Method of treating magnesium oxysulfate or magnesium oxychloride article with water soluble phosphate solution

Abstract

This application relates to a treating method for magnesium oxysulfate cement (MOS) or magnesium oxychloride cement (MOC) articles. The MOS or MOC article is molded, cured and dried, then the article is treated in water soluble phosphate solution for periods of 0.5 to 90 minutes. When the MOS or MOC article is treated in water soluble phosphate solution, phosphate can penetrate into MOS or MOC article, react with the non-active magnesia remaining in MOS or MOC article to form magnesium phosphate cement (MPC). After such treatment, MOS or MOC phase is bonded by magnesium phosphate cement (MPC), many defects for MOS or MOC article, such as cracks, distortion, chalking, effloresce, poor durability, and so on, can be reduced or eliminated.

Claims

1. A method for improving the durability characteristics of a formed article of MOS or MOC, comprising: MOS or MOC article is treated with phosphate solution.

2. The method of claim 1, wherein MOS or MOC article are formed by the intimate mixing of concentrated solutions of magnesium sulfate or magnesium chloride with magnesium oxide. MgO used to prepare MOS/MOC articles was light-burnt magnesia (LBM) powder, in which the active MgO, which can react with MgCl.sub.2/MgSO.sub.4, should be more than 40%. Magnesium sulfate solutions may typically contain 15 to 40 percent by weight magnesium sulfate solids. Magnesium chloride solutions used typically contain from 15 percent to 50 percent by weight solids.

3. The method of claim 2, wherein the mole ratios of active magnesium oxide to hydrated sulfate used to prepare MOS are typically in the range of between 5:1 to 14:1. The mole ratios of magnesium oxide to hydrated magnesium chloride are typically in the range of 5:1 to about 8:1.

4. The method of claim 2, wherein dead-burnt MgO can be used as fillers, the amount of dead burnt MgO is 030%.

5. The method of claim 1, wherein the phosphates can be selected from water soluble acid and/or salt of phosphate, pyrophosphate and polyphosphate, such as KH.sub.2PO.sub.4, NH.sub.4H.sub.2PO.sub.4, NaH.sub.2PO.sub.4, Mg(H.sub.2PO.sub.4).sub.2, Al(H.sub.2PO.sub.4).sub.3, Zn(H.sub.2PO.sub.4).sub.2, Ca(H.sub.2PO.sub.4).sub.2, and so on, in which KH.sub.2PO.sub.4 and NH.sub.4H.sub.2PO.sub.4 are preferred.

6. The method of claim 1, wherein the concentration of phosphate solution is 0.550%, the treating time is 0.590 minutes.

Description

REFERENCE TO DRAWINGS

[0008] FIG. 1 XRD patterns of MOS cement before KH.sub.2PO.sub.4 solution treatment

[0009] FIG. 2 XRD patterns of MOS cement after KH.sub.2PO.sub.4 solution treatment

[0010] FIG. 3 MOS articles without phosphate treatment

[0011] FIG. 4 MOS articles treated with phosphate solution, exposed in exterior for 6 months.

DETAILED DESCRIPTION

[0012] As mentioned above, MPC can be prepared by mixing MgO powders and a soluble acid phosphate together, and non-active MgO (also called dead-burned MgO) is often used. Based on the present reports, it has been demonstrated that the activity of MgO is an very important parameter for MOS and MOC, since only active MgO can convert into MOS/MOC. For example the best light burnt MgO used in MOS/MOC products made in China is called 85-MgO, in which the total MgO content is 85%, but the active MgO content is only 6065%, which means more than 20% of MgO was non-active MgO for MOS/MOC. These non-active MgO can react with CO.sub.2, water in humid environment to form carbonate, Mg(OH).sub.2 and Magnesium carbonate hydroxide. On the other hand, during the curation of MOS or MOC, it is inevitable that MgO hydrates to form Mg(OH).sub.2 in parallel with the formation of the MOC or MOS phase, so the mole ratios between magnesium oxide and sulphate are more than the theoretical ratio, which means more MgO are often added into the slurry. In general, there are many Magnesium compounds for the formation of MPC. In addition, the water absorption for MOS and MOC is 2030 percent, which means MOS and MOC article is porous and the water soluble phophate solution can penetrate into MOS and MOC article to react with non-actice MgO. So the magnesium compounds, including MgO, MgCO.sub.3, Mg(OH).sub.2 and Mg.sub.2(OH).sub.2CO.sub.3, can react with phosphate to form MPC, then the disadvanges of MOS and MOC can be reduced or eliminated.

