines and expansive free CaO and MgO.

BOF slag contains about 20-26 % FeO, 40–45% CaO, 12–15% SiO2, 6–10% MgO, 3–6% Al2O3, 1.0 – 6.0% MnO and 0.1–0.2% S by weight and EAF slag consist of 22-30 % FeO, 35–40% CaO, 15–18% SiO2, 8–9% MgO, 10–15% Al2O3, 1.5–2.0% MnO and 0.1–0.2% S by weight. Non-metallic steel slag is obtained by metal separation followed by screening of the steel slag at ambient temperature at slag processing station. Aged air cooled steel slags have started being used as coarse aggregates in the recent past but after long ageing or weathering process during which the free expansive oxide phases are converted to stable carbonate phases. Rest of the finer fraction, having wide size distribution from 0-10mm is not suitable for any application is dumped thereby affecting the environment. The size distribution of this slag is irregular with presence of fines and coarse particles. These slags do not have expansion issue as a coarse slag as these are naturally aged due to large surface area. If any application can be developed for this finer slag it will help steel makers to generate revenue from waste.

These different size fractions cannot be removed by a single process. Hence some multi-step process must be developed to make the gradation matching to any suitable application. Comparison of unprocessed steel slag properties with river sand is shown in Table 1.
Table 1:
Properties River Sand Non-metallic steel slag
Size IS 383 - Zone II (Fine) Not suitable
Density, Kg/m3 1300 - 1600 1700-1900
Sp Gravity 2.6 - 2.8 3.1
Water Absorption 1 – 3 % 1 - 3 %

In the present invention (-) 10 mm steel slag (BOF and EAF both)is processed through shaping,sizing and fine separation process to convert it into steel slag fine aggregates for replacing river sand in construction.

A major positive point identified with the use of steel slag in construction is the higher bulk density (1700-1900 kg/m3) in comparison to that of river sand (1300-1600 kg/m3) which results in higher strength of concrete. Higher specific gravity also contributed to its properties.

Moreover, Size distribution of conventional steel slag particles showed that the particles are mostly in lumps and almost impossible to use directly as fine aggregates. The coarser steel slag particles have irregular shape and with sharp edges, whereas the finer steel slag particles have irregularities as there is no fix percentage observed.

There has been thus a continuing need in the field of developing substitute aggregate from steel making slag for replacement of natural river sand to provide a process to convert steel slag sand as fine aggregate having modified physical properties especially size distribution and shape as per standard for use in concrete to meet strength and other required properties in construction industry.

Present work is thus focused on developing a multistage processing technology for converting non-metallic steel slag (-10mm) into acceptable fine aggregate. Various tests conducted under this study have confirmed that non-metallic steel slag can be used as 100% replacement of natural sand in concrete mix design if slag gradation is fixed as per IS 383-Zone II.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide processed steel slag sand aggregates as total or partial replacement of river sand and a multistageprocess for converting steel making slag into non-critical or road construction grade fine aggregate for up to 100% replacement of river sand.

A still further objective of the present invention is directed to provide processed steel slag sand aggregatesand a multistage process for producing the same wherein whole of the steel slag is converted into 3 products without any wastage.

A still further objective of the present invention is directed to a process for converting > 65 % of the feed non-metallic steel slags (-10 mm) into construction grade fine aggregate as replacement for river sand.

A still further objective of the present invention is directed to a process for converting non-metallic steel slags (-10 mm) into construction grade fine aggregate as replacement for river sand wherein slag particles are subjected to selective abrasion process using a vertical shaft impactorto meet the shape and size distribution requirement as per applicable standards for ensuring desired minimum properties.

A still further objective of the present invention is directed to a process for converting non-metallic steel slags (-10 mm) into construction grade fine aggregate as replacement for river sand wherein slag particles are subjected to vibratory screen to remove the coarse fractions (>4.75 mm) to fit this product into desired gradation specified by Indian standard.

