Calcium silicate concrete

High density calcium silicate concrete products are made, particularly in Eastern Europe [26.46], by co-grinding silica sand and lime, adding controlled amounts of water and unground sand, and casting the mix into moulds. The products are then autoclaved at elevated temperature and steam pressure to produce hydrated calcium silicate concrete. A range of products, including slabs, beams and lintels, are produced, with and without metal reinforcement. 

The values of the initial shear strength given in Eurocode 6 (2005) depend on the type of unit (clay, calcium silicate, concrete, stone) and on the type of mortar (general-purpose mortar, lightweight mortar, or thin-layer mortar). 

Calcium Silicates. Cements aie hydiated at elevated tempeiatuies foi the commercial manufacture of concrete products. Using low pressure steam curing or hydrothermal treatment above 100°C at pressures above atmospheric, the products formed from calcium siUcates are often the same as the hydrates formed from their oxide constituents. Hence lime and siUca ate ftequendy used in various proportions with or without Portland cement in the manufacture of calcium siUcate hydrate products. Some of these compounds are Hsted in Table 6. 

Beaudoin, J. J. Feldman, R. F. (1975). Mechanical properties of autoclaved calcium silicate systems. Cement and Concrete Research, 5 (2), 103-18. 

Richardson, I. G. Groves, G. W. 1993. The incorporation of minor and trace elements into calcium silicate hydrate (C-S-H) gel in hardened cement pastes. Cement and Concrete Research, 23, 131-138. 

The second part of my talk deals with the surface of a particular solid, a calcium silicate hydrate, called tobermorite. The two main constituents of Portland cements are two calcium silicates, which make up about 75% or more of a portland cement by weight, and both of these silicates produce tobermorite in their reaction with water. This tobermorite is the most important constituent of hydrated portland cement, concrete, and mortar. That is not the reason, however, for my talking about it—the reason is that it is a fascinating substance for a colloid chemist. I will discuss only two properties of the tobermorite surface the surface area and the surface energy. 

Previous investigations of these hydration reactions at room temperature have been reviewed recently (4). Research in this laboratory has included the stoichiometry of the hydration of both silicates, employing different methods of hydration (2, 3, 5, 21), and a determination of the surface energy of tobermorite, the calcium silicate hydrate produced in the hydration of both silicates under most experimental conditions (8). The surface area and the surface energy of tobermorite are briefly discussed by Brunauer (I). These properties play vital roles in determining the strength, dimensional stability, and other important engineering properties of hardened portland cement paste, concrete, and mortar. 

The hardening of concrete or other materials based on calcium silicates may be further accelerated by the use of saturated steam under pressure in an... 

Ft is doubtful whether XRD can distinguish a mixture of tobermorite and C-S-H from a uniform material of intermediate crystallinity the situation may lie between these extremes (A30). This question is discussed further in Section 11.7.4. Crystallization is probably favoured by low bulk density its extent is apparently minimal in calcium silicate bricks (P49), but considerable in aerated concretes (A30). In cement-silica materials, substantially all the AljOj appears to enter the C-S-H, which as its Ca/Si ratio decreases can accommodate increasing amounts of tetrahedrally coordinated aluminium (S70). NMR results (K34) support an early conclusion (K62) that 1.1-nm tobermorite, too, can accommodate aluminium in tetrahedral sites. Small amounts of hydrogarnet have sometimes been detected, especially in products made from raw materials high in AljOj, such as pfa or slag. Minor amounts of tricalcium silicate hydrate (jaffeite C, S2H,) have sometimes been detected (A29,K61). 

Portland cement, a complex mixture of calcium silicates, aluminates, and ferrates, is one of the world s most important construction materials, with annual worldwide production in excess of lO kg. When mixed with water and sand, it changes by slow hydration to concrete. Water and hydroxide link the other components into larger crystals with great strength. 

Large-quantities of quicklime are used in the manufacture of sand-lime bricks and aerated concrete blocks. These are construction materials which are manufactured from lime-containing and silicate-containing raw materials and whose strength is due to the hydrothermal reaction of the raw materials to calcium silicates. 

Cement is a binder that sets and hardens by itself or binds other materials together. The most widely known application of cements is in construction a second one is the area of bone cements. Cements used in construction are characterized as hydraulic or nonhydraulic and mostly for the production of mortars and concrete. Hydraulic cements set and harden after combining with water. Most construction cements are hydraulic and based on Portland cement, which consists of calcium silicates (at least 2/3 by weight). Nonhydraulic cements include the use of nonhydraulic materials such as lime and gypsum plasters. Bone cements and bone cement composites refer to compounds that have a polymer matrix with a dispersed phase of particles. For instance, polymethylmethacrylate (PMMA) is reinforced with barium sulphate crystals (for radio-opacity) or with hydroxyapatite... 

Concrete is a mixture of sand and gravel held together by a binding agent which is a mixtnre of calcium silicate hydrates and calcinm hydroxide. The calcium hydrox-... 

Some of the early uses of lime in construction and building have already been mentioned (section 1.3.2). A major development made by the Romans was a blend of slaked lime and volcanic ash, which would harden under water, called Roman Cement. The volcanic ash contained reactive silica and alumina which combined with the lime in the presence of controlled amounts of water to produce a solid mass bound by calcium silicates and aluminates. Such reactive materials are called pozzolans after Pozzuoli, a city near Naples. Roman Cement was mixed with aggregate to make a time concrete, which was used for a wide range of products and constructions. 

Disposal of the solid residues from the treatment of waste gases is an issue that needs to be addressed, particularly when they contain heavy metals and toxic organic compounds. One possibility is to convert the residues into an alinite cement, composed mainly of calcium silicates and calcium chloride [29.14]. When set, the concrete effectively locks up the toxic components (see also section 28.4.6). 

This chemical equation is not balanced because, in practice, x varies between the approximate limits of 1.8-2.2, y varies around a mean value of 1.0 and aq means that water is also combined in the material in an indeterminate amount. The idealised composition of the calcium silicate hydrate phase is Ca2Si04 aq. The reaction is rapid and continues for up to approximately 20 days. Considerable heat is evolved - in the order of 500 J per gram of powder -and care must be taken to remove this heat when forming large masses of concrete into structures. The reaction gives a high-strength product. 

First Step. When polymer latexes are mixed with fresh cement mortar or concrete, the polymer partides are uniformly dispersed in the cement paste phase. In this polymer-cement paste, flie cement gel is gradually formed by the cement hydration and the water phase is saturated with calcium hydroxide formed during the hydration, whereas the polymer partides dqrosit partially on the surfaces of the cement-gel-unhydrated-cement partide mixtures. It is likely that the calcium hydroxide in the water phase reacts witit a silica surface of the aggre tes to form a calcium silicate layer.I It is confirmed that tire formation of the calcium hydroxide and ettringite in the contad zone between tire cement hydrates and aggregates is attributed to the bond between them.I lPl... 

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