**1. Introduction**

In 2017, the global production of cement, the base component of concrete, amounted to almost 4.65 billion Mg [1]. For the production of 1 Mg of Portland cement clinker, about 1.7 Mg of natural resources are used, mainly carbonate raw materials such as limestone and marl. Thus, as a result of the clinker firing process, huge amounts of CO2 are released into the atmosphere, the source of which is the thermal dissociation of carbonates in the raw material bulk (60%) and the emission of CO2 from the combustion of technological fuel (40%) [2,3]. It is considered that cement production is responsible for about 7.4% of the world carbon dioxide emission (2.9 Mg in 2016) [4]. Therefore, the world cement industry has to meet the constantly growing environmental requirements, which mainly concern the reduction of dust and greenhouse gas emissions [5]. Unfortunately, the production of the basic component of cement, i.e., Portland clinker, is associated with the emission of CO2, which is about 825–890 kg of CO2 per Mg of clinker [6]. The world average is about 840 kg of CO2 but the carbon dioxide emission level should be lower than 400 kg per Mg of cement. It is suggested that the emission levels reach around 350–410 kg per Mg of cement [4].

The possibilities of emission reduction include two solutions in the cement production process [2,7]:


In the case of production of CEM II-CEM V multicomponent cements, the main components are usually by-products of industrial processes such as siliceous fly ash (V), calcareous fly ash from coal dust combustion in the power industry or granulated blast furnace slag from iron metallurgy (S) [2,9,10]. The cements containing significant amounts of fly ashes and slags are characterized by low hydration heat (a feature important in the implementation of massive concrete structures), higher strength after longer curing periods and higher resistance to chemical aggression [2,9–11]. To ensure appropriate durability of concrete made of cement with lower clinker content in the assumed construction environment, the concrete composition (type and amount of cement, w/c ratio, type of admixtures and amount of concrete additives) should be properly designed, so that the concrete is characterized by a tight matrix. Determining the concrete tightness, e.g., by limiting the amount of water in the concrete mix or using cement with mineral additives, results in limiting the capillary porosity of the hardened cement slurry [2,12–14]. On the "macro" scale, it directly affects the depth of penetration of aggressive media and the size of capillary pull, whereas on the "micro" scale, it results in impeding the diffusion of aggressive ions into the cement matrix. However, the availability of fly ash and granulated blast furnace slag, with increasing cement production, is limited [15]; therefore, limestone (LL) is used increasingly often in cement composition. The main advantage of this component is its widespread availability and the fact that it can be obtained from the cement plants own raw material resources [16–18].

Calcium carbonate, the main component of limestone, reacts with calcium aluminates to form hydrated calcium carboaluminates. The presence of hydrated calcium carboaluminates inhibits the transition of ettringite to monosulfate, thus, in hydration products the amount of monosulfate decreases or disappears while the amount of ettringite increases [19]. The fact that calcite reacts with C3A to form carboaluminates means that CaCO3 may play, to a limited extent, the role of a regulator of setting time. This results in the reduction of the amount of gypsum, which is necessary to regulate the setting time [20].

In addition to the reaction with calcium aluminate, the addition of limestone to the cement may accelerate the C3S phase reaction. This effect is explained by the nucleation effect, in which CaCO3 grains act as additional crystallization germs for cement hydration products [19–21]. Limestone is a very soft component in comparison to Portland clinker. After the milling process, it has a much higher specific surface area and, as a micro-filler, influences the properties of cement composites, e.g., by reducing porosity, increasing strength in the initial period of hardening and improving workability, reducing water consumption and reducing water draining from the concrete mixture (so-called "bleeding") [19–24]. Bearing these facts in mind, the European Committee for Standardization CEN has undertaken standardization works aimed at extending the range of cements containing cement components other than Portland clinker in its composition. It is proposed to implement the non-harmonized standard prEN 197-5 [25], which extends the range of Portland multicomponent cements (the possibility of using several main components in the composition of cement) by a group of Portland multicomponent cements CEM II/C-M with a minimum content of Portland clinker of 50% and a newly created group of multicomponent cements CEM VI, in which the share of non-clinker components may be a maximum 65%.

This paper presents the results of research on Portland multicomponent cement CEM II/C-M with 40% of non-clinker main components and multicomponent cement CEM VI with 55% of these components. Ground granulated blast furnace slag (S), siliceous fly ash (V) and ground limestone (LL) were used as non-clinker main components. Concrete tests were performed for the analyzed cements CEM II/C-M and CEM VI. The basic properties of concrete mixture and hardened concrete were determined with a view to future use of cements in construction practice. The level of CO2 emissions originating in the composition of CEM II/C-M and CEM VI cement was also calculated, as well as the level of CO2 emissions from the production of concrete with the use of tested cements.

#### **2. Materials and Methods**

## *2.1. Characteristics of Components and Composition of Tested Cements*

Three types of non-clinker ingredient were used in the study: granulated blast furnace slag from iron metallurgy, siliceous fly ash from the combustion of coal in power plants, and natural limestone.

The chemical composition of the cement components and selected physical properties are given in Table 1. Figures 1 and 2 show diffractograms of ground granulated blast furnace slag (S) and fly ash (V).

In the slag phase composition (S), the dominant component is the vitreous phase, the quantitative content of which (determined microscopically) is 98%. In fly ash, next to the vitreous phase, the main crystalline components identified are quartz, mullite, hematite and magnetite. The Portland cement CEM I is a semi-finished product with an increased SO3 content (5.0%) in order to obtain a normal SO3 content (max. 3.5%) when mixed with the other main components of the cement. Therefore, this cement is a semi-finished product in the process of manufacturing multicomponent cements. The clinker content in the cement was 90%.

**Table 1.** Chemical composition and physical properties of the main components of cement.


**Figure 1.** Diffractogram of granulated blast furnace slag.

**Figure 2.** Diffractogram of siliceous fly ash.

When analyzing the properties of the main components of cement used, attention should be paid to the high specific surface area of limestone of 6150 cm2/g (Table 1). Obtaining such a high specific surface area is relatively easy due to the very good granularity of the limestone. The granulometric composition of non-clinker cement components is shown in Figure 3.

**Figure 3.** Particle size distribution of supplementary cementitious materials.

Two CEM II/C-M cements with a Portland clinker content of 54% and two CEM VI cements with a Portland clinker content of 40.5% were prepared for testing the ternary cements. The composition of the tested cements and CO2 emission levels are given in Table 2. The CO2 emission level from 1 Mg of cement was calculated assuming the average CO2 emission level of production of 1 Mg of clinker at the level of 840 kg [4] and the clinker content in the composition of the tested ternary cements CEM II/C-M and CEM VI (Table 2). In the calculations, the CO2 emission level related to the transport and grinding of the components into cement was omitted. The obtained CO2 emission levels from 1 Mg of tested cements at the level of 340.2–453.6 kg allows the inclusion of CEM II/C-M and CEM VI cements into low-emission cements.

In order to evaluate the synergy effects of cement components, comparative cements containing one non-clinker component—slag cement C(55S), fly ash cement C(40V) and limestone cement C(40LL) were also tested (Table 2).


**Table 2.** Composition of tested cements and CO2 emission level.

(1) comparative cements.

The properties of cements were determined according to the procedures of EN 196 and the density according to the standard on the properties of aggregates EN 1097-7 (Table 3).

**Table 3.** Procedures used to determine the properties of cement.

