Investigation and Characterization of Eco-friendly Limestone-rich Concretes and their Carbonation Behavior

This work results from an interdisciplinary study on the carbonation behavior of clinker-reduced eco-friendly cements and concretes in collaboration with the Institut für Massivbau, Technische Universität Darmstadt. The focus of this study is the carbonation behavior of cements with high limestone content (up to 50 %). The use of limestone powder allows for the reduction of Portland cement clinker, whose synthesis is responsible for the high CO2 emission during the cement production. The reduced Portland cement clinker contents in cement and concrete results in lower amounts of hydration products such as calcium hydroxide and C-S-H phases. However, a minimum amount of Ca(OH)2 and a high density of the hardened cement pastes are necessary to ensure sufficient alkalinity, which is necessary for the passivation of the steel reinforcement. Therefore, questions regarding the durability against carbonation induced corrosion of the reinforcement must be answered before new limestone-rich cements and concretes will be applied in practice. In the framework of this work the reactions of the most important hydration products of cement with CO2 were investigated. In the first part of this work cementitious pastes of commercially available cement with and without limestone addition were prepared and the carbonation reactions investigated. To gain a deeper insight into the reactions occurring in this complex material the most important clinker phases were synthesized with high purity. The carbonation reactions of the hydrated samples were investigated in detail. To understand the time-dependence and the order of the reactions a suitable experimental setup for the accelerated carbonation reaction was established and tested. Additionally, a model cement was synthesized and investigated. The detailed investigations showed, that the carbonation resistance does not depend solely on the amount of Ca(OH)2 initially formed; it reveals the important role of the water/cement-ratio. The time-dependent investigations of the single phases show that Ca(OH)2 and the C-S-H phases react simultaneously with CO2 – differently from well-established models claiming that C-S-H phases only react with CO2 after the complete amount of Ca(OH)2 is consumed