Ground granulated blast furnace slag (GGBFS)

INTRODUCTION

Ground granulated blast furnace slag (GGBFS) has been used for many years as a supplementary cementitious material in Portland cement concrete, either as a mineral admixture or as a component of blended cement.

The use of GGBFS in Portland cement is governed by AASHTO M302.⁽¹⁾ Three types of ground granulated slag cements are typically manufactured. They include Portland cement as covered by AASHTO M85⁽²⁾, Portland blast furnace slag cement (blended cement type IS), and slag cement (slag cement type S) as per AASHTO M240.⁽³⁾

The use of GGBFS as a partial Portland cement replacement takes advantage of the energy invested in the slag making process and its corresponding benefits with respect to the enhanced cementitious properties of the slag. Grinding slag for cement replacement requires only about 25 percent of the energy needed to manufacture Portland cement.

PERFORMANCE RECORD

Four state agencies are reported to be investigating the use of GGBFS as a supplementary cementitious material (Florida, Maryland, New Hampshire, and Oregon).⁽⁵⁾ At least 11 states (Delaware, Florida, Indiana, Maryland, Massachusetts, Michigan, New Hampshire, North Carolina (limited use on experimental basis), Pennsylvania, South Carolina and Virginia) currently have specifications covering the use of GGBFS as a partial replacement for Portland cement. Some agencies have reported durability problems (salt scaling resistance) with exposed concrete containing blast furnace slag where the amount of slag cement exceeds about 25 percent of the total cement.⁽⁶⁾

The Environmental Protection Agency (EPA) has recommended that effective May 1, 1995, procuring agencies specifically include provision in all construction contracts for the use of GBBFS, as appropriate. ⁽⁷⁾

MATERIAL PROCESSING REQUIREMENTS

Dewatering

Moisture in blast furnace slag, which occurs in the granulation process or in pelletized slag, should be removed by drying prior to the use of GGBFS as either an additive to Portland cement or a mineral admixture to Portland cement concrete.

Grinding

Processing for use as a supplementary cementitious material requires grinding of the slag, typically using the same or similar plant and equipment as for Portland cement production.

ENGINEERING PROPERTIES

Some of the engineering properties of GGBFS that are of particular interest when blast furnace slag is used as a supplementary cementitious material in Portland cement concrete include the hydraulic reactivity of the slag and its fineness.

Hydraulic Reactivity: Depending on the quenching process, the structure of pelletized blast furnace slag can range from crystalline (slow quench) to vitreous (rapid quench). Rapid quenching is important if cementitious properties are to be achieved. The chemical composition of GGBFS use in Portland cement concrete must also conform to sulfur and sulfate content limitations outlined in AASHTO M302.⁽¹¹⁾

Fineness: Granulated blast furnace slag is a glassy granular material, and its particle distribution, shape, and grain size vary, depending on the chemical composition and method of production, from popcornlike friable particles to dense, sand-size grains. Pelletized blast furnace slag, in contrast to air-cooled and expanded blast furnace slag, has a relatively smooth texture and rounded shape. Grinding reduces the particle size to a cement fineness for use as a hydraulic cement, which is typically less than 3500 cm²/g.⁽⁸⁾

Some of the properties of concrete mixes containing GGBFS that are of particular interest when it is used as partial cement replacement include strength development, workability, heat of hydration, resistance to alkali-aggregate reactivity, resistance to sulfate attack, and salt scaling.

Strength Development: Concrete containing GGBFS develops strength at a somewhat slower rate than concrete containing only Portland cement, but ultimately can develop equivalent strength. This can be a concern where early strength development is important (staged construction where the first structure must develop strength before the second structure can be placed). Low temperatures (cold weather) generally have a more adverse impact on strength development with concrete containing GGBFS than concrete containing only Portland cement.

Workability: Concrete containing GGBFS as a partial cement replacement has longer-lasting workability and low slump loss during hot weather construction.

Heat of Hydration: Concrete containing GGBFS exhibits a lower heat of hydration than conventional Portland cement concrete.

Alkali-Aggregate Reactivity: The use of GGBFS as a partial replacement for Portland cement can reduce available alkalies and can reduce the reaction between certain siliceous components of concrete aggregates and the alkalies in the concrete.⁽⁹⁾

Sulfate Resistance: Use of GGBFS as a partial cement replacement gives concrete moderate resistance to sulfate attack.⁽¹⁰⁾

Salt Scaling: Concrete containing high concentrations of GGBFS may be susceptible to salt scaling (the loss of surface layers of cement mortar during repeated freeze-thaw cycles). Due to this problem, some agencies limit the amount of slag in a Portland cement concrete mix to 25 percent of the total cement weight.⁽⁶⁾

DESIGN CONSIDERATIONS

Mix Design

The most frequently used proportioning recommendations for GGBFS use in concrete mix designs are covered in ACI 226.1R.⁽¹¹⁾ Some agencies require that a salt scaling test also be completed for selected concrete mixes subjected to deicing salts.^(6,12)

Structural Design

Conventional AASHTO pavement structural design methods are appropriate for concrete mixes containing GGBFS.

CONSTRUCTION PROCEDURES

Material Handling and Storage

GGBFS (or cement containing GGBFS) is handled and stored like conventional Portland cement.

Mixing, Placing, and Compacting

The same equipment and procedures used for conventional Portland cement concrete may be used to batch, mix, transport, place, and finish concrete containing GGBFS.

Curing

The slower strength development of concrete containing GGBFS may require that the moisture be retained in the concrete for a longer period of time than what is normally required for conventional concrete. Scheduling of pavement construction should allow adequate time for the specified strength gain prior to the placement of traffic loads, the onset of freeze-thaw cycles, and the application of deicing salts.

Quality Control

The same quality control procedures used for conventional Portland cement concrete can be used for concrete containing GGBFS.