Investigation of magnesium phosphate cement hydration in diluted suspension and its retardation by boric acid

Cement and Concrete Research 87 (2016) 77–86

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Investigation of magnesium phosphate cement hydration in diluted suspension and its retardation by boric acid Hugo Lahalle a, Céline Cau Dit Coumes a,⁎, Adel Mesbah b, David Lambertin a, Céline Cannes c, Sylvie Delpech c, Sandrine Gauffinet d a

CEA, DEN, DTCD, SPDE, F-30207 Bagnols-sur-Cèze cedex, France ICSM, UMR 5257 CEA/CNRS/UM/ENSCM, Site de Marcoule—Bât. 426, BP 17171, 30207 Bagnols-sur-Cèze cedex, France Institut de Physique Nucléaire, CNRS, Univ. Paris-Sud 11, 91406 Orsay Cedex, France d UMR6303 Laboratoire Interdisciplinaire Carnot de Bourgogne, Université de Bourgogne Dijon, Faculté des Sciences Mirande, 9 Avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France b c

a r t i c l e

i n f o

Article history: Received 11 September 2015 Received in revised form 25 November 2015 Accepted 27 April 2016 Available online xxxx

a b s t r a c t Magnesium phosphate cements (MPCs) are used for rapid repair works, but they may also offer prospects for the stabilization/solidification of deleterious waste. MPCs contain calcined magnesium oxide and a water-soluble acid phosphate, such as potassium dihydrogen phosphate (KH2PO4). The main precipitated hydrate is then Kstruvite (MgKPO4·6H2O). This work aims at giving new insight into the processes involved in its formation. Since cement hydration is very rapid, the second objective is to understand how boric acid, a common admixture for field application, retards cement hydration. A multi-stage process is evidenced in diluted suspension: MgHPO4·7H2O likely precipitates first. This phase is then destabilized to form Mg2KH(PO4)2·15H2O which is finally converted into K-struvite and cattiite (Mg3(PO4)2·22H2O). Boric acid doesn't slow down the initial dissolution of the reactants, but rather retards the precipitation of the products. Besides, it tends to favor the formation of cattiite against that of K-struvite. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Magnesium phosphate cements (MPCs) are now well-known for rapid repair works due to their outstanding properties of fast setting and hardening, good volume stability and excellent bonding to old concrete structures [1–3]. Moreover, they may also offer prospects for the stabilization/solidification of deleterious wastes which must be conditioned in a stable, monolithic and confined form prior to disposal [4–9]. Magnesium phosphate cement includes magnesium oxide calcined at high temperature (hard-burnt or dead-burnt) and acidic watersoluble phosphate salt, generally diammonium hydrogen phosphate [10,11]. When an ammonium salt is used, the main product responsible for setting and hardening is struvite, NH4MgPO4·6H2O. Byproducts such as dittmarite, schertelite, newberyite and magnesium orthophosphate tetrahydrate (Table 1) may precipitate under some conditions [12]. However, the reaction of magnesium oxide with diammonium hydrogen phosphate has the disadvantage of producing ammonia. To avoid the release of noxious gaseous ammonia during the hardening process, ammonium phosphate can be advantageously replaced by potassium

⁎ Corresponding author. E-mail address: [email protected] (C. Cau Dit Coumes).

http://dx.doi.org/10.1016/j.cemconres.2016.04.010 0008-8846/© 2016 Elsevier Ltd. All rights reserved.

dihydrogen phosphate (KH2PO4). The main precipitated hydrate is then K-struvite (MgKPO4·6H2O) [10,11]. MgO þ KH2 PO4 þ 5H2 O → MgKPO4  6H2 O

ð1Þ

Such a material, known as Ceramicrete, has been developed in the United States at the Argonne National Laboratory to stabilize hazardous waste [4,13]. Magnesium phosphate cement prepared from MgO and KH2PO4 in the proportions defined by Eq. (1) has a chemical water demand corresponding to a water/cement weight ratio of 0.51 (cement: MgO + KH2PO4). The pH of the KH2PO4 solution, near 4, increases and stabilizes around 8 after adding MgO [4]. The reaction is highly exothermic. When large volumes of material are prepared, an autocatalytic phenomenon is observed: the heat output increases the temperature of the paste, which further accelerates reaction, so that nearly instantaneous setting can be observed. A retarder such as boric acid (H3BO3) (typically 2–3 wt.% of the binder) must be added to control setting and limit the temperature rise. Several studies focused on the processes involved in the setting and hardening of magnesium potassium phosphate cement. For Mg/P ratios comprised between 4 and 12, C. K. Chau et al. [14] showed that precipitation of K-struvite (MgKPO4·6H2O) was preceded by that of MgHPO4·7H2O, and then of Mg2KH(PO4)2·15H2O. The precipitation sequence depended on pH: MgHPO4·7H2O predominated in acidic

