The influence of the alkaline activator (NaOH, waterglass, or Na 2 CO 3 ) on the structure of the hydrated calcium silicate formed in alkali-activated slag (AAS) cement pastes has been investigated by FTIR, 29 Si and 27 Al magic-angle scattering nuclear magnetic resonance, and TEM/EDX techniques. In all cases, the main product formed after 7 d of activation, with activators giving an Na 2 O concentration of 4%, is a semicrystalline calcium silicate hydrate with a dreierkette-type anion. In these structures, linear finite chains of silicate tetrahedra (Q 2 units) are linked to central Ca-O layers, and tetrahedral aluminum occupies bridging positions in the chains. The main chain length and the amount of aluminum incorporated in the tetrahedral chains depend on the activator used. The detection of Q 3 silicon entities in alkaline-activated slags is discussed in relation to the possible formation of cross-linked structures that may be responsible for increased flexural and compressive strengths in AAS mortars. † Q slag ϭ Q 0 /Q 1 (slag); ‡ Q Total ϭ ⌺Q n where Q n stands for Q 1 , Q 2 , and Q 3 units

A study was conducted on the effect of a polycarboxylate (PC) admixture on the mechanical, mineralogical, microstructural and rheological behaviour of Portland cement pastes. It was observed that the presence of PC admixture retards the initial cement hydration reactions, although this effect may be offset by possible increased diffusion in later stages. Additionally, the PC admixtures produce a few alterations in the structure and composition of the formed C-S-H gel. The addition of 1% PC admixture in the pastes generates a higher percentage of silicate bridge (Si Q 2 units) mainly at 2 days. The admixture used in this study induced microstructural modifications in the pastes which slightly reduced the porosity; however the admixture did not affect the mechanical strength of the pastes at either 2 or 28 days of hydration. Finally, from the results of the rheological studies it was concluded that a low dosage of PC led to a substantial reduction (over 70%) in the yield stress.

The objective of this work was to determine the mechanisms that govern the setting times of alkali-activated slag (AAS) cement pastes, as a function of the nature of the alkaline activator used. For this purpose three different activators were used: waterglass (Na2SiO2.nHaOH with a SiO2/NaO ratio equal 1·5), NaOH and Na2CO3. The concentration of all solutions was constant = 3% of Na2O by mass of the slag. The pastes were studied through isothermal conduction calorimetry, FTIR and 29Si and 27Al MAS-NMR. Results indicate that pastes activated with waterglass develop faster setting due to the formation of an initial calcium silicate hydrate. Setting, in pastes activated with NaOH is due to the formation of a more polymerised calcium silicate hydrate. Pastes activated with Na2CO3 show longer setting times due to the initial formation of a sodium calcium carbonate which retard the reaction processes.

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