V.G. Papadakis

Effect of supplementary cementing materials on concrete resistance against carbonation and chloride ingress

Abstract In this work the durability of Portland cement systems incorporating supplementary cementing materials (SCM; silica fume, low- and high-calcium fly ash) is investigated. Experimental tests simulating the main deterioration mechanisms in reinforced concrete (carbonation and chloride penetration) were carried out. It was found that for all SCM tested, the carbonation depth decreases as aggregate replacement by SCM increases, and increases as cement replacement by SCM increases. The specimens incorporating an SCM, whether it substitutes aggregate or cement, when exposed to chlorides exhibit significantly lower total chloride content for all depths from the surface, apart from a thin layer near the external surface. New parameter values were estimated and existing mathematical models were modified to describe the carbonation propagation and the chloride penetration in concrete incorporating SCM.

Effect of fly ash on Portland cement systems Part II. High-calcium fly ash

Abstract A typical low-calcium fly ash was used as additive in mortar, replacing part of the volume either of Portland cement or aggregate. The development of the strength, heat, porosity, bound water, and calcium hydroxide content was measured. In aggregate replacement higher strengths were observed after 14 days, whereas in cement replacement higher strengths were observed after 91 days. The final strength gain was found to be roughly proportional to the content of active silica in the concrete volume. Bound water content and porosity results showed that fly ash reacts with calcium hydroxide, binding small amounts of water. On the basis of the experimental results, a simplified scheme describing the chemical reactions of the low-calcium fly ash in hydrating cement is proposed. Using the reaction stoichiometry, quantitative expressions for the estimation of the chemical and volumetric composition of a fly ash concrete are proposed. The model expressions can be applied in mix design and concrete performance prediction.

Fundamental Modeling and Experimental Investigation of Concrete Carbonation

The physicochemical processes in concrete carbonation are presented and modeled mathematically. These processes include the diffusion of CO2, in the gas phase of concrete pores, its dissolution in the aqueous film of these pores, the dissolution of solid Ca(OH)2 in the water of the the pores, the diffusion of dissolved Ca(OH)2 in pore water, its ultimate reaction with the dissolved CO2, and the reaction of CO2 with CSH and with the yet unhydrated C3S and C2S. In addition, the parallel processes of production of materials susceptible to carbonation during the hydration and carbonation, are included in the model.

Supplementary cementing materials in concrete: Part I: efficiency and design

Many solid industrial by-products such as siliceous and aluminous materials (fly ash, silica fume, slags, etc.) as well as some natural pozzolanic materials (volcanic tuffs, diatomaceous earth, etc.) may be characterized as supplementary cementing materials (SCM) as they exhibit cementitious and/or pozzolanic properties. Due to plenty of these materials and their large variations on physical and chemical composition, the development of a general design for their use in concrete is required. In this work, the concept of an efficiency factor is applied as a measure of the relative performance of SCM compared with Portland cement. Artificial materials of various compositions and some natural pozzolans were studied. Compressive strength and accelerated chloride penetration tests were performed. With regard to these characteristics, efficiency factors for these materials were calculated. A mix design strategy to fulfil any requirements for concrete strength and service lifetime was developed and it enables concrete performance to be accurately predicted.

Physical and Chemical Characteristics Affecting the Durability of Concrete

The durability of reinforced concrete is influenced by those physical characteristics of concrete that control the diffusion of gases, such as CO2 and O2 or of liquids (mainly water) through its pores, and the diffusion of ions, such as Cl-, dissolved in the pore water. These physical characteristics depend on the composition of concrete, the chemical composition and type of cement, and the relative humidity and temperature of the environment. In the present paper, these characteristics of concrete are determined analytically and/or experimentally in terms of the composition parameters and environmental condtions; the molar concentration of those constituents that are susceptible to carbonation; the porosity and pre-size distribution; the degree of saturation of the pores; and effective diffusivity of gases through the concrete.

