1. Introduction
Portland cement concrete is the most widely used building material in the world, mainly because of its durability, strength, chemical stability, low production cost, and ability to be easily formed into a wide variety of shapes [
1,
2]. For all these reasons, the demand for concrete has increased with the progress and innovation of the construction industry, as there is no other material that can match its various properties [
3,
4]. However, the environmental impact has also increased, as the Portland cement (PC) industry is one of the main protagonists of the greenhouse effect and thus of global warming, due to the calcination of raw materials at temperatures above 1450 °C, which causes the emission of greenhouse gases such as CO
2, SOx, and NOx [
5]. Due to the CO
2 emissions associated with cement production, currently reported around 7–9%, the need for environmentally friendly solutions for a reduction in the carbon footprint is currently an important research topic [
1,
6,
7,
8].
Innovations in Portland cement production processes have evolved to achieve better properties, reduce the environmental impact of the production process, and lower the cost of PC [
9]. One of the most notable innovations is the use of composite cements, which are cements in which a portion of the Portland cement is replaced by industrial by-products [
10,
11,
12], including various types of pozzolanic and hydraulic materials [
13,
14]. Those cements have been used for several reasons, one of them is that these materials have shown highly desirable properties for certain purposes, such as delaying or reducing heat of hydration in massive structures, improving durability and, in some cases, achieving strengths above the normal range [
15,
16]. The use of alternative materials as a partial or total replacement of PC has represented an important advance in the construction industry, as a variety of by-product materials from different industries with high production volumes have been proposed with remarkable results. These materials have a promising future in the construction industry, not only as an ecological alternative, but also as a technological solution since the properties of those composite cements could be greatly improved [
14]. However, with the current restrictions on the use of carbon due to policies or measures in several countries that have promoted the use of alternative energy and aimed at reducing the amount of carbon dioxide and other greenhouse gases emitted by human activities, the availability of such substitute mineral admixtures is beginning to decrease [
5]. Therefore, it is imperative to search for SCMs in nature to expand the availability of these alternative materials.
Over the past few decades, there has been growing acknowledgment of the potential of aluminosilicate materials found in nature or generated as by-products in industries, such as fly ash (FA), ground granulated blast furnace slag (GGBFS), and metakaolin (MK). These materials have demonstrated their ability to serve as sustainable alternatives to ordinary Portland cement, leading to noteworthy environmental advantages [
10,
14,
17]. Moreover, the use of geothermal silica (GS) has been reported by various researchers [
18,
19,
20]. This is a by-product of the process of energy production by steam extraction from the subsurface. GS is obtained as a mixture of geothermal brine and steam that undergoes a series of steps to extract heat. The reports indicated that the GS has a strong pozzolanic behavior producing a densification of Portland cement hydration products matrix [
18]. These benefits include reduced CO
2 emissions and energy consumption associated with Portland cement production, as well as the preservation of non-renewable natural resources [
1,
2,
21,
22,
23,
24,
25]. Numerous studies have shown that these materials have practical utility as construction materials, particularly in the production of sustainable binders such as alkali-activated (AA) binders [
17,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35]. These binders offer several advantageous properties over ordinary Portland cement concrete (OPC). These include rapid high-strength development, excellent durability, and high resistance to temperature and chemical attacks [
24,
28,
36,
37]. Those benefits are attributed to the abundant availability of these materials and the presence of soluble silica and alumina in them [
37,
38]. Through the formation of reaction products and property development, these materials contribute to the strength and stability of these matrices [
38].
The use of alkali-activated (AA) mortar or concrete has been steadily increasing in the construction industry. Extensive evaluations of AA binders, particularly those made from fly ash and ground granulated blast-furnace slag, have clearly demonstrated that these materials, which contain abundant amounts of silica (SiO
2), alumina (Al
2O
3), and/or calcium oxide (CaO), serve as suitable source materials. This is because the alkali activation process involves a chemical reaction between various aluminosilicate oxides and silicate, or the formation of a silica-rich calcium silicate hydrate (C-S-H) gel. These reactions contribute to a deeper understanding of these binder systems and their properties [
32].
