High-Early-Strength Magnesium Phosphate Cement with Fly Ash
The microstructure and properties of a novel magnesia phosphate cement (MPC) with large amounts of fly ash were studied in the present work. For comparison purposes, reference specimens without the incorporation of fly ash were also investigated. The effect of fly ash content on the properties of MPC was studied with two dead burnt magnesia materials with different MgO contents and fineness. The results showed that fly ash can improve the bonding and compressive strength of MPC, even at very early ages. It was found that a fly ash content of 30 to 50% has the best improving effect on MPC, despite the two types of magnesia. Due to the difference of MgO content and particle fineness of two dead burnt magnesia materials, however, MPC mortars with finer magnesia demonstrated higher compressive strength. The incorporation of fly ash does not retard the setting reaction of MPC, but decreases the total heat evolution of MPC. The hydrates and microstructure of MPC paste were examined by an x-ray diffractometer, scanning electron microscopy-energy dispersive x-ray analysis (SEM-EDX), and Fourier transform infrared (FTIR) spectroscopy. The main products formed in MPC paste were crystal magnesium potassium phosphate hexahydrate and amorphous species. It is believed that the particles of fly ash fill the voids of MPC paste and strongly bond together with hydrates of MPC.
Keywords: fly ash; strength; test.
The composition and properties of magnesia phosphate cements (MPCs) have been investigated by many researchers.1-26 MPCs are based on the chemical reaction of an aqueous acidic phosphate solution with a solid basic magnesia powder. The advantages of MPCs are rapid setting and strength development at ambient temperatures, no water needed for curing, and fire-proof behavior. Therefore, MPCs are attractive for applications in rapid repair for deteriorated highways, airport runways, bridge decks, and in the stabilization of radioactive waste.19 MPCs are also called chemically bonded phosphate ceramics (CBPCs) because they have some characteristics of ceramic materials. CBPC has been used for the management of radioactive and toxic industrial wastes.25-27 The MPCs studied by previous researchers can be classified into the following four categories: 1) MPC based on the reaction between magnesium oxide powder and orthophosphoric acid solution; 2) MPC based on the reaction of magnesium oxide and ammonium dihydrogen phosphate, often in the presence of sodium tripolyphosphate, which was mixed with water; 3) MPC based on the reaction between magnesium oxide and diammonium hydrogen phosphate; and 4) MPC based on the reaction between magnesium oxide and ammonium polyphosphate. The hydrates formed by Category 1, however, are water-soluble and therefore are of no practical significance. In the other three MPCs systems, ammonium phosphates were employed. However, ammonia is liberated from the ammonium phosphate-based MPCs, during minding with water and in use. This creates an unpleasant odor, causes corrosion of placing equipment,23,24 and limits the use of MPCs in practice.
In addition, fly ash is frequently incorporated into ammonium-phosphate-based MPC systems to reduce the cost to receive a color similar to that of portland cement. The addition of fly ash, however, generally reduced the strength of MPCs.16 From the mid-1990s, a novel nonammonia phosphate MPC system was developed that was based on the reaction of magnesia and potassium phosphate.23-24 One of the advantages of the new MPC system is the high volumes of industrial waste, such as fly ash, that can be incorporated within. Fly ash is a by-product produced from burning pulverized coal in power stations, is a low-cost material, and is available in large quantities. Therefore, the new MPC is very favorable to sustainable development and environmental protection. Until now, there was no systemic research about the bonding mechanism of MPC with fly ash. To understand the mechanism of fly ash in the new MPC, the effect of fly ash content on the properties of MPC, the hydrates, and microstructure of MPC were investigated in the current research work.
