WANG Nan, YU Hongfa*, BI Wanli, GUAN Yan, GONG Wei,ZHANG Na, WU Chengyou

(1. Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; 2. School of Materials and Metallurgy, Liaoning University of Science and Technology, Anshan 114051, China; 3. School of Civil Engineering, Qinghai University,Xining 810016, China)

Abstract: Sodium dihydrogen phosphate (NaH2PO4) and potassium dihydrogen phosphate (KH2PO4)were selected as additives for magnesium oxysulfate (MOS) cement. The phase composition and the microstructure of MOS cement were characterized using X-ray diffraction (XRD), thermogravimetric analysis(TG-DSC), Flourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). It is found that both NaH2PO4 and KH2PO4 lead to an increase in the compressive strength and an improvement in the volume stability of MOS cement. The XRD, MIP and SEM results show that the addition of NaH2PO4 or KH2PO4 does not change the phase composition of MOS cement but promotes the formation of strength phase of 5Mg(OH)2·MgSO4·7H2O (5·1·7 phase). This phase brings a considerable improvement in the microstructure of MOS cement, which has a positive effect on the properties of MOS cement.

Key words: magnesium oxysulfate cement; phosphates; compressive strength; volume stability;microstructure

1 Introduction

Magnesium oxysulfate cement[1,2]is an airhardening inorganic gelling material, which has gained extensive application in light insulation panels and fireproof material due to its properties of light weight[3],thermal insulation[4]and fire protection[5]. MOS cement avoids the use of chlorides so it can be used for the manufacturing of reinforced concrete. Besides, MOS cement exhibits superior weather resistance compared to magnesium oxychloride (MOC) cement because of less hygroscopicity[6].

MOS cement is a typical ternary system MgO-MgSO4-H2O that obtains the mechanical properties through the chemical reactions between magnesia and magnesium sulphate solution.The basic magnesium sulfate salts in the MgOMgSO4-H2O system are represented by the formula [xMg(OH)2·yMgSO4·zH2O], such as 3Mg(OH)2·MgSO4·8H2O (3·1·8 phase), 5Mg(OH)·MgSO4·3H2O (5·1·3 phase), Mg(OH)·MgSO4·5H2O(1·1·5 phase) and Mg(OH)·2MgSO4·3H2O (1·2·3 phase). According to Demediuk and Cole[7], four basic magnesium sulfate salts (3·1·8 phase, 5·1·3 phase,1·1·5 phase and 1·2·3 phase) were the main sources for the mechanical strength of MgO-MgSO4-H2O ternary system, at temperatures from 30 to 120 ℃. Some researchers considered that the 3·1·8 phase played a key role similar to 3Mg(OH)2·MgCl2·8H2O and 5Mg(OH)2·MgCl2·8H2O[8,9]in MOC cement. Typically,the 3·1·8 phase forms as the following equation:

However, the investigation conducted by Urwongse and Sorrell[10]revealed that it was impossible to prepare pure MOS cement at 23 ℃ which contained the content of the 3·1·8 phase exceeding 50%. Dinnebieret al[11]found that the 3·1·8 phase was metastable under ambient condition, not the stable phase as previously thought. As a consequence of this, pure MOS cement shows a porous and loose microstructure consisting mainly of brucite, which results in the low mechanical strength.

During the past several decades, more and more researchers studied the modification of MOS cement by using additives. Wuet al[12]discussed the influence of amino trimethylene phosphonic acid on the performances of MOS cement prepared using light-burned magnesia and industrial waste sulfuric acid. Results revealed that small addition of amino trimethylene phosphonic acid could promote the strength phase of 5Mg(OH)·MgSO4·7H2O (5·1·7 phase)[13]formed in MOS cement, and increased the 28-day compressive strength from 41.8 to 98 MPa. Guoet al[14]published a paper about the performances of MOS cement modified by sodium malate. It was noted that the flexural strength could reach 22.13 MPa at 28 days when MOS cement was added with 0.25% sodium malate. Also, sodium citrate and citric acid are effective additives for the strength improvement of MOS cement.Wanget al[15]proposed that the mechanical strength and water resistance were significantly improved after adding 0.5% sodium citrate or citric acid.

In this paper, it was aimed to investigate the influence of sodium dihydrogen phosphate (NaH2PO4)and potassium dihydrogen phosphate (KH2PO4) as additives on the compressive strength, volume stability,hydrated products and microstructure of MOS cement.XRD, FTIR and TG-DSC were used to analyze the hydrated products. SEM and MIP were conducted to characterize the microstructure.

