J. Cent. South Univ. (2013) 20: 3729−3735 DOI: 10.1007/s11771-013-1901-4

Effects of H3PO4 and Ca(H2PO4)2 on mechanical properties and water resistance of thermally decomposed magnesium oxychloride cement

WEN Jing (文静)1, 2, YU Hong-fa(余红发)1, 3, 4, LI Ying(李颖)1, 2,WU Cheng-you(吴成友)1, 2,

DONG Jin-mei(董金美)1, 2, ZHENG Li-na(郑利娜)1, 2

1. Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China; 2. Graduate School of Chinese Academy of Sciences, Beijing 100049, China;

3. Department of Civil Engineering, Qinghai University, Xining 810016, China; 4. Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2013

Abstract: The effects of H3PO4 and Ca(H2PO4)2 on compressive strength, water resistance, hydration process of thermally decomposed magnesium oxychloride cement (TDMOC) pastes were studied. The mineral composition, hydration products and hydration heat release were analyzed by XRD, FT-IR, SEM and TAM air isothermal calorimeter, etc. After being modified by H3PO4 and Ca(H2PO4)2, the properties of the TDMOC are improved obviously. The compressive strength increases from 14.8 MPa to 48.1 MPa and 37.1 MPa, respectively. The strength retention coefficient (Kn) increases from 0.38 to 0.99 and 0.94, respectively. The 24 h hydration heat release decreases by 10% and 4% and the time of hydration peak appearing is delayed from 1 h to about 10 h. The XRD, FT-IR and SEM results show that the main composition is 5Mg(OH)2·MgCl2·8H2O in the modified TDMOC pastes. The possible mechanism for the strength enhancement was discussed. The purposes are to extend the potential applications of the salt lake magnesium resources and to improve the mechanical properties of TDMOC. Key words: thermally decomposed magnesium oxychloride cement (TDMOC); H3PO4; Ca(H2PO4)2; strength; water resistance; hydration heat

1 Introduction

Bischofite (MgCl2·6H2O) is a kind of salt lake industrial by-products in the process of producing potassium chloride (KCl). For 1 t of KCl, there is approximately 8−10 t MgCl2·6H2O produced and the present output of MgCl2·6H2O in China is more than 10 million tons [1]. This kind of salt lake magnesium resource causes the so-called “magnesium pollution”, breaks the balance between potassium and magnesium resource and seriously affects the normal production of KCl if it is not utilized appropriately [2]. It is an urgent issue how to utilize magnesium resource: bischofite.

Magnesium oxychloride cement (MOC), also known as Sorel cement, is an air-dried, magnesium- based cementitious material developed not long after the invention of Portland cement (PC) [3]. A lot of research on the reaction mechanism of magnesium oxychloride cement has been done worldwide [4−12]. These studies indicate that the main reaction products of hardened MOC pastes are 3Mg(OH)2·MgCl2·8H2O (abbreviated to

“Phase 3”) and 5Mg(OH)2·MgCl2·8H2O (abbreviated to “Phase 5”). Phase 5, crystallizing more rapidly than Phase 3, is the major strengthening phase in MgO/MgCl2/H2O system. However, after a long period of time, Phase 5 is transformed into Phase 3 which is more easily to react in the CO2 atmosphere to form 2MgCO3·Mg(OH)2·MgCl2·6H2O (abbreviated to “Phase 2116”) when the molar ratio of MgO to MgCl2 is less than 3. There are many advanced performance of MOC such as quick hardening, high early strength, low thermal conductivity, and good resistance to abrasion. However, its application scope is limited greatly owing to its poor water resistance and more complex preparation method compared with PC [13]. Hence, the improvement of the preparation method for traditional MOC is a very urgent issue to make good use of it as a common building material.

SUN et al [14−15] reported a new method to prepare MOC by partially thermal decomposing MgCl2·6H2O that came from Salt Lake in Qinghai Province, China. This cement is called thermally decomposed magnesium oxychloride cement (TDMOC).

