**1. Introduction**

The deteriorating air conditions and the significant environmental impact of the industry that causes further new restrictions on CO2 emissions are introduced to achieve climate neutrality [1]. These restrictions strongly affect the cement industry, owing to the industry's significant carbon footprint [1]. To reduce the consumption of natural raw materials and the emissions needed to produce cement, clinker can be partially replaced by additives such as ground blast furnace slag, fly ash or limestone powder, which can reduce CO2 emissions by up to 8% by 2050 [1]. The use of mineral materials of natural origin allows for higher strength properties, especially after a longer curing period.

One of the materials used to replace part of the cement was limestone powder. This material is obtained by grinding limestone. The main component of limestone used in the cement industry is calcite CaCO3. Because of the high availability of this raw material, ground limestone as a type I additive is widely used in concrete technology departments, in addition to being used as a cement additive. Limestone powder is widely used because it has a beneficial effect on the properties of concrete, mainly owing to its high specific surface area—the additive reduces the distance between particles. The physical filling of the concrete structure makes the finished product stronger and improves its frost resistance and durability. The additive also contributes to the hydration process of the cement. Limestone particles can create additional foci of crystallisation, dynamising the setting time of the cement. The limestone powder in cement mortars disperses the grains of ground Portland clinker, facilitating the access of water to them, which increases their degree of reactivity [2].

**Citation:** Brachaczek, W.; Chlebo´s, A.; Kupczak, M.; Spisak, S.; Stybak, M.; Zyrek, K. Influence of the Addition of ˙ Ground Granulated Blast Furnace Slag, Fly Silica Ash and Limestone on Selected Properties of Cement Mortars. *Mater. Proc.* **2023**, *13*, 32. https://doi.org/10.3390/ materproc2023013032

Academic Editors: Katarzyna Mróz, Tomasz Tracz, Tomasz Zdeb and Izabela Hager

Published: 16 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In the cement industry, silica fly ash is also used, which is mainly obtained from the combustion of hard coal dust at 1300–1450 ◦C in pulverised-gravel furnaces [3]. Silica fly ash is characterised by a high content of glassy phase. It is widely used in the cement industry because of its high fineness and pozzolanic activity. The pozzolanic activity of silica fly ash means that on its own it does not exhibit binding properties, but after grinding on contact with moisture, it reacts with calcium hydroxide Ca(OH)2 derived from the hydration of the silicate phases of Portland clinker to form products with binding and hydraulic properties [4,5]. The main components of fly ash are inorganic carbon constituents derived from the thermal decomposition of certain clay minerals, pyrite and calcite. Silica fly ash, according to the standard [6], is a type II concrete additive which means that it must not exceed 5% by weight of cement. It has the structure of fine dust with a spherical shape [4]. The undesirable components of silica fly ash used as a concrete additive include an excessively high sulphur content, free lime particles, unburned carbon and iron compounds: haematite and magnetite. These are formed on the surface of the fly ash grains and block the access of the liquid to its glassy phase, thus adversely affecting the pozzolanic reaction. The high roasting losses occurring especially in the production of raw material in less efficient older-type power plants and the associated increased content of unburned carbon cause an increase in the water content of the ash and lead to a decrease in the water and frost resistance of the finished cement product [5]. According to [5,7,8], the addition of silica fly ash V has a positive effect on reducing the overall porosity of cement mortars and contributes to a reduction in the dominant pore size. A characteristic feature of mortars with the addition of light silica fly ash is the low calcium hydroxide content [5]. The reduced porosity and Ca(OH)2 content makes cement mortars with this additive show increased resistance to chemical corrosion, which is extremely important especially for materials intended for contact with water.

Ground granulated blast furnace slag according to the standard [6] is a type II concrete additive. Blast furnace slag is obtained during the smelting of pig iron in a blast furnace as a by-product. The component raw materials introduced into the furnace are iron ore, coke and fluxes, which lower the melting point of the ore and help to separate the metal from other admixtures contained in the ore. By burning the coke in hot air, the furnace is heated. This process takes place at a temperature of 1400–1600 ◦C. Molten blast furnace slag is obtained by melting the charge on the surface of the pig iron. The granulation process takes place after separation from the pig iron by rapid cooling with air or water. The formation of a microstructure that ensures an adequate level of activity is obtained by rapid cooling of the liquid slag. The high content of the glassy phase in the slag is responsible for the activity of granulated blast furnace slag [9]. Ground slag is a constituent of Portland multicomponent cements CEM II, metallurgical cements CEM III and multicomponent cements CEM V. It is classified as a material with latent hydraulic properties [9]. Ground granulated blast furnace slag consists mainly of calcium oxide (CaO), magnesium oxide (MgO), silicon dioxide (SiO2) and aluminium oxide (Al2O3) [10]. In terms of radioactivity, according to the classification in the Ordinance of the Council of Ministers of 2 January 2007, Journal of Laws ne 4, item 29, blast furnace slags are classified in Group I, which means that these materials can be used in the production of building materials used in buildings for human and livestock residence [11]. Cements containing ground blast furnace slag are characterised by longer setting times, lower heat of hydration, better workability, significant strength gain over longer hardening periods and higher resistance to chemical aggression [12].

The aim of the study was to assess the feasibility of using ground blast furnace slag, silica fly ash and limestone powder in the production of cement mortars. A comparative analysis of the properties of fresh mortar and after hardening containing these materials was carried out. The design and testing procedures were based on the guidelines in the currently applicable national construction standards. Consistency tests were carried out in accordance with [13] and flexural and compressive strengths in accordance with [14].

### **2. Experimental Part**

The tests were carried out at the Building Materials Laboratory at the University of Bielsko-Biała and at the R&D Laboratory of SEMPRE Farby Company in Bielsko-Biała. The design and testing procedures were based on the guidelines contained in the currently valid national construction standards. Consistency tests were carried out in accordance with the standard [2] and flexural tensile and compressive strength in accordance with [1]. The composition of the mortar on which the tests were carried out is shown in Tables 5–7. In all samples, the granulometric composition of the fine aggregate selected by sieve analysis was kept constant. The mortar binder was Portland cement CEM I 42.5 R Górazd˙ ze. The ˙ physical and mechanical properties of this cement are shown in Table 1.

**Table 1.** Physical and mechanical properties of the used cement.


Because of the study of the impact of the introduced changes mainly in terms of cement mortars, it was decided to use as the main aggregate dried quartz sand with a fraction of 0.0–0.5 mm typical for cement mortars, sourced from sand mine KOTLARNIA S.A. The grain size distribution of the sand is shown in Figure 1.

**Figure 1.** Particle size distribution of aggregates.

A constant quantity of batch water of 270 ml per kg of dry mix was maintained in all samples. Because of the influence of the additives used with different water content, the consistency was adjusted using the superplasticiser Melment F10—a polycondensate of sulphonated melamine and formaldehyde. The superplasticiser was dosed in such a way that the mortar maintained a constant flow measured on the flow table according to the standard [2]. With the use of recycled additives, the mass proportion of cement was reduced so that the sum of the cement and additive masses was constant. The maximum proportion of additives was 10% of the initial cement mass.
