Corrosion inhibiting cementitious compositions

Abstract

Provided herein is a cementitious composition comprising a photoactivator, wherein the photoactivator is capable of converting NOx to NO.sub.2 or NO.sub.3 when exposed to ultraviolet (U.V.) or visible light.

Claims

1. A cementitious composition comprising at least 16 wt. % a hydrated cement comprising of aluminate-ferrite-monosubstituent phase (AFm) and 0.001-5 wt. % of a photoactivator, wherein the photoactivator comprises doped TiO.sub.2 and the photoactivator is capable of converting NO.sub.x to NO.sub.2.sup. or NO.sub.3.sup. when exposed to U.V. or visible light.

2. The cementitious composition of claim 1, wherein the photoactivator is capable of converting NO.sub.x to NO.sub.2.sup. or NO.sub.3.sup. when exposed to visible light.

3. The cementitious composition of claim 1, wherein 50% by weight or more of the photoactivator is present topically in the composition.

4. The cementitious composition of claim 3, wherein the photoactivator is coated.

5. The cementitious composition of claim 3, wherein the photoactivator is layered.

6. The cementitious composition of claim 1, comprising 18-22 wt. % of AFm.

7. The cementitious composition of claim 6, comprising 0.01-4 wt. % of the photoactivator.

8. The cementitious composition of claim 1, comprising at least 16 wt. % and up to 25 wt. % of AFm.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 illustrates an embodiment of a cementitious composition where the photoactivator is substantially present topically. In this embodiment, the cementitious composition comprises layers L1 and L2, where the photoactivator is substantially present in L1 in accordance with this technology. L1 can further contain concrete. L2 contains concrete containing 15-25% of AFm.

(2) FIG. 2 illustrates an embodiment of a cementitious composition where the photoactivator is substantially present topically. In this embodiment, the cementitious composition comprises a coating C and a layer L3. The photoactivator is substantially present in coating C in accordance with this technology. L3 contains concrete containing 15-25% of AFm.

DETAILED DESCRIPTION

(3) The solid phases in hydrated cement systems mainly include portlandite and a gel-like phase, a calcium silicate hydrate termed CSH. Alumina combines with water, calcium and sulfate to form mainly AFt (ettringite) and AFm phases. Portland cement pastes contain 5-15% of combined amount of AFm and AFt.

(4) AFm refers to a family of hydrated calcium aluminate hydrate phases. Its crystalline layer structure is derived from that of portlandite, Ca(OH).sub.2, but with about one third of the Ca.sup.2+ ions replaced by a trivalent ion, such as Al.sup.3+ or Fe.sup.3+. The resulting charge imbalance gives the layers a positive charge which is compensated by intercalated anions (e.g. SO.sub.4.sup.2, OH.sup., Cl.sup., NO.sub.3.sup., etc.); the remaining interlayer space is filled with H.sub.2O. Its general formula is [Ca.sub.2(Al, Fe)(OH).sub.6].Y.yH.sub.2O, where Y represents a monovalent ion or 0.5 of a divalent interlayer anion, as exemplified above, and y represents the number of water molecules. Studies have shown a multiplicity of AFm hydrate states and solid solution formation between various Y anions. Moreover, chloride and sulfate AFm phases form an anion-ordered compound, Kuzel's salt, e.g., with the 2:1 molar ratio of [C/]/[SO.sub.4]. AFm containing Cl.sup., OH.sup., and CO.sub.3.sup.2 are also referred to as hydrocalumite. SO.sub.4-AFm (monosulfoaluminate) is also referred to as kuzelite and Cl-AFm is referred to as Friedel's salt. The AFm content of a cementitious composition can be increased by increasing the amount of alumina present in the cement. The amount of Al.sup.3 ions can be controlled by adding aluminum-containing sources e.g. metakaolin, fly ash or aluminous cements.

(5) AFt (aluminate-ferrite-trisubstituted) refers to [Ca.sub.3Al(OH).sub.6].sub.2(SO.sub.4).sub.3.(24+2)H.sub.2O (also known as C.sub.6AS.sub.3H.sub.32) or ettringite, which is a product of hydration of C.sub.3A and gypsum. C.sub.3A refers to 3CaO.Al.sub.2O.sub.3.

