Chemical reaction hardening binders

Chemical reaction Hardening binders Highly viscous binders Adsorption layers (<3nm)... 

Particulate composites are used in greater volume than any others because concrete is a particular composite. In many ways, concrete is the archetype of this class of composites. It consists of particles or aggregates of various sizes almost always of mineral materials, bonded together by a matrix of an inorganic cement originally mixed with and hardened by its chemical reaction to water. Many types of particles are employed, at least five different types of Portland cements and several other types of inorganic cements act as binders. 

Sinter bridges are formed when particles are partially melted, due to heat, and then resolidify as agglomerates. A second method is by chemical reaction or by the use of a viscous or hardening binder. 

The formation of solid bridges by chemical reactions or hardening binders depends only on the participating materials, their chemical reactivity, and their tendency to harden. Sometimes elevated temperatures and/or pressure improve the binding characteristics and may cause a modified, potentially stronger, bridge structure. Chemical reactions are often activated by the presence of moisture. 

In all other cases shown in Figure 14 the particles can or must be composed of different materials and also the bridges, if present, may have another composition. The condition depicted by case (b) is characterized by a thin, not freely movable layer enveloping the particle which, if certain circumstances prevail, acts as a binder. Particularly, these layers can be the product of chemical reactions as well as hardening or highly viscous binders. Adsorption layers, which below 3 nm thickness are generally immobile, can also participate in adhesion. 

Chemical reaction (1.2) Hardening binders (1.4) Highly viscous binders (11.1) Adsorption layers (<3 nm thickness) (11.2)... 

Chemical Reaction(1.3) Liquid Bridges (H. 1) Hardening Binders (1.4) Hardening Binders (1.4)... 

The formation of solid bridges by chemical reaction or hardening binders de-... 

As the chemical reactions in the paste will continue even after setting, the hardness and strength of the paste—which are low immediately after setting— will increase, and a hardened cement paste will be formed. Ultimately the hardening reaction in the paste comes to an end, owing to the complete consumption of the original binder or insufficient amounts of mixing liquid, and a mature hardened cement paste results. 

As discussed in Chapter 17 under portland cement coating, when water is added to portland cement a chemical reaction takes place during the hardening. This reaction produces calcium hydroxide and tricalcium silicate hydrate. The alkalinity of concrete is provided by the presence of calcium oxide from the cement. Consequently, concrete attack can be due to chemicals that react with the Portland cement binder and form conditions that physically deteriorate the material. Any material that will cause the calcium oxide or hydroxide to be removed, lowering the pH of the cement mix, will cause instability and solution of the cement hydrates. 

The convertible binders undergo chemical reaction in the film. The drying and hardening of the film is catalyzed by driers with oxygen absorption. Hence, there is a limit to the thickness of the film, which will dry. The two-pack materials, notably epoxies and polyurethanes, cure by chemical reaction between two components in the film with no oxygen absorption. 

There are various mineral chemical binders used in refractory products. These react in a complex maimer i.e. by formation of gels which modify their specific stractures at low temperatures and sometimes through direct chemical reaction with the finer part of the aggregate. In the latter case, these ensure hardening around a mean temperature, sometimes even after the service temperature. Generally rather low contents (less than 5%) are added, on account of the not very refractory nature of these binders (most contain alkalis and are fusible at low temperatures). 

Chemical analysis permits us to control the loss of any kind of groups participating in the hardening of binders either directly or upon the release of the byproducts of the reaction (by observing the loss of the methylol groups or the release of such low-molecular products as water and formaldehyde). 

Properties, The special properties of silicate paints are imparted by the binder, generally potassium water glass. The paint coats harden physically by evaporation of water, as well as chemically by several reactions. These stabilization and consolidation processes are termed silicification [2.176]. The water glass reacts as follows with atmospheric carbon dioxide ... 

Mineral additions mnst be distinguished from chemical admixtures, which are added to the binder only in small amounts (usually below 1%), and which influence the properties of the fresh mix or the hardened paste, but do not alter snbstantially the product of the hydration reaction. 

Chemical/physical reactions during agglomeration. In a holt-melt agglomeration process, the hot-melt binder typically is of low viscosity initially but immediately starts to harden on contact with the cooler powder. Hence, the deformability of the agglomerating mass is constantly changing. 

Polyurethane Adhesives. The search for formaldehyde-ffee binders resulted in a new generation of polyurethane-based glues for chipboard production. The usage of polyurethane based adhesives for assembly of wooden materials has also increased. These liquid polyurethane adhesives are single-component adhesives that harden by a chemical cross-linking reaction which is initiated by moisture in the material. The water... 

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