Construction and Building Materials 23 (2009) 3094–3102

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Construction and Building Materials

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Influence of the addition of grinding dust to a magnesium phosphate cement matrix

Daniel Véras Ribeiro *, Marcio Raymundo MorelliDepartment of Materials Engineering, Universidade Federal de São Carlos, Rodovia Washington Luis, Km 235, 13565-905 São Carlos/SP, Brazil

a r t i c l e i n f o

Article history:Received 14 November 2008Received in revised form 18 March 2009Accepted 19 March 2009Available online 14 April 2009

Keywords:CementMgOWasteMicrostructureMechanical strengthSetting time

0950-0618/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2009.03.013

* Corresponding author. Tel.: +55 16 33518508; faxE-mail address: verasribeiro@hotmail.com (D.V. Ri

a b s t r a c t

Magnesium phosphate cement materials were prepared by reacting magnesium oxide with water-sol-uble phosphates such as mono-ammonium dihydrogen phosphate (ADP), which solidified at ambienttemperature through the formation of hydrated phases in the material. Cylindrical specimens of mag-nesium phosphate cement were molded and varying amounts (0–40% weight) of grinding dust, wastegenerated in the clutch disc finishing process by grinding and polishing, were added to the ceramicmatrices. The influence of the addition of grinding dust on the characteristics of the cement compo-sitions was verified in terms of setting time, apparent porosity, density and leaching/solubilizationtests. The setting time was analyzed according to NM 65 (the Vicat needle) and by indirect calorimet-ric measurements, the apparent porosity and density of the materials were analyzed by the waterimmersion method, based on the Archimedes principle. Using an Instron 5500R universal testingmachine, various analyses were made to ascertain how the different waste contents in the composi-tions affected the mechanical strength (axial compression and tensile strength by diametral compres-sion). The results obtained proved highly satisfactory for the application of this waste as an additivein magnesium phosphate mortars. The addition of grinding dust to the magnesium phosphate cementmatrix did not affect the formation of new phases or the setting time to any appreciable extent, butan increase in grinding dust content led to an initial increase in strength up to a given limit (about30% of waste).

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1. Introduction

Ever growing industrial activity around the world and the lackof efficient waste management programs means that increasingamounts of wastes are generated without their being utilized ordisposed of correctly, leading to environmental problems thatendanger the quality of life of future generations. A good part ofthese wastes is hazardous, containing elements harmful to humanhealth and that contaminate soil and water tables. Until just a fewyears ago, the world produced only a few kilograms of wastes perinhabitant each year. Today, however, highly industrialized coun-tries such as Germany and France produce about 4.0 tons/inhabi-tant/year, and the average volume of waste produced in Brazil’slargest cities is currently about 0.95 tons/inhabitant/year [1].

Because of the diversity of the physicochemical compositions ofliquid effluents, no single solidification and stabilization technol-ogy can be used to successfully treat and dispose of these wastes.The stabilization of such wastes requires their contaminants beimmobilized effectively. These contaminants are often volatilecompounds and hence cannot be treated effectively by high-temperature processes. This critical need for a low-temperature

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: +55 16 33615404.beiro).

treatment and stabilization technology to effectively treat second-ary wastes generated by high-temperature treatment processesand wastes that are not amenable to thermal treatment was themain motivation for the development of chemically bonded phos-phate ceramics (CBPCs) [2]. Once developed, CBPCs have foundtheir niche applications in treating most difficult wastes such assalts, ashes, liquids and sludges.

The major attraction of this new moldable type of ceramics isthat reasonable mechanical strength and excellent hardness canbe achieved by processing the material without sintering, an oper-ation that increases the product’s final cost because it consumesmore energy and takes place at high temperatures [2,3]. This typeof cement is economically and technologically attractive because itcombines the advantage of low energy processing of traditional ce-ments with the physical properties of advanced ceramics that al-low for high technology applications. Compared with plastics,these cements possess other advantages, i.e., they are nontoxicand nonflammable [3].

Sant’anna and Morelli [4,5] found that magnesium phosphatecements are porous and composed predominantly of nanometricpores which result from the hydration process. They obtained amaterial with porosity and mechanical strength unheard of inmany manufactured materials. These are cements that not onlyhave excellent mechanical properties but are also easily molded.