[0013] It is well known that the excellent ductibility of MOS and MOC comes from its fiber-like micro-structure. Just like fiber reinforced cement, the hybrid cements, in which fiber-like MOS or MOC particles are bonded by magnesium phosphate cement (MPC), may be found some special properties and new applications. The detail treating method describes as follows:

[0014] Magnesium oxychloride and oxysulfate articles are formed by the intimate mixing of concentrated solutions of magnesium sulfate or magnesium chloride with magnesium oxide. MgO used to prepare MOS/MOC articles was light-burnt magnesia (LBM) powder, in which the active MgO, which can react with MgCl.sub.2/MgSO.sub.4, should be more than 50% so that the strength is enough to be handled. Magnesium sulfate solutions may typically contain 20 to 40 percent by weight magnesium sulfate solids. Magnesium chloride solutions used typically contain from 15 percent to 50 percent by weight solids. The mole ratios of active magnesium oxide to hydrated sulfate used to prepare magnesium oxysulfate cements are typically in the range of between 5:1 to 14:1. The mole ratios of magnesium oxide to hydrated magnesium chloride are typically in the range of 5:1 to about 8:1. It is preferred in manufacturing the magnesium oxysulfate cements to provide an oxysulfate having the formula 5MgO.MgSO.sub.4.7H.sub.2O. The preferred magnesium oxychloride cement is one having the formula 5MgO.MgCl.sub.2.8H.sub.2O. In the MOS/MOC articles, silicate, alumina, EPS, saw dust, fillers and additives can be used. In order to get more MPC after phosphate treatment dead-burnt MgO can be used as fillers since the dead burnt MgO can not react with MgCl.sub.2/MgSO.sub.4, but dead burnt MgO can react with phosphate quickly. The articles can also be enhanced by using glass fiber meshes and fabric non-weave cloth, and so on. The slurry prepared by mixing MgO, magnesium sulphate solution, fillers, additives, sawdusts, scrapes. In one embodiment, MOS or MOC slurry is mixed by directing magnesium chloride/magnesium sulfate, magnesium oxide, additives, fillers and water. The slurry is directed to a mold. The mold is formed with the slurry to form a MOS/MOC article. The MOS/MOC article is then cured and dried at certain temperature and humidity.

[0015] The phosphates can be selected from water soluble acid and/or salt of phosphate, pyrophosphate and polyphosphate, such as H.sub.3PO.sub.4, KH.sub.2PO.sub.4, NH.sub.4H.sub.2PO.sub.4, NaH.sub.2PO.sub.4, Mg(H.sub.2PO.sub.4).sub.2, Al(H.sub.2PO.sub.4).sub.3, Zn(H.sub.2PO.sub.4).sub.2, Ca(H.sub.2PO.sub.4).sub.2, and so on, in which KH.sub.2PO.sub.4 and NH.sub.4H.sub.2PO.sub.4 are preferred since MgKPO.sub.4.6H.sub.2O and MgNH.sub.4PO.sub.4.6H.sub.2O are the most important MPC. The concentration of phosphate solution are not limited. The lower concentration means longer treating time. The preferred concentration is 0.550%. Other additives, such as pH adjustment, surfactant, and so on, can be added into phosphate solution.

[0016] The treatment employed may be rendered in many ways, such as in a bath, or in a transporting line. Typically, the articles are immersed in a phosphate solution bath for the requisite time period. The bath temperature is not limited since MgO can react with phosphate quickly even in a low temperature. The time of immersion is at least 0.5 minute and typically ranges between 0.5 and 90 minutes. Soluble phosphate may be excess when treating time is too long. The articles can also be treated by spraying the phosphate solution onto the articles when they are moving in a transporting line.

[0017] The invention is exemplified by the following, non-limiting examples

Example 1

[0018] MgO used to prepare MOS articles was light-burnt magnesia (LBM) powders with particle size of 180 mesh and provided from Liaoning province, China. Typical chemical analysis of LBM was listed in Table 1. Because only active MgO (a-MgO) can hydrate and convert to magnesium oxysulfate within the setting process of MOS cement, it is necessary to know the content of a-MgO in the LBM. The content of a-MgO used in this work was determined to be 64.10% by the standardized hydration method. Magnesium sulfate (MgSO.sub.4.7H.sub.2O) was analytically pure. Organic acids such as citric acid are additives. Perlite was used as fillers.

TABLE-US-00001 TABLE 1 Chemical Composition of Light Burnt Magnesia(LBM) Powder Chemical composition MgO CaO SiO.sub.2 Fe.sub.2O.sub.3 Al.sub.2O.sub.3 I.L Mass fraction % 84.10 1.20 5.07 0.43 0.15 9.05

[0019] First, get ready for some MOS articles. 500 g light weight magnesia was mixed with 810 g magnesium sulfate solution with 2.5 g citric acid, the molar ratio of between a-MgO and MgSO.sub.4.7H.sub.2O is 5. The mixing time of the paste was 2 min. Then 80 g perlite and 50 g sawdust were added into the paste to form the slurry and further mixed for 5 min. The slurry was cast into 12024012 mm polyethylene molds, cured at temperature of 203 C. for 24 hours, after which the sample is demoulded, left in same conditions at room temperature for 7 days to make all the a-MgO react to form 5.1.7 phase, dried in open condition for 7 days.

[0020] Secondly, the MOS articles are treated in KH.sub.2PO.sub.4 solution bath with mass fraction 20% for 5 minutes, then these MOS articles are cured at room temperature for 24 hours, left in open condition at room temperature for 7 days, dried in open condition for 7 days. The weight gain after treating with water soluble solution is 3.2%.

