Corrosion-preventing additive for reinforced concrete

Abstract

The corrosion-preventing additive for reinforced concrete is a concrete additive for preventing corrosion of steel rebars in steel-reinforced concrete. The corrosion-preventing additive is powdered scoria, including concentrations of about 45 wt % SiO.sub.2, 14 wt % Fe.sub.2O.sub.3, and 15.5 wt % Al.sub.2O.sub.3, with the remainder being standard components found in volcanic rock. The average particle size of the powdered scoria is 45 microns or less. Reinforced concrete treated with the corrosion-preventing additive includes a mixture of an aggregate, water, and cement (such as Portland cement), along with at least one steel rebar embedded in the mixture, and the powdered scoria.

Claims

1. A corrosion-preventing additive for reinforced concrete for providing corrosion resistance to steel rebars in said reinforced concrete, comprising: powdered scoria, the powdered scoria having about 45 wt % SiO.sub.2, 14 wt % Fe.sub.2O.sub.3, and 15.5 wt % Al.sub.2O.sub.3, whereby the powdered scoria and cement in the reinforced concrete form a corrosion-resistant passive film layer in an interfacial area where the powdered scoria and the cement are in direct contact with said steel rebars embedded in the reinforced concrete due to anodic polarization of said steel rebars where the powdered scoria and the cement are in direct contact with said steel rebars, thereby providing corrosion resistance to protect said steel rebars from corrosion.

2. The corrosion-preventing additive for reinforced concrete as recited in claim 1, wherein the powdered scoria has an average particle size of up to 45 microns.

3. The corrosion-preventing additive according to claim 1, wherein said scoria comprises scoria obtained from Harrat Rahat, Harrat Habesha or Harrat Hutaymah on the Arabian Peninsula in Saudi Arabia.

4. Reinforced concrete treated with a corrosion-preventing additive for providing corrosion resistance to steel rebars in said reinforced concrete, comprising: a mixture of an aggregate, water, and cement; at least one steel rebar embedded in the mixture; and a corrosion-preventing additive added to the mixture, the corrosion-preventing additive comprising powdered scoria, the powdered scoria having about 45 wt % SiO.sub.2, 14 wt % Fe.sub.2O.sub.3, and 15.5 wt % Al.sub.2O.sub.3, whereby the powdered scoria and cement in the reinforced concrete form a corrosion-resistant passive film layer in an interfacial area where the powdered scoria and the cement are in direct contact with said at least one steel rebar embedded in the reinforced concrete due to anodic polarization of said at least one steel rebar where the powdered scoria and the cement are in direct contact with said at least one steel rebar, thereby providing corrosion resistance to protect the at least one steel rebar from corrosion.

5. The reinforced concrete with a corrosion-preventing additive as recited in claim 4, wherein the powdered scoria has an average particle size of up to 45 microns.

6. The reinforced concrete according to claim 4, wherein said scoria comprises scoria obtained from Harrat Rahat, Harrat Habesha or Harrat Hutaymah on the Arabian Peninsula in Saudi Arabia.

7. The reinforced concrete with a corrosion-preventing additive as recited in claim 4, wherein the cement has a wt/wt ratio to the powdered scoria between 90:10 and 70:30.

8. The reinforced concrete with a corrosion-preventing additive as recited in claim 4, wherein the cement has a wt/wt ratio to the powdered scoria of 80:20.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an X-ray diffraction (XRD) diffractogram for samples of processed pozzolan (PP) of a corrosion-preventing additive for reinforced concrete.

(2) FIG. 2 is a scanning electron microscope (SEM) micrograph of particles of a sample of PP of the corrosion-preventing additive for reinforced concrete.

(3) FIG. 3 is a composite electrochemical impedance spectroscopic (EIS) or Bode plot (log of impedance modulus v. log of frequency) comparing control samples of steel rebar embedded in concrete (SPSL) with samples of steel rebar embedded in concrete and treated with the corrosion-preventing additive (SPSL+PP).

(4) FIG. 4 is a composite potentiodynamic EIS anodic polarization plot (potential v. log of current density) comparing control samples of steel rebar embedded in concrete (SPSL) with samples of steel rebar embedded in concrete and treated with the corrosion-preventing additive (SPSL+PP).

