1. Introduction
Under sustainable development conditions in the field of construction, researchers’ attention has focused on the manufacture of new ecological building materials.
Many researchers have focused on utilizing different industrial by-products in the production of sustainable, ecological cement and concrete [
1,
2].
Portland cement is a binding material which provides very good mechanical characteristics compared with several other materials. The worldwide production of cement amounts to almost 5 Gt. Due to the high calcination temperature, the production of cement requires significant energy inputs, being responsible for almost 10% of worldwide CO
2 emissions [
3].
The increased use of industrial wastes and by-products as supplementary cementitious materials to produce concretes and blended cements represents a way to obtain net zero CO
2 emissions [
4].
Incorporating a wide range of wastes (wood-based fly ash, wheat straw ash, granite sawdust waste, palm oil fuel ash etc.) as cement substitutes in concrete might, in addition to improving mechanical properties and durability, favor the removal of noxious materials and prevent massive amounts of waste being deposited in the environment [
5,
6,
7,
8,
9,
10,
11].
It is necessary to search for other materials susceptible to activation for use as active pozzolans in cementitious systems. A pozzolanic material is generally defined as a natural, synthetic, or secondary raw material, containing high percentages of active SiO
2 and Al
2O
3 [
12,
13,
14,
15,
16].
According to Sicakova and Spak, mineral additives may be categorized as follows—chemically active mineral admixtures (highly reactive pozzolan) and micro-filler mineral additives (low to moderately reactive pozzolan). The character depends on the physical (particle size, particle shape, specific surface area, etc.) and chemical properties (chemical and mineralogical/phase composition, ratio of hydraulic oxides); the source from and process by which these components are obtained are responsible for the particular character [
17].
Metal waste recovery plays a key role in the task of complying with the ambitious environmental objectives set by the European Union, with a significant impact on greenhouse gas emissions and, therefore, on global climate change.
The European Union finances some significant projects aiming to provide energy and resource flexibility to energy intensive industries (the CIRMET Project - Innovative and efficient solution, based on modular, versatile, smart process units for energy and resource flexibility in highly energy intensive processes) and to study the possibilities of processing metals and recovering metal wastes in a more sustainable way (the DIGISER++ project - Innovative waste recycling for the copper industry) [
18,
19].
Industrial by-products and waste result from different industrial processes, such as the steel manufacturing process. In steel industry, by-products are represented by steel slag.
The surface treatments of steel, such as pickling, involve cleaning to remove oxides, mill scale and dust. This process is performed by hot immersion in acid solution. The evacuated baths are treated by chemical methods and the quality of effluents should be ensured in accordance with the standards.
The treatment methods involve precipitation of heavy metals, flocculation, sedimentation and deposition. Each stage is conducted in a separate container and the entire process requires several pH adjustments, as well as coagulant and lime addition.
The process leads to the generation of high amounts of sludge, with a major environmental and economic impact. Currently, sludge-after neutralization-is stored in authorized external deposits. Due to the continuous increase in sludge amounts, as well as associated economic and environmental factors, solutions are searched to recover the neutralization sludge and to avoid its storage in deposits.
There are many methods for sludge management. It could be used in agriculture, building materials, wastewater treatment reagents, sludge dewatering or land filling [
20,
21,
22,
23,
24]. Such methods, in addition to their advantages, economic savings, and environmental sustainability, have some limitations, such as the complexity of the method and problems that can be caused by the pollutants present in the sludge [
22].
Ilutiu-Varvara et al. studied the possibility of using the metallurgical wastes (steel mill scales) in the composition of mortar. The recycling of the oily mill scale in mortar compositions is a form of sustainable manufacturing through the conservation of raw material [
25].
Following the studies performed, ground granulated blast furnace slag was proven to have a pozzolanic and cement-like behavior, and is therefore being used in the composition of various types of cement [
26,
27,
28,
29]. In addition, there are a few studies regarding the use of steel sludge in concrete.