MPCs have been attracting more and more attention in the rapid repair of concrete due to their properties of rapid setting and high early strength. The MPC studied herein incorporates a large amount of fly ash and low-quality magnesia and has values as both engineering and sustainable materials. The novel MPC also overcomes the limitations of traditional MPCs such as the liberation of ammonia gas. The present research studies the effect of fly ash content on the properties of the new MPC material, the hydrates, phase, and microstructure of MPC by strength evaluation, x-ray diffraction (XRD) and scanning electron microscopy-energy dispersive x-ray analysis (SEM-EDX). The results provide a better understanding of the strengthening mechanism of fly ash in this new MPC.
Table 1 lists the chemical composition and average particle size of the raw materials. As shown in Table 1, two kinds of dead burnt magnesia and an ASTM C618 Class F fly ash were used in the present work. The chemical composition was determined by x-ray fluorescence spectroscopy (XRF) and the particle size was carried out using a particle size analyzer. The particle size distribution is shown in Fig. 1. The magnesia containing 89.51% magnesium oxide was named M9, and the other containing 71.5% magnesium oxide was named M7.
The dry MPC binder was composed by magnesia, monopotassium phosphate (MPP, KH^sub 2^PO^sub 4^), fly ash, and a small amount of borax (Na^sub 2^B^sub 4^O^sub 7^ · 10H^sub 2^O)-a retarding agent, at 4% by weight of magnesia material. The mole ratio of phosphate to magnesia was 1:4. Tap water was used as the mixing water, and the water-binder ratio (w/b) was maintained at 0.17. The fine aggregate was quartz sand, and the sandbinder ratio (s/b) was 1:1. The content of fly ash in these two systems composed by M9 and M7 was 0, 10, 20, 30, 40, 50, and 60%. The mortar samples made from M9 magnesia with different fly ash contents were denoted as M9F0, M9F1, M9F2, M9F3, M9F4, M9F5, and M9F6, respectively. The mortar samples made from M7 magnesia with different fly ash content were denoted as M7F0, M7F1, M7F2, M7F3, M7F4, M7F5, and M7F6, respectively.
The setting time of the MPC mortar was carried out using a Vicat apparatus, according to the procedure of BS 4550. The specimens for the compressive strength test were formed in the mold of 40 × 40 × 40 mm cube. The static elastic modulus was determined using a B15 rock mechanics test system (MTS), and the MPC mortar specimens for elastic modulus were 40 x 40 x 80 mm. The hydration process of MPC was exothermic and the temperature rise in a small (100 g) specimen of mortar was recorded by a digital multimeter.
All of the specimens were cured and tested in the lab air environment (room temperature 25 ± 2 °C with a relative humidity of 50 ± 5%). The compressive strength was determined using a universal testing machine. The loading rate was 0.375 MPa/s. The specimens were tested to failure after 3, 7, and 28 days, respectively. The early strengths of four samples at 1, 4, 7, and 24 h were tested. The reaction products were analyzed by XRD analysis using an x-ray diffractometer with a scanning rate of 0.5° 2 θ/min, and Fourier transform infrared (FTIR) spectroscopy. The sample for XRD and FTIR analysis was a powder with the particle size of approximately 20 µm milled from the hardened paste. To observe the element distribution across the interface of fly ash and hydrates, the small hardened paste lump was analyzed with an SEM-EDX.
RESULTS AND DISCUSSION
Compressive strength and elastic modulus
Compressive strength versus fly ash content for MPC mortars at 3, 7, and 28 days is presented in Fig. 2. It can be seen that for both M9 and M7 series, the MPC mortars with 30 to 50% fly ash exhibited higher strength than the sample without fly ash, and the highest strength occurred at the samples with 40% fly ash. For the mortars made from M9, the strength gradually increased with the addition of fly ash from 10 to 40%, at all ages (except M9F1 at 28 days, which had lower strength than that of M9F0). When the fly ash content surpassed 40%, the strength decreased, but the strength of sample with 50% fly ash was still comparable to that of the sample with 30% fly ash.