(5) FIG. 5 is a composite EIS or Bode plot comparing control samples of steel rebar embedded in concrete with samples of steel rebar embedded in concrete and treated with the corrosion-preventing additive for reinforced concrete (PP—20% by weight Portland cement replaced by pozzolan).

(6) Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) The corrosion-preventing additive for reinforced concrete is a concrete additive for preventing corrosion of steel rebars in steel-reinforced concrete. The corrosion-preventing additive is processed scoria (a highly vesicular volcanic rock), which, when blended with or when partially replacing Portland cement in concrete mixtures, improves the protective properties of the passive film formed on the surfaces of the steel reinforcement bars. The scoria is mined from a volcanic rock field, which is located at Harrat Rahat, Harrat Habesha and Harrat Hutaymah in Saudi Arabia. The scoria is then milled to a particle size of 45 microns or less. This powdered scoria includes 45 wt % SiO.sub.2, 14 wt % Fe.sub.2O.sub.3, and 15.5 wt % Al.sub.2O.sub.3, with the remainder being standard components found in volcanic rock, including CaO, MgO, SO.sub.3, TiO.sub.2, Na.sub.2O, K.sub.2O, and chlorides. The concentrations of SiO.sub.2, Fe.sub.2O.sub.3 and Al.sub.2O.sub.3 in the powdered scoria meet the ASTM C618 requirements for a pozzolanic material, which require the total combined wt % of SiO.sub.2, Fe.sub.2O.sub.3 and Al.sub.2O.sub.3 to be at least 70 wt %. The powdered scoria is mixed with hydraulic cement, such as Portland cement, as an additive or partial replacement therefor in the concrete mix.

(8) For convenience, hereinafter, the processed scoria having concentrations of 45 wt % SiO.sub.2, 14 wt % Fe.sub.2O.sub.3, and 15.5 wt % Al.sub.2O.sub.3, as described above, will be referred to as processed pozzolan (PP). The concentrations of the SiO.sub.2, Fe.sub.2O.sub.3 and Al.sub.2O.sub.3 in the PP were determined by X-ray diffraction (XRD). The results of the XRD analysis are shown in FIG. 1, particularly indicating the major phases of sodium silicate and sodium calcium aluminum silicate in the PP.

(9) The morphologies of the particles present in the PP were examined by scanning electron microscope (SEM), and an SEM image of the particles of the PP is shown in FIG. 2. FIG. 2 shows an average particle size of between 5 μm and 25 μm. Additionally, the pH of the PP in water was examined by blending 5 g of PP with 100 mL of distilled water, followed by agitation at a high stirring rate for 72 hours in a sealed, air-tight container. Table 1, below, shows the results of measuring the pH at room temperature. As can be seen in Table 1, the PP is low in alkalinity and has little effect in raising the pH of distilled water. It should be noted that in other types of common pozzolanic materials, the increase in pH is much higher. For purposes of further comparison, Table 2 below shows the results of making similar pH measurements with a lime-saturated solution, where the results of adding 50% PP were measured. For this test, the mixture was vigorously stirred in a sealed container for 192 hours and the pH was measured at room temperature (25° C.).

(10) TABLE-US-00001 TABLE 1 Measured pH of Distilled Water at Room Temperature pH Substance After 2 hours After 24 hours After 72 hours Distilled water 6.05 6.05 6.05 Distilled water 9.71 9.08 8.38 with 5% PP

(11) TABLE-US-00002 TABLE 2 Measured pH of Lime-Saturated Solution at Room Temperature Substance pH 300 mL of lime-saturated solution 12.50 200 mL of lime-saturated solution 11.27 with 100 g of PP

(12) It can be seen in Table 2 that the PP will not impart alkalinity to the pore solution of concrete/mortar. The siliceous materials of the PP react with lime to form C-S-H gel, which is beneficial for the cast concrete. Table 3 shows the measured results of metallic cations released (measured in ppm) after 50% weight/volume (w/v) pozzolan was mixed and stirred for 72 hours in lime-saturated solution in a sealed, air-tight container at 25° C. From these results, it can be seen that siliceous material is leached out in concrete pore solution from the PP. Further, the calcium ion, after blending of the PP in the lime-saturated solution, is reduced nearly by one-half, which indicates that the lime reacted with the PP to form insoluble C-S-H gel.