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Table 1 Magnesium phosphate minerals investigated in this study. Name

Formula

Name

Formula

Struvite Dittmarite Schertelite Phosphorösslerite Newberyite

NH4MgPO4·6H2O NH4MgPO4·H2O (NH4)2Mg(HPO4)2·4H2O MgHPO4·7H2O MgHPO4·3H2O

K-struvite – Cattiite Magnesium orthophosphate tetrahydrate Lünebergite

MgKPO4·6H2O Mg2KH(PO4)2·15H2O Mg3(PO4)2·22H2O Mg3(PO4)2·4H2O Mg3B2(PO4)2(OH)6·6H2O

solution (pH ~ 4), K-struvite formed at pH above 7, and Mg2KH(PO4)2· 15H2O was observed at intermediate pH values (6 b pH b 8). By monitoring the electrical conductivity of cement suspensions at lower Mg/P ratio (equal to 1), Le Rouzic [15] reported only a two-step mechanism, involving the initial precipitation of newberyite (5 ≤ pH ≤ 8), and the subsequent formation of K-struvite (6 ≤ pH ≤ 9). Recently, Gardner et al. [16] characterized the phase assemblage of hydrated MPC blended with fly ash or blastfurnace slag. They showed that in addition to K-struvite, the main hydrate, the materials also comprised an amorphous orthophosphate mineral, potentially a potassium aluminosilicate phase, resulting from the partial reaction of fly ash or slag. Given the high reactivity of MPCs, boric acid is extensively used to delay cement hydration [17,18]. The process involved in retardation is however controversial. Several assumptions have been postulated: (i) precipitation of a coating layer of lünebergite (Mg3 B2 (PO 4 )2 (OH) 6·6H2 O) at the surface of the cement grains, which would slow down their dissolution [19,20], (ii) adsorption of B(OH)3 at the surface of the MgO grains, which would also slow down their dissolution [19], or (iii) stabilization of aqueous magnesium by formation of a magnesium borate complex in solution [16,19]. The objective of this study was thus twofold: – give new insight into the processes leading to precipitation of K-struvite; and – explain the retardation effect of boric acid.

The focus was placed on a binder comprising equimolar amounts of MgO and KH2PO4. Excess of MgO was avoided to discard the risk of deleterious expansion of the material in the long term due to slow hydration of residual MgO into Mg(OH)2. Experiments were performed on diluted suspension. This method can bring valuable information on the hydration process of cement as long as saturation of the solution with respect to the cement hydrates can be reached. It has already been successfully used to investigate the different stages of Portland cement hydration, and the acceleration or retardation effect of various admixtures [21,22]. It makes possible a regular titration of the magnesium, potassium, phosphate and borate ions resulting from the dissolution of MgO, KH2PO4 and B(OH)3 respectively, which would be more difficult in a cement paste. Moreover, it increases the duration of the different stages of hydration, which is of great help to investigate highly reactive systems. Species retarding cement hydration often influence the dissolution of cement anhydrous phases or the nucleation and growth of cement hydrates and most processes are interfacial ones, involving sorption of ions or molecules onto surfaces of minerals [21,23,24]. The

important parameter is then the ratio between the amount of sorbing species and the surface of reacting minerals. In this study, it was thus decided to investigate B/Mg molar ratios representative of those encountered in cement pastes, which resulted in lower concentrations of boric acid than in the pastes because of the higher water content. 2. Experimental 2.1. Materials The raw materials used in this study were hard-burnt magnesia (MgO: MAGCHEM 10 CR from M.A.F. Magnesite BV; particle size distribution: d10 = 4.8 μm, d50 = 18.9 μm, d90 = 45.6 μm; specific surface area ≈ 0.9 m2/g, purity 98.3%, fire loss 0.25%), potassium dihydrogen phosphate KH2PO4 (VWR, purity N98%, d10 = 175 μm, d50 = 365 μm, d90 = 594 μm) and deionized water. Five cement suspensions were investigated. The Mg/P molar ratio was kept to 1 and the water-to-cement weight ratio was fixed to 100. This value resulted from a compromise aiming at: - increasing the number of aliquots to be sampled for analysis without changing significantly the water-to-cement ratio, - increasing the duration of the different stages of hydration, and - reaching saturation of the solution with respect to the cement hydrates.

