Categories
Modelling

Two main methods to model cement-based materials

With the development of computer technology, increasing attention is being paid to the modeling work of cementitious-based materials. Many researchers have been carrying out the research to better model the hydration of cement-based materials in the last decades. It is obvious that a cogent simulation of the hydration of cement-based materials has crucial role in construction industry, such as reducing tedious process of trial and error when doing experiments, facilitating the research of durability of concrete.

Many models are available to model the hydration process and microstructure of cement-based materials. According the simplifications of hydration models, two main approaches could be classified, viz. continuous approach and discretization approach. Because of the complexity of real hardening cement-based materials, each model is based on certain assumptions that aim to simplify the complexity of real cement microstructure and its interactions.

HYMOSTRUC
HYMOSTRUC (by TuDelft)

The advantages of continuous approach are resolution free and no requirement to divide features of microstructure into smaller elements. Particles are normally assumed as sphere shapes. The assumption of sphere demands less information to describe particles when modeling compared with other shapes, thus resulting in the calculation of the distance between two particles fast. This method also preserves information about features that scale over several orders of magnitude, viz. from micrometers to millimeters. However, the drawback of this method is that the geometry of microstructure is altered.

CEMHYD3D
CEMHYD3D (by NIST, USA)

The principle of discretization approach is similar to finite element method. The entire volume of the microstructure is filled with smaller elements which are placed with their faces in contact. By means of a certain algorithm, the hydration and microstructure development of cement could be simulated.

Categories
Blended cement

Brief introduction of limestone cement

Limestone has been utilized as a mineral addition in cement industry for a long time. The latest European Standard (EN 197-1-2000) allows up to 5% limestone as a minor additional constituent, and also identifies four types of Portland limestone cement containing 6-20% limestone (types II/A-L and II/A-LL) and 21-35% limestone (types II/B-L and II/B-LL), respectively. The ASTM Standard (C150-04) allows up to 5% of limestone filler. Since the early 1980s, the Canadian cement standard has allowed the inclusion of up to 5% limestone addition to Portland limestone cement.

Similar trend could also be found in Latin-American countries such as Argentina, Brazil, and Mexico (Menendez). According the review Cement Standards of the World (CEMBUR-EAU 1997), more than 25 countries allow the use of between 1% and 5% limestone in their Portland cement. Many countries even allow up to 35% replacement in Portland composite.

limestone-cement-composition
Compositions analysis of several typical limestone

The main composition of limestone is calcite (CaCO3), with smaller proportion of quartz or amorphous silica and sometimes of dolomite. Table above is composition analysis of several typical limestones. Though the LOI content of limestone is up to 42.5% or even more, they must be low in clay minerals and organic matter to guarantee the water demand and setting time. As for the quality requirement, according to ASTM Standard (ASTM C-150-4), the calcium carbonate (CaCO3) content calculated from the calcium oxide content should be at least 70% by mass.

Limestone is softer than cement clinkers, thus could be grind into finer particles. When it interground with clinkers, for an overall specific surface area (Blaine) of 420 m2/kg, 50% of the limestone can be below 700nm, compared with 3 µm for the corresponding clinker. Addition of finer limestone particles to clinker completes the fine fraction in the granulometric curve of cement without an increment on water demand, and improves the cement packing and blocks the capillary pores.

Categories
Blended cement

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.