For the MPC mortars samples made from M7, there was a similar trend to M9 series. With the exception of Samples M7F1 and M7F2, which had lower strength than that of M7F0 at 3 and 7 days, they showed higher strength at 28 days. Comparing the samples of the M7 and M9 series, it was found that M9 series mortars had higher absolute strength than that of the M7 series. Sample M7F4 reached 50 MPa at 28 days, but M9F4 could reach 70 MPa at the same age. The first reason was the different fineness of the two types of raw materials, M7 and M9. The former had a mean particle size of 59.8 µm, but the latter had a mean particle size of 30.6 µm. The larger the grains, the slower the reaction speed, and the less the cementing matter produced during a certain period of time. The second reason might be the different magnesia content in the two raw materials. M7 contained less MgO content than that of M9. It was possible that the cementitious reaction products in the samples made from M7 were less than that of from M9.
The compressive strength of MPC mortar at 1, 4, 7, and 24 h is shown in Fig. 3. For comparison, only the mortar samples containing 40% fly ash and mortars without fly ash were chosen in this test. From Fig. 3, one could see that, after 1 h, the four MPC samples had similar initial strengths. After 4 h, however, the samples containing 40% fly ash developed strength much faster than the samples without fly ash. The former samples had nearly two times the strength of the latter samples. From 4 to 7 h, the strength of samples containing fly ash continuously increased with at a faster rate, though all the samples had a similar strength increase rate from 7 to 24 h. It could be deduced that in the first hour, the interaction of fly ash and MPC paste was negligible. However, after 1 h, the interaction of fly ash and MPC accelerated and such a result prompted the increased strength development at early age. If fly ash replacement in portland cement is over 20%, its early strength would decrease very significantly compared with the portland cement without fly ash. In contrast, fly ash had an evident strengthening effect in MPC. By determining the elastic modulus of MPC mortars M9F0 and M9F4 at 7 days, it was found the elastic modulus of M9F0 and M9F4 is 27.47 and 28.85 GPa, respectively.
Setting time and temperature rise
Because the interval between initial and final setting times of MPC was very short, only the final setting time was measured in the current work. The setting time of MPC made from M7 and M9 were very similar-around 9 min. With an increase in fly ash content, the setting time was not retarded. In other words, the addition of fly ash does not retard the reaction of MPC. The same phenomenon was observed in MPC based on ammonium phosphate.15
The maximum temperature T^sub max^ of MPC reaction reached was demonstrated in Fig. 4 for both M9 and M7 series. The two MPC series samples were similar in maximum temperature rise T. T^sub max^ decreased with the addition of fly ash (from 57 °C in M9F0 to 42 °C of M9F6). However, there was a sudden change of T^sub max^ observed for the samples with 40% fly ash- M9F4 and M7F4. The results showed that fly ash could reduce the total heat evolution of MPC (due to the dilution effect), but had little effect on its reaction speed. The results also indicated that there was no additional heat evolution caused by the interaction of fly ash in MPC.
Analysis of hydrates and microstructure
Figure 5 is the XRD patterns of MPC paste at 28 days, together with the raw materials, fly ash, and M9. Fly ash was mainly amorphous-only a little quartz and mullite crystallite in the fly ash. However, M9 was basically composed by pericalse (or MgO crystal). The JCPDS card number of magnesia is 45-0946, characteristic peaks d = 2.106, 1.496, and 0.942. The XRD spectra showed that the main crystallite product inside MPC was magnesium potassium phosphate hexahydrate (MgKPO4 · 6H2O-MKP). JCPDS card number of MKP is 35-0812, and the characteristic peaks are d = 4.241, 2.899, and 4.123. The following reaction might occur during the hydration reaction of phosphate and periclase
MgO KH^sub 2^PO^sub 4^ 5H^sub 2^O = MgKPO^sub 4^6H^sub 2^O (MKP)
At the same time, there were diffuse diffraction peaks (those peaks with very small intensity) in M9F0 and M9F4; this means that amorphous gel formed inside the MPC paste matrix. From XRD analysis, it was also found that there was unreacted magnesia inside the cement paste matrix. Although both samples, M9F0 and M9F4, had similar diffraction peaks, the diffraction intensity of MKP in Sample M9F0 was higher than that of Sample M9F4. It indicates that there was more amorphous mass in Sample M9F4 paste due to the amorphous fly ash. For M9F4, there was a quartz characteristic peak that was brought by fly ash. The result of XRD analysis showed that, after fly ash was added into MPC, there was no new crystallite phase formed. However, the intensity of diffraction peaks of MKP and periclase decreased.
