In earlier work, we have observed discrepancies relating to the early hydration of calcium aluminate cement (CAC) when comparing data from heat flow calorimetry of CAC paste with results from mortar strength tests using the crushing method. Here, we investigated on this phenomenon and found that the sand which is used as a filler exerts a major influence on CAC hydration resulting in acceleration. Furthermore, in particular fine filler materials such as, for example, microsilica, fine limestone powder, and especially α-and γ-Al 2 O 3 also produced a strong hydration accelerating effect which is dependent on their specific surface area. The mechanism underlying the acceleration is that under alkaline conditions their negative surface charge attracts calcium ions as was confirmed via inductively coupled plasma atomic emission measurements. Such a layer generates favourable conditions for the nucleation of CAC hydration products (C-A-H phases). The resulting crystalline hydrates which form on the surface of the filler particles submerged in CAC cement pore solution were visualized via SEM imaging. This way, specifically selected fillers can significantly accelerate CAC hydration and save precious lithium salts which are commonly used to boost the early strength of CAC.
Following
a previous study where we presented on the surprising
accelerating effect of alginates on calcium aluminate cement (CAC),
we now systematically screened specific groups of biopolymers, hoping
to identify other polysaccharides that also can accelerate CAC. Testing
of pure, noncompounded alginates from different species/genera of
algae revealed comparable strong acceleration, while chemical modification
via esterification, decarboxylation, or sulfatation reduced the accelerating
effect, thus highlighting the importance of high anionic charge and
the presence of carboxylate groups as key structural features. Furthermore,
biopolymers possessing a “cavity” that effectively can
chelate and capture Ca2+, such as alginate, were found
as another key structural unit; hence, pectin and ι- as well
as κ-carrageenan were identified as biopolymers that possess
a similar accelerating effect to alginate. Among other natural, synthetic,
or semisynthetic biopolymers, karaya, gellan, and xanthan gum as well
as agarose produced a slight accelerating effect, whereas konjac gum,
hydroxypropyl guar, and methyl hydroxyethyl cellulose ether either
perform neutral or retard CAC hydration. A preliminary mechanistic
study revealed that effective accelerators reduce the concentration
of free Ca2+ present in the cement pore solution and that
a combination of high anionic charge, presence of a Ca2+ capturing cavity, and a high M
w is required
for a biopolymer to act as an accelerator in CAC. Our concept of using
biopolymers such as alginate allows us to replace at least partially
lithium salts (e.g., Li2CO3), which are currently
applied to accelerate CAC but are much needed for the production of
Li-ion batteries that are necessary for widespread electromobility.
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