2 Experimental

2.1 Materials

The main raw materials are light-burned magnesia,magnesium sulphate heptahydrate (MgSO4·7H2O)and additives. In this work, the light-burned magnesia was calcinated at 800 ℃ from Haicheng, Liaoning Province, China. The chemical composition of lightburned magnesia containing 62.00% active magnesia is listed in Table 1. The particle size distribution curve of light-burned magnesia (Fig.1) was measured by the laser particle size analyzer. Analytical reagents of MgSO4·7H2O, NaH2PO4and KH2PO4were purchased from Nanjing Chemical Reagent Co. Ltd.

Table 1 Chemical composition of light-burned magnesia

Fig.1 Particle size distribution for light-burned magnesia

2.2 Specimen preparation

MOS cement specimens with dimensions of 40 mm×40 mm×40 mm and 40 mm×40 mm×160 mm were produced according to a MgO:MgSO4:H2O molar ratio of 10:1:20. Firstly, the homogeneous magnesium sulphate solution was made by mixing MgSO4·7H2O with water containing NaH2PO4or KH2PO4(0.5%by weight of light-burned magnesia). Then, the preweighed light-burned magnesia was stirred with the magnesium sulfate solution at low speed for 1 min and then at high speed for 2 min. The cement pastes were demolded after casting for 24 hours, and were cured under the condition of 20±2 ℃ and 60±5% RH. NPMOS and KP-MOS represent the MOS cement with NaH2PO4and KH2PO4, respectively, while pure MOS cement is denoted by P-MOS.

2.3 Test methods

The compressive strength of MOS cement with dimensions of 40 mm×40 mm×40 mm was tested in accordance with GB/T 17671-1999[16]at the ages of 1,3, 7 and 28 days. The final result was the average value of three specimens for each kind of MOS cement.

The volume stability of MOS cement with dimensions of 40 mm×40 mm×160 mm was measured using a shrinkage dilatometer (SP-175)[17]. The weight loss of MOS cement was conducted on an electronic balance (accuracy of 0.01 g). The shrinkage ratio(∆l) and weight loss (∆m) atndays were obtained as follows:

whereL0is the initial length;Lnis the length atndays;cis the length of measuring head that is placed in the paste;M0is the initial weight;Mnis the weight atndays.

The hydrated products of MOS cement specimens with particle size under 75 μm were identified by X-ray diffraction (XRD, D8 Advance, Bruker) over a 2θrange from 5° to 70° with step size of 0.02°. The pH changes of MOS cement paste in early time were monitored on a pH meter. A thermo scientific nicolet iS10 spectrometer with a resolution of 4 cm-1was used to collect the FTIR spectrums in the range of 400-4 000 cm-1. Thermogravimetric analysis (TGDSC) was performed using a NETZSCH SAT449F3 instrument. MOS cement powder was heated from 25 to 1 100 ℃ at a heating rate of 10 ℃/min under N2environment. Investigation of pore structure was detected using a mercury intrusion porosimetry (MIP,AutoPoreIV9510, Micromeritics). Before MIP tests,MOS cement specimens were cut into small pieces and vacuum-dried to keep constant weight at 60 ℃. The morphologies of hydrated products in MOS cement were characterized by a scanning electron microscope(SEM, ΣIGMA HD, ZEISS) after spray gold treatment.

3 Results and discussion

3.1 Compressive strength and volume stability

The experimental results in this paper are similar to those that have been reported in the literature[18].From Fig.2, it can be observed that P-MOS presents the lowest compressive strength at all curing ages, and only gains the compressive strength of 33.3 MPa at 28 days.After 3 days, the compressive strength of NP-MOS and KP-MOS can reach 39.7 and 42.2 MPa, respectively.With the increasing of curing age, the compressive strength of NP-MOS and KP-MOS are 97.7% and 90.9% at 28 days higher than the compressive strength at 3 days, respectively. It is generally considered that the higher content of the 5·1·7 phase results in the denser microstructure and higher compressive strength of MOS cement[19]. The effects of NaH2PO4and KH2PO4on the hydrated products and microstructure of MOS cement will be discussed in next sections.

The volume stability of MOS cement with and without phosphates is depicted in Fig.3. As can be seen in the picture, the shrinkage ratio of all the cement specimens increases with the prolonging of curing age and tends to be stable after 14 days. When the curing age is the same, the shrinkage ratio of NP-MOS and KP-MOS is lower than that of P-MOS. Fig.4 shows a linear correlation between the shrinkage ratio and weight loss[20]. That is, the lower the weight loss rate is, the lower the shrinkage ratio is. Therefore, it can be concluded that the addition of NaH2PO4or KH2PO4causes a reduction in the shrinkage and weight loss of MOS cement, which is conducive to the volume stability of MOS cement.