The FTIR spectra of fly ash for Samples M9F0 and M9F4 are shown in Fig. 6. For the curves of M9F0 and M9F4, the absorption band of approximately 2900 cm^sup -1^ is attributed to the absorption band of H^sub 2^O (O-H stretching), the other band of H^sub 2^O was 1670 to 1600 cm^sup -1^, the valley of 2368 cm^sup -1^ could be attributed to the hydrogen bond ((p)-O-H stretching). The absorption bands attributed to PO^sub 4^^sup 3-^ (v^sub 3^(PO4), asymmetric stretching of vibration)28 were 1003, 1020, and 569 cm^sup -1^. For the FTIR spectrum of fly ash, due to the amorphous aluminosilicate glass, the absorption band SiO^sub 4^^sup 2^- (Si-O) was widened at 1084 cm^sup -1^. Compared to Sample M9F0, the M9F4 curve had lower absorption values. It was observed that after fly ash was added, the absorption bands of v^sub 3^(PO4) at 1003 cm^sup -1^ was widened. Meanwhile, the absorption bands of PO^sub 4^^sup 3-^ (v^sub 3^(PO4)) in M9F4 had weaker intensity as compared to M9F0. It means that the crystallite of MKP would be affected by fly ash. Because of the similarity of the tetrahedron of [SiO4]^sup 4-^ and [PO^sub 4^]^sup 3-^, it is possible that they would intersubstitute each other to a certain degree.29,30
The micro-morphology of MPC paste after 1 day of hydration is shown in Fig. 7. From the SEM pictures, it appears that there were aggregated clumps of hydration products, MKP in Sample M9F0 paste, and there is a small amount of amorphous gel filled among these clumps. In contrast, more amorphous hydrates formed inside the matrix of Sample M9F4. It appeared that the fly ash grains combined with the matrix tightly. Therefore, after the addition of fly ash, the MPC paste became denser, and thus had higher strength.
Figure 8 and 9 show the micrographs of phosphate-bonded cement paste and the elemental distributions from SEM analysis. Figure 8 is for Sample M9F0 and Fig. 9 is for M9F4. In M9F0, the amorphous gel is filled among unreacted magnesia grains. The amorphous gel can lead to a cementitious property due to its huge specific area (high surface energy).
Figure 9 illustrates that, after fly ash was added, the distribution of Mg along the interface between fly ash particles and hydrates became continuous, suggesting that Mg may have diffused into the surface of fly ash particles. At the same time, Si and Al diffuse from the fly ash into hydrates. Fly ash, being an amorphous material, is thermodynamically unstable. Therefore, there would be a strong interaction between fly ash and phosphate gel in an MPC matrix. In fact, fly ash had been widely used in phosphate immobilization because of its strong tendency to absorb phosphate.31
After fly ash was incorporated into the MPC system, the strength of the composite improved. The reason might include two aspects. The first is the physical filling of fly ash. From the physical view, the addition of fly ash in the hydration system increased the particle concentration in the cement paste. More fly ash grains in the paste matrix decreased the gaps of all solid grains (including unreacted magnesia grains and fly ash grains), so the filling effect can be enhanced. When the fly ash is in an appropriate content range (for example, approximately 40% by weight), the filling effect would reach an optimum particle packing. Furthermore, the fly ash can fill the voids of hardened paste and make the pores finer. The properties of cement stone are relative with respect to the porosity and to pore size distribution. Research results have proved that, under the same porosity, increasing the amount of smaller pores and decreasing the amount of larger pores may improve the properties of cement stone. The fly ash grains can fill those pores and densify the microstructure. Finally, the strength of cement paste was increased.
