1. Introduction
Because the building sector has been recognized to be one of the major contributors to global warming, finding alternatives to conventional building materials is receiving increasing attention. Alkali-activated materials (AAMs) present promising substitute materials as their lower energy demand during production causes a smaller CO
2 footprint. In the most general description, AAMs are inorganic systems consisting of two main components: a reactive solid precursor such as metakaolin, slags or ashes, and an alkaline activator solution such as Na
2SiO
3, K
2SiO
3, NaOH, or KOH [
1]. Adding a foaming agent to this basic mixture leads to materials denoted as alkali-activated foams (AAFs) [
2,
3]. AAFs represent higher added value products due to their low production temperature of well below 100 °C, but still show properties comparable to foamed glasses or ceramics which are produced at above 900 °C.
AAFs can find various applications as catalysts, adsorbents, bone scaffold materials, filtration membranes, or thermal/acoustic insulators [
2,
4] and can be produced by different routes [
2]. Of these, direct foaming is most commonly applied; here foaming agents such as Al, SiC, Si, NaOCl, FeSi alloys, NaBO
3, or H
2O
2 are added to the alkali-activated slurry to trigger a chemical reaction which releases gaseous products. The gasses are trapped in the material’s structure during hardening which results in a highly porous material [
5,
6]. Surfactants or stabilizing agents such as sodium oleate, sodium dodecyl sulfate, triton, or stearic acid are added to the slurry to stabilize the pores and control their size [
7,
8]. However, several studies have shown that the compressive strength of AAFs, usually ranging from 1 MPa–10 MPa with densities of 360–1400 kg/m
3, decreases with a density reduction independent of the production method or used additives [
9,
10]. Lightweight aggregates (LWAs) are construction materials with a reduced bulk density, and their key physical properties are their bulk density, specific gravity, unit weight, porosity, and water absorption [
11]. They are widely applied for geotechnical fills, insulation products, soil engineering, hydroculture, drainage, roof gardens, or filters in several industries [
12]. LWAs can either be produced using natural rock by crushing and sieving scoria, pumice, breccias, tuff, or volcanic cinders or by thermally treating naturally occurring materials (e.g., vermiculite, clay, perlite, shale, slate) or industrial by-products (e.g., fly ash, blast furnace slag, industrial waste, sludge) [
11]. Artificial LWAs can be manufactured by either expansion or agglomeration. Expansion, as in expanded glass, occurs when a material is heated to a fusion temperature where it becomes pyro-plastic with a simultaneous formation of gas, released from added or intrinsic foaming agents. During agglomeration, the powdered material is bound together by either sintering mechanisms or cold bonding processes including additive binders [
13,
14,
15,
16]. Currently, the most used and valued manufactured LWAs from natural source materials are shale and expanded clay. However, LWAs can also be produced by applying the alkali-activation process to industrial waste such as fly ash, ground granulated blast slag, or rice husk ash [
17]. A polymer LWA extensively used in building and construction is expanded polystyrene (EPS). Its manufacturing process begins with small polystyrene beads ca. 200 µm in diameter which are permeated with a foaming agent, most commonly pentane, and expanded using steam [
15]. EPS is widely used in construction for external thermal insulation panels due to its energy efficiency, but it also finds application as the aggregate in lightweight concrete, decorative tiles and molding, panels, and embankment backfilling [
16,
18].
Several LWAs, such as recycled lightweight blocks [
19], Petrit T [
20], pumice aggregates [
21], vermiculite [
22], cork [
23], Etna volcanic aggregates [
24], and water reservoir sediments [
25] have been combined with AAMs. Optimized foamed thermal insulation materials produced by the alkali-activation process using Na
2SiO
3 and unexpanded ground waste, perlite, and rock wool showed a low thermal conductivity of 0.040–0.060 W/mK, a low density of 0.1–0.2 g/cm
3, and compressive strengths from 0.09 to 0.60 MPa [
26]. Foamy alkali-activated materials have been produced from nonexpanded perlite and show a thermal conductivity of 0.030 W/mK, a compressive strength of 0.78 MPa, and superior fire resistant properties, i.e., they are 100% noncombustible and categorized as the fire class A1 [
27]. A similar material (density 0.46 g/cm
3, thermal conductivity 0.084 W/mK, compressive strength 1.6 MPa) was produced using expanded perlite and K
2SiO
3 as the activator [
28].