Foundation item: Project(B0210) supported by One Hundred Talent Project of Chinese Academy of Sciences; Project(2008-G-158) supported by Science

and Technology Tackling Key Program of Qinghai Province, China Received date: 2012−07−27; Accepted date: 2013−01−11 Corresponding author: YU Hong-fa, Professor; Tel: +86−13912962664; E-mail: yuhongfa@nuaa.edu.cn

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It is composed of the thermal decomposition mixture of MgCl2·6H2O (MgO and anhydrous MgCl2) by mixing with a specified volume of water. The preparation method of TDMOC is more simple and easier than traditional MOC. However, during the hydration process of TDMOC, owing to the hydration heat released from hydration reaction and the heat from the dissolving of anhydrous MgCl2, pastes interior could reach very high temperature [16−17]. As a result, great temperature difference between the interior and the outer of pastes can cause serious thermal stress [18−22], weakening the strengths of the pastes.

Taking all above-mentioned issues into account, we investigate the effects of H3PO4 and Ca(H2PO4)2 on the compressive strength, water resistance, hydration heat, hydration products, and microstructure of TDMOC pastes. The final objective is to improve the strength and water resistance, decrease the hydration heat release of TDMOC so that this kind of TDMOC material can be widely and effectively utilized in building industry. 2 Experimental 2.1 Raw materials

The raw materials used for this experiment were obtained from the thermal decomposition products of MgCl2·6H2O coming from Salt Lake in Qinghai Province, China at decomposition temperature of 850 °C [14]. Figure 1 shows the XRD pattern of the raw material. From Fig. 1, the peaks of MgO and MgCl2·4H2O can be seen. The MgCl2·4H2O might be produced by the moisture absorption of anhydrous MgCl2 decomposed by MgCl2·6H2O. 1% H3PO4 and Ca(H2PO4)2 (analytically pure) by the mass fraction of raw materials were added respectively in the tests.

Fig. 1 XRD pattern of TDMOC raw material

2.2 Methods 2.2.1 Mechanical properties tests

The constant water to solid mass ratio (w/s) of 1.33

was used throughout the investigation. The pastes were mixed in a mechanical mixer and the specimens were cast in 40 mm×40 mm×40 mm and 40 mm×40 mm× 80 mm moulds. The fresh samples were covered with poly-ethylene sheet to prevent evaporation. After 24 h, the samples were removed from the mould and cured in air. The compressive strength and flexural strength of pastes were tested at 3, 7 and 28 d using a concrete compression machine (TYE-300). Each compressive strength value was the average of three samples.

After air curing of 28 d, part of the specimens were soaked in distilled water for 28 d. Then, residual strength of the specimens after water soaking was measured and compared to the reference strength of the specimens cured in air for the calculation of the strength retention coefficient. The larger the strength retention coefficient is, the better the water resistance. In this work, the strength retention coefficient is calculated by the following formula:

Kn=Rn/R0 where Kn is the strength retention coefficient, in which n is the days of specimen immersing in water; Rn is the strength of the wet specimens after immersion in water for n days; R0 is the strength of the dry specimens cured for 28 d before immersion in water. 2.2.2 XRD, FT-IR and SEM analysis

The crystalline phases of the TDMOC pastes after 28 d curing were identified by X-ray powder diffractometer made in Holland, using Cu target, radiation at 40 kV and 30 mA. Rietveld method was employed to perform qualitative analysis [23−24] using Topas 3.0 software [25].

For the infrared spectroscopic analysis (FTIR), 1 mg of sample was homogenized with 0.2 g KBr and the disks formed were examined in a FTIR spectrophotometer (Nicolet NEXUS, USA) in an absorbance range between 4 000 and 400 cm−1.

The microstructure of the TDMOC pastes after 28 d curing was characterized by scanning electron microscope (SEM, JSM-5610LV) on a fractured surface with gold coating. 2.2.3 Hydration heat test

An eight-channel isothermal calorimeter (TAM Air, Swiss) was used to measure the hydration heat evolution of TDMOC. The w/s ratio was 1.33 and experimental temperature was (20 ± 0.1) °C. Cement and water were tempered for several hours before mixing, then the water was injected into the reaction vessel and the samples were stirred in the calorimeter for several minutes. This procedure had a process of monitoring the heat evolution from the very beginning when water was added to cement. Data logging was continued for about 24 h.

