Mineral Binders: chemical and physical challenges in cement and concrete

Title

Mineral Binders: chemical and physical challenges in cement and concrete

Abstract

Concrete is generally considered to be the most widely-used manufactured material on Earth.  Global demand is now of the order of 10 cubic km per year. This requires the manufacture of about 4 Giga- tonnes of cement per year, accounting for about 8% of global anthropogenic CO₂ emissions, despite the fact that concrete is probably the most eco-efficient of all man-made construction materials.  Evidently, even small improvements in cement and concrete technology can have a significant effect on global CO₂ emissions as well as on other globally-important sustainability indices. 

Hydraulic cements are so called because they react with water to form stable hydrated compounds capable of bonding under wet conditions. Almost all modern concrete construction is made using hydraulic cements in which the main ingredient is a man-made mineral called “Portland cement clinker” (PCC). The most important components of PCC are calcium silicates which hydrate to give a poorly-crystalline calcium silicate hydrate referred to as C-S-H. This complex solid-solution phase constitutes the main binder in ordinary concretes, and has the great advantage of low cost; but its chemical and physical properties are complex and still rather poorly understood due to its poor crystallinity and the difficulty in analyzing its complex structure on the atomic-, nano- and meso-scale. Some recent novel hypotheses about its formation and structure will be discussed.

Many other mineral-based binders are possible, and some may be almost as inexpensive to make as C-S-H but with a lower carbon footprint. So a better general understanding of the way such materials form and bond to each other could help us better predict critical phenomena such as volume change, creep and permeability, which can strongly influence the durability of concrete structures. Some examples will be presented to illustrate the scientific challenges. Ultimately, solution of such challenges might allow us to develop a wide range of useful alternative mineral binder technologies.

Binder (material)

A binder or binding agent is any material or substance that holds or draws other materials together to form a cohesive whole mechanically, chemically, by adhesion or cohesion.

In a more narrow sense, binders are liquid or dough-like substances that harden by a chemical or physical process and bind fibres, filler powder and other particles added into it. Examples include glue, adhesive and thickening.

Examples of mechanical binders are bond stones in masonry and tie beams in timber framing.

Classification

Binders are loosely classified as organic (bitums, animal and plant glues, polymers) and inorganic (lime, cement, gypsum, liquid glass, etc.). These can be either metallic or ceramic as well as polymeric depending on the nature of the main material. For example, in the compound WC-Co (Tungsten Carbide used in cutting tools) Co constitutes the binding agent for the WC particles.

Based on their chemical resistance, binders are classified by the field of use: non-hydraulic (gypsum, air-cements, magnesia, hydrated lime), hydraulic (Roman cement, portland cement, hydraulic lime), acid-resistant (silicon fluoride cement, quartz cement), and autoclavable (harden at 170 to 300°С i.e. 8-16 atm pressure and, e.g., comprise CaSiO3 materials).

Physical properties

Some materials labeled as binders such as cement have a high compressive strength but low tensile strength and need to be reinforced with fibrous material or rebar if tension and shear forces will be applied.

Other binding agents such as resins may be tough and possibly elastic but can neither bear compressive nor tensile force. Tensile strength is greatly improved in composite materials consisting of resin as the matrix and fiber as a reinforcement. Compressive strength can be improved by adding filling material.

In art, binders have use in painting, where they hold together pigments and sometimes filling material to form paints, pastels, and other materials. Binders used include wax, linseed oil, gum arabic, gum tragacanth, methyl cellulose, gums, or protein such as egg white or casein. Glue is traditionally made by the boiling of hoofs, bones, or skin of animals and then mixing the hard gelatinous residue with water. Natural gum-based binders are made from substances extracted from plants.^[1] Larger amounts of dry substance are added to liquid binders in order to cast or model sculptures and reliefs.^[2]

In cooking, various edible thickening agents are used as binders. Some of them, e.g. tapioca flour, lactose, sucrose, microcrystalline cellulose, polyvinylpyrrolidone and various starches are also used in pharmacology in making tablets. Tablet binders include lactose powder, sucrose powder, tapioca starch (cassava flour) and microcrystalline cellulose.

In building construction, concrete uses cement as a binder. Asphalt pavement uses bitumen binder. Traditionally straw and natural fibres are used to strengthen clay in wattle-and-daub construction and in the building material cob which would otherwise become brittle after drying. Sand is added to improve compressive strength, hardness and reduce shrinkage. The binding property of clay is also used widely to prepare shaped articles (e.g. pots and vases) or to bind solid pieces (e.g. bricks).

In composite materials, epoxy, polyester or phenolic resins are common. In reinforced carbon–carbon, plastic or pitch resin is used as a source of carbon released through pyrolysis. Transite, hypertufa, papercrete and petecrete used cement as a binder.

In explosives, wax or polymers like polyisobutylene or styrene-butadiene rubber are often used as binders for plastic explosives. For polymer-bonded explosives, various synthetic polymers are used.

In rocket fuels, polybutadiene acrylonitrile copolymer was used in 1960-70's big solid-fuel booster rocket fuels.

Organic binders, designed to disintegrate by heat during baking, are used in sintering.

^{Mineral materials. Flashcards}