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They can be manufactured with the incorporation of pigment, andaccept polymeric paint and coatings that adhere to various materi-als, particularly to steel, thus opening up a wide range of possibil-ities for combinations in metallic structures [6,7].

Phosphate cements possess mechanical and chemical proper-ties that are superior to those of normal hydraulic (or Portland) ce-ments in general, thanks to innovations in their processing andmicrostructural handling and to compositional modifications. Thebonds in such chemically bonded phosphate ceramics (CBPCs) isa mixture of ionic, covalent and van der Waals bonds, with a pre-dominance of ionic and covalent bonds; in traditional cementhydration products, van der Waals and hydrogen bonds predomi-nate [8–10].

The reaction between magnesia and acid ammonium phosphateis very rapid and exothermic, and the materials cannot be practi-cally used as such. Thus, the use of calcined or dead-burned magne-sia is suggested. Today’s commercial materials are formulated withmany inexpensive inert materials such as sand, fly ash, etc. to con-trol the rate of reaction or setting time while simultaneously pro-viding high early compressive strength. Phosphate cements forcommercial applications are generally based on reactions betweena metallic oxide and an acid salt or derivative of phosphoric acid [8].

Immediately upon mixing the raw materials at ambient tem-perature, exothermic reactions occur and ammonium and magne-sium phosphate phases are formed. These phases are hydrated andinsoluble in water.

The main product of these reactions is a crystalline phase calledstruvite (NH4MgPO4 � 6H2O), which results from the followingreaction:

MgOþ NH4H2PO4 þ 5H2O!MgNH4PO4 � 6H2O ð1Þ

Other phases such as dittmarite (NH4MgPO4 � H2O), schertelite[(NH4)2MgH2(PO4)24H2O], and stercorite [Na(NH4)HPO4 � 4H2O]are ordinarily observed during hydration, while hanaite[(NH4)2Mg3(HPO4)48H2O], newberyite [MgHPO4 � 3H2O] and otherphosphate hydrates are found less frequently [11].

Struvite is a thermally stable phase in air up to a temperature of55 �C, at which point it decomposes through the loss of H2O andNH3 molecules from its structure, forming an amorphous composi-tion that corresponds chemically to MgHPO4. In the presence ofwater at ambient temperature, this composition can be rehydrated,forming the original phase (struvite) and other amorphous and/orcrystalline phases.

Struvite and dittmarite are chemically and structurally similarand the transformation of one into the other does not cause anymicrostructural damage to molded specimens. Dittmarite is thepredominant phase when the reaction is fast, while struvite pre-dominates when the reaction rate is slow [7].

ðNH4Þ2MgðHPO4Þ2 � 4H2OþMgOþ 7H2O

! 2ðNH4ÞMgPO4 � 6H2O ð2Þ

A recent study by Soudée and Péra [11] suggests a mechanismof formation of these materials, whereby, immediately followingthe addition of water to the mixture, the ADP goes into solutionup to saturation while magnesia starts to be wetted. The drop inpH that occurs during this period leads to the dissociation of mag-nesia through an acid–base reaction. Crystallization of the hydratephases begins when Mg+2 ions react with six molecules of water toform MgðH2OÞ2þ6 compounds. These compounds replace watermolecules in the wetting process of magnesia. Their size preventsnew water molecules from being adsorbed onto the MgO surface.The MgðH2OÞ2þ6 complexes remain attached to the surface, coveringit progressively. PO3�

4 and NHþ4 ions and MgðH2OÞ2þ6 complexes cantherefore develop a struvite network. Wagh et al. [10] considered

that the kinetics of these transformations is very similar to thatof the conventional sol–gel process of ceramics fabrication. Fig. 1gives a step-by-step illustration of the kinetics of CBPC formation.

� Dissolution of oxides and formation of sols by hydrolysis.

When metal oxides are stirred into an acid solution, theydissolve slowly, releasing their own metal which containscations and oxygen-containing anions (Fig. 1a – dissolutionstep). The cations react with water molecules and form pos-itively charged ‘‘aquosols” by hydrolysis (Fig. 1b – hydrationstep). Dissolution and hydrolysis are the controlling steps inthe formation of CBPCs.