Boric acid was added at different concentrations (0 mmol/L, 4.17 mmol/L, 10 mmol/L, 20 mmol/L and 41.7 mmol/L). The 4.17 mmol/L concentration corresponded to a boric acid-to-cement weight ratio of 2.5%, which is a typical dosage in MPC pastes or mortars. The mixture proportions are presented in Table 2. 2.2. Characterizations Hydration of cement suspensions was investigated using a Multicad CDM 210 conductimeter. Experiments were performed in a thermoregulated vessel (25 °C) under mechanical stirring. The vessel was tightly closed to avoid carbonation and evaporation. The cell was calibrated with a standard KCl solution (12.888 mS/cm at 25 °C) before every trial. Cement suspensions (~250 mL) were generally prepared according to the following protocol: (i) dissolve KH2PO4, and possibly H3BO3, in 250 g of demineralized water under mechanical stirring (≈200 rpm), and (ii) add MgO and maintain stirring until the end of the experiment. At constant temperature, the electrical conductivity of the suspension depends on the number and type of ions in solution.

Table 2 Composition of diluted MPC systems (for 250 g of water). System reference

Water/cement wt. ratio

Mg/P ratio

MgO (g)

KH2PO4 (g)

H3BO3 (g)

[H3BO3] (mmol/L)

H3BO3/cement ratio (wt.%)

#1 #2 #3 #4 #5

100 100 100 100 100

1 1 1 1 1

0.570 0.570 0.570 0.570 0.570

1.930 1.930 1.930 1.930 1.930

0 0.0645 0.154 0.309 0.645

0 4.17 10 20 41.7

0 2.58 6.16 12.36 25.80

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steps with 50 s measurement time per step. The data were then refined by the Rietveld method using the Fullprof_suite package [25]. A pattern of pure silicon was also collected under similar conditions, and then used to extract the instrumental function. During the Rietveld refinement of each powder pattern, the standard profile and structural parameters were allowed to vary (except the atomic positions). Moreover, an anisotropic size model was applied to each phase to simulate the microstructural effect. Thermogravimetric analyses were carried out under N2 atmosphere on 50 ± 2 mg of ground sample using a TGA/DSC Netzsch STA 409 PC instrument at 10 °C/min up to 1000 °C. 3. Results and discussion 3.1. MPC hydration in the absence of boric acid Fig. 1. Evolution of pH and electrical conductivity over time for diluted MPC suspension #1 comprising MgO, KH2PO4 and H2O (w/c = 100).

Variations in the conductivity thus indicated changes in the chemical composition of the aqueous phase. The pH of the cement suspensions was also recorded every 1 min with a pH electrode (Mettler Toledo Inlab Expert Pt1000 pH 0–14 T 0–100 °C) calibrated using standard buffers between 4.1 and 9.2. The composition of their aqueous phase was determined after increasing periods of hydration. 1 mL aliquots were sampled using a syringe, filtered at 0.2 μm and diluted with a 2 wt.% HNO3 solution before analysis by ICP-AES. The analytical error was ±5%. Cement hydration was stopped after fixed periods of time for mineralogical characterizations. The suspensions were filtered under vacuum on a Buchner funnel. The solid phases were rinsed with isopropanol, and dried at 22 ± 2 °C and 20% relative humidity. Crystallized phases were studied by powder X-ray diffraction (PXRD) using the PanAlytical X'pert Pro diffractometer equipped with X'celerator detector and having copper radiation (λKα1,2 = 1.54184 Å) in the Bragg Brentano geometry. All the PXRD patterns of pastes ground by hand to a particle size less than 100 μm were collected in the range of 2θ = 5° to 70° in 0.017°