The other reason should be chemical effect. Fly ash itself is an amorphous aluminosilicate material, which is active in an alkaline environment (the hydrated MPC system is alkaline, with a pH of 10 to 11). The temperature during hydration provides a favorable environment for the interaction. Around the surface of fly ash particles, there is evidence that Mg diffuses into fly ash particle, and Si and Al diffused into hydrates. Hence, a diffusion layer would be formed between the interface of fly ash grains and hydrates. It could be deduced that the diffusion layer around fly ash particles built a strong linkup between fly ash and hydrates.
According to Wilson,32 a very strong phosphate bonded dental cement material can be formed by the reaction of aluminosilicate glass and phosphoric acid. The chemical composition of the aluminosilicate glass used was 41.6% SiO^sub 2^, 28.2% Al^sub 2^O^sub 3^, 8.8% CaO, and 13.3% Fe^sub 2^O^sub 3^. It is believed that this cement was formed by an aluminium phosphate gel (other than silicate gel). The metal ions, Ca^sup 2 ^ and Al^sup 3 ^, extracted from the glass, migrated into the aqueous phase, then precipitated as insoluble salts as the pH increased, leading to the formation of an aluminium phosphate gel. Fly ash is also an aluminosilicate glass, which consists of 50.54% SiO^sub 2^, 26.69% Al^sub 2^O^sub 3^, 8.07% CaO, and 7.76% Fe^sub 2^O^sub 3^. It was possible there was a similar reaction between the fly ash and the phosphate solution.
It is also known that phosphoric acid can stabilize claytype soil. Demirel, Ddah-Yinn, and Boybay33 proposed that the cementing substance formed from clay soils was a gel, which was produced from the reaction of phosphoric acid and aluminosililcates contained in soils. High-strength cement can be formed by the reaction of fly ash with concentrated aqueous phosphoric acid. Therefore, it is believed that similar cementing gel would be produced from the reaction of phosphoric acid with aluminosilicates contained in fly ash. Demirel, Ddah-Yinn, and Boybay33 thought the cementing gel was a complex compound such as aluminum phosphate or iron phosphate. XRD analysis indicated the hydration products would be Al^sub 3^H^sub 14^(PO^sub 4^)^sub 8^·14H^sub 2^O and Fe^sub 3^H^sub 14^(PO^sub 4^)8·14H^sub 2^O. The compressive strength of the reaction system of MgO and aluminum phosphate, Al^sub 2^(H^sub 2^PO^sub 4^)^sub 3^ was found to improve significantly by adding fly ash. Tomic’s experimental evidence5 indicated that fly ash can react with aluminum phosphate to form a cementitious composition, which was favorable to the strength of a hydration system. Like Wilson, Tomic also thought the amorphous cementing substances were formed by the reaction of aluminum phosphate solution and aluminosilciates contained in fly ash. It could be therefore inferred that a similar reaction may happen in the MgO and KH^sub 2^PO^sub 4^ hydration system, after fly ash was blended into the system. The cations, Ca^sup 2 ^, Al^sup 3 ^, and Fe^sup 3 ^, may be dissociated from aluminosilicate glass (fly ash), and cementing gel would be formed by the reaction of phosphates and calcium, aluminium, and iron. From the XRD spectra of MPC paste, no new crystal product was found in the MPC matrix. Because the newly formed substance was amorphous, it could not be examined by XRD analysis. These newly formed gels can fill the voids and make the cement structure denser than the cement system without fly ash. Therefore, the strength of MPC can be enhanced when fly ash is at a suitable content. In the current research, the optimum content of fly ash was approximately 40% by weight in dry binder. When the addition of fly ash was too low or too high, the strength of the hardened cement was reduced because the most reasonable microstructure was not formed.