Experimental investigation and mathematical modeling of the concrete carbonation problem

Abstract Carbonation of concrete is the major time-limiting factor for the durability of reinforced concrete structures. The carbonation reaction between atmospheric CO 2 and Ca(OH) 2 of the concrete mass destroys the high pH environment of surrounding concrete which protects the steel bars of reinforced concrete from corrosion. In this paper we present experimental results obtained in an accelerated carbonation apparatus using a variety of techniques, including TGA, and we extend the mathematical model developed recently to include the entire range of ambient relative humidities.

Experimental investigation and theoretical modeling of silica fume activity in concrete

Abstract Silica fume was used as additive in mortar, replacing part of the volume of either Portland cement or aggregate. In a series of experiments, the development of the strength, porosity, bound water, and calcium hydroxide content was measured. The silica fume addition in both cases gave higher strengths than the control mixture. The bound water content and porosity were finally the same as for the control mixture in the case of the aggregate replacement. On the basis of the experimental results, the chemical reaction of the silica fume in hydrating cement is proposed. Using this equation and the cement hydration reactions, quantitative expressions for estimation of the chemical composition, porosity, and, indirectly, strength and durability of a silica fume–concrete are proposed.

Support-induced promotional effects on the activity of automotive exhaust catalysts: 1. The case of oxidation of light hydrocarbons (C2H4)

Abstract The kinetics of oxidation of a light hydrocarbon (C 2 H 4 ) were studied on catalysts comprising of combinations of one of three metals, Pt, Pd or Rh supported on five different supports, that is, SiO 2 , γ-Al 2 O 3 , ZrO 2 (8% Y 2 O 3 ), TiO 2 or TiO 2 (W 6+ ). Significant variation of turnover frequency with the carrier was observed, which cannot be explained by structure sensitivity considerations and is attributed to interactions between the metal crystallites and the carrier. The catalytic activity of these metal-support combinations was investigated over a wide range of partial pressures of ethylene and oxygen. In a separate set of experiments, the kinetics of C 2 H 4 oxidation were also investigated on polycrystalline Rh films interfaced with ZrO 2 (8 mol% Y 2 O 3 ) solid electrolyte in a galvanic cell of the type: C 2 H 4 , O 2 , Rh/YSZ/Pt, air, during regular open-circuit conditions as well as under Non-Faradic Electrochemical Modification of Catalytic Activity (NEMCA), that is, closed-circuit conditions. Up to 100-fold increase in catalytic activity was observed by supplying O 2− ions to the catalyst surface via positive potential application to the catalyst. The observed kinetic behavior upon increasing catalyst potential parallels qualitatively the observed alteration of turnover frequency with variation of the support of the Rh crystallites.

An AFM-SEM investigation of the effect of silica fume and fly ash on cement paste microstructure

Atomic force microscopy (AFM) was used to observe particle shape and surface texture details of normal portland cement and supplementary cementing materials (silica fume, low-calcium fly ash, and high-calcium fly ash). The latter materials mixed with cement were examined after prolonged hydration. Significant innovative information on particle shape and hydrated paste microstructure was obtained. Conventional microscopy techniques, such as scanning electron microscopy (SEM), cannot provide such detailed images and surface texture characteristics of the fine materials (especially silica fume) and of the product microstructure. AFM showed, for the first time, that silica fume particles are primarily composed of two complimentary parts (hemispheres or semicylinders). Nano-size particles were found in all materials. A relatively smooth product surface was observed in the hydrated cement paste. The hydrated surface of the addition-cement pastes presented small spheroid bulges, giving an additional roughness as was measured by AFM. A sufficient correlation of this microscopical quantitative information with macroscopical engineering and durability properties of cement products is also presented.

Effect of lime putty addition on structural and durability properties of concrete

The effect of lime putty addition on main structural and durability properties of concrete was studied. Different types of cement were used for concrete preparation: a Portland cement, a pozzolanic cement and a Portland cement with the addition of 20% fly ash. The measured concrete properties were compressive strength, setting times, length change, porosity, carbonation depth and degree of steel bar corrosion. It was found that the lime putty addition has a positive effect on the properties of concrete that contain pozzolans and a slightly negative effect on the properties of pure Portland cement. This behavior was correlated with the availability of active silica of cementitious materials. The active silica of pozzolans reacts with the added calcium hydroxide giving constituents, which improve the concrete stability and durability.