Fig.2 Compressive strength of MOS cement at 1, 3, 7 and 28 days

Fig.3 Shrinkage ratio of MOS cement with curing age

Fig. 4 Relationship between the shrinkage ratio and weight loss

3.2 Hydrated products

Fig.5 XRD analysis of three kinds of MOS cement at different curing ages

Fig.5 shows the effects of NaH2PO4and KH2PO4on the hydrated products in MOS cement from 8 hours to 28 days. After hydration for 8 hours, the hydrated product of the 5·1·7 phase can be detected with a large amount of unreacted MgSO4·7H2O existing in all the specimens. The diffraction peak intensity of the 5·1·7 phase becomes stronger in NP-MOS and KP-MOS compared with P-MOS. Moreover, the diffraction peak intensity of the 5·1·7 phase in NP-MOS and KPMOS increases continuously up to 28 days, showing the promoting effect of KH2PO4and NaH2PO4on the nucleation and growth of the 5·1·7 phase. At the same time, MgSO4·7H2O gradually disappears in all the specimens. It can be also noted that the 1·1·5 phase is present at 1 day and no longer found at 3 days, which demonstrates that the 1·1·5 phase is a metastable phase under ambient temperature[21].

As to NP-MOS and KP-MOS: (1) there are no apparent differences in the XRD patterns. The development of compressive strength also presents a similar trend; (2) after 28 days, the diffraction peak intensity of the 5·1·7 phase is much higher than P-MOS; (3) the diffraction peak intensity of brucite is lower than P-MOS. The results indicate that both KH2PO4and NaH2PO4can promote the formation of the 5·1·7 phase instead of brucite, which benefits the early and later compressive strength of MOS cement.

3.3 Hydration

According to previous studies[21-23], the hydration process of magnesia in the magnesium sulfate solution can be summarized as follows: (1) when magnesia contacts with the solution, the hydration layer and OH-ions start to take place (Eq.(4)). This reaction causes the dissolving of magnesia and the increase of pH value;(2) with the progress of hydration, Mg(OH)+may react with OH-ions to form the precipitation of brucite (Eq.(5)), and thus the pH value drops down obviously; (3)the new surface of magnesia is exposed to the solution and continuous to hydrate due to the destruction of the hydration layer on surface. The value of pH gradually tends to be a constant level at this stage.

In the presence of NaH2PO4or KH2PO4, the dissociation reactions can take place as shown in Eqs.(6), (7) and (8). These phosphate anions are rapidly adsorbed on the magnesia surface, forming a protective layer, as schematically illustrated in Fig. 6. The protective layer may retard the further hydrolysis of magnesia and inhibits the nucleation sites for brucite.For NP-MOS and KP-MOS, the nucleation and growth of the 5·1·7 phase (Eq.(9)) can increase the mechanical strength of MOS cement. Fig.7 shows the pH changes of P-MOS, NP-MOS and KP-MOS in early time. It can be seen that both KH2PO4and NaH2PO4lead to a reduction in the value of pH peak and the decrease rate of pH, which confirms the above speculation:

Fig.6 Schematic representation of magnesia hydration in (a) P-MOS; (b) NP-MOS or KP-MOS

Fig.7 pH changes for MOS cement with and without phosphates

Fig.8 Infrared spectrums of MOS cement with and without phosphates at 1 day

Fig.8 displays that the infrared spectrums of P-MOS, NP-MOS and KP-MOS are similar, except the 3 710 cm-1peak. In the low-frequency range, the 452,990 and 1 100 cm-1peaks are assigned to the vibrations of the sulfate anion. The absorption peaks at 1 448 and 1 644 cm-1are attributed to the stretching vibration of carbonate group and the bending vibration of water molecules, respectively. In the high-frequency range,the absorption peaks at 3 634 and 3 710 cm-1are characteristic for the stretching vibration of hydroxyl group, while the broad absorption peak at 3 397 cm-1is caused by the stretching vibration of water molecules.The intensity of hydroxyl group at 3 710 cm-1gets weak in NP-MOS and KP-MOS, indicating that the formation of bruite is hindered by adding KH2PO4or NaH2PO4. This is in good agreement with the results of pH and XRD.

