Microstructure and Microchemistry of Slag Cement Pastes

Abstract

Pastes of a portland cement (60%) blended with a granulated blastfurnace slag (40%) were examined, principally by SEM with EDX analysis. Reaction rims around slag particles and relicts of fully reacted slag particles had compositions compatible with mixtures, in varying proportions, of C-S-H having Si/Ca -0.62 and a phase of the hydrotalcite family having Al/Mg -0.38. Calculations taking into account relevant densities and water contents indicated that replacement of the slag by its in situ hydration products entails little or no change in the numbers of Mg and 0 atoms per unit volume, but that substantial proportions of the Ca, Si and Al are released and an equivalent amount of H gained. In other respects, the microstructures qualitatively resembled those of pure portland cement pastes of similar ages, but less CH was formed and the C-S-H not formed in situ from the slag had a Si/Ca ratio of 0.56, higher than that of 0.50 to 0.53 found in the absence of slag. None of the individual phases in the slag cement pastes showed significant compositional variation with time in the 28 day to 14 month period studied. The relative amounts of Ca, Si and Al expelled from the slag are such that, in order to form C-S-H and AFm phase, more Ca is required. It is obtained partly at the expense of CH formation, and partly through increase in the Si/Ca ratio of the C-S-H formed from the clinker phases. Mass balance, volume composition and bound water content were calculated for the 14 month old paste and compared with corresponding results for the pure portland cement.

The hydration of slag, part 2: reaction models for blended cement

“…Cement 3 originates from the study of Harrisson et al [13]. Pastes of blended cement containing 40% slag were examined using SEM fitted with an energy dispersive X-ray (EDX) system.…”

“…However, the C/S ratio in C-S-H is still higher than the C/S ratio in the pure slag (about 1.0-1.1), indicating additional calcium demand by the slag hydration. This calcium demand is supplied either by the CH formed by the Portland cement hydration [12], or by the lower C/S ratio in the C-S-H [13]. In this respect, the CH produced by the hydration of Portland cement acts as not only an activator or catalyst to the slag hydration, but also as a reactant.…”

“…Summarizing, the hydration products present in the hydrating slag-blended cement paste include C-S-H, M 5 AH 13 , C 6 A " S 3 H 32 , CH and C 4 AH 13 . The Fe-containing hydrogarnet and other products are not considered due to their trace levels.…”

Three-dimensional computer modeling of slag cement hydration

“…The composition of C-S-H governs its various properties, including both physical and chemical properties. It is the subject of numerous researches, and one general conclusion is that replacing Portland cement with slag clearly decreases the C/S ratio in C-S-H [7,[48][49][50]. Increasing the slag proportions in the slag cement decreases the C/S ratio in C-S-H.…”

Recycling of the product of thermal inertization of cement–asbestos for the production of concrete

“…The larger amorphous fraction may be related to the formation of a hydrotalcite-like phase of very low crystallinity from the hydration of the Mg-rich phases (such as periclase) conveyed by KRYÁAS in the reacting system. Such pseudo-amorphous hydrotalcite-like phases are commonly formed by hydration of slag cements [29,35]. These phases have a composition similar to meixnerite Mg 0.75 Al 0.25 (OH) 2 (OH) 0.25 (H 2 O) 0.5 K 30 , which may also occur as mineral phase [29].…”