3.1.1. Electrical conductivity and pH evolution Fig. 1 shows the evolution of the electrical conductivity and pH of suspension #1 with ongoing hydration. The electrical conductivity increased rapidly during the first 1 h 10 min, reached a maximum, decreased from 1 h 10 min to 5 h, increased again from 5 h to 7 h, and finally decreased until the end of the experiment. The pH increased from 4.3 to 10.4 over the duration of the test, but in a nonmonotonous way: two transient plateaus were noticed at 1 h 10 min and around 5 h. 3.1.2. Mineralogical evolution The phase assemblage evolution of system #1 was characterized by X-ray diffraction (Figs. 2, 3-a) and thermogravimetry (Fig. 4-a) with ongoing hydration. MgO was rapidly consumed from 50 min to 2 h. Mg2KH(PO4)2·15H2O precipitated first at 1 h 30 min and was subsequently converted, from 5 h, into K-struvite and Mg3(PO4)2·22H2O (cattiite). It was fully exhausted at 30 h. When hydration was over, the solid fraction comprised 81% K-struvite, and 19% cattiite. Two weight losses were evidenced by TGA: below 100 °C (from 50 min), when the samples contained Mg2KH(PO4)2·15H2O or Mg3(PO4)2·22H2O, and at 136 °C (from 1 h 30 min), which could be attributed to the dehydration of K-struvite [15]. The 50 min-old sample already exhibited a small

Fig. 2. X-ray diffraction patterns of the solid fraction of system #1 at different stages of hydration ([B] = 0 mmol/L).

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with more bound water than K-struvite. Note that the amount of bound water calculated from the phase assemblage determined by Rietveld analysis fitted rather well with that determined experimentally by thermogravimetry (total weight loss at 500 °C), which validated the Rietveld quantification method (Fig. 4-c). 3.1.3. Chemical evolution of the solution Solid phase characterization of the cement suspensions was supplemented by analyses of the liquid phase by ICP-AES. Fig. 5 shows the evolution over time of the Mg, K and P concentrations in solution for system #1. From 0 to 30 min, the K and P concentrations remained constant, equal to their initial values (taking into account the measurement uncertainties), whereas the Mg concentration increased sharply. This first stage thus corresponded to dissolution of MgO. Then, from 30 min to 1 h, the K concentration still remained constant, whereas the Mg and P concentrations started to decrease. This result suggested the precipitation of a potassium-free magnesium phosphate mineral. However, such a compound could not be detected by X-ray diffraction, either because it was formed into too small amounts, or because it was poorly crystallized. From 1 h to about 5 h, a simultaneous decrease in the concentrations of P and K was observed. The Mg concentration slightly increased from 1 h to 2 h, before decreasing again afterwards. The ratio between the slopes of the regression lines fitting the K and P concentration curves was close to 0.5 (calculated value of 0.55 ± 0.5), which was consistent with the precipitation of Mg2KH(PO4)2·15H2O identified by X-ray diffraction and thermogravimetry. The Mg concentration resulted from a competition between two types of reactions: precipitation of Mg2KH(PO4)2·15H2O which consumed Mg2+ ions, and dissolution of MgO and possibly of the magnesium phosphate hydrate previously formed, which released Mg2+ ions in solution. From 1 h to 2 h, the latter predominated over the former. Rietveld analysis of the solid fraction confirmed the abrupt decrease in the MgO content during this period. From 5 h to 8 h, the K and P concentrations exhibited a transient increase, whereas the Mg concentration remained almost constant. XRD analysis showed a progressive dissolution of Mg2KH(PO4)2·15H2O durions in solution. ing this period, which released Mg2+, K+ and HPO2− 4 The amount of potassium released in solution exceeded that of phosphate, which was not consistent with the stoichiometry of Mg2KH(PO4)2·15H2O. This result could be explained by assuming an incongruent dissolution of Mg2KH(PO4)2·15H2O, or, more likely, the precipitation in the same time of another phosphate-containing but potassium-free mineral. The only compound identified by X-ray diffraction during this period was MgKPO4·6H2O. However, Mg3(PO4)2·22H2O was detected later, from 16 h. Precipitation of cattiite might in fact have started earlier, from 5 h, but into too small amounts to be detected by X-ray diffraction. After 8 h, the concentrations decreased again. The slope ratio between the regression lines fitting the K and P concentration curves between 10 and 35 h was slightly lower than unity (calculated value of 0.85 ± 0.05), showing that a potassium-free phase could precipitate in addition to MgKPO4·6H2O. This phase was identified as Mg3(PO4)2·22H2O by X-ray diffraction.

