**2. Low Carbon Concrete**

There are several terms referring to concrete with a lower content of clinker. In this study, we will keep to the term "low carbon concrete." As indicated, the design of LCC is based on direct clinker reduction (linked with CO2), resulting in binder volume reduction, and therefore the mix design process deviates from standard concrete design practices. Mix design is more demanding, and its optimization is required. The main compounds of concrete are aggregates (fine and coarse) and binder (cement, supplementary cementitious materials, water, and eventually superplasticizer and air), and their proportion defines the fresh and hardened properties of final concrete. New trends connected to various aggregate types such as crushed sand, recycled aggregates, or artificial aggregates are becoming popular as deposits of natural sand and good-quality natural aggregates are shrinking. In this study, the focus will be given mainly to binder composition and its contribution to concrete´s carbon footprint as expressed by the environmental product declaration (EPD) value [1]. Supplementary cementitious materials have a significant impact on both fresh and hardened properties, and therefore it is important to have good knowledge of their characteristics.

Throughout the whole publication, a demonstration series of three mixes will be used as an example for mix design, characterization, and environmental assessment. The series consists of a reference mix (REF) and two different low-carbon concretes (LCC1 and LCC2). See Table 1.


**Table 1.** Concrete proportions in volume (%).

All designed mixes belong to the same strength class C45/55 according to European standard EN206 [2], with a minimum cube strength of 55 MPa after 28 days (see Table 2). Concrete mixes were designed to be air-entrained for exposure classes X0, XC1-4, FX1, XD1-3, XA1-4, and XS1-3. The testing of fresh concrete consisted of workability evaluation by a slump test and air content determination by a pressure test.

**Table 2.** Concrete characteristics.