� Acid–base reaction and formation of gel by condensation.

Sols react with aqueous phosphate anions (Fig. 1c) to formhydrophosphate salts, while the protons and oxygen reactto form water. The newly formed hydrophosphate saltsmake up a network of molecules in the aqueous solution,leading to the formation of a gel (Fig. 1d).

� Saturation and crystallization of the gel into a ceramic.

As the reaction proceeds, this process introduces increasingamounts of reaction products into the gel, which is thusthickened. When sufficiently thick, the gel crystallizesaround the unreacted core of each grain of the metal oxide,forming a well-connected crystal lattice that grows into amonolithic ceramic (Fig. 1e) [10].

The objective of this paper is to study the effectiveness of themagnesium phosphate cement matrix in encapsulating hazardouswastes without losing its characteristics of high performance.Thus, this new cementicious matrix may offer an alternative forthe treatment of wastes that cannot be encapsulated in othercementicious matrices such as, for instance, Portland cementmatrix.

2. Materials and methods

2.1. Materials

2.1.1. Magnesium oxide (MgO)The present study used a commercial dead-burned magnesium oxide calcined

at 1600 �C, with a MgO content of 88% to 93%. Dead-burned calcined magnesia isproduced at temperatures above 1400 �C and is characterized by its low chemicalreactivity and high resistance to basic slags. The magnesium oxide presented a spe-cific surface area of 0.81 m2/g, unitary mass of 1.34 kg/dm3 and specific gravity of3.61 kg/dm3.

2.1.2. Ammonium dihydrogen phosphate (ADP)The ammonium dihydrogen phosphate (NH4H2PO4), or ADP, reacts with magne-

sium oxide in the presence of water in an acid–base reaction.The recent literature reports that the amount of hydrates decreases with the

amount of NH4H2PO4 and, therefore, the strength of magnesium phosphate cement(MPC) theoretically decreases. From the proportions of MPC it can be calculatedthat MPC paste consists mainly of dehydrated grains of MgO and phosphate hy-drates, and the strength of MgO grains is much higher than that of phosphate hy-drates. Therefore, the lower the amount of ADP the higher the strength of MPCpaste, provided the amount of phosphate hydrates is sufficient to surround theMgO grains thoroughly [12]. The ADP presented a surface area of 0.68 m2/g, unitarymass of 0.79 kg/dm3 and specific gravity of 1.87 kg/dm3.

2.1.3. Grinding dustThe automotive industry generates large and variable quantities of wastes.

These include grinding dust, a hazardous waste (class I) generated in the clutch discfinishing process by grinding and polishing.

Table 1 presents the results of leaching and solubilization tests that classify thegrinding dust as ‘‘hazardous” (class I). The main problem of this waste is its leadcontent, which is about eightfold higher than the maximum allowed in the leachingtest according to the Brazilian NBR 10004 standard. Sodium, cyanides, phenols, sul-fates and surfactants also show levels exceeding the maximum allowed in the sol-ubilization test.

The material is particulate and quite complex due to its diverse constituents(metallic and polymeric fibers as well as fiberglass). This type of waste was usedhere. The scanning electron microphotographs exhibiting morphology of grindingdust is given in Fig. 2.

Fig. 1. Pictorial representation of the formation of CBPCs: (a) Dissolution of oxides; (b) formation of aquosols; (c) acid–base reaction and condensation; (d) percolation andformation of gel; (e) crystallization and saturation of gel into a ceramic [3].

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Grinding dust was supplied from ZF Sachs of Brazil factory, São Bernardo doCampo, Brazil. It had a surface area 1.32 m2/g, specific gravity 2.08 kg/dm3. Fig. 3corresponds to the obtained diffraction spectrum that allows identifying the differ-ent hydroxides present.

2.1.4. Boric acidRetardants have been found to increase the setting time and reduce the inten-

sity of exothermic reactions during the initial setting and hardening stages. There-fore, chemical retardants are utilized in large-scale mixing operations [13]. Boricacid containing about 99.5% of H3BO3 was used in this study.

When boric acid containing MgO is mixed into a phosphate solution, luneberg-ite [Mg3B2 (PO4)2(OH)6 � 6H2O] is formed around the MgO grains, preventing them

Table 1Result of the leaching and solubilization test, according to the NBR 10004/2004standard, which classifies the grinding dust as a hazardous waste (class I).