A still further objective of the present invention is directed to a process for converting non-metallic steel slags (-10 mm) into construction grade fine aggregate as replacement for river sand wherein slag particles are subjected to removal of excessive fines (-150 µm) from steel slags using a dry air classifier to fit this product into desired gradation specified by Indian standard.

A further object of the present invention is directed to a multistage process for converting steel slags into construction grade fine aggregate which will have bulk density of slag particles minimum 1700 kg/m3, specific gravity up to 3.1, and with reduced water absorption.

A still further object of the present invention is directed to a three stage process for converting non-metallic steel slags into construction grade fine aggregate as replacement for river sand wherein the density, size distribution and shape is modified to meet minimum applicable standards.

A still further object of the present invention is directed to a multistage process for converting steel slags into construction grade fine aggregate as replacement for river sand wherein highly rough and non-uniform structure of slags is converted into fine aggregate for use as replacement of river sand.

A further object of the present invention is directed to a multistage process for converting steel slags into construction grade fine aggregate as replacement for river sand wherein flow characteristics of the mortars with processed steel slag sand are equally good or marginally better than the mortars with river sand, the compressive strength of mortars using steel slag sand is much higher than the strength of mortar with river sand, also the Flexure bond strength of masonry using mortars having steel slag sand as aggregate is higher when compared to masonry flexure bond strength with mortar using river sand.

A still further object of the present invention is directed to utilize the finer fraction (<-150 µm) generated in the process in cement making.

A still further object of the present invention is directed to utilize the coarser fraction (> 4.75 mm) generated in the process as grit in road making.

SUMMARY OF INVENTION
The basic aspect of the present invention is directed to processed non-metallic steel slag based particulate comprising abrasion based rounded/blunted shaped non-metallic steel slag particulates including selectively anyone or more of (i) sand particulates of substantially rounded /blunted particulates free of sharp edges as construction grade fine aggregates free of clay and silt (ii) Non-metallic steel slag larger particulates in the size range of > 4.75 mm suitable as grit for road making; (iii) Non-metallic steel slag finer particulates in the size range of < -150 µm suitable for cement manufacture.

A further aspect of the present invention is directed to processed non-metallic steel slag based particulate wherein said construction grade fine aggregates have bulk density >1700 kg/m3preferably 1700-1900 Kg/m3 ,water absorption 1-3 % and specific gravity of 3.0 to 3.3 suitable as replacement for river sand.

A still further aspect of the present invention is directed to processed non-metallic steel slag based particulate wherein said construction grade fine aggregates have comparative strength with respect to conventional natural sand as hereunder:

Fine Aggregate 7th day Strength(Mpa) 28th day Strength, Mpa
100% Natural Sand 30-33 48-51
100% Processed steel slag sand 35-37 56-59

Another aspect of the present invention is directed to processed non-metallic steel slag based particulate wherein said construction grade fine aggregates have compact structure and size distribution as per IS-383 zone –II suited for upto 100 % replacement of natural/river sand in mortar, concrete and road making.

Yet another aspect of the present invention is directed to processed non-metallic steel slag based particulate wherein said construction grade fine aggregates provide for compressive strength of mortars of much higher in the range of 56 to 59 Mpavis a vis mortar with river sand.

A further aspect of the present invention is directed to processed non-metallic steel slag based particulate wherein said non-metallic steel slag based particulate is obtained of selectively (a) BOF slag comprising of about 20-26 % FeO, 40–45% CaO, 12–15% SiO2, 6–10% MgO, 3–6% Al2O3, 1.0 – 6.0% MnO and 0.1–0.2% S by weight and (b) EAF slag comprising of 22-30 % FeO, 35–40% CaO, 15–18% SiO2, 8–9% MgO, 10–15% Al2O3, 1.5–2.0% MnO and 0.1–0.2% S by weight.