However, we refer to the study conducted by Nurul et al., regarding the performance of steel slag and sludge in concrete by evaluations of the pozzolanic activity [
30]. The materials used in this study were: Portland cement type 1 according to the American Society for Testing and Materials (ASTM), steel slag, steel sludge, aggregates, and water.
While steel slag was obtained through the melting of metal residues in an electric arc furnace, steel sludge-referred to in the literature-was obtained from steel cable production, generated during the remodeling of cables to obtain the desired shape and size [
30].
The results obtained by the mentioned team show that including both the slag and the sludge in the cement of concrete improves its compressive strength. Although 10% slag addition seemed to be the optimal dose, adding a percentage of up to 15 and 20% of sludge and slag, respectively, induced a superior increase in strength compared to the control (standard) mixture, particularly at longer setting times (over 60 days) [
30].
Concrete containing steel sludge showed improvements in the flexural strength. However, the development of the flexural strength, in the case of sludge in particular, was slow compared to that of the control (standard) mixture. Nevertheless, a percentage of up to 20% steel slag and 5% steel sludge could be applied to cement without the risk of it affecting the flexural strength quality of the concrete [
30].
The presence of oxides such as CaO, SiO
2, FeO and Al
2O
3 make metallurgical wastes useful as clinker materials in cement production [
31].
This study aimed at investigating the sue of sludge as a possible addition to ordinary Portland cement in order to obtain a sustainable, ecological cement that takes advantage of the waste resulting from the pickling of steel pipes that, until now, has generated large volumes without a specific use.
2. Materials and Methods
“Structo Plus” cement type: CEM II/B-A1(S-LL) 42.5N, SR EN 197-1:2011 manufactured by Holcim (Alesd, Romania); steel sludge; sand with a grain size of up to 2 mm, and water were used.
The CEM II/B-A1(S-LL) cement is a variant of CEM II/B-LL (SR EN 197) cement, which is a mixed cement: Portland with limestone, whose “main component (without a clinker) is limestone with organic carbon that does not exceed 0.2% (LL)”, having the 42.5 N strength class.
The sludge samples were taken from a metallurgical plant, which produces different domestically made steel trademarks, located in Salaj County, Romania.
The plant produces cold drawn pipes from hot rolled tubes, which are pickled with sulfuric acid before being processed and finished. The waste generated from the pickling, are treated by precipitation of heavy metals, flocculation, sedimentation and deposition. Each stage is conducted in a separate container and the entire process requires several pH adjustments, as well as coagulant and lime addition. The process leads to the generation of high amounts of sludge.
In order for the sampling to be representative of the whole manufacturing process, a sample was taken once a week for twelve weeks and the resulting samples were merged in a homogenous sludge sample.
Initially, the sludge samples were in the form of plates of variable sizes, with a fragile consistency, which allowed them to be easily ground (
Figure 1a).
These samples were dried in the oven at a temperature of 105 °C and were subsequently ground. Grinding was performed in a ball mill, and a material with a fine grain size was obtained (
Figure 1b).
After grinding, the sludge was sieved with a mesh of 0.125 mm (
Figure 1c).
The resulting powder was analyzed for determination of its element content (%), using the X-ray fluorescence (XRF) method, with an INNOV-X ALPHA 6500 analyzer, and for confirmation, with a simultaneous-detection inductively coupled plasma-optical emission spectrometer ICP-OES (SpectroFlame FMD 07, Kleve, Germany).
The chemical composition was also studied using the X-ray diffraction (XRD) method with a D8 ADVANCE analyzer-Da Vinci Design-manufacturer Bruker AXS GmbH, Karlsruhe, Germany.
The mix proportions in which in the composition of standard mortar (RS) cement is replaced by 10% (R10C), 20% (R20C), 30% (R30C), and 40% (R40C) steel sludge, respectively, are presented in
Table 1.