Parameters Solubilization Leaching

Result (mg/L) MVA* (mg/L) Result (mg/L) MVA* (mg/L)

Barium 0.3 0.7 0.1 70.0Cadmium <0.002 0.005 <0.002 0.5Lead <0.01 0.01 7.8 1.0Copper 0.09 2.0 x #Selenium <0.01 0.01 <0.01 1.0Sodium 340 200.0 x #Zinc 0.1 5.0 x #Cyanide 0.4 0.07 x #Chlorides 24.9 250.0 x #Phenols 1.6 0.01 x #Fluorides <0.01 1.5 <0.01 150.0Nitrates 0.1 10.0 x #Sulfates 250.1 250.0 x #Surfactants 0.5 0.5 x #

x = not required by the NBR 10004/2004 standard.# = absence of a limit established by the NBR 10004/2004 standard.* MVA = maximum value allowed by the NBR 10004/2004 standard.

from dissolving in the acid solution [10]. Subsequently, as the oxide powder ismixed into the acid solution, however, the coating itself dissolves and exposesthe oxide particle surface to the acid solution, and magnesium oxide then starts dis-solving into the solution [2].

2.1.5. Sodium tripolyphosphate (STPP)The incorporation of sodium tripolyphosphate (Na5P3O10), or simply STPP, into

the magnesium phosphate mixture has a beneficial effect. The deflocculating char-acteristics of tripolyphosphate ions suggest that they may play a significant role inenabling improved compaction of the wet mix, reducing the porosity of the hard-ened material [8].

Moreover, Abdelrazig et al. [14] reported that the incorporation of STPP in mor-tars brought about an increase in compressive strength and a decrease in total andcoarse pore volumes.

2.2. Methods

2.2.1. Dosage and molding of specimensThe raw materials were physicochemically characterized and the relation

among the components was determined, in weight. The ‘‘reference proportion”used in the molding of the specimens was 1.0:0.75:0.10:0.30:0.50 (MgO: ADP: boricacid: STPP: H2O). Cylindrical specimens (50 mm in diameter and 100 mm in length)of magnesium phosphate cement containing different amounts of grinding dust (0–40% in weight) were molded.

2.2.2. Setting timeThe setting time was determined by the Vicat method, according to NM 65, and

by indirect calorimetric measurements. Recent studies [14–16] have shown that thesetting time of magnesium phosphate cements determined indirectly by calorime-try is correlated to the setting time measured by the Vicat needle; hence, the forma-tion of hydrated phases in magnesium phosphate cements is an exothermicreaction.

As a further qualitative measure of the reaction rate, the setting time of themortar was monitored using a Raytek laser pyrometer with two characteristictimes, denoted t1 and t2. These times served to indicate the heat evolution rate, t1

being the time at which the maximum rate of temperature increase occurred,and t2 the time at which the maximum temperature Tmax was attained, as illus-trated in Fig. 4 [15].

Fig. 2. SEM of grinding dust (scattered morphology).

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Because the CBPC formation reaction is sufficiently exothermic, the amount ofmaterial directly affects in the reaction kinetics. During the calorimetric measure-ments, the compositions were molded in the same Vicat method mold, accordingto NM 65.

2.2.3. Apparent porosity and densityThe apparent porosity and density were verified using the technique based on

the Archimedes principle. The samples were weighed while they were still dry(Ms). They were then left immersed in water for 24 h until they became fully satu-rated, after which the immersed mass (Mi) and the wet mass (Mu) were determined.Thus, the apparent porosity (PA) and the apparent density (DA) were calculatedaccording to equations below

%PA ¼ 100� ðMu �MsÞ=ðMu �MiÞ ð3Þ

DA ¼ qL � ðMsÞ=ðMu �MiÞ ð4Þ

The qL is the liquid density (in this case, the water, qL = 1.0 g/cm3 at 25 �C).

2.2.4. X-ray diffraction (XRD)The identification of the crystalline phases of a material by X-ray diffraction is

based on the incidence of a monochromatic X-ray beam with an k wavelength,which is diffracted by periodically distributed planes of high atomic concentrationin the sample, with destructive or constructive interferences occurring between thediffracted waves.