A still further aspect of the present invention is directed to a process for manufacture of processed non-metallic steel slag based particulate comprising

the step of converting non-metallic steel making slag into fine aggregate involving subjecting the said non-metallic steel making slag to processing for changing shape and structure without breaking grains such as to generate selectively anyone or more of (i) sand particulates of substantially rounded/blunted particulates free of sharp edges as construction grade fine aggregates free of clay and silt (ii) Non-metallic steel slag larger particulates in the size range of > 4.75 mm suitable as grit for road making; (iii) Non-metallic steel slag finer particulates in the size range of < -150 µm suitable for cement manufacture.

A still further aspect of the present invention is directed to a process comprising:

providing steel making slag and carrying out processing involving aged/unagedstable slag granules having compact structure and

subjecting the obtained slag granules to abrasion process for desired blunted/round shape and stable structure of granules avoiding elongated or flaky shape with sharp edges and obtaining therefrom the processed steel slag sand based dense and compacted constructional material like natural/river sand.

A still further aspect of the present invention is directed to process comprising:

subjecting slag granules to abrasion process with controlled parameters in vertical shaft impactor to modify the shape and structure of granules to have rounded shape without breaking grains, avoiding elongated, irregular or flaky shape with sharp edges;

subjecting the removal of coarser fractions (> 4.75 mm) using a vibratory screen to match the gradation.

subjected to removal of excessive fines (-150 µm) from steel slags using a dry air classifier.

Another aspect of the present invention is directed to said process wherein said abrasion process with controlled parameters in vertical shaft impactor comprises feed rate of 1.1 – 1.3 T/min and rotor speed of 2700-2900 rpm resulting in tip velocity in the range of 30-45 m/s whereby the slag particles do not break and only change to rounded shape.

Yet another aspect of the present invention is directed to a process wherein said angle of vibratory screen maintained between 12 - 20o.

A further aspect of the present invention is directed to a process wherein said processing step one of abrasion and screening, the bulk density of the slag is optimized at 1700 kg/m3 and water absorption at< 3% due to reduction in porosity.

A still further aspect of the present invention is directed to a process wherein said non-metallic steel making slag used is selected from (a) BOF slag comprising of about 20-26 % FeO, 40–45% CaO, 12–15% SiO2, 6–10% MgO, 3–6% Al2O3, 1.0 – 6.0% MnO and 0.1–0.2% S by weight and (b) EAF slag comprising of 22-30 % FeO, 35–40% CaO, 15–18% SiO2, 8–9% MgO, 10–15% Al2O3, 1.5–2.0% MnO and 0.1–0.2% S by weight.

A yet further aspect of the present invention is directed toa system to carry out process for manufacture of processed non-metallic steel slag based particulate comprising

a vertical shaft impactor for subjecting the slag grains to abrasion process for modifying elongated, flaky shape of the particles to desired size and shape to uniform, cubical/rounded product;
a vibratory screen for screening out the larger (> 4.75 mm) sized unbroken slag particles;
an air classifier for removal of ultra-fines specially <150 µm, generating output after this processing as the processed slag sand having size distribution conforming to IS 383 - Zone II.
The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: Size distribution curves of the steel slags, processed steel slag sand and River sand.
Fig 2: Schematic Process layout of converting non-metallic steel slag (-10 mm) into construction grade fine aggregate for replacing natural/river sand or road construction or cement making based on size fraction.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS AND EXAMPLES

The present invention relates to fine aggregates of processed steel slag sand as total or partial replacement of river sand and a multistage process for converting non-metallic steel slag (-10 mm) into construction grade fine aggregate for up to 100% replacement of river sand for mortar/concrete used in civil construction. It is an object of the present invention to provide a three-step method to convert non-metallic steel slag into fine aggregate having the attributes comparable with river sand. Present invention also generates steel slag coarse fractions (> 4.75 mm) which can be used grit in road construction and steel slag fines fractions (-150 µm) which can be used in cement making.