Fig. 3. Rietveld quantification of the crystallized phases in the MPC suspensions. (a) System #1 ([B] = 0 mmol/L); (b) System #2 ([B] = 4.17 mmol/L); (c) System #5 ([B] = 41.7 mmol/L).

weight loss at 80 °C, suggesting the presence of Mg2KH(PO4)2·15H2O into too small amount to be detected by X-ray diffraction. The solid fractions of the suspensions exhibited a more important total weight loss from 3 h 30 min to 16 h than at the end of the experiment (30 h). This resulted from their high content in Mg2KH(PO4)2·15H2O, a phase

3.1.4. Discussion Hydration of MPC in diluted suspension occurred in three steps: 1/ likely precipitation of a magnesium phosphate hydrate free from potassium; 2/ precipitation of Mg2KH(PO4)2·15H2O, and 3/ precipitation of K-struvite and cattiite. According to Chau et al. [14], the magnesium phosphate hydrated formed initially may be phosphorösslerite MgHPO4·7H2O. Le Rouzic et al. [15] rather reported the formation of newberyite MgHPO4·3H2O. It should be noted however that phosphorösslerite is very unstable and easily transforms into newberyite [26]. It is then possible that phosphorösslerite precipitates in the cement paste or suspension, but is later converted into

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Fig. 4. Thermogravimetry analysis of the solid fraction of systems #1 to #5 at different stages of hydration. (a) Derivative weight loss of system #1 ([B] = 0 mmol/L); (b) Evolution of the bound water, calculated from the total weight loss at 500 °C, for sytems #1 ([B] = 0 mmol/L), #2 ([B] = 4.17 mmol/L) and #5 ([B] = 41.7 mmol/L); (c) Bound water content of system #1: comparison between experimental data measured by TGA and calculated values based on the phase assemblage determined by Rietveld analysis.

newberyite when the solid samples are dried prior to mineralogical characterization. Dissolution of MgO (Eq. (2)) and balance equation (Eq. (3)), describing the formation of phosphorösslerite from MgO and H2PO− 4 , consumed some protons and could thus explain the initial pH increase. þ

2þ

MgO þ 2H → Mg

þ H2 O

ð2Þ

MgO þ H2 PO4 − þ Hþ þ 6H2 O → MgHPO4  7H2 O

ð3Þ

By contrast, conversion of phosphorösslerite into Mg2KH(PO4)2· 15H2O (Eq. (4)) released some protons, which could explain why the pH increase strongly slowed down around 1 h, time at which potassium ions started to be consumed from the solution to form Mg2KH(PO4)2·15H2O. 2MgHPO4  7H2 O þ Kþ þ H2 O → Mg2 KHðPO4 Þ2  15H2 O þ Hþ

ð4Þ

In the same way, conversion of Mg2KH(PO4)2·15H2O into K-struvite (Eq. (5)) also yielded some protons, which could account for the slight decrease in pH observed from 3 h 20 min to 7 h 20 min. Mg2 KHðPO4 Þ2  15H2 O þ Kþ → 2MgKPO4  6H2 O þ 3H2 O þ Hþ

ð5Þ

The final pH (10) was higher than that reported for cement pastes (typically between 7 and 8 [4]). This could result from different phase assemblages. In suspension, cattiite formed in addition to K-struvite, possibly because of the excess of water. In paste, K-struvite was the sole detected hydrate. The Chess software [27] and its thermodynamic database supplemented with phosphate minerals (Table 3) were used to calculate the pH of a solution in equilibrium with (i) K-struvite, or (ii) with a mix of K-struvite and cattiite. The calculated values were respectively 7.88 and 10.01, which fitted well with the experimental data reported for both systems.

Fig. 5. Composition evolution of the aqueous fraction of system #1 with ongoing hydration ([B] = 0 mmol/L).

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Table 3 Boron-containing aqueous species and phosphate minerals added to the Chess database. Species Aqueous

Minerals

B(OH)− 4 B3O3(OH)− 4 B5O6(OH)− 4 B4O5(OH)2− 4 + MgB(OH)4 MgKPO4·6H2O Mg3(PO4)2·22H2O

Formation reaction

Log K (T = 25 °C, P = 1 bar)

Ref.