Recently, there have been reports on the development of hybrid cements that combine ordinary Portland cement (OPC) with alkali-activated cements [
32]. These hybrid cements are obtained by blending OPC clinker in proportions of less than 30% by weight [
34], together with supplementary cementitious materials (SCMs) in proportions of more than 70% by weight, and the addition of an activator in proportions of about 5% [
29,
39]. The importance of these mixtures, with their numerous advantages, has attracted considerable attention from the scientific and technological communities [
29]. These mixtures show remarkable versatility, as they exploit the inherent properties of the raw materials used [
29]. Hybrid cements have the advantage of accommodating significant amounts of mineral admixtures, such as industrial by-products or natural pozzolans, which require alkaline activation. In addition, they require only small amounts of cement to facilitate the hardening of the mix when mixed with water. This has the notable advantage of reducing the clinker content in the mix, resulting in energy savings and conservation of natural resources. In the hydration process of hybrid cements based on GGBFS and PC, it has been observed that alkaline activation produces reaction products very similar to those formed during the hydration of conventional Portland cement. The primary reaction product formed is the C-A-S-H gel, which has a composition and structure different from the C-S-H gel formed by conventional Portland cement [
35]. Furthermore, the hydration process of hybrid cements combining fly ash and Portland cement shows a different mechanism. FA, characterized by low CaO content and high SiO
2 and Al
2O
3 content, follows a model proposed by Palomo and Fernández-Jiménez [
35,
40]. This mechanism leads to alkaline activation, resulting in an amorphous matrix with cementitious properties, known as a N-A-S-H gel. In studies of supplementary cementitious materials (SCMs), researchers have observed the simultaneous formation of N-(C)-A-S-H, C-(N)-A-S-H, and C-A-S-H gels within the cementitious matrix. The specific types and amounts of precursors used contribute to the presence of these different gel formations [
27,
32,
41,
42].
Due to their high substitution content, this type of material must be combined with an alkaline solution for blending. Hybrid cements are included in the category of environmentally friendly cements since they combine the positive properties provided by an OPC with the properties of alkaline activated materials [
41], generally resulting in a material with good mechanical properties [
43]. Nevertheless, these hybrid cements are not well understood yet, and in order to see the effect of cement quantity on their properties, here is proposed up to 50% of replacement level.
With the aim of providing an alternative to the environmental impact caused by the production of PC and the growing demand for it, the main objective of the research was to evaluate composite pastes in which partial substitution of ordinary Portland cement (OPC) was made by a mixture of FA, GGBFS, and GS, with and without external alkaline activation utilizing water glass, sodium hydroxide and sodium sulfate, analyzing the compressive strength and the microstructures obtained. The formulations were prepared using different alkaline activators to improve the mechanical properties of the composite cements. The hydration products of the cements were then characterized to evaluate the interaction and hydration kinetics of the materials used at different curing ages. Consequently, the main novelty of this research is the comparison of the development of new composite cements containing different types of pozzolanic and hydraulic materials with and without alkaline activation.
2. Materials and Methods
The materials used in this investigation were: (a) Alkaline activators: Three different types of alkaline activators were used: (i) sodium hydroxide, (ii) Sodium silicate (water glass) with a ratio SiO2/Na2O = 2.0, and (iii) sodium sulfate. NaOH solutions with the required concentration were prepared by dissolving NaOH pellets in distilled water, leaving the solution to cool down for at least 24 h. Sodium hydroxide solution was used to adjust the Na2O content in the activating solution. (b) Precursors: The materials used in this study were obtained from different suppliers in Mexico. A commercial Ordinary Portland cement (OPC), compatible with ASTM type I Portland cement, from Cemex Mexico was used for the mixes. In addition, ground granulated blast furnace slag (GGBFS) from the steel company Altos Hornos de México (AHMSA), fly ash (FA), and geothermal silica (GS), both from the Mexican Federal Electricity Commission, were used as supplementary cementitious materials (SCM).
The FA and GGBFS were sieved through a 75 μm mesh before use [
25], with special care for geothermal waste; the GS is a material containing sodium and potassium chlorides, which must be removed by washing, as they can have a negative effect on the behavior of the replaced cement pastes. The treatment was carried out with potable water at 90 °C, in a ratio of 3:1 water:GS; between each washing, a chloride analysis was carried out by titration. When the content was minimal (0.02%), the material was able to be used as a substitute for OPC [
18]. The material was then dried at 110 °C for 24 h to remove all water.
Table 1 lists some of the physical properties of the cementitious materials and shows that GGBFS and FA have the lowest Blaine fineness and GS has the highest, followed by OPC. The cementitious materials were also characterized by X-ray fluorescence spectroscopy and X-ray diffraction (XRD).
Table 2 shows the chemical composition of the cementitious materials used in this research. Together with particle size, chemical composition is a key factor influencing the pozzolanic and hydraulic action of cementitious materials [
44,
45,
46].