This technique was used to characterize the grinding dust and compositions(3, 7 and 28 days old), using a Rigaku Geigerflex ME 210GF2 diffractometer, withcopper targets of 40 kV and 40 mA, and a monochromatic graphite filter system. Adiffraction spectrum was obtained in the range of 2h 10�–80� at 2�/min. Thephases in the samples were identified using DIFFRAC plus-EVA software, whosedatabase follows the JCPDS system (Joint Committee on Power DiffractionStandards).

2.2.5. Mechanical strengthThe values of axial compression and tensile strength under diametral com-

pression correspond to the average of three values for each magnesium phos-phate composition and mortar age (3, 7 and 28 days after molding), and were

Fig. 3. X-ray diffraction analysis of grinding dust showing the identified speciespresent. x-axis corresponds to Bragg’s angle 2u.

Fig. 4. Schematic illustration of indirect measurement of setting time by calorim-etry [15].

Fig. 5. Results of setting time and calorimetry analyses of magnesium phosphatecement as a function of grinding dust content.

Fig. 6. Maximum temperature of the reaction of magnesium phosphate composi-tions prepared with various amounts of waste.

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obtained with an Instron 5500R universal testing machine and a load of 1.5 mm/min. The specimens that presented an error of more than 5% were excluded andreplaced by others, following the procedure established by the Brazilian NBR7222 standard (Mortar and concrete – Determination of the tensile strength ofcylindrical specimens subjected to diametral compression – Test method).

2.2.6. Leaching/solubilization testsThe leaching/solubilization tests were carried out following the NBR 10.004

standard, which classifies wastes and materials as hazardous (class I) or non-haz-ardous (II-A non-inert, and II-B inert).

3. Results and discussion

3.1. Determination of setting time

The measured setting times indicated that the addition of thiswaste to the magnesium phosphate cement matrix considerablyaugmented the hardening reaction. The setting time measured bythe Vicat needle method showed a reduction of about 33%, accord-ing to Fig. 5, due the high sodium content in the grinding dust.However, the addition of increasing amounts of waste hardly af-fected the setting time.

Fig. 5 shows that the setting time curve obtained by the Vicatneedle is similar to the curve that represents the values of maxi-mum temperature time (t2), in the composition containing 10% ofwaste, which is congruent with the results observed by Hall [15].Nonetheless, this similarity can only serve as a qualitative measureof the setting time, since there is no obvious correlation betweenthe two methods used in this work and those used in previousstudies [15,16].

Another behavior that was verified by the setting time analyzedby calorimetry was that the gradual addition of the waste to thephosphate matrix also gave rise to a gradual increase in the maxi-mum temperature of reaction, as indicated in Fig. 6.

This behavior, which was attributed to parallel reactions be-tween the phosphate and other metallic ions present in the waste,became more evident with increasing amounts of grinding dust.The high reaction temperature probably contributed to reducethe setting time, since large amounts of released heat acceleratethe setting rate and the interval between the initial and final set-ting time is very short [11,17].

3.2. Apparent porosity and density

Porosity is detrimental to the strength of ceramic materials be-cause it reduces the area of the transversal section where loads areapplied, thus decreasing the loads the material can bear [18].Hence, the apparent porosity is an important factor that affectsthe strength of these materials. Fig. 7 presents the results of appar-ent porosity.

An increase in waste content caused a gradual reduction inporosity. The unreacted water was eliminated during the timethe sample remained in the furnace. Fig. 7 shows that the influ-ence of loss of free water cannot be taken into consideration andthat the process is influenced solely by the ‘‘filler effect”, withstrong interactions between waste particles and matrix.

Fig. 7 also indicates that the results were practically the same,regardless of the age of the compositions (3, 7 or 28 days). This is

Fig. 7. Apparent porosity of magnesium phosphate cement compositions as afunction of the addition of different amounts of grinding dust waste.

Fig. 9. DRX of magnesium phosphate cement compositions as a function ofgrinding dust content at (a) 3 days, (b) 7 days and (c) 28 days (� MgO; � Struvite; +Dittmarite).

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due to the fact that the material was sufficiently hydrated after3 days and, consequently, the amount of unreacted water in thecompositions (and lost in the furnace) was practically the same.