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3 Results and discussion 3.1 Mechanical properties

Compressive strength is one of the important indices to evaluate product quality in the application of cementitious material [26]. The 3 d, 7 d, and 28 d compression strengths of TDMOC pastes before and after modification by H3PO4 and Ca(H2PO4)2 are shown in Fig. 2. The 28 d compressive strength and flexural strength of the TDMOC before modification are only 14.8 MPa and 2.09 MPa, respectively. After 1% H3PO4 and Ca(H2PO4)2 are added, respectively, in the TDMOC pastes, the rate of strength gain of the TDMOC pastes is significantly improved and the 28 d compressive strength (flexural strength) of the TDMOC pastes increase to 48.1 MPa (6.4 MPa) and 37.1 MPa (3.6 MPa), respectively. The results of the strengths indicate that H3PO4 and Ca(H2PO4)2 could obviously enhance the strengths of the TDMOC pastes.

Fig. 2 Compressive strength (a) and flexural strength (b) of

TDMOC pastes

3.2 Water-resistance

MOC has a poor water resistance [27], and the influences of H3PO4 and Ca(H2PO4)2 on improving the water resistance of MOC were studied. Table 1 shows

the compressive strengths of the pure and modified TDMOC pastes before and after immersion in water for 28 d. After being modified by H3PO4 and Ca(H2PO4)2, the strength retention coefficients (Kn) increase from 0.38 to 0.99 and 0.94, respectively, which suggests that H3PO4 and Ca(H2PO4)2 are effective admixtures to improve the water resistance of TDMOC. Table 1 Compressive strengths of TDMOC before and after

soaking (n=28 d)

Cement Compressive strength/MPa

KnR0 Rn

No additive 14.8 5.7 0.38

Adding 1% H3PO4 48.1 47.5 0.99

Adding 1% Ca(H2PO4)2 37.1 35.0 0.94

3.3 Hydration heat tests

To reflect the effect of H3PO4 and Ca(H2PO4)2 on the hydration process of TDMOC, the heat liberation of hydration was primarily measured during 24 h, as shown in Fig. 3 and Table 2. From Table 2, it can be seen that H3PO4 and Ca(H2PO4)2 reduce the hydration heat and hydration reaction rate. The 24 h total hydration heat of the TDMOC pastes is reduced by 10% and 4% with H3PO4 and Ca(H2PO4)2 and the hydration reaction ratio

Fig. 3 Hydration heat (a) and hydration reaction rate (b) of

TDMOC

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at the peak is reduced from 69.62 J/(g·h) to 26.94 J/(g·h). The occurring time of hydration peak is delayed from 1.44 to about 10 h, avoiding the TDMOC pastes from cracking in the hydration process, which provides guarantee for the enhancement of strength of the TDMOC pastes.

Table 2 Hydration heat released and reaction ratio of TDMOC

pastes

Cement

Total 24 h

hydration heat/

(J·g−1)

Reaction ratio at peak

Occurring

time/h

Reaction rate/

(J·g−1·h−1)

No additive 783.88 1.44 69.62

Adding 1% H3PO4 701.88 10.00 27.04

Adding 1%

Ca(H2PO4)2 749.52 9.83 26.94

3.4 FT-IR analysis

Figure 4 shows the FTIR spectra of TDMOC before and after modification by H3PO4 and Ca(H2PO4)2. In the IR spectra, the shapes and positions of bands at 3 710 cm−1 and 3 614 cm−1 are attributed to the oxhydryl band stretching vibration (νO—H) in Mg(OH)2 and Phase 5, respectively [28]. In Fig. 4(a), the peak intensity at 3 614 cm−1 is much lower than that at 3 710 cm−1, which indicates that more Mg(OH)2 and less Phase 5 present in the TDMOC pastes with no additive. Contrarily, by comparing the peak intensity at 3 614 cm−1 and 3 710 cm−1 in Figs. 4(b) and (c), it can be seen that more Phase 5 and less Mg(OH)2 present in the TDMOC pastes

