Hydration of Slag

Dissolution is an important process of slag hydration, particularly at early age. The sodium, potassium and aluminum ions, as well as the other basic elements of glass silicate network are released first to the solution [1, 2], The concentration of calcium ions, silica and alumina in the liquid phase is low (Fig. 8.1) [2]. 

However, Kondo [3 ] is of the opinion that the hydrolysis of the glass in water occurs and the calcium ions are released initially to the liquid phase. Simultaneously on the surface of slag grains an acid, colloidal shell of silica-alumina gel is formed. This shell has low permeability and hence the further slag reaction with water is hindered. In the presence of Ca(OH)2, added as alkaline activator, the siUcon and aluminum from the shell are released to the solution (Fig. 8.2). The solubiUty of aluminum compounds becomes considerably increased in the solution of pH higher than 12.5 because in this condition the Al(OH) ions are formed [4]. The concentration of aluminum in the liquid phase is increasing because the calcium aluminates crystallize a httle later, primarily the C-S-H (1) is formed. Simultaneously the solubility of hydrates formed in this condition is reduced. 

The examination of chemical composition of slag glass surface at the early age of hydration has shown that it is modified immediately after the contact with the Uquid phase [6]. As the result of incongment dissolution of slag grains on their surface the l er of C-S-H is formed, with, however, lower C/S ratio than in the Portland cement paste. When the hydration of slag is activated by alkalis, this phase contains the Na+ orK+ ions. 

According to Dron [8], the hydration of vitreous slag consists in its dissolution and crystallization of hydrates from the hquid phase. The dissolution occurs in the alkaline environment and the OH ions play a decisive role. The change of free enthalpy of this system is depending on the solubilities of the anhydrous phase and of the hydrates-reaction products. 

Because the solubility products of activators [Ca(OH)2, CaS04-2H20] and hydrates (C-S-H, ettringite, C4AH ) are constant at given temperature, the change of 

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Battagin, A.F. (1992) Influence of degree of hydration of slag cement, in Proceedings 9th ICCC New Delhi, Vol. 3, pp. 166-172. 

This reaction - in contrast to OPC - is slow and slag cement needs an activator, usually NaOH, KOH or sulfates. In blast furnace slag cement, a mixture of OPC and up to 70% slag, these activators are present. The hydration of slag does not produce Ca(OH)2 but rather consumes it, incorporating the reaction products into the cement gel. The quantity of free lime in hydrated... 

Thermal techniques such as DTA and conduction calorimetry have been applied extensively for a study of the mechanism of hydration of slag, divitrilication in slags, the rate of hydration of slags under different conditions, the identification of compounds, and the effect of various activators. 

The rate of hydration of slag cements has been followed by conduction calorimetry, TG, and The temperature of curing... 

Superplasticizing admixtures are used widely in high performance concrete production. Not only do they influence the rheological parameters, but also the setting characteristics. These effects depend on the type and dosage of the admixture. In Fig. 17, the role ofthree types of superplasticizers, viz., 0.5% Ca-SNF, commercial SMF, or Na-SNF on the hydration of slag cements is examined. Addition of the superplasticizer results in the retardation in terms of the time of appearance of the exothermal peak and also a decrease of the peak intensity. Na-SNF retards most of the superplasticizers studied. 

Mascolo, G. Marino, O. 1980. MgO-bearing phases in the hydration products of slag cement. 

C-S-H = poorly crystalline or amorphous calcium silicate hydrate of unspecified composition. Ggbfs = ground granulated blast furnace slag. Hep = hardened cement paste. Pfa = pulverised fuel ash (fly ash). 

Another type of product with similar properties is represented by fired bricks additionally impregnated with tar. The operation comprises evacuation and subsequent forcing of the tar into the pores under pressure. Also in this case, the residual carbon from the tar improves resistance to corrosive and erosive effects of slag. With dolomite materials, the tar also effectively protects the material from hydration. 

Figure 1.7 Microstructure of hydration of Portiand cement [a), and cements with addition of fiy ash (fa) and blast furnace slag (c) (Bakker from [5])...

The resistivity of concrete is an important parameter used to describe, for example, the degree of water saturation, the resistance to chloride penetration or the corrosion rate. The resistivity of concrete may have values from a few tens to many thousands of n m (Table 2.3) as a function of the water content in the concrete (relative humidity), the type of cement used (Portland or blended cements), the iv/c, the presence of chloride ions or whether the concrete is carbonated or not At early ages, the resistivity of concrete is low and considerable increases occur due to hydration of the cement AU of these factors can be rationalised on the basis of ion migration in the porous and tortuous concrete microstructure a high relative humidity increases the amount of water-filled pores (decrease of resistivity), the iv/c ratio and type of cement determine the pore volume and pore-size distribution (less but more coarse pores with pure Portland cement more but finer pores and less interconnectivity of pores with blast furnace slag or fly ash) chloride ions increase the conductivity of the pore solution and carbonation decreases it. An increased resistivity is accompanied by a reduced corrosion rate [38]. Table 2.4 shows resistivities determined for mature concrete in various chmates [39-41]. 

CO2 (molecular weight 44) that can react with a concrete produced with 300 kg/m Portland cement that we can suppose is composed by 64% of CaO (molecular weight 56) is 300 X 0.64 X 44/56 150 kg/m. In the case of blast furnace slag cement with 70 % of GGBS, the percentage of CaO is only 44%. For other blended cements, the quantity of CaO is somewhere between these two values [3]. For blended cement, hydration of pozzolanic materials or GGBS also leads to a lower Ca(OH)2 content in the hardened cement paste which may increase the carbonation... 

X-Y Wang and H-S Lee, Modeling the hydration of concrete incorporating fly ash or slag. Cement and Concrete Research, in press 2010. 

The method of differential calorimetry is one of the best method of cement heat of hydration measurements especially at early period [39]. In Ihe world standards the method of the dissolution heat is also very popular [40]. This method is particularly suitable for long period of hydration, even for one year [41-43]. It caimot be used in the case of cements with the addition of slag or fly ash, which are not totally dissolved in the acids mixture. 

Fig. 5.67 Reduction of capillary porosity in the paste as a result of C-S-H phase formation, at addition of slag or fly ash (schematically) a capillary pores in cement paste after 15 day of hydration. b modification of pore structure as a result of C-S-H phase formation after longer time, (according to [140])...

Granulated blastfurnace slag can hydrate without activation too [2]. The calcinm hydroxide necessary to activate the reaction of slag glass is probably formed as a result of calcium sulphide hydrolysis ... 

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