T. Sugama

Magnesium monophosphate cements derived from diammonium phosphate solutions

Abstract Rapid-setting magnesium monophosphate cementitious materials were prepared by mixing calcined magnesium oxide (MgO) powder with an aqueous solution of diammonium phosphate (ADP) at 24°C. The activation energy for the curing reaction of the cement paste was determined to be 30.29 kcal/mole, and at age 1 hr the compressive strength was ≈900 psi (6.2 MPa). X-ray diffraction studies of the cured cement indicated that the major reaction product was magnesium orthophosphate tetrahydrate [Mg3(PO4)2·4H2O]. Magnesium ammonium phosphate hexahydrate [MgNH4PO4·6H2O] and Mg(OH)2 were also detected. Subsequent heating of the cement to 1300°C resulted in the conversion of the three compounds to a single phase of anhydrous magnesium orthophosphate [Mg3(PO4)2]. The resultant product had a compressive strength of 7000 psi (48.23 MPa) and was thermally stable in air at temperatures >1000°C.

Hot alkali carbonation of sodium metaphosphate modified fly ash/calcium aluminate blend hydrothermal cements

Sodium metaphosphate-modified fly ash/calcium aluminate blend (SFCB) cements were prepared by autoclaving for 1 day at 300 C and their resistance was evaluated in a highly concentrated Na{sub 2}CO{sub 3} solution at 300 C. The hydroxyapatite and analcime phases formed in the autoclaved SFCB cements played an essential role in conferring resistance to the degradation of cements caused by alkali carbonation. Although the carbonating reaction of the analcime phase led to the formation of cancrinite, this analcime cancrinite transformation did not show any influence on the changes in the mechanical and physical properties of the cements. Additionally, there was no formation of the water-soluble calcium bicarbonate in the cements exposed for 28 days. Contrarily, the conventional class G cement systems were very vulnerable to a hot alkali carbonation. The major reason for the damage caused by carbonation of the cements was the fact that the xonotlite phase formed in the 300{degree} autoclaved cements was converted into two carbonation products, calcite and pectolite. Furthermore, the reaction between calcite and carbonic acid derived from Na{sub 2}CO{sub 3} led to the formation of water-soluble calcium bicarbonate, thereby causing the alteration of dense structures into porous ones and the loss of strength of cements.

Characteristics of magnesium polyphosphate cements derived from ammonium polyphosphate solutions

Magnesium polyphosphate cement pastes prepared by mixing MgO powder and ammonium polyphosphate (AmPP) solutions yielded early strengths of 2000 psi (13.78 MPa) at an age of 1 hr. The major reaction products responsible for the initial strength development at room temperature were found to be ternary phases of NH4MgPO4·6H2O and Mg3(PO4)2·4H2O. The former exhibited morphological features resembling interlocking crystals composed of thin plates ∼ 7 μm in length. The use of sodium tetraborate decahydrate (borax) as an additive to reduce the rate of reaction between MgO and AmPP was demonstrated. The inclusion of 20% borax by weight of AmPP extended the reaction time to 20 min, compared with a reaction time of <3 min for specimens without borax.

Carbonation of hydrothermally treated phosphate-bonded calcium aluminate cements

Abstract To evaluate the potential of phosphate-bonded calcium aluminate cements (PBC) as wet CO 2 -resistance cementitious materials in geothermal wells, specimens that had been autoclaved at 200°C or 300°C were exposed to Na 2 CO 3 -laden water at 100°C or 200°C. The phase compositions of the hydrothermal reaction products formed in the autoclaved PBC consist mainly of hydroxyapatite (HOAp) and boehmite. The Na 2 CO 3 -induced carbonation of autoclaved PBC yielded two different carbonate species, CaCO 3 as the major species, and CO 3 2− -incorporated HOAp as the minor one. The former species resulted from the carbonation of HOAp and free Ca dissociated from the calcium aluminate reactants; the latter was derived from the substitution of CO 3 2− for OH − in HOAp. However, the rate of CaCO 3 -related carbonation was very small, suggesting that the PBC can be categorized as a CO 2 -resistant cement.