Fig. 4 FTIR spectra of TDMOC pastes before and after

modification by H3PO4 and Ca(H2PO4)2

modified by H3PO4 and Ca(H2PO4)2. 3.5 XRD analysis

The crystalline phases of the TDMOC pastes before and after modification by H3PO4 and Ca(H2PO4)2 after 28 d, as identified by XRD, are shown in Fig. 5. The phase compositions are presented in Table 3, which was analyzed by Rietveld method [23−24]. It can be seen from Fig. 5 that the TDMOC pastes before modification after 28 d curing are primarily composed of the hydration products of Phase 3, Mg(OH)2 with minor amounts of Phase 5 and carbide Phase 2116. After being modified by H3PO4 and Ca(H2PO4)2, the main products of the TDMOC pastes at 28 d curing are Phase 5 with minor amounts of Mg(OH)2 and unhydrated MgO. This does not indicate that H3PO4 and Ca(H2PO4)2 yield new hydration products in TDMOC but change the content of hydration products. From Table 3, we can see that the TDMOC pastes with H3PO4 and Ca(H2PO4)2 generate more Phase 5 and less Mg(OH)2 than the pure TDMOC pastes. Moreover, the quantity ratios of Phase 3 and Phase 5 to Mg(OH)2 of 27.7 and 8.24 in the TDMOC pastes with H3PO4 and Ca(H2PO4)2 are greater than 0.92 for the corresponding pure TDMOC pastes. XRD analysis results reveal that the interaction of H3PO4 and Ca(H2PO4)2 with TDMOC not only prevents the formation of Mg(OH)2 and promotes the formation of Phase 5 but also prevents the conversion of hydration phase from Phase 5 to Phase 3 and carbonation of Phase 3 in the TDMOC pastes. Phase 5 is responsible for the strength of MOC, but Mg(OH)2 has a negative effect

Fig. 5 XRD patterns of TDMOC pastes

Table 3 Crystalline phases and Rietveld results of specimens for 28 d curing (mass fraction, %)

Cement Phase 3 Phase 5 Mg(OH)2 Phase 1126 MgO R

No additive 39.22 8.24 51.51 1.03 0 0.92

Adding 1% H3PO4 0 96.25 3.47 0 0.28 27.7

Adding 1% Ca(H2PO4)2 0.28 85.02 10.35 0 4.35 8.24

R: Quantity ratio of Phase 3 and Phase 5 to Mg(OH)2 in specimen.

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on strength [9, 12]. More Phase 5 and less Mg(OH)2 make the strength of the modified TDMOC pastes higher than that of the pure TDMOC pastes.

Figure 6 shows the XRD patterns of the pure and modified TDMOC pastes before and after immersion in water after 28 d. The phase compositions are presented in Table 4. From Fig. 6(a), it can be seen the main hydration products of the pure TDMOC pastes are Phase 3 and Mg(OH)2. After soaking in water for 28 d, Phase 3 has largely disappeared and been decomposed to Mg(OH)2, which leads to the strength reduction. From Fig. 6(b), it can be seen that, no matter immersion is in water or not, the composition of the TDMOC pastes modified by Ca(H2PO4)2 are mainly Phase 5 and minor amounts of Mg(OH)2. In other words, when the TDMOC pastes with small quantity of Ca(H2PO4)2 are immersed in water, Phase 5 will not be decomposed by water,

Fig. 6 XRD patterns of pure (a) and modified (b) TDMOC

paste before and after soaking

Table 4 Crystalline phases and Rietveld results of specimens

after soaking (mass fraction, %)

Cement Phase 3 Phase 5 Mg(OH)2 Phase 1126 MgO

No additive 7.63 1.02 89.41 1.94 0

Adding 1%

Ca(H2PO4)2 0 86.01 13.66 0 0.33

which may be the reason of no reduction of the strength of TDMOC pastes. 3.6 SEM analysis

Figures 7−9 illustrate the SEM micrographs of the TDMOC pastes before and after modification by H3PO4 and Ca(H2PO4)2 for 28 d, respectively. It can be seen that the TDMOC pastes before modification has a more unconsolidated microstructure than the modified TDMOC pastes. The unconsolidated microstructure, which is harmful for the strength, is possibly led by more Mg(OH)2 and less Phase 5 from the XRD analysis results. When 1% H3PO4 and 1% Ca(H2PO4)2 are added to the specimens, respectively, the microstructure becomes more compact, because of more Phase 5 and less Mg(OH)2 in the modified TDMOC pastes from the XRD analysis results. At the same time, it can be also seen that H3PO4 and Ca(H2PO4)2 make the crystal become more perfect and slender. This more compact and perfect structure makes the modified TDMOC have higher strengths than the pure TDMOC.