(6) In some embodiments of this technology, photoactivators include without limitation, doped TiO.sub.2 such as, mesoporous carbon doped TiO.sub.2 nanomaterials with anatase phase (see, Dong et al., J. Phys. Chem. C, 2009, 113, 16717-16723), lanthanum and iodine doped TiO.sub.2 (see, He et al., J. Phys. Chem., C 2008, 112, 16431-16437), doped TiO.sub.2 prepared by direct hydrolysis of tetrabutyl titanate through iodine-doping (see, Hong et al., Chem. Mater., 2005, 17, 1548-1552), boron doped TiO.sub.2 (see, In et al., J. Am. Chem. Soc., 2007, 129, 13790-13791), polyaniline modified TiO.sub.2 nanoparticles (see, Li et al., Applied Catalysis B: Environmental, 81 (2008) 267-273), nitrogen doped TiO.sub.2 (see, Burda et al., Nano Lett., 2003, Vol. 3, No. 8, 1049-1051, Livraghi et al., J. Am. Chem. Soc., 2006, 128, 15666-15671 and Peng et al., Journal of Physics and Chemistry of Solids, 69 (2008) 1657-1664), bromine and chlorine doped TiO.sub.2 (see, Luo et al., Chem. Mater. 2004, 16, 846-849), anionic sulfur, carbon, and nitrogen doped TiO.sub.2 (see, Reddy et al., Journal of Solid State Chemistry, 178 (2005) 3352-3358), copper nitrogen doped TiO.sub.2 (see, Song et al., J. Am. Ceram. Soc., 91 (4) 1369-1371 (2008)), iodine doped TiO.sub.2 (see, Song et al., Applied Catalysis A: General, 378 (2010) 169-174), carbon modified TiO.sub.2 (see, Xu et al., Applied Catalysis B: Environmental, 64 (2006) 312-317), and nitrogen and carbon doped TiO.sub.2 (see, Zhang et al, Angew. Chem., 2011, 123, 7226 7230). Each of these references are incorporated herein by reference. Also useful according to the present technology are photoactivators which are capable of providing nitrite and nitrate species in aqueous solution by atmospheric NO.sub.x conversion.

(7) In some embodiment, AFm is characterized as containing OH.sup. and SO.sub.4.sup.2 in anion positions. In some other embodiments, such as under service conditions or when calcium carbonate is added, the AFm phase may also contain carbonate. According to the present technology, the AFm phases are utilized to serve as a smart sink for NO.sub.2.sup.NO.sub.3.sup. species generated during NO.sub.x oxidation, which can then be released into the pore solution during Cl.sup. ingress. The presence of elevated levels of aqueous NO.sub.2.sup. and/or NO.sub.3.sup. species then inhibits steel corrosion. This approach which includes topical (e.g. coated or layered) and integral methods of anatase deployment is contemplated to be effective against steel corrosion as it is: (1) regenerative due to the continuous provisioning of NO.sub.2.sup./NO.sub.3.sup. species and (2) tunable: as linked to the NO.sub.x oxidation efficiency (related to the TiO.sub.2 structure/properties) and cement chemistry which controls the AFm phase balance (i.e., content) in the system. The approach is applicable to high value structural elements, such as reinforced concrete bridge/decks with a large surface to volume ratio (i.e., exposed surface area) that are exposed to moisture and salt action and whose repair or replacement is expensive.

(8) The light sensitivity and physical properties of the anatase are linked to the chemistry of the cement to develop an auto adaptive material which exhibits smart ion exchange behavior. An intelligent materials engineering (IME) program is followed to aid in materials design. Here, numerical tools including: (1) Gibbs free energy minimization methods are used to predict cementitious phase assemblages and anion exchange capacities, (2) reactive transport models are used to predict coupled fluid and ion effects as relevant to describe NO.sub.2.sup. NO.sub.3.sup. and Cl.sup. transport and corrosion inhibition and (3) nanoparticle aggregation models are used to describe the role of TiO.sub.2 particle properties/dispersion on photoactivity. Experimental studies are applied to describe the role of the overall cement composition, partial pressure (e.g., of NO.sub.x, O.sub.2, H.sub.2O), electrochemical corrosion potential and fluid/ion transport. Further efforts evaluate the effect of topical or integral application of TiO.sub.2 in organic/inorganic coatings or in the concrete bulk in terms of the efficiency/efficacy of each method. Spectroscopic methods are used to track interfacial changes and products on the TiO.sub.2 and steel surface to describe TiO.sub.2 fouling or steel corrosion.

(9) The corrosion inhibitory efficacy of the compositions provided here are tested by various methods known in the art as illustrated and not limited by the following example.

EXAMPLE

(10) Mixtures of C.sub.3A, CaSO.sub.4, CaCl.sub.2, and the photoactivator are reacted at molar ratios indicated below under various partial pressures of NO.sub.x and O.sub.2 with water at 25 C. and agitated in stoppered containers for 45 days. The container is made of a material that allows sufficient amount of visible or U.V. radiation to pass through it and reach the photoactivator inside it. Solutions and solids for analysis are obtained by separating the solid and liquid phases by filtration.

(11) TABLE-US-00001 Photoactivator Composition C.sub.3A CaSO.sub.4 CaCl.sub.2 (wt. % of total # (moles) (moles) (moles) composition) 1A 0.01 0.01 0 0.5 2A 0.01 0.01 0.005 0.5 3A 0.01 0.01 0.0075 0.5 4A 0.01 0.01 0.01 0.5
% of a photoactivator, while keeping the other components same as tabulated above. Various photoactivators, e.g. those capable of converting NO.sub.x to NO.sub.2.sup./NO.sub.3.sup. under U.V. or visible light are useful. The amount of NO.sub.2.sup. or NO.sub.3.sup. in the solid and the solution shows the conversion of NO.sub.x to NO.sub.2.sup./NO.sub.3.sup. under U.V. or visible light in the compositions provided herein and correlates to the corrosion protecting effect of the compositions provided herein.