Agents for removing stains and discoloration caused by metal ion oxidation on and in clay and loam building materials

Abstract

Soluble metal ions contained in clay and loam can migrate to the surface with the moisture present in the clay building materials and react there with oxygen. This can lead to the formation of manganese dioxide and thus to discoloration of the building material surface. This reaction can be reversed using reducing agents such as HDEP and hydrogen peroxide and effectively prevented by the use of chelating agents such as HEDP or citric acid.

Description

[0001] The invention relates to the components and the production of a primer for eliminating color changes due to oxidation on clay and loam building materials, as well as its preventive use in the construction process, according to the preamble of the independent claims. When processing earth-colored clay plasters, dark discolorations occasionally occur after completion. This can happen soon after completion or even years later.

[0002] This is not efflorescence or mold, but rather dark, brown or blackish, cloudy discoloration ranging from a light haze to quite dark patches. Non-capillary-active materials are visible. Screw heads or reinforcing mesh are clearly recognizable in their original color. The clay plaster, however, which is homogeneous to its core, is darkened. Until now, experts have been baffled and have tried all possible methods to solve this problem by chipping or sealing the surface. As a rule, these discolorations appear relatively soon after exposure to high humidity, for example, after the installation of screed or heavy plaster coatings, coinciding with the general drying process of the building.

[0003] However, there were also cases that developed several years later after long, intense heat waves. The substrate often involved clay building panels from various suppliers and only rarely pure clay plaster surfaces on different substrates.

[0004] ClayTec has therefore added a supplement to its "Information Sheet on Clay Building Panels" stating that ClayTec clay building panels should be installed after a screed has been laid and without any subsequent exposure to moisture. On another construction site from 2017, an expert removed the reinforcing mortar from a damaged white decorative plaster at a test site and replaced it with new plaster, hoping to solve the problem in the same way as with sacrificial plaster in cases of salt damage. On this site, almost all clay plaster surfaces on clay panel walls showed varying degrees of discoloration, without any discernible pattern, while the joint between the clay panels was often unaffected or only significantly less so. However, the same clay plaster remained completely undamaged on the straw bale walls in the same room.On other construction sites, white clay plaster on OSB boards with a clay plaster coating discolored four years after completion, brown clay plaster on localized wood fiber insulation darkened after six months, ground floors with clay drywall became patchily discolored, while the attic on wood fiber insulation with identical clay plaster remained completely undamaged. Plaster surfaces were removed and replastered, others were sealed with vapor-impermeable coatings, or bird flux, lime, and water repellents were applied. Silicate paints were then used.

[0005] The invention is based on the objective of preventing color changes in clay building materials during and after installation, whereby the color change is due to oxidation of metal ions, which are generally always present in clay and loam-containing building materials.

[0006] What is the problem? Clay plaster contains metal oxides and metal ions, which are present in clay and loam in varying degrees of oxidation due to geological conditions. These oxides very often determine the color of the clay and loam. Depending on the oxidation state, iron ions produce a yellow, red, brown, or even black hue; manganese is colorless or brown / black, depending on its oxidation state; magnesium is usually white or silvery; titanium oxide is white; and potassium and sodium are also white or yellowish. These are most of the coloring substances regularly found to varying degrees in clay and therefore also in loam, and which are regularly detected in chemical analyses. Iron and manganese remain as the primary coloring agents, which is why most clays are beige, yellow, red, gray, and black, and only highly purified clays, due to the absence of iron and manganese, are white and therefore relatively rare.

[0007] When this clay and loam are processed into clay plaster, oxidation and the upgrading of lower-valent metal ions sometimes occur. In the case of divalent manganese, which is colorless, this can lead to a blackish-brown discoloration due to the upgrading to tetravalent manganese. This may result in a uniform appearance during drying, which in itself is not problematic, as most clay plasters already have a brown hue. However, if significant moisture is introduced again after completion, either through construction work or climatic conditions, which can penetrate deeply due to the excellent properties of the clay plaster and be temporarily stored in the clay minerals, this unfortunately leads to the subsequent transport of soluble metal ions such as divalent manganese upon drying. This oxidizes on the surface of the wall, forming tetravalent manganese and thus causing blackish-brown stains.This MN2+ also migrates through highly purified clay plasters (containing white bentonite) and is therefore even more unsightly and impossible to overlook.

[0008] Chemical reaction: Manganese(IV) reacts with sodium sulfite (NaSO3) and sulfuric acid (H2SO4) in a redox reaction. In this process, manganese(IV) (in the form of, for example, manganese(IV) oxide, MnO2) is reduced, while sulfite (SO3) is removed. 2- ) to sulfate (SO4 2- ) is oxidized.