To examine if any reaction occurred between fly ash and the saturated MPP solution, the two were mixed uniformly and cast into a 40 × 40 × 40 mm mold. After several days, no hardening evidence was observed in the specimen. This indicated no bonding occurred between fly ash and the MPP solution. After magnesia was added, however, the paste hardened quickly. It can be seen that the reaction of magnesia and phosphate is the main strength source of the cement paste. It is possible that the cementitious gel could be formed between the phosphate solution and fly ash only when magnesia occurred in the reaction system. In the other words, the gel may be a complex mixture of magnesia, alumina, silicate, and phosphate. In fact, the commercial zinc phosphate cement powder often contains 3 to 10% by weight of MgO. Alumina and silica are sometimes to be found. The satisfactory cements could be produced only if aluminum was incorporated into the phosphoric acid solution.1 Therefore, the simultaneous occurrence of MgO and Al^sub 2^O^sub 3^ in the phosphate bonded systems may very favorable.
The compressive strength of MPC mortars is improved by the addition of fly ash. With the increase of fly ash content from 10 to 40%, the strength also increased gradually. The strength reached its maxim with 40% fly ash content. Thereafter, the strength decreased. The M7 series mortars had lower compressive strengths than those of M9 series samples, possibly caused by its coarser grain and lower magnesia content.
The addition of fly ash to MPC does not retard the setting time of MPC, but reduces the total heat evolution. For early strength, the 1 h compressive strength of MPC mortars with or without fly ash had the similar values. However, there was a large difference in strength after 4 h. MPC mortars with 40% fly ash had much higher strength than those without fly ash.
The hydrates of MPC were crystalline hexahydrate phosphate and amorphous species. There was some unreacted magnesia inside the pure MPC paste. Due to the addition of fly ash, the samples with 40% fly ash had more amorphous species than the sample without fly ash. There was no new crystal substance formed after adding fly ash according to XRD analysis.
The micro-analysis showed that the strengthening of fly ash to the cement might come from both physical and chemical effects. The fly ash particles might fill the voids in the MPC matrix and make the structure of MPC denser. For the chemical effect, it was possible that the strong interaction would happen at the interface of fly ash grains and phosphate gel. SEM-EDS line scanning analysis showed that Mg diffuses into fly ash particle surfaces. Meanwhile, the Si and Al diffused into MPC paste from fly ash particles. FTIR analysis suggested that a certain degree of interaction would occur between phosphate and silicate.
Financial support from the Hong Kong Grant Council (HKUST6226/01E) is greatly acknowledged.
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Zhu Ding is an associate professor at Shenzhen Municipal Key Laboratory of Durability for Civil Engineering Structures, Shenzhen University, Shenzhen, China. He received his PhD from the Hong Kong University of Science and Technology. His research interests include cement chemistry, properties, structures, ecological building materials, new phosphate bonded cements, water-resistant magnesia chloride cement, and durability of concrete.
ACI member Zongjin Li is an associate professor of civil engineering at the Hong Kong University of Science and Technology. He received his BE from Zejiang University, Hongzhou, China, and his MS and PhD from Northwestern University, Evanston, Ill. His research interests include bond property studies of fiber/cement or aggregate/ cement interface, high-performance concrete, corrosion detection of reinforcing steel in concrete using acoustic emission technique, composite reinforced concrete, debonding detection of ceramic tile systems, cement-based functional materials, and property investigation for cement-based materials at early ages.
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Bibliography for: “High-Early-Strength Magnesium Phosphate Cement with Fly Ash”
Ding, Zhu “High-Early-Strength Magnesium Phosphate Cement with Fly Ash”. ACI Materials Journal. FindArticles.com. 23 Sep, 2011.
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