The crystal structure of a new calcium aluminate phase containing formate

Abstract

A new calcium aluminate phase containing formate ions was synthesized and its crystal structure determined. This new phase is indicated as M-phase and was firstly observed in Portland cement pastes hydrated in presence of Ca-formate and in excess of water. The crystal structure of the M-phase was successfully solved in the R-3 space group of the trigonal system on the basis of synchrotron X-ray single crystal diffraction data. The structural model was confirmed by Rietveld refinement of the powder diffraction data acquired on the synthesized pure sample. The crystal structure of the M-phase is similar to that of ettringite, being characterized by columns of AlO₆ octahedra alternating with groups of three edge-sharing CaO₇ polyhedra. The formate ions (HCOO)⁻ share two oxygens with Ca polyhedra and are located in the interspace between the columns. The crystal structure of the M-phase testifies the strong interaction occurring between small organic molecules as formate and the calcium aluminate components of Portland cement.

Introduction

The use of organic admixtures to tailor the performance of concrete is a consolidated practice whose development dates back to the beginning of the 1950s [1]. Nowadays, the concrete industry uses a wide range of admixtures to accelerate or retard setting, to modify the rheological behaviour, to entrain air or to control the hardening process. Calcium formate belongs to the category of non-corrosive accelerators of cement setting and hardening and it is extensively used in prestressed and reinforced concrete applications [2].

It is known that calcium formate induces an acceleration of C₂S and C₃S hydration at early stages leading to a more rapid development of calcium silicate hydrate (C-S-H) and consequently reducing the setting time of the cement paste [3,4]. The effect on calcium silicates is attributed to the higher diffusion rate of (HCOO)⁻ ions which facilitate the dissolution of C₃S and C₂S [5,6]. A different interpretation was proposed by Lota [7] who observed that calcium formate is likely reacting with alkalis in the pore solution precipitating Ca(OH)₂ (CH in cement-chemistry notation). The early precipitated CH acts as nucleation seeds thus promoting the further precipitation of CH. Lota's hypothesis is based on the assumption that formate ions in solution combine with alkali to precipitate K- or Na-formate and the remnant Ca ions forms CH, however he did not consider the different solubility of the involved salts, being Ca-formate less soluble than K- or Na-formate. We can hypothesize that the excess of Ca ions released in solution by Ca-formate contributes to supersaturate the solution with respect to CH thus promoting the early precipitation of CH. Lota also observed a retarding effect of calcium formate at increasing additions, invoking a poisoning effect of formate incorporated onto C-S-H surface according to the model proposed by Jennings et al. [8]. Considering the different interpretations proposed in the literature, it emerges that a comprehensive understanding of the mechanism through which calcium formate operates is still to be accomplished. It is worth mentioning that the use of Ca-formate has been established on the basis of empirical optimization of the mixtures whereas the formate-cement interaction at molecular scale has not been deeply investigated.

Lota et al. [7] pointed the attention to calcium aluminate components of cement, suggesting that calcium formate readily reacts with C₃A to form C₃A·Ca(HCOO)₂·nH₂O, a calcium aluminate phase with a layered structure hosting formate ions in the interlayer (formate analogous of AFm phase). It is also suggested that, depending on the C₃A over Ca(HCOO)₂ ratio, a calcium aluminate phase containing six moles of formate ions (C₃A·3Ca(HCOO)₂·nH₂O) could form. This phase is described as the formate analogue of AFt phase [6,7]. These authors prospect the possible formation of calcium aluminate phases with formate ions, but no experimental data on their identification and characterization are reported. Going back to early literature on cement, Jones reports that needle-like crystals of C₃A·3Ca(HCOO)₂·nH₂O were prepared through precipitation from calcium aluminate solution [9]. No crystal structure characterization by X-ray diffraction was performed at that time.

An extensive work on the synthesis and characterization of calcium aluminate phases containing formate was presented by H. Pöllmann and co-workers, who observed that, depending on the concentration of formate and condition of hydration, layered formate-AFm or solid solutions of AFm-phases containing formate or formate-AFt with an ettringite-like structure can crystallize [[10], [11], [12], [13]].

In this work we present the crystal structure of a calcium aluminate phase containing formate with structural features resembling that of an AFt-phase. This new phase has been indicated as the M-phase and was obtained from a Portland cement paste with the addition of a high dosage of calcium formate. The M-phase was additionally obtained by reacting C₃A with calcium formate in excess of water. The crystal structure of the M-phase gives evidences of the strong interaction occurring between small organic molecules as formate and the calcium aluminate components of the cement phases. The proposed structural model of M-phase fills a gap in the knowledge of the possible hydration products of Portland cement and on their interaction with formate ions.