Nature of interfacial interaction mechanisms between polyacrylic acid macromolecules and oxide metal surfaces

The mechanism for the adhesion of polyacrylic acid (PAA) coatings to oxidized metal surfaces has been studied. The work entailed studies of the mechanical and chemical interactions occurring at the interfaces between PAA polyelectrolyte macromolecules and iron (III) orthophosphate dihydrate or zinc phosphate hydrate (hopeite) crystalline films that were deposited on the metal surfaces. With respect to mechanical interactions, it was determined that the surface topography of the highly crystallized hopeite layers consisted of an open microstructure. This resulted in enhanced wettability of the oxide film by the polyelectrolyte macromolecules, thereby increasing the mechanical interlocking bond forces. Studies of the interfacial chemical reactions indicated that the conformation changes in the PAA macromolecules relate directly to the frequency of the magnitude of acid-base and divalent metallic ion crosslinking interactions between the proton-donating pendent COOH groups in PAA molecules and polar OH groups at hydrated oxide surface sites. Namely, the presence of numerous free nucleophilic ions existing on the deposited oxide film leads to a substantial increase in the coil-up and entanglement macromolecule density. These entangled complex macromolecules at the interfaces result in a decrease in the degree of chemisorption at the oxide film surfaces, whereas regularly oriented COOH groups produce strong interfacial chemisorption with the polar OH groups. Since the polyelectrolyte macromolecules have hydrophilic pendent COOH groups, the polymer structure which appears best for use as an adhesive and coating should have only enough hydrophilic COOH groups to occupy all available polar OH groups at the oxide metal surface sites.

Study of interactions at water-soluble polymer/Ca(OH)2 or gibbsite interfaces by XPS

In an attempt to better understand interactions occuring at hydrated cement/organic polymer interfaces, the reaction mechanism and products formed at the interfaces between poly(acrylic acid), p(AA) or poly(acrylamide), p(AM), and Ca(OH)2 or gibbsite, Al2O3·3H2O, were explored using x-ray photoelectron spectroscopy (XPS). It was estimated that at p(AA)/Ca(OH)2 interfaces, a Ca-complexed carboxylate interfacial reaction product was formed by an ionic reaction between the COOH in p(AA) and Ca2+ ions from Ca(OH)2. A similar reaction product was formed at p(AM)/Ca(OH)2 interfaces as a result of an inter-facial transformation of amide in p(AM) into carboxylic acid, caused by the alkali-catalyzed hydrolysis of the amide. The proton-accepting hydroxyl groups existing at the outermost surface sites of Al2O2·3H2O react favorably with proton-donating COOH groups in p(AA). This acidbase interaction at the p(AA)/Al2O3·3H2O joint formed hydrogen bonds. Whereas, when the p(AM) was applied on Al2O3·3H2O surfaces, interfacial electrostatic bonds were formed through charge-transferring reaction mechanisms in which the charge density was transferred from the Al in Al2O3·3H2O to the C=0 oxygen in p(AM).

Sodium phosphate-derived calcium phosphate cements

Abstract Calcium phosphate cements (CPC) were synthesized by the acid-base reaction between sodium phosphate, NaH2PO4 or -(-NaPO3-)-n, as the acid solution, and calcium aluminate cements (CAC) as the base reactant at 25 °C. The extent of reactivity of -(-NaPO3-)-n with CAC was much higher than that of NaH2PO4, thereby resulting in a compressive strength of > 20 MPa. Sodium calcium orthophosphate (SCOP) salts as amorphous reaction products were responsible for the development of this strength. When this CPC specimen was exposed in an autoclave, in-situ amorphous → crystal conversions, such as SCOP → hydroxyapatite (HOAp), and Al2O3· xH2O → γ-A100H, occurred at ≈ 100 °C, while the rate of reaction of the residual CAC with the phosphate reactant was increasingly accelerated by hydrothermal catalysis. Based upon this information, we prepared lightweight CPC specimens by hydrothermally treating a low-density cement slurry (1.28 g/cc) consisting of CAC powder, -(-NaPO3-)-n solution, and mullite-hollow microspheres. The characteristics of the autoclaved lightweight specimens were a compressive strength of > 9.0 MPa, water permeability of ≈ 5.0 × 10−3 milli darcy, and a low rate of alkali carbonation. The reasons for such a low carbonation rate reflected the presence of a minimum amount of residual CAC, in conjunction with the presence of HOAp and γ-A100H phases that are unsusceptible to wet carbonation.

