Variation in hybrid cements over time. Alkaline activation of fly ash–portland cement blends
Introduction
Concerns around the CO2 emitted during portland cement manufacture have driven the scientific community to seek alternative materials with the same features as portland cement but which can be sustainably manufactured.
Portland cement has been effectively replaced by SCMs (supplementary cementitious materials such as slag, natural pozzolans or ash) for many years, as amply documented in the literature [1], [2], [3], [4], [5], [6], [7], [8]. This practice has, moreover, been acknowledged by legislation the world over as a way of generating different types of cement. All authors have generally reported that the use of SCMs leads to significant improvements in cementitious systems whilst contributing to sustainability.
The use of fly ash as a partial replacement for portland cement, in addition to effectively reducing cement consumption, provides a solution for recycling this industrial by-product. A considerable number of references can be found in the literature on water-hydrated OPC–FA systems in which OPC replacement by FA varies over a wide range [9], [10], [11], [12]. European standard EN 197 limits the replacement ratio to under 35% for type II cements and under 55% for type IV cements [13], primarily because at higher percentages the early age mechanical strength of the cement declines substantially (when used alone, fly ash exhibits no hydraulic behaviour).
One proposal (and one of the principal aims of the present study) for raising the ash content in OPC blends to 70% without compromising initial mechanical strength development is the alkaline activation of the materials at ambient temperature.
The alkali activation of fly ashes (aluminosilicates) is a line of research that is obtaining very promising results in terms of the properties of the cements produced [14], [15]. The main differences between this procedure and normal portland cement hydration are the high alkaline content and the substantially lower total calcium content in the materials. In these systems the reaction product to which mechanical strength development and durability are attributed is a N-A-S-H gel, a three-dimensional alkaline aluminosilicate structure [14], [15], [16] very different from the C-S-H gel, the calcium silicate hydrate obtained in OPC hydration [17].
The reaction products forming during the alkaline activation of cement and ash blends is an area of keen scientific and technological interest, and the compatibility between the two cementitious gels, N-A-S-H and C-S-H (the main hydration products in the OPC–fly ash–alkali system), is the object of considerable research today [18], [19], [20]. Earlier studies have shown that the co-precipitation of these two gels in hybrid cements is possible [21], [22], [23], [24], [25], [26], although recent research has revealed that the two products do not develop separately as two distinct gels, but that they interact, undergoing structural and compositional changes in the process [20].
Recent studies on synthetic samples to analyse C-S-H/N-A-S-H compatibility in greater depth showed that the stability of the N-A-S-H structure in the presence of calcium depends heavily on the pH in the medium [20]. In the presence of sufficient calcium, pH values of over 12 favour the formation of a C-A-S-H rather than a N-A-S-H gel [19], [20]. The experiments yielding these findings were conducted in equilibrium conditions, however, which are not normally in place during binder hydration, particularly in the early stages of the reaction. The scientific evidence would appear to show that whilst reaction kinetics stimulates the early age formation of both types of gels, over time a C-A-S-H type gel would be the most thermodynamically stable product. Nonetheless, since the co-precipitation of the two gels (C-S-H + N-A-S-H) has been described for short reaction times (28 days at most) only [21], [22], [23], no information on the variations in these blends at longer reaction times (such as 1 year) has been forthcoming. In light of that knowledge gap, the present study aimed to determine the variations over time in the main cementitious gels formed in non-equilibrium conditions during the alkaline activation of hybrid cements consisting of 70% FA and 30% OPC.
Section snippets
Materials
The prime materials used in this study were commercial 52.5 NSR portland cement, supplied by MOLINS SA (Spain), and a type F fly ash (ASTM C618-94) [27] from the coal-fired steam plant at Puentenuevo, likewise in Spain. The chemical composition of the materials given in Table 1 was determined as specified in the European standard EN 196-2 [28] for the cement and as recommended in the Spanish standard UNE 80-225-93 [29] for the fly ash (wet chemistry method).
The hydration liquids used were
Mechanical strength
Fig. 1 shows the variations in compressive strength over time in the water-hydrated (MW) and the alkali-activated (MA) blends. As expected, mechanical strength rose with time in both systems.
Both the 28- and 365-day pastes hydrated in the presence of the alkaline activator had higher mechanical strength than observed in the respective water-hydrated pastes. As discussed below, this finding suggests that all the traditional products formed during water hydration of these mixes differ from the
Discussion
The significant conceptual differences between using alkaline solutions or water to hydrate cementitious materials explain the differences observed in the mineral composition and nanostructure of the hardened matrices. In materials hydrated with water (MW), normal portland hydration yields a C-S-H gel, although in the longer term the presence of a second C-S-H gel is detected (a product of the pozzolanic reaction); this gel is richer in silicon and contains some aluminium [7]. Since the
Conclusions
The main conclusions to be drawn from the present study are as follows:
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Binders with high initial (and longer term) mechanical strength can be manufactured using over 70% fly ash and on the order of 30% cement, if hydrated with alkaline activators.
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At early ages these hybrid cements generate a mix of C-S-H/N-A-S-H gels. These gels do not precipitate in a pure state, however, but are affected by the presence of dissolved species.
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N-A-S-H gel interacts with Ca over time. The polarising effect of Ca2 +
Acknowledgements
This research project was funded by the Spanish Ministry of Science and Innovation under project BIA2010-17530. The support provided by the Spanish Ministry of the Economy and Competitiveness in the form of a Post-graduate Studies Council grant, co-funded by the European Social Fund, is gratefully acknowledged.
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