Tobermorite gel

Calcium siHcate hydrate is not only variable ia composition, but is very poody crystallised, and is generally referred to as calcium siHcate hydrate gel or tobermorite gel because of the coUoidal sizes (<0.1 fiva) of the gel particles. The calcium siHcate hydrates ate layer minerals having many similarities to the limited swelling clay minerals found ia nature. The layers are bonded together by excess lime and iatedayer water to form iadividual gel particles only 2—3 layers thick. Surface forces, and excess lime on the particle surfaces, tend to bond these particles together iato aggregations or stacks of the iadividual particles to form the porous gel stmcture. 

The important compounds in Portland cement are dicalcium silicate (CazSi04) 26%, tricalcium silicate (CasSiOj) 51%. tricalcium aluminate (Ca3Al206) 11% and the tetracalcium species Ca4Al2Fe2 Oio (1%). The principal constituent of moistened cement paste is a tobermorite gel which can be represented schematically by the following idealized equations ... 

The form of silica in the matrix is at present unknown. In the freshly prepared cement there are appreciable amounts of silicic acid present which decline as the cement ages (Crisp, Lewis Wilson, 1976d). In the set cement silica could be present as a polymeric silicic acid, a siliceous gel or even a hydrated silicate gel, such as the tobermorite gel present in Portland cements (Taylor, 1966). 

The differences in chemical composition are accompanied by differences in the morphology of the tobermorite gel. Spicular or cigar-shaped rolled sheets are formed in the normal plain hydrated cement paste, whilst in the presence of calcium chloride, thin crumpled sheets or foils are formed. It has been suggested [16] that either the high lime content or adsorbed chloride prevents rolling of the sheets. 

When the hydraulic radii of tobermorite gel pores are examined using adsorption of water vapor, very little difference is found between plain pastes and pastes containing calcium chloride [20], as shown in Fig. 5.13. 

This difference between H O and N adsorption data has been attributed to either the accessibility of water to interlayer spaces in the tobermorite gel or to the presence of ink bottle pores with narrow necks and wide bodies. The considerable increase (4-5 times) in pore surface and pore volume available to nitrogen in pastes containing calcium chloride suggests that the crumpled pore type of morphology is more open than the spicular type [20],... 

The way in which salts such as calcium chloride and calcium formate operate is not fully understood, but it is clear that the mechanism involves an acceleration of the C S and C S hydration. It has been proposed [23] that the initial products of cement hydration form a sort of membrane which acts as a restraint to the diffusion process which in turn leads to the dormancy period . It seems likely that the chloride ion, by virtue of its small size and high mobility, is able more easily to penetrate the pores of the restraining layer allowing the diffusion process to proceed more rapidly. The resultant tobermorite gel has a higher lime-silica ratio and a more open, accessible structure, based on a crumpled foil morphology rather than the usual spicular. The considerable reaction with,... 

At one time, the relation between C-S-H gel and tobermorite was widely believed to be very close, and the name tobermorite gel was in common use. The Ca/Si ratio and poor crystallinity of C-S-H gel do not justify this name. Assuming the idealized formula CjSgHg for 1.4-nm tobermorite, removal of... 

The CsAH phase or ettringite is in the form of rods and fibers that interlock. The second reaction, which causes the cement to harden, is slower. It starts after about 10 hours, and takes more than 100 days to complete. The product is tobermorite gel, a hydrated calcium silicate (Ca3Si207-3H20), which bonds everything together. 

TLV. Threshold Limit Value. Tobermorite. A hydrated calcium silicate approximating in composition to 5CaO.6SiO2.5H2O. Tobermorite gel is... 

Tricalcium silicate hydrate (tobermorite gel) is the compound most responsible for the mechanical strength and dimensional stability of the hardened concrete. The pore space within the hardened concrete leading to its adsorption, porosity, and drying shrinkage is the result of the tobermorite gel structure. 

Tobermorite gel particles have an average particle size of approximately 1000 times that of the original unhydrate cement grain of 10 p,m. This large... 

The volume of pore spaces is also the result of the tobermorite gel. In addition to these pores there are voids caused by capillary movement of excess water and air entrapped by the mixing operation. The resulting concrete can have a water adsorption capacity in excess of 20% of the apparent volume of the mass. 

With a CaO/SiO ratio of 1.0, the products were a tobermorite gel, CSH(I), tobermorite solid solution, xonotlite, hydrogarnet solid solution, y-AlO(OH) and anhydrite. The tobermorite substituted about 15 mol% Al O and the crystal had a plate-like shape. 

Additions of 5 to 10% Al O and 5% CaSO -2H20 to the 0.5 CaO/SiO mixture helped the formation of a compact structure made up of tobermorite gel, and contributed to strength development. ... 

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