3.4 Thermal stability

The TG-DSC curves of MOS cement with and without phosphates at 28 days from 25 to 1 100 ℃ are shown in Fig.9. According to the XRD results, P-MOS,NP-MOS and KP-MOS are mainly composed of the 5·1·7 phase, bruite, magnesite, magnesia and quartz at 28 days. The minor amounts of magnesite, magnesia and quartz originate from the raw materials. It can be seen from the DSC curves of Fig.9 that there are four major endothermic peaks positioned in the temperature ranges of 80-90 ℃, 120-140 ℃, 370-420 ℃ and 940-960 ℃. Zhanget al[24]suggested that the decomposition process of the 5·1·7 phase in MOS is characterized by four steps. When temperature raises from 25 to 180℃, 5Mg(OH)2·MgSO4is formed by removing seven water molecules from the 5·1·7 phase in two steps.The reaction equations are given by Eqs.(10) and (11).Then 5Mg(OH)2·MgSO4loses its structural water and produces magnesia between 180 and 1 100 ℃ in two steps as well. The reaction equations are shown as Eqs.(12) and (13):

Therefore, the endothermic peaks of NP-MOS and KP-MOS at 80-90 ℃ and 120-140 ℃ are mainly considered as the removal of crystal water from the 5·1·7 phase. Due to the formation of few 5·1·7 phase,the endothermic peaks of P-MOS at 80.7 ℃ and 128.1 ℃ are possibly caused by the 5·1·7 phase and undetectable MgSO4·nH2O. More research is needed to confirm this. The endothermic peak for NP-MOS and KP-MOS at 376.6 and 375.9 ℃ is mainly attributed to the loss of structural water within the 5·1·7 phase, while the endothermic peak of P-MOS at 419.8 ℃ mainly results from the thermal decomposition of brucite (Eq.(14)). The endothermic peak for P-MOS, NP-MOS and KP-MOS occurs at nearly 945 ℃ that arises from the release of sulfate group from magnesium sulphate.

Fig.9 TG-DSC curves of MOS cement with and without phosphates at 28 days

In Fig.9, the weight loss of NP-MOS is almost consistent with that of KP-MOS, but the weight loss of NP-MOS and KP-MOS is different from that of P-MOS. Moreover, the weight loss of P-MOS is found to be smaller than that of NP-MOS and KP-MOS.The possible reason is that the addition of NaH2PO4or KH2PO4into MOS cement promotes the nucleation and growth of the 5·1·7 phase, forming more 5·1·7 phase instead of brucite.

3.5 Microstructure

The pore structure is a key factor for the mechanical properties of cementitious materials.The MIP method can be used to measure the pores in the range from 3.5 nm to 500 μm[25], and provides the parameters of pore structure,e g, the pore size distribution, total porosity and average pore diameter.

Table 2 Total porosity, average pore diameter and pore size distribution

As shown in Fig. 10 and Table 2, the incorporation of NaH2PO4or KH2PO4influences not only the total porosity but also the pore size distribution. Compared with P-MOS, NP-MOS and KP-MOS have lower total porosity and larger average pore size of 20-30 nm.The main type of pore in P-MOS is the gel pore with diameter <10 nm that is as high as 43.06%; the main type of pore in NP-MOS and KP-MOS is the capillary pore in the range from 10 to 100 nm that occupies more than 70%. Also, the volume proportion of harmful macropore with diameter >100 nm decreases with the incorporation of NaH2PO4or KH2PO4.These differences are attributed to the changes in the proportion of hydrated products and microstructure between MOS cement with and without phosphates. In MOS cement, the gel pore is mainly produced from the poorly crystallized hydrated products such as brucite and MgSO4·nH2O, while the capillary pore is mainly produced from the 5·1·7 phase[15,19]. From Figs.11(b)and (c), it can be observed that there is a large amount of the rod-like 5·1·7 phase in the matrix as well as the needle-like 5·1·7 phase in the pores of NP-MOS and KP-MOS. The bonding of such crystals with each other generates a dense microstructure, which leads to an improvement in the compressive strength and volume stability of MOS cement[26]. Fig.11(a) shows that P-MOS has a loose structure established mainly by brucite, resulting in an increase in the volume proportion of gel pore and total porosity.

Fig.10 Effects of NaH2PO4 and KH2PO4 on the pore structure of MOS cement at 28 days

Fig.11 SEM images of MOS cement with and without phosphates at 28 days

4 Conclusions

a) NaH2PO4and KH2PO4as additives are used to prepare MOS cement. The early and later compressive strength of MOS cement are improved by adding KH2PO4or NaH2PO4.

b) All hardened MOS cement shrinks during the whole hydration process. The addition of NaH2PO4or KH2PO4leads to a reduction in the shrinkage ratio of MOS cement.

c) For MOS cement, the complexation of phosphate ions absorbed on the magnesia surface can hinder the further hydrolysis of magnesia and the formation of brucite.

d) In the presence of KH2PO4or NaH2PO4, the formation of large amounts of the 5·1·7 phase and the densification of microstructure are conducive to the compressive strength and volume stability of MOS cement.