The density of the samples was then analyzed. As observed inthe porosity analysis, the free water did not affect the results toany significant degree since the water was lost while the sampleswere being dried in the oven. The density initially decreased inthe presence of the waste (qwaste < qmixture). However, startingfrom 10% of waste content, the density increased as a function ofthe amount of waste added (filler effect). The results are shownin Fig. 8.

3.3. X-ray diffraction (XRD)

The 3, 7 and 28-day-old compositions were analyzed by X-raydiffraction. The X-ray spectra of the various compositions pre-sented slight alterations, which did not suggest the formation ofnew phases in response to the addition of waste in the phosphatemagnesium matrix.

As indicated in Fig. 9, the phases identified at the three ageswere magnesium oxide (MgO), struvite (NH4MgPO4 � 6H2O) anddittmarite (NH4MgPO4 � H2O).

The only alterations shown in Fig. 9 are a reduction of the peakintensity corresponding to nonreacted MgO and a slight displace-ment of the diffractograms, which is more clearly depicted inFig. 10. These results are not directly attributable to the additionof waste.

Fig. 8. Density of magnesium phosphate cement compositions as a function of theaddition of different amounts of grinding dust waste.

Fig. 10. Overlapping of the spectra of magnesium phosphate cement compositionswith different amounts of added waste at 7 days of age.

3.4. Mechanical strength

The initial mechanical strength of the magnesium phosphatecements with added grinding dust increased up to a limit (about30%) with increasing waste content. This behavior held true for

Fig. 11. Axial compressive mechanical strength of magnesium phosphate cementas a function of grinding dust contents.

Fig. 12. Tensile strength by diametral compressive resistance of magnesiumphosphate cement as a function of grinding dust contents.

Fig. 13. Relationship between the axial compressive mechanical strength and theapparent porosity of magnesium phosphate cement.

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both axial compressive strength and tensile strength by diametralcompression, as illustrated in Figs. 11 and 12.

A similar behavior was reported by Zangh et al. [19], who ob-served an increase of 30% to 50% in the strength of composites con-taining fly ash, which reached a maximum at 50% and declinedthereafter with the addition of 60% of waste.

This behavior can be attributed to a combination of two factors:

(1) Reduction of the water/solids relation (w/s) due to a pro-gressive increase of waste content, generating an increasein resistance [14,20], which can result from the increase indensity of the compositions as a function of the increase ingrinding dust content, due to better particle packing.

(2) Interaction between the microcracks and the particles of theinorganic phase – elongated rods (waste). Microcracks nor-mally begin in the matrix phase, and their propagation isobstructed or retarded by waste particles. These particlescan inhibit the propagation of microcracks by deflectingtheir tips, forming bridges through the microcracked phaseand absorbing energy and/or inducing a redistribution ofstresses in regions adjacent to the tips of the microcracks[18,21].

The second aspect was aided by the fact that the specimens con-taining waste did not show catastrophic breaking, as occurs withtraditional hydraulic cement mixtures such as Portland cement.The specimens continued bearing a large part of the load afterbreaking, giving the impression that the body was ‘‘stopped” byinternal bridges (inorganic fraction – glass fibers – such as rod-likewaste particles).

However, the addition of 40% of waste led to a reduction in themechanical strength, because this amount of added waste affectsthe formation of CBPCs, negatively affecting the material’s struc-ture (not percolating and/or packing interference). Although thevalues decrease, they are still higher than those of the mixturewithout the waste, mainly in terms of tensile strength by diametralcompression.

It was also observed that the material was not completely sta-bilized after 28 days of analysis, as shown in Figs. 11 and 12. Addi-tional tests were conducted at 90 days of age to verify the behaviorof the composites, which yielded results similar to those reportedby Abdelrazig et al. [14], who also observed constant mechanicalstrength between 28 and 90 days of age.

As can be seen from the results (see Fig. 13), there is no clearrelationship between strength and apparent porosity. However,the ‘‘optimal” value of apparent porosity that satisfies the requiredstrength is about 37%. Values above or below this level are associ-ated with lower strengths.

3.5. Environmental performance analysis

After concluding the technical performance analysis, leachingand solubilization tests were carried out according to the NBR10,004/2004 standard to analyze the environmental performanceof the compounds produced.

