ICE - Institution of Civil Engineers - Virtual Library

Various methods of measuring the initial and final setting time of cements have been proposed and widely used. None of these, however, is satisfactory for a more scientific and fundamental investigation of the hydration process, since the hardening process could not be followed continuously. These indirect and empirical measurements led to many misconceptions on the hydration process and on the effect of accelerators and retarders, etc. The measurement of conductance provides a non-destructive and continuous method for studying the hydration of cements. The kinetics can be described by two parameters: an induction period, t₀, and one or two first-order reaction rate constants, k and k′, for the decrease of conductance. The influence of additives on both parameters has been studied extensively.

Neural network analysis was used to construct models of setting time as a function of mix composition and addition concentration using existing data for calcium aluminate cements. The models were able to represent the non-linear temperature dependency of setting times and generalised from the literature data to find exponential relationships between setting time and addition concentration. An induction time estimated from the results of different types of setting measurements was used in modelling to determine the following ranking of induction times: Ba > Ca ≥ Sr ≥ Mg ≥ K ≈ Na ≥ control ≫ Li and citrate ≥ Br⁻ ≫ SO42− > C1⁻ > control ≈ CO32− ≥ OH⁻ > PO43− ≈ NO3− > AlO2− High variability of the results reported in the literature resulted in a root-mean-square error of 174% for the best model, which may provide a method-independent estimate of the interlaboratory reproducibility for setting time measurement.

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.

Hydration of an ordinary Portland cement (OPC) with highly concentrated alkaline solutions has been carried out. Two different alkaline solutions were used: NaOH (10 M) and Na₂CO₃ (3·8 M). Mineralogical composition of the compounds formed after hydration process were determined by FTIR; XRD; DTA/TG. Soluble salts in water and in methanol–salicyclic acid were extracted and the amount of sodium fixed by the CSH gel was deduced. The main conclusion of this work is that if high concentration of NaOH is present in the hydrating solution, CSH gel breakdown is produced. Additionally, it has been showed that gypsum reacts with sodium producing Na₂SO₄. Finally, in the carbonation process a calcium±sodium carbonate is formed.

The results of scanning electron micrographs (SEM) obtained from samples of Pulverised Fuel Ash (PFA)-lime, PFA-lime-gypsum and PFA-cement mixtures subjected to various curing times are presented and interpreted in the context of a mechanism previously developed for the hardening behaviour of cementitious materials. Additionally energy dispersive X-ray analysis (EDAX) has also been carried out on the same samples. The micrographs suggest that the strength of the materials increase with time, partly as a result of the growth of cementitious materials in places where PFA particles are in contact with each other, and partly due to the needle-like growths which eventually link the PFA particles. Using EDAX it is found that when the beam of electrons is focused on the cementitious material a higher proportion of aluminium, potassium and silicon ions is found compared with that in a more overall field of view. The results suggest that the rate of hardening of these materials would be, in ascending order, PFA-lime, PFA-lime gypsum and PFA-cement.