N. Carciello

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.

Oxidation of carbon fiber surfaces for improvement in fiber-cement interfacial bond at a hydrothermal temperature of 300°C

Carbon fiber oxidized by a hot naoh solution appears to have potential for use as reinforcement in high-temperature cementitious material systems. It has been determined that active carboxylic acid and na-labeled carboxyl functional groups introduced on the fiber surfaces by extensive oxidation react preferentially with calcium ions released from cement in a hydrothermal environment at 300 deg c. This interfacial interaction leads to a linkage between the fiber and the cement matrix, thereby enhancing the bond strength. (Author/TRRL)

Interfacial reactions between oxidized carbon fibers and cements

Abstract Interactions between thermally oxidized carbon fibers and cements were studied using x-ray photoelectron spectroscopy as the primary analytical tool. It was determined that the heat treatment of the fiber in air at temperatures ≥450°C introduces additional functional COOH and C=0 groups onto the fiber surface. These functional organic groups react preferentially with the Al dissociated from aqueous cement solutions. Presumably this reaction occurs by a mechanism involving electron transfer from the Al to the electron accepting oxygen portion of the organic functionaries. Although a chemical bond resulting from the interaction between the Ca(OH) 2 and the COOH is favored, the extent of electrostatic bond formation yielded by the Al- functionary change transfer reaction was markedly greater than that of the Ca-OOC chemical bond formation. Thus, a large diffusion of Al on the oxidized fiber surfaces contributes to increase the cement-fiber interfacial bonding.

Aspects of the adhesion and corrosion resistance of polyelectrolyte-chemisorbed zinc phosphate conversion coatings

The ability of polyelectrolyte macromolecules to suppress the crystal growth of zinc phosphate (Zn · Ph) conversion coatings depends primarily on the functional pendant groups. The extent of segmental chemisorption of macromolecules having carboxylic and sulphonic acid groups on the embryonic crystal faces was found to be considerably higher than that of macromolecules containing amine groups. The reaction products formed by intermolecular reactions between amide groups in polyurethane coatings and carboxylic acid groups on the outermost surface of polyelectrolyte-modified Zn · Ph in Zn · Ph-to-polymer adhesive joint systems played an essential role in developing interfacial adhesive forces. A highly dense precipitation of Zn · Ph derived from a zinc orthophosphate dihydrate-based phosphating solution contributed significantly to reducing the corrosion rate of cold-rolled steel. It also was determined that the presence of an internally diffused polyelectrolyte in the Zn · Ph layers further enhances the resistance to corrosion of Zn · Ph itself.

Interface between zinc phosphate-deposited steel fibres and cement paste

In an attempt to protect steel fibre reinforcements from corrosion and improve their adherence to cement pastes, we deposited a zinc phosphate (ZnPh) conversion coating on the surface of the fibres. At the interfacial contact zones between the cement paste and ZnPh, alkali-induced dissolution caused the dissociation of abundant PO43− ions from the ZnPh. The interaction of PO43− ions with Ca2+ ions from the pastes led to the formation of hydroxyapatite and brushite in the vicinity of the dissolved ZnPh surface. Such intermediate calcium phosphate compounds played important roles in (1) improving the cement-fibre interfacial bonds, and (2) repairing the damage of the ZnPh surfaces dissolved by alkali. These processes protected the steel fibre from corrosion.

Carbonation of geothermal grouts — Part 2: CO2 attack at 250°C

Exposure of cement grouts containing a range of added silica contents to CO2 dissolved in water at 250°C has shown that the preferred binding phases xonotolite and truscottite are reasonably resistant to CO2 attack. However, their carbonation products are permeable so that some carbonation occurs through the center of samples, reducing alkalinity and the ability to protect well casings. Specimens which contain less than 10% added silica form hydrates, which when carbonated, form an impervious layer of calcite which slows CO2 penetration. Ultimately, all portland cement based grouts will carbonate and allow penetration of ions such as chloride. Corrosion will occur in fluids that are under-saturated with respect to calcium carbonate.

Mullite microsphere-filled lightweight calcium phosphate cement slurries for geothermal wells: Setting and properties

Abstract The chemical factors affecting the setting properties of microsphere-filled lightweight calcium phosphate cement (LCPC) slurries, ranging in density from 1.32 to 1.12 g/cc, at high hydrothermal temperature were investigated. The LCPC slurries consisted of calcium aluminate cement (CAC) as the base reactant, a (NaPO 3 ) n solution as the acid reactant, and mullite-shelled hollow microspheres as the lightweight additive. Two major aspects were studied; one was the importance of the chemical constituents of CAC, and the other was the rate of the hydrothermal reaction between the mullite shell and the Na ions dissociated from (NaPO 3 ) n . For the former, the use of CAC having C 2 AS and CA phases as the major chemical components shortened the thickening time of LCPC slurries, whereas CACs with CA, CA 2 , and α-Al 2 O 3 phase components extended the thickening time. For the hydrothermal reaction, an excess of zeolite X type formed by the uptake of Na by mullite microspheres reduced the thickening time.