Microstructural development and sulfate attack modeling in blended cement-based materials

Blending portland cement with pozzolanic admixtures is an effective way to improve the strength and durability of concrete. Additional benefits of this approach are that many pozzolanic materials used for blending today would be otherwise discarded in landfills. This reuse and recycling approach contributes to the solution of major environmental problems. To determine the role of a given type of mineral admixture in concrete, research efforts were carried out in two directions: characterization of the microstructural and macrostructural properties of candidate materials used for blending and the effect of microstructural changes on the durability of the material. In order to achieve the second task, a comprehensive model for the effects of external agents on the integrity of the material is needed. Copper slag was selected as a potential candidate and important source of mineral admixture for the fabrication of blended cements. Preliminary experimental results indicated that use of copper slag may result in an improvement in the resistance to sulfate attack. From the characterization and hydration viewpoint, this study presents several aspects of the role played by copper slag in the properties of concrete. Characterization studies describe the chemical, physical and mineralogical composition of the copper slag using quantitative X-ray diffraction, Differential Thermogravimetry, and Raman Spectroscopy. The potential densification and increase of strength due to calcium hydroxide was examined by analyzing pastes made of calcium hydroxide and slag, and pastes made of portland cement and slag. It was concluded that the increase in strength and durability of cement-based materials with copper slag is due to a reduction in the capillary porosity, and improved by the minor pozzolanic properties. A model for the external sulfate attack of concrete was also developed. The physicochemical properties of hardened cements are used as inputs to predict the undesirable expansion of cement-based materials subjected to sulfate attack. This model is based on a numerical solution of the diffusion-reaction equation. An innovative concept of moving boundary due to mechanical damage is introduced. Damage accumulation due to cracking results in a progressive increase of the diffusivity in the zone comprised between the surface exposed to sulfates and the internal moving boundary. A well defined boundary separates the uncracked and cracked zones. Cracking is the consequence of the expansion of ettringite in an initially microcracked brittle material. Expansion is modelled as the change in volume of calcium aluminates (residual tricalcium aluminate, calcium aluminate, monosulfate hydrate, and tetracalcium aluminate hydrate) when they are transformed into ettringite. Cracking causes softening of the material in the cracked zone, leading to a reduction of the global stiffness of the body subject to the attack. The outputs of the model are compared to experimental data from the literature. The diffusion coefficient of the mortar or concrete and the tricalcium aluminate content of the cement appear to be the most important parameters with respect to the rate and amplitude of expansion.