+ B(OH)3(aq) + H2O → B(OH)− 4 +H + 3B(OH)3(aq) → B3O3(OH)− 4 + H + 2H2O + 5B(OH)3(aq) → B5O6(OH)− 4 + H + 5H2O 4B(OH)3(aq) → B4O5(OH)2− + 2H+ + 3H2O 4 + Mg2+ + B(OH)3 + H2O → MgB(OH)+ 4 +H K+ + Mg2+ + HPO2− + 6H2O → MgKPO4·6H2O + H+ 4 3 Mg2+ + 2HPO2− + 22H2O → Mg3(PO4)2·22H2O + 2H+ 4

−9.24 −7.21 −7.01 −15.89 −7.35 10.762 −1.596

[28–32] [33,34] [29,30] [29,35–37] [38–41] [42] [38]

3.2. Influence of boric acid on MPC hydration 3.2.1. Electrical conductivity and pH evolution Fig. 6 compares the electrical conductivity and pH of suspensions with and without boric acid. During the first 30 min, all systems exhibited very similar evolutions. In particular, the initial slope of the curves plotting electrical conductivity versus time remained constant, meaning that the initial dissolution rate of MgO did not depend on the boric acid concentration. Retardation in the systems containing boric acid was noticeable afterwards. Conductivity and pH exhibited non-monotonous variations with time, which indicated again a multi-stage process. Raising the initial boric acid concentration slowed down the hydration process, increased the area of the second peak of electrical conductivity and the residual conductivity value at the end of the experiment, but decreased the final pH (pH 8.8 at 32 h for system #5, against 10.2 for system #1).

Fig. 6. Influence of the initial boric acid concentration on the electrical conductivity (a) and pH (b) of MPC suspensions (C–C0 stands for the electrical conductivity, corrected from its initial value before the addition of MgO).

3.2.2. Mineralogical evolution The phase assemblage evolution of systems #2 ([B(OH)3] = 4.17 mmol/L) and #5 ([B(OH)3] = 41.7 mmol/L) were characterized by X-ray diffraction (Figs. 3-b and -c, 7, 8) and thermogravimetry (Fig. 4-b). System #2 exhibited the same phase evolution as the reference: Mg2KH(PO4)2·15H2O was the first hydrate to be detected by X-ray diffraction. It was subsequently converted into K-struvite and Mg3(PO4)2·22H2O. When the boric acid concentration was raised to 41.7 mmol/L, phosphorösslerite was transiently observed, in addition to Mg2KH(PO4)2·15H2O, at 2 h. Note that the initial precipitation of MgHPO4·7H2O was also postulated for reference system #1, based on the solution analyses. Increasing the boric acid concentration had two striking effects: dissolution of Mg2KH(PO4)2·15H2O was delayed, as well as precipitation of K-struvite. Moreover, at the end of the experiment, the fraction of cattiite tended to increase whereas that of K-struvite decreased. The systems with boric acid did not contain any crystallized boron-containing mineral, such as lünebergite (Mg3B2(PO4)2(OH)6·6H2O) mentioned in the literature [18,19], in sufficient amount to be detected by X-ray diffraction. It cannot be excluded that, because of the high w/c ratio investigated in this study, saturation with respect to lünebergite was never reached in our systems. As expected, the fraction of bound water increased faster in reference system #1 than in systems #2 and #5 retarded by boric acid (Fig. 4-b). However, the higher the initial boric acid concentration, the higher the fraction of bound water at the end of the experiment. This resulted from the increased content of cattiite, a more hydrated phase (7.3 mol of H2O per mol of Mg) than K-struvite (6 mol of H2O per mol of Mg). 3.2.3. Chemical evolution of the solution Fig. 9 compares the evolution over time of the B, Mg, K and P concentrations in solution for systems #1 to #5. Regardless of the initial boric acid concentration, the boron concentration remained constant during the whole experiment, equal to its initial value. The initial dissolution of MgO was not influenced by the presence of boric acid: during the first 30 min, all systems exhibited the same Mg concentration in solution. However, differences were noticed afterwards. The Mg concentration reached a higher value in the presence of boric acid, and then decreased less rapidly. It was governed by two competing processes: MgO dissolution (which released Mg2 + ions in solution), and precipitation of MgHPO4·7H2O and Mg2KH(PO4)2·15H 2O (which consumed Mg2 + ions from the solution). Since dissolution of MgO did not seem to depend on the boric acid concentration, it was rather the precipitation of the hydrates which was affected. Thermograms at 50 min and 1 h 30 min confirmed that the amount of precipitated hydrates was much smaller in the samples containing boric acid than in the reference. During this period, their aqueous concentrations of phosphates and potassium were also higher. The transient increase in the Mg, P and K concentrations occurred later in the presence of boric acid, but was higher. These concentrations depended on the dissolution rate of Mg2KH(PO4)2·15H2O, and on the precipitation rates of K-struvite and cattiite. Accumulation of ions in solution either resulted from a faster dissolution of Mg2KH(PO4)2·15H2O,

