Reaction mechanism of magnesium potassium phosphate cement with high magnesium-to-phosphate ratio
Introduction
Magnesium phosphate cements, also known as magnesium phosphate ceramics, are clinker-free cements that have strong chemical bonding and high mechanical strength through acid-base reactions between magnesia and phosphate acid (H3PO4) or soluble acid phosphates, such as ammonium dihydrogen phosphate (NH4H2PO4), sodium dihydrogen phosphate (NaH2PO4), or potassium dihydrogen phosphate (KH2PO4) [[1], [2], [3], [4]]. Magnesium potassium phosphate (MKP) cement which uses KH2PO4 has been intensively studied in the past two decades [[4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]]. Rehabilitations of infrastructure [[4], [5], [6], [7], [8], [9], [10],[13], [14], [15],[19], [20], [21], [22], [23],[30], [31], [32], [33], [34]] and waste stabilization/solidifications [18,[27], [28], [29]] are the two main practical applications of MKP cement.
MKP cement is a ternary system consisting of magnesia, KH2PO4 and water. The principal hardening reaction of MKP cement is the following
As indicated in Eq. (1), magnesium potassium phosphate hexahydrate (MgKPO4·6H2O), or K-struvite, is the principal reaction product that provides mechanical strength of MKP cement-based composites. However, the reaction path that leads to the final precipitation of K-struvite is much more complicated than represented by Eq. (1). Variations of the ratios among magnesia, KH2PO4 and water can lead to different reaction products depending on time [10,25,28], as well as changed material performance, such as setting time, mechanical strength, and volume stability [7,8,[12], [13], [14],19,29,33]. Thus a number of mix proportions have been used for fabricating MKP cement-based composites in the past two decades. The ternary phase diagram of MgO-KH2PO4-H2O in Fig. 1(a) shows the mix proportions of MKP cement systems reported in open literature [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36]]. As displayed in Fig. 1(a), the distribution of these mix proportions can be generally divided into two zones.
Zone 1 includes mix proportions with the magnesium-to-phosphate (Mg/PO4) molar ratios ranging from 1 to 12 and high water-to-solid (w/s) mass ratios (w/s > 1). These mix proportions were employed for making MKP cement suspensions that mainly aimed to study the reaction mechanisms [10,25]. The mix proportions in zone 2 have the Mg/PO4 molar ratio ranging from 1 to 14, but small w/s ratios which were mostly lower than the theoretical value (0.51) for producing K-struvite (Eq. (1)). Normally, they were adopted for fabricating MKP cement-based composites that aimed for potential practical applications [[5], [6], [7], [8], [9],24,26,27,[29], [30], [31], [32], [33], [34], [35], [36]]. Moreover, as displayed in Fig. 1(b), optimum Mg/PO4 molar ratios for fabricating high strength MKP cement-based composites were in the range from 4 to 8 [7,8,19,33,34].
Understanding reaction mechanisms of MKP cement is of significant importance, as it is closely related to the optimum design of MKP cement-based composites. Wagh [3] proposed a three-step mechanism, which could be described as follows: (i) the first formation of aqueous Mg(H2O)62+, when magnesia is mixed with KH2PO4 solution; (ii) the percolation and formation of K-struvite gel as the aqueous magnesium reacts with the released K+ and PO43− ions in solution; (iii) saturation and crystallization of K-struvite gel. Recently, the reactions involved in setting and hardening of MKP cement have been investigated in more detail. It was found that different solids formed consecutively before the precipitation of K-struvite [10,25,28]. As displayed in Fig. 2(a), Chau et al. [10] reported the formation of two intermediate reaction products, MgHPO4·7H2O (phosphorrösslerite) and Mg2KH(PO4)2·15H2O, before precipitation of K-struvite in a MKP cement suspension with a Mg/PO4 molar ratio of 4 and a w/s ratio of 10. In a highly diluted MKP cement suspension with a Mg/PO4 molar ratio of 1 and a w/s ratio of 100, Lahalle et al. [25] observed Mg2KH(PO4)2·15H2O as the only intermediate reaction product, which was subsequently transformed to K-struvite and Mg3(PO4)2·22H2O (cattiite), as schematically demonstrated in Fig. 2(b). Rouzic et al. [28] studied the reaction evolution of a MKP cement paste with a Mg/PO4 molar ratio of 1 and a low w/s ratio of 0.2 and found the intermediate reaction product, MgHPO4·3H2O (newberyite), before the formation of K-struvite. These studies suggest that the Mg/PO4 ratio, the availability of water and the pH values [10,25,28] determine which hydrates precipitated during the reaction of MKP cement-based composites.
As discussed above, current studies of the reaction mechanism of MKP cement have been limited to those with low Mg/PO4 molar ratios (Mg/PO4 ≤ 4) [10,25,28]. In practice, higher Mg/PO4 molar ratios (Mg/PO4 > 4) are more frequently used for making MKP cement-based composites, particularly those for infrastructural applications [[5], [6], [7], [8], [9], [10],[13], [14], [15],[19], [20], [21], [22], [23],[30], [31], [32], [33], [34]]. However, their reaction mechanism is still not fully explored. In this study, two MKP cement systems with a high Mg/PO4 molar ratio of 8 and with two different w/s ratios of 0.5 and 5 were studied. Changes of the solid and liquid phase in these two systems during reaction were determined by various techniques.
Section snippets
Materials and mix design
Dead-burnt magnesia, KH2PO4, and deionized water were used as raw materials. Chemical compositions of magnesia and KH2PO4 were determined by X-ray fluorescence (XRF) analysis, and are given in Table 1. Besides MgO, Mg2SiO4 and CaMgSiO4 are present as minor phases in the magnesia. The mean particle size and BET surface area of the magnesia are 19.0 ± 0.3 μm and 0.55 ± 0.08 m2/g, respectively.
The reactivity of the dead-burnt magnesia was evaluated by using the acetic acid method [19,37] at acid
pH and electrical conductivity
The changes in pH and electrical conductivity of the MKP cement suspension are shown in Fig. 5. It can be seen in Fig. 5(a) that both pH and conductivity reach stable values after 3 h, suggesting a completion of the main chemical reactions. Compared with the highly diluted MKP cement system with a Mg/PO4 molar ratio of 1 and a w/s ratio of 100 reported in [25], the reaction rate of this MKP cement system with a Mg/PO4 molar ratio of 8 and a w/s ratio of 5 is much faster due to the higher
Conclusions
In this study, the reaction mechanisms of two MKP cement systems with a high Mg/PO4 molar ratio of 8 and two different w/s ratios of 0.5 and 5 were investigated. Based on the presented experimental findings, the following conclusions can be drawn.
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Compared with MKP cements with low Mg/PO4 molar ratios, the use of the high Mg/PO4 molar ratio (Mg/PO4 = 8) suppresses the formation of potassium-free magnesium phosphate hydrates such as MgHPO4·7H2O, MgHPO4·3H2O and Mg3(PO4)2·22H2O, while mainly
Acknowledgements
The authors express their thanks to Saint-Gobain Recherche, France (5211.01192), for the financial support, to J. Yammine-Malésys, R. Leiva Muňoz and N. Brielles for helpful discussions. Luigi Brunetti, Josef Kaufmann and Latina Nedyalkova (Empa, Laboratory for Concrete and Construction Chemistry) are acknowledged for their help with the experimental part of the work.
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