[0009] The reaction equation looks like this: MnO2 + Na2SO3 + H2SO4 → MnSO4 + Na2SO4 + H2O

[0010] The redox reaction is balanced and leads to the formation of manganese(II) sulfate (MnSO4), sodium sulfate (Na2SO4) and water (H2O).

[0011] This reducing agent is not environmentally neutral and, due to the use of sulfuric acid, is dangerous and unhealthy. Even in low concentrations, the sulfuric acid fumes are extremely concerning. While this agent does remove stains, it also bleaches colored, pigmented clay plasters. Therefore, this agent is impractical and certainly not environmentally neutral.

[0012] Other ways to reduce MN4+ would be: 1. Reduction with sulfite ions (SO3P / 2 -) • Sulfite ions (as in sodium sulfite, Na2SO3) can reduce manganese(IV) to manganese(II). • The reaction is often carried out in an acidic medium, where MnO2 is reduced as a manganese(IV) compound: MnO2 + SO3 / 2 - + H+→Mn 2 + SO4 / 2 - + H2O 2. Reduction with iron(II) ions (Fe 2 +) • Iron(II) ions can reduce manganese(IV) and are themselves oxidized to iron(III) in the process. will be: MnO2 + 2Fe 2 + 4H+ → Mn 2 + + 2Fe 3 + + 2H2O • This reaction also takes place in an acidic environment, as protons (H+) are required. 3. Reduction with hydrogen peroxide (H2O2) • Hydrogen peroxide is a strong reducing agent in acidic solutions and can reduce manganese(IV) to manganese(II): MnO2 + H2O2 + 2H+ → Mn 2 + + 2H2O 4. Reduction with oxalic acid (C2H2O4) • Oxalic acid can reduce manganese(IV) to manganese(II) in an acidic environment, releasing carbon dioxide: MnO2 + C2H2O4 + 2H+ → Mn 2 + + 2H2O 5. Reduction by elemental hydrogen (H2) In a special, controlled environment, elemental hydrogen can reduce manganese(IV): MnO2 + 2H2 → Mn + 2H2O • This reaction requires high temperatures and is primarily used in industrial processes. used. 6. Reduction by (-Hydroxyethylidene)-1,1-diphosphonic acid HDEP • HEDP is often used to prevent the formation of deposits and the oxidation of metals, and in this case it acts as a reducing agent for Mn 4+ . • The reduction of manganese(IV) by HEDP proceeds in an acidic environment, which H + -ions are available for the redox reaction. The exact mechanism can be complex, as phosphonic acids like HEDP are strong ligands and can stabilize the manganese ion by complexing with it. • The simplified reaction equation looks like this: MnO2+HEDP+H+→Mn2++Oxidation products of HEDP+H2O • Oxidation of HEDP: The HEDP molecules are oxidized and converted into simpler organic acids or phosphonates. Reduction of Mn 4+ on Mn 2+Manganese(IV) is reduced to manganese(II). • In the presence of protons (in acidic solution) and HEDP, MnO2 is reduced, with the HEDP molecules donating their electrons to the manganese. This is a typical redox reaction in which electrons are transferred from the HEDP to the manganese. MnO2+HEDP+2H+→Mn2++Oxidized products of HEDP+H2O Prevention of re-oxidation

[0013] In all cases, the tetravalent manganese is reduced, becoming colorless again, but this carries the risk of re-oxidation and thus staining. This is prevented by using a chelating agent in the reducing agent or by adding an additional ligand. There are several such agents available, some more reliably than others, and some that cause varying degrees of environmental damage. 1. EDTA (ethylenediaminetetraacetic acid)

[0014] EDTA is one of the most commonly used chelating agents and works well in the neutral pH range. EDTA can efficiently bind manganese and keep it in solution, thus preventing precipitation. Reaction equation: Mn 2+ + EDTA 4- → [MnEDTA] 2- 2. DTPA (Diethylenetriaminepentaacetic acid)

[0015] DTPA is similar to EDTA, but offers additional binding sites and forms even more stable complexes. It is also effective in neutral solutions.

[0016] Advantage: DTPA is particularly useful in cases where the pH fluctuates slightly, as it remains stable over a wider pH range (pH 4 to 9).

[0017] Reaction equation: Mn 2 + + DTPA 5- ⇌ [Mn(DTPA)] 3-

[0018] This results in a very stable chelate that binds manganese in the neutral pH range. 3. Citric acid (citrate)

[0019] Citrate (the anion of citric acid) can also act as a chelating agent. It is less potent than EDTA or DTPA, but it is biodegradable and works in neutral and slightly acidic environments.

[0020] Advantage: Citrate is natural and biologically compatible, making it ideal for applications where environmental friendliness plays a role.