(13) TABLE-US-00003 TABLE 3 Measured Metallic Cations Concentration of Metallic Cations (ppm) Solution Al Ca S Si Na K Lime-saturated solution — 0.85 — —  29  22 Lime-saturated solution 69.4 0.4  19.08 433.5 166 230 blended with 50% PP

(14) The polarization resistance of the rebars was measured by electrochemical impedance spectroscopy (EIS). In this technique, a sinusoidal voltage of 10 mV was introduced at the corroding interface at their corrosion potentials. The frequencies of the sinusoidal voltage were varied between 100 KHz to 0.001 Hz. The resulting impedance and shift in phase with changes in frequencies were monitored using a potentiostat. For determination of polarization resistance and other impedance parameters of the corroding surfaces in the presence (and the absence) of admixtures, a constant phase element (CPE) model was used to extract data. Polarization resistance measured by this technique is inversely related to the corrosion current density (I.sub.corr) and follows the Stern-Geary equation:
I.sub.corr=B/R.sub.p,
where B is a constant and R.sub.p is the polarization resistance (measured in Ω.Math.cm.sup.2). The Stern-Geary equation shows that the corrosion current density, and thus the corrosion rate of a corroding metal-electrolyte interface, has an inverse relationship with R.sub.p.

(15) The passive film of the steel rebars provides a measure of protection against chloride corrosion, and the addition of PP to the concrete mix is found to improve this protection. The steel rebars embedded in mortars/concrete remain immune to corrosive attack due to the high alkalinity of pore solution imparted by the Portland cement. The addition of the PP modifies the pores of the cast, making the cast concrete more compact and dense, and reducing the diffusion of chloride, oxygen, moisture and other acidic gases through the concrete to reach the surface of the embedded steel bars. In addition to having a pozzolanic effect, the PP also improves the protective properties of the passive film formed on the surface of the rebars, which can be seen through electrochemical impedance spectroscopy and polarization studies performed on steel rebars directly exposed to concrete pore solution. The electrochemical impedance spectroscopic (EIS) plots of FIG. 3 show that adding PP to the synthetic pore solution (SPSL) improves the corrosion resistance of the steel rebar's passive film to a considerable extent.

(16) In the anodic polarization plots of FIG. 4, it can be seen that the surface of the steel rebar gains anodic protection in the presence of PP in concrete pore solution+0.6M Cl ions. Apart from corrosion current density, the most noticeable effect is on pitting potential (E.sub.pit). For a control solution (without PP), the rebar surface experiences pitting attack at 0.81 V (SCE). After blending with the PP, the E.sub.pit value becomes more positive (0.88V (SCE)), which indicates that blending of the PP in the concrete and mortar improves the pitting resistance of steel rebar. This clearly shows the inhibitive role of the PP on pitting attack of chloride ions on steel rebars. FIG. 4 particularly shows the effect of the PP in improving the polarization and pitting (E.sub.pit) resistance of steel rebar exposed for 768 hours in concrete pore solution+0.6M Cl ions.

(17) For the EIS plots of FIG. 5, two types of the mortars were cast: one with ordinary Portland cement (OPC), and the other replacing 20% (by mass) of OPC with PP. In both cases, the water-to-cement ratio was maintained at 0.5. Mild steel rebars with diameters of 16 mm were embedded in both types of mortar. After 28 days of curing via the standard procedure, the mortars were subjected to a wet/dry cycle in 5% sodium chloride solution, with 7 days held in the dipped state, and three days of drying at 55° C. After 7 cycles of testing, the electrochemical impedance spectra for the embedded rebars were recorded. The plots, showing log frequency vs. log modulus of impedance, are shown in FIG. 5, where the conjoint effect of the pozzolan, by pozzolanic reactions and an improvement in passive film of the embedded rebars in the cast mortars, can be seen.

(18) It should be understood that other types of pozzolanic material may be used to improve the pitting and corrosion resistance of steel rebar embedded in concrete, such as silica flume, fly ash, and the like.

(19) It is to be understood that the corrosion-preventing additive for reinforced concrete is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.