Cement-aggregate reaction

A core sample was loaned by the Building Research Establishment and has been examined by optical microscopy, BSEM and ATEM. The objective was to characterize cement paste gels and the products of cement/rock or cement/ aggregate reactions in greater detail than has been done before in materials of this age, principally by ATEM. It should be noted that an inherent sampling bias is incurred by sampling... 

There is some evidence for cement/aggregate reaction. Some quartz, potassium feldspar and calcareous material has undergone dissolution... 

Potential volume change of cement—aggregate combinations Accelerated detection of potentially deleterious expansion of mortar bars due to alkali-silica reaction... 

Concrete is made of cement aggregate and water mixed together to form a paste. The aggregate is usually a tiller material composed of inert ingredients such as sand and rocks. When water is added, the components of cement undergo a chemical reaction known as hydration. As hydration occurs, the silicates are transformed into silicate hydrates and calcium hydroxide (Ca OII 2), and the cement slowly forms a hardened paste. 

Special requirements on the chemical composition of the cement may be necessary for certain applications, such as those requiring higher resistance to sulfate attack, reduced heat evolution or where using aggregates susceptible to alkali-aggregate reaction caimot be avoided [4]. 

As a recent trend, there have been a few attempts to use polymer latexes with chemical admixtures such as superplasticizers,alkyl alkoxy silane,and amino alcohol derivativeP I at low polymer-cement ratios of 5.0% or less. A trial to inhibit alkali-aggregate reaction with chemical admixtures and SBR latex has also been done.l When such attempts are completed, new polymeric chemical admixtures for ordinary cement concrete may successfully be developed in the near future. 

Shayan, A., Diggins, R., and Ivanusec, I. (1996) Effectiveness of fly ash in preventing deleterious expansion due to alkali-aggregate reaction in normal and steamcured concrete. Cement and Concrete Research 26, 153-164. 

Another use of pozzolans and blast furnace slag is to reduce the available alkalinity of cement systems to prevent the occurrence of alkali aggregate reaction (see later). 

Levels of particulate emissions from exhaust gases range from 0.3 to 1.0 kg ton , with much of this kiln dust being first collected into fabric filter baghouses and then reintroduced into the kiln feed. The dust is normally very rich in sodium and potassium chlorides that have vaporization temperatures of only 883 °C and 774°C, respectively. In the past, before any concerted efforts were made to capture the particulate emissions, the sodium and potassium plumes from cement plant chimneys settled over the countryside, helping to combat acid rain and also acting as fertiUzers in the soil. Today, however, the dust is mainly carried out in the clinker stream, where it creates problems with alkaU aggregate reactions (Bremner, 2001). 

Selected groups of problems related to durability and the long-term behaviour of concrete-like composites are examined below. Particular questions related to the alkali-aggregate reaction (AAR) in concrete and concerning the durability of glass fibres in the Portland cement matrix are described in Sections 4.2.2 and 5.3 respectively. 

Advanced modelling methods are used to a large extent. Simnlating models are developed to represent various processes in microstrnctnre, such as hydration of cement, progress of corrosion and of alkali-aggregate reaction, etc. Two- and three-dimensional simnlations not only allow the verification of the inflnence of different conditions on examined processes in qnick and inexpensive operations, but also provide the ability to demonstrate them to students in an extremely instructive way. 

Silica and Alumina. The manufacture of Pordand cement is predicated on the reaction of lime with siUca and alumina to form tricalcium sihcate [12168-85-3] and aluminate. However, under certain ambient conditions of compaction with sustained optimum moisture content, lime reacts very slowly to form complex mono- and dicalcium siUcates, ie, cementitious compounds (9,10). If such a moist, compact mixture of lime and siUca is subjected to steam and pressure in an autoclave, the lime—silica reaction is greatiy accelerated, and when sand and aggregate is added, materials of concrete-like hardness are produced. Limestone does not react with siUca and alumina under any circumstances, unless it is first calcined to lime, as in the case of hydrauhc lime or cement manufacture. 

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