[0021] Disadvantage: The stability of the complex is lower than with EDTA or DTPA, which means that the Mn 2+ The complex is not as stable in the neutral pH range and dissociates more readily. Reaction equation: Mn 2 + + Cit 3- ↔ [Mn(Cit)]- 4. Amino acids (e.g. glycine)

[0022] Amino acids such as glycine can also chelate manganese in the neutral range. Amino acids have both an amino group (-NH2) and a carboxyl group (-COOH) that can bind with manganese.

[0023] Advantage: Amino acids are biological and environmentally friendly, making them ideal for biological applications. They can also be used to supply manganese to living systems or for dietary supplements.

[0024] Disadvantage: Chelation stability is not as high as with EDTA or DTPA, but this may be sufficient for biological or medical applications.

[0025] Reaction equation: Mn 2+ + n Glycine ⇌ [Mn(Glycine)n] 2+ 5. 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP)

[0026] It is a very good chelating agent and is frequently used to bind metal ions. It belongs to the group of phosphonic acids, which are particularly effective at binding polyvalent metal ions. The key functions of HEDP as a chelating agent are: Strong binding to metal ions: HEDP can form stable chelate complexes with metal ions such as calcium, magnesium, iron, copper, and manganese. This makes it useful for preventing deposits in industrial systems, as it suppresses the formation of sparingly soluble metal compounds. Effective across a wide pH range: HEDP remains effective even under acidic and alkaline conditions, making it versatile for use in applications such as cooling water, cleaning, and water treatment systems. It effectively binds metal ions even at slightly acidic to neutral pH levels. Thermal and chemical stability: HEDP is resistant to hydrolysis and exhibits good thermal stability, making it suitable for applications at higher temperatures. Prevention of oxidation and corrosion: By chelating metal ions, HEDP prevents the metals from participating in oxidative reactions. Ecology:

[0027] Logically, a sustainable, ecological building material like clay and loam should use an equally environmentally neutral chelating agent. Comparing the environmental impacts of chelating agents reveals the following: 1. EDTA is problematic due to its poor biodegradability, the mobilization of heavy metals, and its accumulation in the environment. These properties make it harmful to aquatic life and potentially toxic to aquatic organisms and the entire ecosystem. 2. DTPA (diethylenetriaminepentaacetic acid) is used in agriculture, medicine and industry, is more biodegradable than EDTA, but can also collect heavy metals and release them again under unfavorable conditions. 3. Citric acid is very readily biodegradable, decomposing into water and carbon dioxide and does not promote eutrophication. 4. Glycine and other amino acids are natural components of proteins and are therefore found in all living organisms. Biodegradability: Because they are already components of natural cycles, amino acids are generally very biodegradable and are quickly broken down by microorganisms. Low persistence: Due to their rapid degradability, they do not accumulate in the environment and therefore have low persistence. Low toxicity: Amino acids are hardly toxic to living organisms at the concentrations in which they typically occur in the environment.

[0028] Advantages over synthetic chelating agents • Environmental friendliness: Due to their natural origin and rapid degradability, amino acids are significantly more environmentally friendly than synthetic chelating agents. • Biocompatibility: Their biological compatibility makes them interesting for certain applications, such as in medicine or the food industry.

[0029] restrictions • Lower complex stability: The complexes formed by amino acids with metal ions are generally less stable than those formed by synthetic chelating agents. This can lead to limitations in certain applications that require very high complex stability. • Higher costs: Natural amino acids can be more expensive to produce than synthetic chelating agents.

[0030] 5. The oxidation products of HEDP become phosphates, which promote algal growth and can lead to eutrophication of bodies of water. However, the quantities used are not comparable to fertilizer use in agriculture and are therefore probably negligible.

Claims

This concerns a mixture of reducing agents and chelating agents that reverse color changes caused by oxidized metal ions in clay and loam building materials after installation and prevent re-staining. is characterized by the fact that this mixture is liquid or powdery when used. is characterized by the fact that this mixture primarily contains hydrogen peroxide and HEDP, or alternatively sulfite ions, iron ions, oxalic acid, which reduces the oxidized, stain-causing manganese and iron ions. is characterized by the fact that these reducing agents react at normal indoor temperatures of 0 to 40°C. is characterized by the fact that no further components or catalysts are required for the reaction. is characterized by the fact that no pollutants are produced that impair the use of the rooms or the recycling of the building materials. is characterized by the fact that renewed oxidation of the metal ions is excluded by the use of ligands and permanent chelation. is characterized by the fact that these ligands are primarily HEDP, citric acid, amino acids such as glycine, or alternatively DTPA or EDTA. is characterized by the fact that both reducing agents and ligands are combined in one mixture, but can also be used individually. is characterized in that the invention can be added preventively during the consumption process of clay and loam building materials.