Accordingly, the first aspect of the invention resides in improving the shape and size of the slag grainsby modifying elongated, flaky shape of the particles (1) by subjecting it to customized shaping process.The purpose was to control and convert individual particles into rounded, cubical shape without breaking grains.It was envisaged that the slag particles should be subjected to abrasion process and not grinding to meet the shape and size requirement.As no such customized equipment was available for abrasion process, it was decided to use a vertical shaft impactor (2) for achieving the desired function. Normally a vertical shaft impactor is used for crushing stones. In a shaft impactor, feed material drops through the feed tube onto the impeller table or enclosed rotor which, through centrifugal force, throws the material against stationary anvils made up of composite metal alloys. When the rock particles impact the anvils, it shatters along natural stress lines, creating a uniform, cubical/roundedproduct. This method of shaping is simple and economical to operate. In an impactor, the particle’s projectile velocity and impact force with which it hits the anvils decides whether the particle will get shaped, crushed or grinded. With some experiments, it was found that by manipulating the rotor speeds the feed material can be subjected to only shaping. Series of experiments were conducted by varying these variables and at an optimized range of feed rate of 1.1 – 1.3 T/min and rotor speed of 2700-2900 rpm were found out. At these parameters, the resulting tip velocity is in the range of 40-45 mps where the slag particles do not break and only change the shape. The impactor processed slag granules were of rounded shape but the size distribution required some alteration. Somelarger shaped particles need to be removed and thus send to second step of processing.

The second aspect of the invention resides in screening out the larger (> 4.75 mm) sized unbroken slag particles. However due the previous abrasion process, the fines also increased. Hence selected removal of coarser particles from a normal screen is not possible as the fines choke the screen and stops the operation. Normal screening is also affected in this material with increased moisture during rainy season. To address this, issue a vibratory screen (3) was employed.As compared to conventional screening methods which are limited to producing unacceptable sizes in the products, Vibratory screens can produce more efficient sizing to obtain a finer product. Firstly, it screens out the coarse particles and recirculates them back to the Impactor. The vibration frequency of the screen is mainly controlled by an electromagnetic vibrator which is mounted above and directly connected to the screening surface. The controlling step in this operation is the angle of inclination which maximizes the separation at an optimized angle. For the output of impactor, the optimized angle was found to be between 12o - 20o. This step generated two products steel slag and grit. Grit (6) is having size in the range of 4.75 – 10 mm and can be utilized for road making. The sand produced was having significant amount of micro fines specially <150 µm, which cannot be used as aggregates. Hence a third step of processing was required to remove the fines to convert the slag granules to match the standard requirement of fine aggregate.

The third aspect of invention resides in screening out the smaller (> 4.75 mm) sized ultrafine slag particles. Since there is wide distribution of particles in the step two product, the removal of ultra-fines specially <150 µm was not possible by normal screen. Hence an air classifier (4) was introduced into the circuit which removes the ultrafine. It works by injecting the slag particles to be separated into a chamber which contains a column of rising air. Inside the separation chamber, air drag on the particles supplies an upward force which counteracts the force of gravity and lifts the slag particles to be sorted up into the air. Due to the dependence of air drag on object size and shape, the objects in the moving air column are sorted vertically and can be separated. Internal baffles apply drag forces to the coarse particles while allowing the air to pass through them for separation of the fines. The heaviest particles drop to the bottom of the classifier. Fines are carried by the exiting air to a fabric filter for final recovery (7). In this process the air flow is the controlling step which governs the cut point of the separation.In this step the flow rates vary depending upon the distribution and moisture content. The outputs after this third step of processing, includes the sand (5) matching to the standard requirement and slag fines which are suitable material for use in cement making.

The complete layout of the process is shown in Fig 2. The developed process is highly efficient as whole of the steel slag is converted into 3 products without any wastage. The primary product is construction grade fine aggregate which can be used as replacement for river sand and fits into desired gradation specified by Indian standard. The second product is the coarser fraction (> 4.75 mm) of sand (6) which can be used as grit in road making. The third product is the finer fraction (<-150 µm) of sand (7) which can be used in cement making. The generation of the primary product – slag sand is > 65% of the initial feed. Generation of coarser fraction (> 4.75 mm) of slag to be used as grit is 10-15 % of the initial feed. Generation of finer fraction (<-150 µm) of slag to be used in cement making is 15 - 20 % of the initial feed.