X-Ray diffraction patterns obtained from the precursors are shown in
Figure 1.
Figure 1 shows the diffraction pattern of the OPC, including its characteristic phases: alite, belite, aluminate, ferrite, and gypsum. The amount of each phase was calculated by Rietveld analysis and is shown in
Table 3. GGBFS is a calcium silicoaluminate material which, due to its processing, tends to be highly vitreous [
19], as can be seen from the diffraction pattern also in
Figure 1; an important characteristic of this material is the appearance of an amorphous halo between 20°–40°2θ, it is also observed that it shows some weak reflections corresponding to akermanite, a typical phase of GGBFS. In the XRD pattern of the FA, the amorphous characteristics of aluminosilicates can be observed in 20 and 30°2θ, as well as the crystalline phases quartz, mullite, and calcite [
15]. Additionally, the main characteristics of GS are published [
13,
15,
47], showing only an amorphous halo between 20°–30°2θ. Moreover, for SCM, an amorphous structural state is generally required, i.e., with high internal energy and therefore thermodynamically unstable and highly chemically reactive [
48,
49]. The percentage of crystalline phases in the GGBFS and FA were analyzed using Rietveld, and the percentage of amorphous fraction was calculated by difference (see
Table 4 and
Table 5). The amorphous properties of the materials used were within acceptable limits and that adequate cementitious properties can be expected [
50].
The experimental program consisted of two stages (a) alkali-activated pastes prepared in the first stage, based on a 50% replacement of OPC with GGBFS, FA, and GS, and (b) 30% replacement without activation (see
Table 6). The water/binder ratio was set at 0.4, giving good mechanical strength but poor workability, which was improved using polycarboxylate-based superplasticizer (SP) from Sika at a fixed proportion of 0.3% of the solids. The dosage of the alkaline activators was set to 4% and 7% of Na
2O equivalent (contained in) to sodium hydroxide, sodium silicate and sodium sulfate in order to avoid the low rate of setting time. Pastes without alkaline activation were prepared by adding distilled water and SP in a blender with a planetary movement and stirred during 30 s for complete incorporation. For the mixtures that required external alkaline activation with NaOH (NH), Na
2SiO
3 (SS), and Na
2SO
4 (NS), the activator was first dissolved in the water and the SP was added seconds before mixing to avoid degradation due to the high pH of the solution. The GS was mixed with the mixing water in the mixer at slow speed for 1 min and the OPC, GGBFS, and FA, previously dry homogenized, were added. Pastes were poured into cubic 50 mm plastic molds. The molds were vibrated on a vibrating table to eliminate any entrapped air, and then sealed with plastic film to prevent water evaporation during the first 24 h in a room at 20 ± 2 °C. Samples were cured under calcium hydroxide saturated water for up to 90 days. After reaching the respective curing age, compressive strength tests (CS) were performed following the ASTM C109 standard [
51]; values were estimated from the average of four specimens. Then, fragments of the crushed specimens were submerged under acetone to stop the hydration reaction, during 72 h and dried at a vacuum oven at 40 °C for 24 h [
52,
53]. The dried samples were further ground in a zirconia mill, the resulting powder was sieved on a #200 mesh (74 μm) at various intervals until all the material passed through the mesh. The powder samples were used for X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Some unground broken fragments were used for scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS).
XRD analyses were performed on selected samples after 3 and 90 days of curing for OPC and activated composites and for 3 and 28 days of curing for non-activated composite cements and those activated with Na2SO4. XRD patterns were recorded on an Xpert MPD Phillips diffractometer using pure copper K-Alpha 1 radiation at a wavelength of 1.54 A. The X-ray generator was operated at 40 kV and 40 mA. The settings were as follows: the recorded angular range was 10° to 90°(2θ) with a step close to 0.01°; step time 0.05 s and step size 0.01°. For the TGA samples of around 30 mg were heated from 25 to 950 °C at 10 °C/min in a nitrogen environment in a thermal analyzer. The mass loss at different temperatures was used to quantify the reaction product formation. SEM and EDS were performed to identify the changes in the microstructure and elemental composition of the selected samples. SEM images and EDS data were obtained on gold–palladium coated samples using a JEOL 6490 LV scanning electron microscope. The solid pieces of the selected pastes were mounted in epoxy resin, ground with SiC grit paper and then polished with diamond paste up to 0.25 microns. The accelerating voltage was set at 20 kV. The characterization by EDS was performed by collecting 30 spot analyses per sample to determine the elemental analysis of the samples.