Figure 10 shows the SEM image of modified TDMOC after immersion in water. It can be seen that the modified TDMOC crystal after soaking is also perfect and slend, which can prove the strength of the TDMOC pastes remaining unchanged in water. 3.7 Relations between hydration heat and strength

H3PO4 and Ca(H2PO4)2 dissociate resulting negative ions (PO4

3−) in aqueous medium. At the stage of MgO

Fig. 7 SEM images of hydration products of pure TDMOC

pastes

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Fig. 8 SEM images of hydration products of TDMOC modified

by H3PO4

Fig. 9 SEM images of hydration products of TDMOC modified

by Ca(H2PO4)2

hydration, Mg2+ species form on the surface of the particles. As the PO4

3− rapidly adsorbs, a low solubility magnesium phosphate protective coating forms. The coating may prevent the hydration of MgO and reduce

Fig. 10 SEM image of hydration products of modified TDMOC

after soaking the concentration of Mg2+ hydrolyzed by MgO. The hydration heat is decreased by the coating. When the hydration heat is high, it is bad for the production of Phase 5 owing to the high temperature [29]. When H3PO4 or Ca(H2PO4)2 is added in the TDMOC pastes, the hydration heat can be released slowly, which is beneficial for the production of Phase 5. So more Phase 5 and less Mg(OH)2 are detected in the modified TDMOC pastes. Because Phase 5 is the major strengthening phase in MgO/MgCl2/H2O system, the TDMOC with H3PO4 and Ca(H2PO4)2 has higher strength than the pure TDMOC. 3.8 Water-resistance mechanism

As shown in Table 1, the strength retention coefficients (Kn) of the TDMOC without additive and the TDMOC with 1% of Ca(H2PO4)2 are 0.38 and 0.94, respectively. On the other hand, the XRD analysis results given in Table 3 and Table 4 illustrate that before the TDMOC with no additive is immersed in water, the main phases are Mg(OH)2 and Phase 3, but after being immersed in water for 28 d, Phase 3 has totally disappeared and Mg(OH)2 is mainly observed. Whereas, Phase 5 in TDMOC with Ca(H2PO4)2 remains almost unchanged after being immersed in water for 28 d. These results clearly indicate that the phase compositions in the pure TDMOC are totally changed by water. Thus, the strength retention coefficient is very small (Kn=0.38). However, the phase compositions of modified TDMOC remain unchanged in water. Thus, the strength retention coefficient is large (Kn=0.94). So, the water resistance of TDMOC pastes is related to the change of the phase compositions caused by water. The reason may be that Ca(H2PO4)2 is soluble and can be ionized in water to form such phosphate radical anions as H2PO4

−, HPO42−,

and PO43−. These anions can decrease the lowest

concentration of Mg2+ ions in the solutions required by the formation and stable existence of the main 5 phases in MOC pastes [30]. Thus, when the TDMOC modified by Ca(H2PO4)2 is immersed in water, the Phase 5 in it

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will not be decomposed by water, which makes the strength of the TDMOC pastes remain unchanged in water and provides the guarantee for the good water resistance. 4 Conclusions

1) H3PO4 and Ca(H2PO4)2 , as additives of TDMOC, can effectively reduce the 24 h hydration heat of TDMOC and delay the appearing time of hydration peak. The 24 h hydration heat release decreases to 10% and 4%, and the time of hydration peak appearing is delayed from 1 h to about 10 h. The 28 d compressive strength of the TDMOC can increase from 14.8 MPa to 48.1 MPa.

2) H3PO4 and Ca(H2PO4)2 not only prevent the formation of Mg(OH)2 and promote the formation of Phase 5, but also prevent the conversion of hydration phase from Phase 5 to Phase 3 and carbonation of Phase 3 in the TDMOC pastes.

3) H3PO4 and Ca(H2PO4)2 can not only make the surface morphology of the TDMOC pastes change from unconsolidated to compact, but also make the crystal become more perfect and slender than that of the pure TDMOC pastes.

4) H3PO4 and Ca(H2PO4)2 are effective admixtures to prevent the decomposition of Phase 5 in water and improve the water resistance of TDMOC. The strength retention coefficients increase from 0.38 to 0.99 and 0.94, respectively.

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(Edited by YANG Bing)