Tag Archives: Slag cement

Hydration products of slag in blended cement

Many XRD analysis results have shown the main hydration products of slag blended cement are essentially similar with that of pure Portland cement, except the amounts of CH found by this method or other are in varying degrees and less than those that should be given by the pure Portland cement constituent if the slag part did not participate the reaction.

The main hydration products of the slag-cement are C-S-H gel, Ca(OH)2, the sulpho-aluminate hydrate phases AFt and AFm and a Mg, Al-rich hydroxide phase.

TEM slag
Fig. 1. Transmission electron micrograph showing foil-like Op C-S-H in a water-activated slag paste hydrated for 3 1/2 years at 40°C (W/S = 0.4) (by I.G Richardson: The nature of C-S-H in hardened cements).

TEM cement slag
Fig. 2. A TEM micrograph that illustrates fine, dense Op C–S–H in the paste containing 75% slag. (by I.G Richardson: Composition and microstructure of 20-year-old ordinary Portland cement–ground granulated blast-furnace slag blends containing 0 to 100% slag).

In the case of C-S-H, its morphology and composition may be modified by partial accommodation of M and A within the micro- or nanostructure, its Ca/Si ratio is then lowered (e.g., 1.55) than that formed from alite and belite (e.g. 1.7). Hydrotalcite-like phase with approximate composition Mg6Al2(OH)16(CO3)·4H2O is formed from the MgO content of BFS, typically 5-9%.

As stated above, the hydration products can be classified into inner product and outer product. Inner product C-S-H from cement grains has a Ca/(Si+Al) ratio similar to outer product C-S-H, Meanwhile, Katoite (C3ASαHβ, α < 1.5) has also been suggested as a slag product, but it is less documented than the former mentioned species.

Introduction of slag cement

Blast-furnace slag (BFS) is formed from a liquid at 1350-1550 °C during the manufacture of iron. When reaching the bottom of the furnace, the liquid slag forms a layer on top of the molten iron due to its lower density. Being separated from the molten iron, the liquid slag is cooled in the air or with water. Depending on the way of cool process, three main categories of BFS could be produced, which are namely ground granulated BFS, pelletized slag and air-cooled slag.

Ground granulated blast-furnace slag (GGBFS), which normally contains up to 95% of glass, is formed by cooling the liquid slag with large amount of water and subsequently ground to fine powder.

If the liquid slag is partially cooled with water and then flung to air, pelletized slag could be thus produced. Comparing with GGBFS, pelletized slag contains much low glass content, e.g. 50%, therefore it is normally used as concrete aggregate or as raw materials to produce cement clinkers.

When cooled and solidified in the air, air-cooled slag is produced. During the production process, a water spray procedure is sometimes applied to accelerate the cooling process. Because of the hard and dense property, air-cooled slag is utilized in road bases, asphalt paving or as concrete aggregate, etc.

Among these three kinds of slag, GGBFS is the most valuable one in cement industry. Due to its cementitious properties mixing with lime, alkalis or Portland cement, GGBFS is often used to make blast-furnace slag cement. The history of the production of slag cement in Germany, France, Luxembourg and Belgium has been more than one century.

The usage of slag cement, which based on partial replacement of Portland cement by slag, has mainly three advantages. First, it offer cost reduction because blast-furnace slag is a byproduct of steel industry, which makes it cheaper than Portland cement on the one hand, though the increasing demand increases the market price more or less; on the other hand, utilization of byproducts from steel industry also protects environment. Second, the manufacture of slag cement requires 75% less energy than that needed for the production of Portland cement. The less energy demand brings environmental benefit, cost save, and less emission of carbon dioxide that causes the planet warmer. Third, the use of slag cement can arguably improve the properties of cement products, e.g. better concrete workability, lower permeability, improved resistance to aggressive chemicals, higher compressive and flexural strengths, etc.