Pyrogenic polygermanosiloxane coatings for aluminum substrates

The factors governing the film-forming performance of pre-ceramic polygermanosiloxan (PGS) coatings for aluminum (Al) substrate surfaces were investigated. The coatings were prepared through the hydrolysis-dehydrochlorinating and dehydrating condensations-pyrolysis reactions of a sol-precursor solution consisting of N-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole, Ge(OC2H5)4, water, CH3OH, and HCl. Six factors were important in obtaining a good film: (1) the high spreadability of the sol solution on the Al surfaces: (2) the formation of organocpolygermanosiloxane at sintering temperatures of 150° C; (3) the pyrolytic conversion at 350°C into an amorphous PGS network structure in which the SiOGe linkages were moderately enhanced; (4) the persistence of only minimum amounts of organic by-products; (5) the non-crystalline phases; and (6) the formation of interfacial oxane bond between PGS and aluminum oxide.

Oxidized potato-starch films as primer coatings of aluminium

Potato-starch (PS) films for use as primer coatings of aluminium substrates were prepared in two steps, chemical-thermal-catalysed oxidation routes. The PS was modified with cerium (IV) ammonium nitrate (CAN) as a chemical oxidizer, followed by thermal oxidation at 150°C in the presence of atmospheric oxygen; this led to the formation of a functional carbonyl derivative caused by cleavage of the glycol C-C bonds in glycosidic rings, thereby resulting in the ring openings. Increasing oxidation by raising the temperature to 200 and 250°C promoted the conversion of carbonyl into carboxylate derivatives, while facilitating the breakage of C-O-C linkages in the open rings. The latter phenomenon reflected the formation of another carboxylate. The intermediate carboxylate derivatives favourably reacted with Ce4+ ions released from CAN to form cerium-bridged carboxylate complexes. Cerium-complexed carboxylate films used as primer coatings not only afforded some protection of aluminium substrates against corrosion, but also displayed excellent adhesion to both the polyurethane (PU) top-coating and aluminium sites. The latter demonstrated that the loss of adhesion at PU/primer/aluminium joints occurs in the PU layers, representing the mode of cohesive failure.

Pyrolysis-induced polymetallosiloxane coatings for aluminium substrates

Inorganic amorphous polymetallosiloxane (PMS) coating films on aluminium substrate were produced through the polycondensation-pyrolysis reaction mechanisms of a sol-precursor solution. The precursor solution was formed by HCl-catalysed hydrolysis of a mixture ofN-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole (TSPI) and M(OC3H7)n, (M=Zr, Ti and Al,n=3 or 4). The TSPI/Zr(OC3H7)4 or Ti(OC3H7)4 precursor systems formed higher quality thin coating films, compared to the /Al(OC3H7) system. This was because of the critical formation of the polyorganosiloxane terminated by end groups containing zirconium and titanium oxides. These end groups were derived by a dechlorinating reaction between the Cl, bonded to the propyl C in organosilane, and the hydroxylated Zr or Ti compounds in the sintering stages of the film production. Good film-forming performance resulted from moderate degrees of cross-linking of metal oxides to polysiloxane chains and of the densification of metal -O-Si linkages in the pyrolysis-induced PMS network. In addition to these factors, the formation of an oxane bond at the interface between PMS and aluminium provided corrosion protection for the aluminium substrate.