Graniterock Offers Another Way to Go Green
By Josh Patterson and Mike McGrath, Graniterock Research and Technical Services
Slag cement is Graniterock’s newest recycled product acceptable for use in concrete. Slag cement has been used successfully in green concrete throughout Europe for many years, but its use has been fairly uncommon in California. Slag cement is also known as ground granulated blast furnace slag (GGBFS) and is governed by ASTM C989 and AASHTO M302. Slag cement is environmentally friendly because it reduces the need to produce Portland cement and, since slag is a by-product from the manufacturing of iron in a blast-furnace, it recycles a previously unusable material. The production of Portland cement releases harmful greenhouse gas emissions, including the currently political unfavorable CO2.
Slag is similar to fly ash, in the way that it gains higher later age strengths; however, slag is not a pozzolanic material (fly ash) because it has cementitious properties by itself. Slag has a specific gravity between 2.85–3.00 as compared to cement’s 3.15, so it occupies more absolute volume than the same mass of cement. Slag hydration is activated by alkalis, sulfates, and high temperature. Slag also reacts with a byproduct of cement hydration, the chemical reaction of Portland cement and water. Portland cement hydration mainly produces calcium silicate hydrate (C-S-H) and calcium hydroxide (C-H). C-S-H is primarily responsible for strength in Portland cement concrete; however, C-H does not produce any significant strengths and is in fact seen as a nuisance. C-H, also known as lime, is the white powder you may have seen permeating through concrete that is a few days to a month or two old. Slag will react with that lime activator and water to produce additional calcium silicate hydrate and eventually stronger concrete with other beneficial properties.
In the blast furnace, iron ore, scrap metal, fuel, and limestone or dolomite (fluxing stone) are loaded in layers. Liquid (molten) iron collects at the bottom and slag floats on the top of it. The molten slag, at about 1500°C, is periodically removed from the blast-furnace and cooled. The three cooling processes are dumping the slag in water, cooling with water sprays as the slag falls over a vibrating feed plate (pelletizer or air granulator), and hitting the slag with large amounts of water using high-pressure jets (jet process granulator). The rapid cooling (granulation) is important to obtain cementitious properties. When the slag is rapidly quenched, it forms a non-crystalline (amorphous) material, called glass. The slag granules are de-watered and dried and then ground to a fineness similar to or greater than that of Portland cement—450 to 650 m²/kg Blaine, depending on the grade. Slag granules are angular in shape.
Slag is defined in three performance grades according to the slag activity index. The three grades are 80, 100, and 120. Graniterock uses Grade 120 slag. Grade 120 is the highest grade and means that a 50% substitution of Portland cement for slag results in a mortar that is 15% stronger than pure Portland cement alone at 28 days. Grades 100 and 80 have less stringent grinding and strength requirements.
The use of slag cement in concrete can be divided up into three categories: Type I (SM) is a blended cement that contains less than 25% slag; Type IS is a blended cement that contains 25%–70% slag; and Type S is a blended cement that contains 70% slag or more. Type IS is by far the most commonly used, but Graniterock can produce redi-mix concrete with any of the three blended cement types.
Concrete containing slag will have a slower strength development than concrete containing only Portland cement, but it can ultimately develop higher later age strengths. Slag should not be used in construction applications in which early strength development is important (staged construction, post-tensioned slabs, highways or roadways, etc.). Lower ambient temperatures generally have an adverse impact on strength development with concrete containing slag and will retard the strength gain as well. However, concrete with slag can reach later age strengths (at 28 days and beyond) greater than equivalent Portland cement mixes.
The use of slag cement in concrete has numerous desirable effects on fresh concrete. Slag cement has a lower water demand than fresh concrete, so the amount of water needed to reach a target slump can be decreased. This means that an equivalent cement content will have a lower water-cement ratio or the design water-cement ratio can be reached using less cement. Slag cement has a finer particle size than Portland cement which results in increased workability and pumpability of fresh concrete. The amount of bleed water depends upon the fineness or grade of the slag cement. The use of slag cement in mass concrete will lower the heat of hydration and increase the set time, which is an advantage to some contractors during the summer. Slag cement mixes are cohesive but not sticky like concrete with silica fume.
Slag cement also has desirable effects on hardened concrete; one of slag cement’s greatest advantages is reduced permeability. Slag cement produces a denser past than pure Portland cement mixes which reduces the permeability in concrete. Permeability plays an important role in making concrete long lasting and more durable. Concrete with high permeability will allow fluids to pass into and affect the internal structure of the concrete as well as the rebar. Water reaching the reinforcing steel can promote corrosion and ultimately damage the concrete. Concrete with low permeability will resist the penetration of fluids and be more durable. One significant cosmetic difference in slag cement mixes is that a greenish blue color may appear on the surface during the first week. This is related to sulfides in slag. The color fades after exposure to air and sunlight. After a few months, slag concrete will generally appear lighter in color than straight Portland cement mixes.
Slag cement has many other advantages over traditional Portland cement. Not only will the use of slag cement in concrete increase the ultimate compressive strength, but it will also improve flexural strength. Type IS and S slag mixes will reduce the potential for Alkali-silica reaction. Slag cement mixes will also have improved aggregate bonding and reduce the likelihood of pop-outs. Slag cement concrete will also assist in sulfate attack, chemical attack, freezing and thawing, and deicing salts.
Slag cement concrete has identical curing requirements to the of conventional Portland cement concrete. Curing is essential to maintaining a satisfactory amount of moisture in finished concrete. The strength and durability properties of slag cement concrete are directly proportional to the quality of the curing.
Saw jointing of slag cement concrete needs to be delayed approximately 30 minutes for every 10% of slag cement replacing Portland cement. Joints should be cut after the concrete strength is high enough to support the saw and keep the saw cut from raveling but before internal stresses in the concrete become great enough to initiate an uncontrolled crack.
Slag cement concrete mixes will offer Graniterock customers an alternative cementitious material that will not only improve many concrete properties, both fresh and hardened, but will also reduce environmental concerns in many ways.