Influence of supplementary cementitious materials on hydration, microstructure development, and durability of concrete

In recent years the use of supplementary cementitious materials in the production of concrete has become an ever more frequent trend, since such use contributes to a sustainable concrete industry. The main reason for this lies in the reduction of the specific energy requirement and of carbon dioxide emissions in the production of cement (OPC). One such environmentally friendly product is fly ash (FA), which occurs as a by-product of coal-fired thermal power plants. In the first part of the thesis the hydration of OPC and FA at early ages, as well as at later ages, was monitored by means of calorimetry and thermogravimetry. During the first hours the FA retarded the hydration of OPC, particularly the belite hydration. Up to an age of 28 days, the FA exerted a physical nucleation effect on the OPC hydration, which did not compensate for the dilution effect. Furthermore, between 21 and 28 days, a decrease was observed in the amount of calcium hydroxide (CH), and a corresponding increase in the amount of bound water relative to the OPC content, due to a pozzolanic reaction, indicating a change in the hydration products that are formed in the FA blended cement, i.e. less CH and more C-S-H and AFm phases relative to the OPC content. Comparing the two investigated types of FA, siliceous and calcareous FA, no clear difference can be observed with regard to the decrease in the amount of CH. On the other hand, the amount of hydrated products increased significantly up to 28 days, and then not changed much up to one year in the case of the cement paste with siliceous FA. In contrast, in cement paste made by using calcareous FA, the amount of hydrated products increased gradually up to one year. Over a period of one year the consumption of CH was 50% greater in the case of cement pastes containing 30% FA than in the case of cement paste without such an addition. This phenomenon was reflected over time in the observed increase in the mechanical strength of the binder. After 90 days, the compressive strength of concrete in which 20% of the OPC was replaced by FA exceeded the compressive strength of the unmodified concrete. Furthermore, when the replacement level was 50%, the 90 day compressive strengths were almost comparable. When characterizing the mechanical properties of two concrete mixtures made with different types of carbonate aggregate but with identical mix designs, it was found that, in the case of concrete made by using dolostone aggregate, there was a considerable increase in the compressive strength (24%) and modulus of elasticity (13%), compared with the values which corresponded to concrete made by using limestone aggregate. These differences initiated a further investigation into the alkali-carbonate reaction (ACR) which occurs when carbonate minerals react in an alkaline environment. The first process of the mechanisms of ACR is called dedolomitization. In contrast to statements which can be found in the literature, in the case of the used high-purity dolostone aggregate, which did not contain any reactive silica, the dedolomitization was observed after a 6-month exposure period in water. That means that no accelerators in the form of highly alkaline solutions or reactive components are needed to initiate the reaction. Moreover, a new phase which was rich in Si, Al and Mg atoms, was observed between the dedolomitizated aggregate and the secondary calcite. It is supposed that this phenomenon may be responsible for the improvement in the mechanical characteristics of the concrete made by using dolostone aggregate, due to the denser interfacial transition zone and better interlocking of the cement binder and the aggregate grains. The behaviour of the FA modified concrete in aggressive environments was also investigated, and compared to that of unmodified concrete. The study of chloride ingress showed that the FA modified concretes achieved smaller penetration depths, and thus required smaller cover depths. The test results indicated a beneficial effect of FA modified concrete, even when 50% of the OPC was replaced by FA. The results correspond well with measured porosity by means of mercury porosimetry. A higher percentage of ink-bottle porosity was confirmed experimentally in the case of the FA modified concrete. This leads to a lower effective porosity, which means that a higher resistance of the concrete to chloride penetration was achieved. Not only the porosity but also the chemical composition of the FA plays an important role in chloride diffusion in concrete. The concrete with siliceous FA, which had a lower content of calcium, showed better resistance to chloride penetration up to 90 days than the concrete with calcareous FA. On the other hand, after 126 days the concrete with -vicalcareous FA showed better chloride resistance characteristics than the concrete with siliceous FA. This phenomenon seemed to correlate with experimentally observed compressive strength results. The chloride binding was also tested by means of the differences between the amount of water and total-soluble chloride content. The effect of the replacement of OPC by FA on chloride binding capacity was not particularly pronounced. However, the assumption that FA blended cements containing higher amounts of AFm phases relative to the OPC content, which are favourable for chloride binding, was confirmed by calorimetry. The performance of FA modified concrete was poorer in the case of carbonation and frost/salt attack by de-icing salts. With regard to carbonation, the carbonation depth increased with increasing FA content in the concrete mixture that was exposed for 18 weeks to a 10 vol% CO2 environment. Nevertheless, the depth of carbonation at the end of the FA modified concrete’s life could still be acceptable in normal environments in the cases when 20% of the OPC is replaced by FA. In the case of concrete structures that are exposed to the combined action of frost and de-icing salts, the addition of FA should not be greater than 20%. Although FA modified concrete mixtures can achieve a high compressive strength class, the concrete mixture containing 50% of FA proved to have poorer resistance to frost/salt attack. Also the impact of the type of chloride salts on frost/salt scaling was discussed. The test results indicated that sodium, magnesium, and calcium chloride at solution concentrations of 3% showed no significant differences of the unmodified concrete mixture. Otherwise, more rapid scaling rate was observed in the case when the solution concentration of CaCl2 was increased to 24%.