Alkali-activated slag cement

A Fernandez-Jimenez. and F.Puertas, Alkali-activated slag cements Kinetic studies Cement and Concrete Research, Vol. 27, N°3 (1997) p. 359... 

Fig. 6.54 Shrinkage of the samples from alkali activated slag cement paste without Portland eement elinker, with Na SiO addition in concentrated MgClj solution. (After [246])...

Fig. 6.56 Pores size distribution in the alkali activated slag cement paste after 2 years of curing in the magnesium chloride solution of high concentration. (According to [245])...

Concentration of these ions in pore solution of concrete plays a decisive role in the formation of efflorescence. Pore solution, in the case of ordinary cements, is always satinated with calcium hydroxide— the basic somce of efflorescence. The concentration of potassium and sodium is important too the potassium content is always several times higher. These both components occm in pore solution in the form of hydroxides, as a result of reaction with calcimn ions. Particularly sodium is a very mobile ion and in the case of concrete produced from alkali activated slag cement, with sodimn compound addition and without Portland cement clinker, Na+ ions migrate to the smface and the white sodimn carbonate effloreseenee is formed. 

Also in other countries alkali activated slag cements, with Na2Si03 and gypsum [101], as well as cement F with 5-8% sodiiun activator and Ugnosulphonate as a plasticizer [102] are produced. 

The structure of C-S-H phase formed in alkali activated slag cements was studied by Deja [98], He found the elongation of sihcate chains, that is the ordering of the stmcture with its transformation closer to tobermorite, as a function of lydration time. [98]. The aluminate ions occur in the paste of this cement primarily in C-S-H (even about 7 % AI2O3), however, they can form also hydrogamet phase rich in silicon [98]. 

Alkali-activated slag cements (called also AAS cements) consist of ground granulated blast furnace slag and an alkaline activator whose role is to activate the Itydration of the... 

The water requirement of alkali-activated slag cements is relatively low, owing to the plasticizing effect of the alkali compounds that are present, resulting in a lower total porosity of the hardened material, as compared with Portland cement mixes. The produced fresh AAS cement based concrete mixes exhibit a distinct thixotropy and require continuous mixing, to prevent a quick slump loss and setting. Owing to their thixotropic properties even stiff mixes may be compacted if vibration is applied. 

At comparable consistencies of the starting mix, alkali-activated slag cement pastes exhibit lower porosity than comparable Portland cement pastes, owing to the lower initial water/solid ratio. The proportion of pores with r<10 nm is usually higher in hardened AAS pastes (Shi et al, 1992) however, the actual pore size distribution also depends on the activator used. In a comparative study, mixes produced with sodimn silicate exhibited the finest and those made with NaOH the coarsest pore stmcture (Shi, 1996, 1997). The specific surface area of AAS cement pastes is higher (by about 35-55%) than that of comparable ordinary Portland cement pastes (Tailing and Brandstetr, 1993). 

The liberation of heat in the hydration of alkali-activated slag cements is eharacterized by one or two peaks within the first few minutes of hydration and an additional peak within a few hours after mixing. The overall heat evolution within the first 24 hours may or may not surpass that of Portland cement (Shi and Day, 1995a, 1995b, 1996). 

Femandez-Jimenez, A., and Puertas, F. (1997a) Alkali-activated slag cements kinetic studies. Cement and Concrete Research 27,359-368. 

Shi, C. (1997) Early hydration and microstructure development of alkali-activated slag cement, in Proceedings 10th ICCC, Gdteborg, paper 3ii099. 

Wang, S.D. et al. (1995) Alkali-activated slag cement and concrete. A review of properties and problems. Advances in Cement Research 7,93-102. 

Finally, a separate category of low-energy cements are represented by the alkali-activated cements (see section 15.7), including alkali-activated slag cement (see section 8.5) and geopolymeric cement (see section 15.7.1). 

Under equal conditions the diffusion coefficients for d vaiy greatly in different cements (see Table 20.4). In general, blended cements that contain blast furnace slag, fly ash or silica fume exhibit a lower permeability to chloride ions and are more suitable for protecting the steel reinforcement from corrosion (Jensen and Pratt, 1989 Aiya et al, 1990 Schiessl and Raupack, 1992 Deja, 1997 El Sayed et al, 1997 Wiens and Schiessl, 1997). An extremely low permeability was also found in alkali-activated slag cement activated with water glass (Deja, 1997). 

Alternative sources of gypsum such as plasterboard waste and red gypsum can be used with slag and other sources of alkalis such as slaked lime or cement kiln dust to produce sulphate-alkali activated slag cement [8, 9]. 

MAS NMR study of high-pressure hydrous magnesium silicates has been published. NMR data have also been reported for MgAl204, f Al), CaAUO , Mg-Ca-Si-Al-O-N glasses, f Al), " magnesium hydrotalcite, f Al), " alkali-activated slag cement, f Al, acid-activated Ca-bentonite,... 

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