TESTS AND RESULTS
The physical characteristics of slag in bulk samples have greatly improved due to the processing methodology adopted. The particles shapes have improved. The particles had blunter edges as compared to elongated or flaky shape with sharp edges in non-metallic steel slags. The processed steel slag sand was similar to 100% river sand with marginally high bulk density. The shapes of the natural river sand and processed steel slag sand particles resemble closely. Size distribution of the processed steel slag sand was also similar to river sand. Processed steel slag sand matched the minimum requirement of physical properties of fine aggregate to be used in concrete which is IS-383 Zone-II. Size gradation of processed steel slag sand is shown in Figure 1. In addition to properties the key advantage of processed steel slag sand over river sand is the absence of impurities like clay and silt.
Processed steel slag sand was then experimented for mortar and concrete cube testing, to check its compressive strength when used with cement (PSC). The flow characteristics clearly indicate that the mortars with processed steel slag sand are equally good or marginally better than the mortars with river sand. The compressive strength of mortars using processed steel slag sand is much higher than the strength of mortar with river sand. The comparison of strength values with natural sand are shown in Table 2. Flexure bond strength of masonry mortars having processed steel slag sand as aggregate is higher when compared to masonry flexure bond strength with mortar using river sand. It is possible to achieve good workability for the concrete using processed steel slag sand as fine aggregate. The results clearly show that with processed steel slag sand as fine aggregate it is possible arrive at mix proportion which yields desirable strength and slump, and meeting the IS 456 requirements. The slightly angular shape of some slag particles increases the amount of surface area for bonding with cement paste and reduces the high internal stress concentrations leading to higher strength values. The bond strength between the rebar and concrete with river sand and processed steel slag sand as fine aggregate was higher in the later case. Tests conducted at applicant’s test facility have shown that processed steel slag sand can be used 100 % individually and also in combination with Natural sand, Crusher Dust and M-sand. This controlled impactor processed slag followed by elutriation process was finally matching the specifications of the fine aggregate standard. This innovative processing route developed for steel making slag has converted a waste into a commercial product.
Table 2:
Fine Aggregate 7th day Strength(Mpa) 28th day Strength, Mpa
100% Natural Sand 30-33 48-51
100% Processed steel slag sand 35-37 56-59

It is thus possible by way of the present invention to providefine aggregates of processed steel slag sand and a process for converting steel slag into such fine aggregate for use as partial/100% replacement of natural/river sand in mortar, concrete and road making. The present invention relates to a multi-step processing technique that has been developed for converting non-metallic steel making slag into fine aggregate to be used as an eco-friendly alternative to 100 % replacement of river sand in construction. This three-stage processing involves change in structure and shape of slag granules and matching the granulometry to the standard requirement. The fine aggregate of steel slag sand is compact and dense having sp. gravity preferably about 3.2 and water absorption preferably about < 3% and bulk density >1750Kg/m3. The present invention also provides a multistage process with selective parameters for converting steel slag into fine aggregate for changing the structure, texture and shape of the slag granules by subjecting to an abrasion process in a vertical shaft impactor, screening the oversized material in a vibratory screen and blowing out the fines using a dry air classifier. The developed process converts whole of the steel slag into 3 products without any wastage.The developed process converts > 65 % of the feed to produce steel slag sand of desired fineness/size distribution having rounded shape without breaking grains meeting the specifications for use in road construction, non-critical applications and underground works as partial or total replacement of natural river sand.In addition to the sand, the under and oversized material can also be utilized. The finer fraction (<-150 µm)has application in cement making and coarser fraction (> 4.75 mm) has application as grit in road making.