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Fig. 7. X-ray diffraction patterns of the solid fraction of system #2 at different stages of hydration ([B] = 4.17 mmol/L).

or more likely, from a slower precipitation of K-struvite and cattiite. The first assumption is indeed contradicted by Rietveld analysis of the solid fractions, showing a slower consumption of Mg2KH(PO4)2·15H2O in system #5 than in system #1. After 20 h, the residual aqueous concentration of potassium, and to a lesser extent that of phosphate, tended to increase with the initial boric acid concentration. 3.2.4. Discussion The monitoring of the electrical conductivity (Fig. 6) and the analysis of the Mg concentration released in solution at early age (Fig. 9) showed that the initial dissolution rate of MgO was not influenced by the

concentration of boric acid. Complementary experiments were carried out to investigate the dissolution rate of KH2PO4: 1.93 g of KH2PO4 were dissolved under constant stirring in 250 mL of a solution containing boric acid at a concentration of 0, 4.17, 10, 20 or 41.7 mmol/L. Whatever the boric acid concentration, the electrical conductivity of this suspension increased rapidly and reached a plateau after 30 s, corresponding to full dissolution of KH2PO4 (Fig. 10). Thus, retardation of MPC by boric acid could not be explained by a slower initial dissolution of the reactants (MgO and KH2PO4). Similarly, the assumptions of a delay due to precipitation of a coating layer of lünebergite at the surface of the cement grains or due to adsorption of B(OH)3 at the surface of the MgO grains could be ruled out.

Fig. 8. X-ray diffraction patterns of the solid fraction of system #5 at different stages of hydration ([B] = 41.7 mmol/L).

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Fig. 9. Influence of the initial boric acid concentration on the chemical composition of the aqueous fraction of MPC suspensions.

Indeed, the total boron concentration in solution showed no variation with ongoing hydration. Moreover, lünebergite was never detected by X-ray diffraction. Boric acid seemed to retard the precipitation of the hydrates, either directly or indirectly by stabilizing Mg in solution. Soudée [20] postulated that the MgB(OH)+ 4 complex might be involved. Its formation constant was measured by several authors [34–37] (Table 3). The solution speciation of systems #2 and #5 was calculated using the Chess software and its enriched database after 30 min and 1 h respectively, when the total magnesium concentration reached its maximum. In both cases, the MgB(OH)+ 4 concentration was negligible (1.9 μmol/L

for system #2, 26 μmol/L for system #5) (Table 4). Thus, MgB(OH)+ 4 didn't seem to play a key role in these systems. Boric acid not only retarded the precipitation of the cement hydrates, but also changed the composition of the final phase assemblage. Increasing the boric acid concentration promoted the precipitation of cattiite against that of K-struvite. It also increased the residual concentration of potassium in solution. This could be simply explained by electrical balance. At pH ~ 10, boric acid was partly ionized, mainly as − B(OH)− 4 (pKa B(OH)3/B(OH)4 = 9.24 at 25 °C, see Table 3). These

Table 4 Thermodynamic calculation of the solution speciation for systems #2 and #5 after 30 min and 1 h of hydration respectively.

Total concentrations measured experimentally

Calculated speciation

Fig. 10. Influence of the initial concentration of boric acid on the dissolution of KH2PO4. (C–C0) represents the electrical conductivity, corrected from its initial value before the addition of KH2PO4.

System #2

System #5

[Mg] = 14.4 mmol/L [P] = 58.3 mmol/L [K] = 57.4 mmol/L [B] = 4.17 mmol/L pH 6.7 [K+] = 52.8 mmol/L [H2PO− 4 ] = 27.2 mmol/L [HPO2− 4 ] = 15.8 mmol/L [MgHPO4] = 8.7 mmol/L [Mg2+] = 4.3 mmol/L [B(OH)3aq] = 4.1 mmol/L [KH2PO4] = 2.6 mmol/L [KHPO4−] = 2.0 mmol/L [MgH2PO+ 4 ] = 0.7 mmol/L [MgP2O2− 7 ] = 0.5 mmol/L [MgPO− 4 ] = 0.1 mmol/L [MgB(OH)+ 4 ] = 1.9 × −3 10 mmol/L

