Solid hydration products

When water comes into contact with the cement (stage I), wetting of the highly hygroscopic cement particles and the solubilization of a variety of ionic species, e.g. Na+, K+, Ca++, S04-, OH-, by complete or selective solubilization (surface hydrolysis) of the various phases present in cement occurs. Surface hydrolysis quickly leads to the formation of a thin layer of both amorphous and gel products. Beyond the initial solubilization phenomena, the formation of any of the solid hydration products (Table 7.31) will be governed by nucleation processes which may occur homogeneously from the solution phase or heterogeneously at a solid-solution interphase [64, 125],... 

The alkali cations in pfa normally occur almost entirely in the glass, and when the latter reacts may be presumed to enter the alkali-rich silicate that appears to be the initial product. When this phase is decomposed by reaction with Ca, they will be distributed, like alkali cations from any other source, between the solution and the solid hydration products, on which they are probably adsorbed (Section 7.3.2). The C-S-H tends to take them up more strongly as its Ca/Si ratio decreases (BI58,G63) consequently, the alkali cations released from the pfa are less effective in raising the OH concentration of the pore solution than are those released from the cement. The method outlined in Section 7.5.2 for calculating the OH concentration in the pore solution of Portland cement mix was extended to cover Portland-pfa cement mixes taking this into account (T37). 

The alkaline constituents of concrete are present in the pore liquid (mainly as sodium and potassium hydroxides. Section 2.1.1) but also in the solid hydration products, e. g. Ca(OH)2 or C-S-H. Calcium hydroxide is the hydrate in the cement paste that reacts most readily with CO2. The reaction, that takes place in aqueous solution, can be written schematically as ... 

Figure 3.2 Principle of the development of the volumetric proportions of the constituents of the cement paste during hydration. CS chemical shrinkage. Volumes is for cement V p, capillary water gel water, V, solid hydration products. (Based on Powers, T. C., Industrial and Engineering Chemistry, 27(7), 790-794, 1935.)...

Abdelrazig, Sharp El-Jazairi (1988, 1989) prepared a series of mortars based on a powder blend of MgO and ADP with a quartz sand filler. They were hydrated by mixing with water. A mortar I (MgO ADP silica water = 17T 12-9 70-0 12-5), with a water/solid ratio of 1 8, formed a workable paste which set in 7 minutes with evolution of ammonia. The main hydration product, struvite, was formed in appreciable amounts within 5 minutes and continued to increase. Schertelite also appeared, but only in minor amounts, within the first 5 minutes and persisted only during the first hour of the reaction. Dittmarite appeared in minor amounts after 15 minutes, and persisted. 

The overall process of cement hydration and setting results from a combination of solution processes, interfacial phenomena and solid-state reactions which lead to the formation of complex products. Some of the hydration products formed from the different mineral components of cement are shown in Table 7.31 [125]. Admixture-cement interactions are essentially interactions between admixtures and the initially formed cement hydrates the influence of admixtures on cement hydration is best considered by reference to the evolution of the reaction with time. Five stages can be identified [125, 126] ... 

As discussed in the previous section, trace elements are essentially retained in the solid combustion products and, because many are present on the surfaces of the particles, they are potentially leachable. Our data show the elements Mo, As, Cu, Zn, Pb, U, Tl, and Se will be readily accessible for leaching. A significant fraction of the V, Cr, and Ni, and a minor proportion of the Ba and Sr will also be potentially leachable because of the surface association, but most of these elements appear to be located in particles and will be released more slowly as the dissolution of the glass and other phases takes place. Rubidium, Y, Zr, Mn, and Nb are contained almost entirely within the particles and dissolution is potentially slower. The extent to which elements are leached also depends on their speciation and solubility in the porewaters, and the pH exerts a major control. In oxidizing solutions, elements such as, Cd, Cu, Mn, Ni, Pb, and Zn form hydrated cations that adsorb onto mineral surfaces at higher pH values and desorb at lower pH values. In contrast, the elements As, U, Mo, Se, and V, under similar Eh conditions, form oxyanions that adsorb onto mineral surfaces at low pH values and desorb at higher values (Jones 1995). 

Tho inventor prefers to apply this mode of concentration for obtaining soda from vat liquors, to such liquors as have been already deprived of a considerable portion of die carbonate of soda previously contained therein,. Tho circulation of these liquors through the tower Is continued until it ts found that tho whole, or nearly the whole, of the carbonate of soda is removed, when the liquor is separated from the deposited salts, and by further evaporation in an iron pan solid hydrate of soda is obtained, Tho separatod Saits, after being washed, are transferred to a reverberatory furnace and completely dried. The product of those Operations, whichever of the foregoing be adopted, is the white ash or soda ash of commerce. When properly prepared it contains fifty-three or fifty-four per cent, of available soda, two or three of which exist in the form of hydrate,-and the rest as carbonate of soda, A little sulphide of sodium is also present, and some Sulphate of soda, chloride of sodium, and minute quantities of other salts,. 

The NaHS solution is filtered to remove the sulfides of heavy metals, such as Fe, Hg, Ni, Mn, and Cu. The clear filtrate may be sold as a 44-46 percent solution of NaHS or evaporated in stainless steel equipment to crystallize a solid hydrate containing 70-72 percent NaHS, which is sold as a flake product. NaHS is easily converted to Na2S by further reaction with caustic ... 

The Step 2 product (0.40 mmol) suspended in methyl alcohol/water, 10 1, was treated with p-toluenesulfonic acid hydrate (0.04 mmol), then stirred 18 hours at ambient temperature, and then diluted with EtOAc. The organic solution was washed twice with saturated NaHC03 solution, once with brine, dried using Na2S04, filtered, and concentrated. The residue was recrystallized from EtOAc and heptane and the solid washed with heptane/CH2Cl2, 1 1. The solid was dried in vacuo at 60 C and the product isolated in 70% yield as a white solid. The product shrinks at 90.8°C, mp= 115-117°C. 

In this model, hep is assumed in the general case to comprise three components from the volumetric standpoint, viz. (a) unreacted cement, (b) hydration product and (c) capillary pores. Individual solid phases are not considered, whether in the cement or in its hydration products, which were collectively called cement gel . This term may be found confusing, because it includes the CH, which forms relatively large crystals and cannot reasonably be considered part of a gel we shall substitute the term hydration product . The water present in the paste was categorized as evaporable or non-cvaporable, the latter being defined in the later work (P34) as that retained on D-drying. Evaporable water, when present, was considered to reside partly in the capillary pores, and partly in so-called gel pores within the hydration product. This latter part was called gel water. 

The hydration product occupies more space than the cement from which it is formed, and the capillary pores were regarded as the remnants of the initially water-filled space. Their volume thus decreases, and that of the gel pores increases, as hydration proceeds. Evidence from water vapour sorption isotherms indicated that the hydration product was composed of solid units having a size of about 14 nm, with gel pores some 2 nm across (P34). The width of the capillary pores could not be determined from the available data, but they were considered to be generally much wider than the gel pores, though tending to become narrower as the water-filled space was used up, and thus in some regions indistinguishable from gel pores. 

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