Table 2 presents the results of the leaching and solubilizationtests on the magnesium phosphate cement containing grindingdust.

As can be seen in Table 2, all the contents of the elements wereapproved in terms of leaching, classifying this material as class II(non-hazardous), according to the Brazilian NBR 10004/2004standard.

In the solubilization test, all the elements were approved for thecomposites containing 10% grinding dust, classifying these com-pounds as Class II-B (non-hazardous inert). The composites con-taining 20% and 30% of grinding dust were classified as Class II-A

Table 2Results of the leaching and solubilization test of the magnesium phosphate cement compositions as a function of grinding dust contents, according to NBR 10004/2004 standard.

Parameters Solubilization Leaching

10% (mg/L) 20% (mg/L) 30% (mg/L) MVA* (mg/L) m (mg/L) 20% (mg/L) 30% (mg/L) MVA* (mg/L)

Barium 0.3 0.4 0.4 0.7 0.2 0.2 0.2 70.0Cádmium <0.002 <0.002 <0.002 0.005 <0.002 <0.002 <0.002 0.5Lead <0.01 <0.01 <0.01 0.01 <0.01 <0.01 <0.01 1.0Copper 1.2 1.1 1.3 2.0 x x x #Selenium <0.01 <0.01 <0.01 0.01 <0.01 <0.01 <0.01 1.0Sodium 170 190 240 200.0 x x x #Zinc 0.1 0.1 0.1 5.0 x x x #Cyanide 0.02 0.02 0.03 0.07 x x x #Chlorides 49.3 50.2 58 250.0 x x x #Phenols <0.001 <0.001 <0.001 0.01 x x x #Nitrates 0.1 0.2 0.2 10.0 x x x #Sulfates 202 307 335 250.0 x x x #Surfactants 0.4 0.4 0.4 0.5 x x x #

x = not required by the NBR 10004/2004 standard.# = absence of a limit established by the NBR 10004/2004 standard.* MVA = Maximum Value Allowed by the NBR 10004/2004 standard.

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(non-hazardous non-inert) because they were not approved interms of their sodium and sulfate content.

These results demonstrate the effectiveness of this matrix inencapsulating highly hazardous wastes.

4. Conclusions

The analyses of the setting time, apparent porosity and density,mechanical strength and phase formation in a CBPC matrix usingX-ray diffraction led to the following conclusions:

� The addition of grinding dust to the magnesium phosphatecement matrix accelerated the hardening reaction, decreasingthe setting time.

� Successive additions of grinding dust to the magnesium phos-phate cement matrix did not affect the setting time to anyappreciable extent. Moreover, the gradual addition of waste alsoallowed for a gradual increase in maximum reactiontemperature.

� The calorimetric measurements only served as a qualitativeanalysis of the setting time.

� The presence of grinding dust initially reduced the porosity ofthe magnesium phosphate matrix due to particle packing. How-ever, in the early stages (first 24 hours), the porosity increasedwith the gradually increment of waste content. The gradualincrement in waste content caused the density to increase, indi-cating that the grinding dust physically filled the voids.

� The formation of new phases as a result of the presence of grind-ing dust in this matrix was not detected.

� An increase in grinding dust content led to an initial increase instrength up to a given limit (about 30% of waste). At the age of28 days the mechanical strength of these composites had notyet stabilized. Their full mechanical strength was only attainedat the age of 90 days.

� An increase in the porosity of the cementicious matrix caused areduction of the mechanical strength, while an increase in theweight content of the grinding dust caused both porosity andsetting time (hence, workability) to decrease. Therefore, the‘‘optimal” grinding dust weight content that satisfies bothstrength and workability requirements was found to be 20%.

� According to the leaching and solubilization tests, the magne-sium phosphate cement matrix proved highly effective in encap-sulating grinding dust, especially for a content of 10%, classifiedas ‘‘non-hazardous inert”.

Acknowledgements

FAPESP–Fundação de Amparo à Pesquisa do Estado de São PauloPPG-CEM/UFSCar—Programa de Pós Graduação em Ciência e

Engenharia de Materiais da Universidade Federal de São Carlos.ZF SACHS of Brazil Ltda.

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