Environmentally friendly, lightweight foamed geopolymer composites have also been produced as a thermal insulating material using H
2O
2 as the foaming agent, fly ash and metakaolin as precursors, and expanded polystyrene as LWAs [
29]. They showed densities of 0.30 to 0.65 g/cm
3, compressive strengths of 2.0 to 5.5 MPa, and thermal conductivities of 0.122 to 0.195 W/mK. [
30]. Exposing fly ash-based geopolymer concretes containing quartz aggregates or expanded clay to temperatures of up to 750 °C showed that the dehydration of capillary water caused cracking accompanied by a loss of strength below 300 °C whereas temperatures above 500 °C caused a sintering-promoted strength increase [
31]. Monolithic geopolymer-expanded glass composites have been prepared for the methylene blue removal from wastewaters [
32]. Here, adding expanded glass positively affected the removal efficiency.
Although composites of LWAs and cement are being applied on an industrial scale, composites of LWAs and AAMs are still under development. Adding LWAs to cements has been shown to counteract shrinkage [
33] and comparable benefits are to be expected in LWA–AAF composites. Replacing cements by AAMs and using waste materials as LWAs significantly lowers the environmental impact of these materials. Superior properties can be expected if a chemical bonding reaction or mechanical interlocking occurs between their components. This should increase their relative mechanical strength, allowing lower densities and with that, enhanced thermal isolation and lighter building components.
The interface between AAMs and aggregates has barely been analyzed and the literature presenting such interfaces usually features dense AAMs and relatively dense aggregates as their interfacial transition zone (ITZ) is easier to analyze. Just as cements, AAMs can form a chemically and structurally modified ITZ to aggregates they are in contact with. The ITZ in an AAM was found to be comparably dense, free of unreacted binder grains due to the “wall effect” and composed of a Na
2O–CaO–Al
2O
3–SiO
2–H
2O (N–C–A–S–H) gel [
34]. A gradual enrichment of Si and Na has been measured at the interface to quartz sand aggregates spanning 20–50 µm [
34]. Low Ca alkali-activated cements where the raw materials contained less than 4 wt% Ca did not form a discernible Ca-enriched ITZ [
35]. Another alkali-activated cement did contain high levels of Ca, but an enrichment at the ITZ was not detected [
36].
The work presented here is aimed at developing and characterizing LWA–AAM composite materials competitive to some commercially available products. They are energetically advantageous as they are manufactured below 100 °C and based on waste materials instead of cement. The LWAs expanded glass (EG), expanded clay (EC), expanded polystyrene (EPS), and expanded perlite (P) are included to reduce their overall densities and increase insulation while ensuring a sufficient mechanical strength. The performed analyses provide a first insight of the detailed microstructure at the interface between the well-known LWAs and an AAF. Furthermore, possible chemical interactions are analyzed and discussed.
2. Materials and Methods
Electric arc furnace slag (slag A), ladle slag (slag R), and fly ash (FA) were used as raw materials. The slags were received as aggregates from Slovenian metallurgical steel and iron plants and milled into powders with a grain size of less than 63 µm. Previously characterized FA from a Slovenian thermal power plant containing akermanite-gehlenite, quartz, anhydrite, hematite, magnesioferrite, and mullite [
37] was also used. It contained more than 70 ma% of an amorphous phase suitable for alkali activation [
37]. These raw materials were weighed using scale 1 (XPE205, Mettler-Toledo, Trzin, Slovenia, ±0.0001 g), heated to 950 °C for 1 h in a 25 mL Pt crucible and then weighed again to determine their loss on ignition (LOI) components, which amounted to 14.15 ma% for slag A, 20.47 ma% for slag R, and 0.51 ma% for the FA.