[Mg] = 18.8 mmol/L [P] = 56.3 mmol/L [K] = 56.6 mmol/L [B] = 41.7 mmol/L pH 6.8 [K+] = 52.3 mmol/L [B(OH)3aq] = 41.4 mmol/L [H2PO− 4 ] = 22.2 mmol/L [HPO2− 4 ] = 17.8 mmol/L [MgHPO4] = 9.9 mmol/L [Mg2+] = 4.4 mmol/L [KHPO− 4 ] = 2.2 mmol/L [KH2PO4] = 2.1 mmol/L [MgP2O2− 7 ] = 0.7 mmol/L [MgH2PO+ 4 ] = 0.6 mmol/L [B(OH)− 4 ] = 0.2 mmol/L [MgPO− 4 ] = 0.2 mmol/L [MgB(OH)+ 4 ] = 2.6 × 10−2 mmol/L

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additional negative charges needed to be compensated by cations. Thermodynamic calculations showed for instance that: – for a pH of 9.6 and total P, Mg and B concentrations of 9.04 mmol/L, 0.43 mmol/L and 4.17 mmol/L respectively (values measured experimentally at 24 h for system #2), a total K concentration of 21.1 mmol/L was needed to keep the electroneutrality of the solution; and – for a pH of 8.7 and total P, Mg and B concentrations of 18.79 mmol/L, 1.26 mmol/L and 41.7 mmol/L respectively (values measured experimentally at 24 h for system #5), a total K concentration of 46.9 mmol/L was needed to keep electroneutrality.

These calculated values were in rather good agreement with those measured experimentally (24.5 mmol/L for system #2, 43.9 mmol/L for system #5) given the experimental uncertainties. The increased solubility of potassium in the presence of boric acid could explain why cattiite, a potassium-free hydrate, precipitated in higher amounts. 4. Conclusion This work, focused on the hydration of magnesium phosphate cement in diluted suspension (w/c = 100) and on its retardation by boric acid, led to the following conclusions. 1. Hydration of MPC is a multi-step process. At w/c = 100 (diluted cement suspension), MgHPO4 ·7H 2 O first precipitates. This phase is then destabilized to form Mg2 KH(PO 4 )2 ·15H 2 O which is finally converted into K-struvite (MgKPO4·6H2O) and cattiite (Mg3(PO4)2·22H2O). 2. Under the conditions of the study (diluted suspension), boric acid retards MPC hydration, as previously observed with cement pastes. Boron is not precipitated within the cement hydrates, nor adsorbed at the surface of the MgO grains, but rather remains in solution. Boric acid does not slow down the initial dissolution of the reactants (MgO and KH2PO4), but retards the precipitation of the products. It cannot be excluded that a complex, stabilizing Mg in solution, might be involved in the retardation process. However, it seems unlikely that the Mg(BOH)+ 4 species, previously postulated in the literature, plays a key role. 3. Boric acid modifies the composition of the hydrated cement. In basic medium, boric acid is dissociated into anionic forms (B(OH)− 4 , and polyborates at high boron concentration). These negative charges are compensated by an increase in the aqueous concentration of potassium, which in turn tends to favor the formation of Mg3(PO4)2·22H2O against that of K-struvite. Future work will be focused on the influence of the w/c ratio on the mechanism by which boric acid retards MPC hydration. There is still an open question about the possible precipitation of lünebergite at lower w/c ratio and on its influence on the rate of hydration. Acknowledgments This work was supported by the interdisciplinary NEEDS project funded by ANDRA, CNRS, EDF, AREVA, CEA and IRSN. Pascal Antonucci (CEA) is greatly acknowledged for his help with XRD characterizations. References [1] J. Bensted, Rapid setting magnesium phosphate cement for quick repair of concrete pavements – characterization and durability aspects – discussion, Cem. Concr. Res. 24 (1994) 595–596. [2] Q. Yang, B. Zhu, X. Wu, Characteristics and durability test of magnesium phosphate cement-based material for rapid repair of concrete, Mater. Struct. 33 (2000) 229–234. [3] F. Qiao, C.K. Chau, Z. Li, Property evaluation of magnesium phosphate cement mortar as patch repair material, Constr. Build. Mater. 24 (2010) 695–700.

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