Alkali-activated cements

Roy, D.M., Silsbee, M.R., and Wolfe-Confer, D. (1990) New rapid setting alkali-activated cement composition. Materials Research Society Symposium Proceedings 179,203-218. 

Brough, A.R. et al. (1995) Microstractural aspects of zeolite formation in alkali activated cements containing high levels of fly ash. Materials Research Society Symposium Proceedings 310, 199-208. 

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). 

Handbook of alkali-activated cements, mortars and concretes... 

Jiang, W Wu, X. Roy, D. M. 1993. Alkali activated fly ash - slag cement based nuclear waste forms. Materials Research Society Symposium Proceedings, 294, 255-260. 

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). 

Bakharev, T., Sanjayan, J.G., and Chen, Y.-B. (1999) Alkali activation of Australian slag cements. Cement and Concrete Research 29, 113-120. 

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

Shi, C. (1996) Strength, pore stmcture and permeability of alkali-activated slag mortars. Cement and Concrete Research 26,1789-1799. 

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

Song, S., and Jennings, H.M. (1999) Pore solution chemistry of alkali-activated ground granulated blast-furnace slag. Cement and Concrete Research 29,159-170. 

Talhng, B., and Brandstetr, J. (1993) Chnker-free concrete based on alkali-activated slag. n Mineral Admixtures in Cement and Concrete (ed. S.N.Ghosh), ABI Books, New Delhi, pp. 296-341. 

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. 

Katz, A. (1998) Microscopic study of alkali-activated fly ash. Cement and Concrete Research 28,197-208. 

Portland cement and related binders should not be used in applications where the concrete surface is permanently exposed to pH values lower than 7. Calcium aluminate cement (see section 10) resists acids somewhat better than Portland cement, and may be applied down to pH values as low as 4. Supersulfated cement (see section 8.4) also performs relatively well in diluted acid solutions down to pH=4. Binders highly resistant to acid solutions include alkali silicate cement (see section 15.3), geopolymer cements (see section 15.7.1), and alkali-activated fly ash-slag cement (see section 9.1.5, and Lu and Li, 1997). 

Solutions of alkali hydroxides generally do not exhibit a corrosive action towards pastes of hardened Portland cement and related binders. Only at very high concentrations may a moderate corrosion become apparent, probably owing to degradation of the hydrated aluminate phases. Also highly resistant to alkaline solution are alkali-activated slag binders (see section 8.5). In contrast, hydrated calcinm aluminate cement may be attacked by high-pH solutions. 

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). 

As an alternative to Portland cement, alkali-activated blended cements (see section 8.5 and 9.1.5) have also been suggested as a binder in wood-based materials (Lin et al, 1994). The hardening of these binders is little affected by the quality of the wood employed. 

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