The precursors “slag A-p”, “slag R-p” and “FA-p” were produced in batches containing 0.946(9) g of the respective raw material and 9.469(0) g of the flux agent FX-X50-2 (i.e., 50% Li-tetraborate and 50% Li-metaborate, Fluxana GmbH & Co. KG, Bedburg-Hace, Germany) weighed using scale 1. Some of the mixture was placed in a 25 mL Pt crucible and heated to 1100 °C in an XRF xrFuse1 electric furnace (Thermo Fisher Scientific Inc., Ecublens, Switzerland), where it was held for 5 min and shaken for another 8 min before the furnace was turned off, allowing the batches to cool. Then, the chemical compositions of the raw materials were determined using a ARL PERFORM’X sequential X-ray fluorescence (XRF) Spectrometer (Thermo Fisher Scientific Inc., Ecublens, Switzerland) using the UniQuant 5.00 software (Thermo Fisher Scientific Inc., Walthem, MA, USA).
The preparation of these composites is also described in the Slovenian patent No. SI 26042 (A) [
38]. Dry mixtures of slag powders (grain size < 90 µm) with the optimized slag A-p/slag R-p = 1/1 ratio reported in Ref. [
39], FA-p and sometimes polypropylene fibers (Belmix, Mouscron, Belgium) with an average length of 11 mm and a density of 0.94 g/cm
3 were added. These were mixed with sodium water glass Crystal 0112 (Na
2SiO
2 containing 30.4 ma% SiO
2, 15.4 ma% Na
2O, and 54.2 ma% H
2O, Tennants distribution, Ltd., Manchester, UK) and solid NaOH (Donau Chemie, Vienna, Austria) before stirring the batch to homogenize it as well as possible. Then the foaming agent, solid sodium perborate (Belinka Perkemija, Dol, Slovenia), or liquid H
2O
2 (Belinka Perkemija, Dol, Slovenia), and the stabilizing agent liquid Triton™ X-100 (Merck, Darmstadt, Germany) were added. Finally, the LWAs expanded clay (Glinopor Vetisa d.o.o., Zalec, Slovenia), perlite (Njiva d.o.o., Zalec, Slovenia), expanded polystyrene (JUB, Dol, Slovenia) or expanded glass (Glasopor AS, Oslo, Norway) presented in
Figure 1 was mixed into each batch.
Samples were produced by casting these mixtures into silicone molds and drying them for three days in a WTB laboratory dryer chamber (Binder, Tuttlingen, Germany) at 70 °C and ambient humidity. The components were weighed using scale 2 (Exacta 2200 EB, Tehtnica, Trzin, Slovenia, ±0.01 g) and combined to produce each sample according to
Table 1.
The flexural and compressive strength were determined using a Toninorm press (Toni Technik, Berlin, Germany, force detection limit 100 N) with a force application rate of 0.05 kN/s by the standard method [
40] and averaged from four test specimens of 20 × 20 × 80 mm
3. Geometrical densities were determined by weighing individual samples (size of 20 × 20 × 80 mm
3) and dividing their weight by their volume. Sample dimensions were measured using a Vernier Calliper (Mitutoyo, Neuss, Germany) with a precision of ±0.01 mm. Thermal conductivities were measured using a HFM 446 (Lambda Small, Stirolab, Sezana d.o.o., Slovenia, ±1%), according to EN 12667 and ASTM C518 in ISO 8301.
Optical microscopy of the material cross-sections was performed using a SMZ25/SMZ18 stereo microscope (Nikon, Minato, Japan) at a working distance of 60 mm, images were captured using a digital MikroCamII Microscope Camera (Leica, Wetzlar, Germany). Cross-sections of selected samples were cut and embedded in EpoThin resin (Buehler, Leinfelden-Echterdingen, Germany), cured at 50 °C, and polished using decreasing grain sizes to a final step of ca. 10 min on a SiC Buehler Micro Cut plate 30-10-4000 (Buehler, Leinfelden-Echterdingen, Germany, ca. 5 µm grain size). Scanning electron microscopy (SEM) was performed using a JSM-IT500 (Jeol, Tokyo, Japan) in low vacuum mode. Energy dispersive X-ray spectroscopy (EDXS) was performed using an Ultim Max 65 detector (Oxford Instruments, Abingdon, UK) and the software Aztec 5.0 (Oxford Instruments, Abingdon, UK). SEM figures and EDXS maps were acquired using an acceleration voltage of 15 kV whereas EDXS spot measurements were performed using 10 kV to reduce the information volume.