[0021] The compressive strength of MOS sample was tested on a testing machine with maximum force of 300 kN according to cement strength test method standard ASTM-C109. Triplicate samples were tested. The crushed cement was reduced to a powder D90<30 m for crystal phase composition analyzed on an X-ray diffractometer with Cu target, and XRD spectra were fitting to determine whether there are MPC after treated in soluble phosphate bath. The composition was also analyzed by X ray Fluorescence. To evaluate the water resistance of MOS specimens, specimens were dipped in water at 203 C. The liquid-solid mass ratio keeps at 20:1, and water was changed once every 7 days. The compressive strength of the samples after different immersion time in water was measured and used to calculate the softening coefficient.

[0022] FIG. 1 shows the XRD patterns of MOS cement. As shown in FIG. 1, MOS article before treated in KH.sub.2PO.sub.4 solution mainly consists of 5.1.7 phase, Mg(OH).sub.2, MgO and MgCO.sub.3. FIG. 2 shows the XRD patterns of MOS cement after treated KH.sub.2PO.sub.4 solution, peaks of MgCO.sub.3 disappear, and the peaks of MgO and Mg(OH).sub.2 become lower which means the amounts of MgO and Mg(OH).sub.2 reduce to a low level. It also means the amount of MgO is excessive for the reaction of MPC and no water soluble phosphate leaves after the reaction finish, which is very important for MPC articles. On the other hand, the peaks of MgKPO.sub.4.6H.sub.2O (MPC) are found. The composition of MOS article was analysized by X ray Fluorescence and shown in Table 2. From Table 2 it can be seen that the amount of P.sub.2O.sub.5 is about 2.22%. Table 3 shows the compressive strength changes of MOS cement before and after phosphate treatment. The compressive strength increased 25%, the bending strength increased 30%, and softening coefficient in water 28 days increased from 0.85 to 1.03.

TABLE-US-00002 TABLE 2 The composition of MOS article analysized by X ray Fluorescence Other MgO SiO.sub.2 SO.sub.3 Al.sub.2O.sub.3 P.sub.2O.sub.5 K.sub.2O CaO Fe.sub.2O.sub.3 compositions 52.7782 21.5181 10.8045 6.0952 2.2214 2.1462 1.8216 1.7501 Balance

TABLE-US-00003 TABLE 3 The strength changes of MOS cement before and after phosphate treatment. The The bending compressive strength softening coefficient in MOS article strength (MPa) MPa water 28 days Before phosphate 56.6 14.5 0.85 treatment after phosphate 71.2 18.9 1.03 treatment.

[0023] In order to evaluate the ageing stability, the MOS articles were exposed in an outdoor environment (Chicago, Ill.), starting on September, 2017. The MOS article without phosphate treatment cracks after 2 months and the picture shown in FIG. 3. FIG. 4 shows the appearance of MOS sample treated with phosphate treatment after 6 months, there is no cracks and its surface is still very smooth, even the cut edge and fracture surface show no change.

Example 2

[0024] A MOC article is prepared by mixing a calcined caustic magnesia with particle size of 180 mesh, an aqueous solution of MgCl.sub.2, phosphoric acid, fine sawdust and perlite, having a mix formulation expressed as MgO 32.2%, MgCl.sub.2 12.6%, H.sub.3PO.sub.4 0.5%, fine sawdust 6%, H.sub.2O 40.7%, and perlite 8%. After mixing by a planetary mixer for 5 minutes, the resulting slurry is cast in a mould. Curing is performed at room temperature in covered condition for 1 day, after which the sample is demoulded, left in same conditions at room temperature for 7 days, dried in open condition for 7 days. Then the MOC article is treated in KH.sub.2PO.sub.4 solution bath with weight concentration 20% for 5 minutes, then the MOC article is recured at room temperature for one day, left in open condition at room temperature for 7 days.

[0025] The composition of MOS article was analysized by X ray Fluorescence and shown in Table 4. From Table 4 it can be seen that the amount of P.sub.2O.sub.5 is about 2.76%. Table 5 shows the strength changes of MOC cement before and after phosphate treatment. The compressive strength increased 26.6%, the bending strength increased 38%, and softening coefficient in water 28 days increased from 0.72 to 0.99.

TABLE-US-00004 TABLE 4 The composition of MOS article analysized by X ray Fluorescence Other MgO SiO.sub.2 Cl Al.sub.2O.sub.3 P.sub.2O.sub.5 K.sub.2O CaO Fe.sub.2O.sub.3 compositions 67.7583 9.8172 13.0787 1.0432 2.765 1.4029 2.3164 0.9144 Balance

TABLE-US-00005 TABLE 5 The strength changes of MOC cement before and after phosphate treatment. The The bending compressive strength softening coefficient in MOS article strength (MPa) MPa water 28 days Before phosphate 52.3 12.5 0.72 treatment after phosphate 66.2 17.3 0.99 treatment.