1. Introduction
Fabrication of earth blocks and soil stabilization are procedures in which ordinary Portland cement (OPC) is normally used as a cementing component [
1]. Despite the low OPC proportion added to soil (5–10%), the consumption of this cement type has major environmental consequences: the cement industry is responsible for more than 7% of global CO
2 emissions and 12–15% of the total energy consumed by industrial activities worldwide [
2]. In some cases, supplementary cementing materials have been added to enhance soil stabilization: Basha et al. [
3] demonstrated that the addition of rice husk ash (RHA) to OPC increases the compressive strength of stabilized soil. Additions of 6% of RHA and 8% of OPC yielded 18–25-fold compressive soil strength [
4]. The use of pozzolans, such as RHA, reduced the CO
2 emissions associated with the binder, although this value is still very high.
In recent years, special focus has been placed on developing new binders with a lower environmental impact [
5]. Alkali-activated cements (AACs) are promising substitutes for OPC. These new cements are prepared by mixing a precursor (Al-Si- or Ca-Al-Si-based mineral admixtures, such as fly ash, metakaolin or blast furnace slag) and a highly concentrated alkaline solution (sodium/potassium carbonates, hydroxides and silicates). Blast furnace slag (BFS) activation is a very interesting proposal because this precursor requires a small amount of chemical activator and it performs very well mechanically [
6]. It is worth noting that these chemical activators are normally synthetic products, and high CO
2 emissions are associated with their production.
Some interesting data have been reported on replacing these chemical activators with alternative and more sustainable by-products or waste. For instance, alkali silicates (K
2SiO
3, Na
2SiO
3) have been replaced with mixtures of KOH or NaOH and biomass ashes like RHA [
7,
8], sugarcane bagasse ash [
9], sugarcane straw ash [
10], or industrial waste, such as spent diatomite from the wine and beer industries [
11], glass waste [
12] or soda residue from ammonia soda process for Na
2CO
3 synthesis [
13,
14]. Very recently, some examples of AACs have been reported in which no commercial reagents are used. For instance, high-calcium content wood ash (61%CaO, 12%K
2O) activates coal fly ashes [
15]. Peys et al. [
16] reported alkaline ashes (30%K
2O) from stalk and cob corn for activating metakaolin. Soriano et al. [
17] demonstrated the feasibility of preparing alkali-activated BFS by adding almond-shell biomass ash.
Soil has been used for several thousands of years as a construction material and is still widely used today, especially in developing countries. Using soil as raw material in construction has several advantages: recyclability, no toxicity, no pollution, low energy use in manufacturing, cheaper than other alternatives and local production (avoiding transport), good hygrothermal behavior, among others [
1]. Obviously, there are some facts that limit its use: few specific regulations, low level of training for engineers, very intensive labor technique during the construction process, seismic behavior, durability in wet climates, and water erosion, among others.
Addition of OPC to soil enhances some properties of stabilized soil blocks, mainly strength and durability. Alternatives are being proposed that employ alkali-activated cement: a significant number of reports/papers have been published in which BFS and coal fly ash were used as precursors. In many cases, the activating solution is highly concentrated: 3–18 M NaOH solution for activating precursors [
18]. This has one major consequence from the sustainability point of view: the consumption of reagents and their carbon footprint. It has been reported [
19] that employing NaOH in BFS activation is more sustainable than Portland cement for soil stabilization (23% less CO
2 emissions).
The aim of this research is to use an alternative and more sustainable activator to prepare soil- compacted blocks: the selected activator is the olive stone biomass ash (OBA). This waste, which derives from the combustion of olive stones, has been tested previously in BFS activation, and very good results were obtained in the strength and microstructure development of OBA/BFS mortars [
20,
21,
22]. The chemical composition of OBA presents high proportions of CaO and K
2O, and it has been reported [
21] that the compressive strength for 4 M NaOH-activated BFS is equivalent to the 15.8% replacement of BFS with OBA.
In this research, the preparation of dolomitic soil blocks and the mechanical, chemical, physical, microstructural and waterproofing characterization were carried out by comparing OPC and OBA/BFS soil-compacted blocks. The OBA/BFS blocks yielded excellent performance as regards to mechanical, waterproofing and environmental characteristics.
2. Materials and Methods
Soil was dolomitic in nature and was supplied by Pavasal Company (Quart de Poblet, Spain). This soil was dried at 105 °C for 48 h before being used. The main mineral phases were dolomite and calcite, and the minor components were quartz and muscovite. The largest particle size was 4 mm, and it had a granulometric distribution (% passing), as shown in
Figure 1a. The blast furnace slag (BFS) was supplied by Cementval (Puerto de Sagunto, Spain). It was ground in a laboratory mill for 30 min, its mean particle size was 26.0 µm and
Figure 1b shows its granulometric distribution. Chemical composition of BFS is summarized in
Table 1. Olive stone (OS) was supplied by Sahuco Aceites S.L. (La Gineta-Albacete, Spain). This sample was dried at 105 °C for 48 h. Olive stone biomass ash (OBA) was supplied by Almazara Candela (Elche, Spain). Ash was dried at 105 °C for 48 h and then ground in a laboratory mill for 10 min. The mean particle size was 22.7 µm and
Figure 1c shows its particle distribution.
The binder (BFS+OBA) had the proportion summarized in
Table 2. The amount of water in the mixed soil was determined by means of the mini Harvard modified proctor (ASTM STP479, [
23]) and in accordance with Spanish standard UNE 103501 [
24] (compaction energy 2632 J/cm
3). The maximum dry density of the mixture was obtained for ca. 8% of moisture, as
Figure 1d shows.
The mixing procedure was followed by using a mortar mixer according to UNE-EN 196-1 [
25]. The rotation rate of the mixing paddle was 140 ± 5 rpm and the planetary movement was 62 ± 5 rpm. The steps included: (a) OBA (40 g) and water (91 g) were mixed for 2 min; (b) the BFS (100 g) was added and mixed for 2 min; (c) the soil (1000 g) was added and the mixture was stirred for 3 min.
The final mix was compacted in the mold shown in
Figure 2a. The stabilized soil was compacted in three layers by an Army-type hammer (1.5 kg;
Figure 2b). The applied energy was 2632 J/cm
3 (19 knocks, 20 cm high). Cubic samples (40 mm size) were obtained (
Figure 2c), coated with a plastic film and stored at room temperature (20–23 °C). Film was withdrawn for the samples tested in compression, samples were left in a laboratory atmosphere for 2 days; for the samples tested in absorption or submerged in water, samples were dried until (laboratory atmosphere) constant weight (ca. 7–10 days). Compressive strength was applied by means of a universal INSTRON model 3382. Samples were tested with a displacement of 1 mm/min in an adapted device (
Figure 2d). The absorption test was carried out in accordance with UNE 41410 [
26]. Submersion in water was performed according to NTC 5324 [
27].
Microscopic studies were carried out by field emission scanning electron microscopy (FESEM) in a ZEISS Supra 55 equipment. The stabilized soil samples were carbon-coated and observed using 2 kV. Energy dispersive spectroscopy (EDS) analyses were carried out with an extra tension of 15 kV (working distance of 6–8 mm). Thermogravimetric studies were done with a Mettler-Toledo 850 ultrabalance: the 65 °C dried samples were placed inside aluminum crucibles (sealed with pin-holed lids). A heating rate of 10 °C/min with an air flow of 75 mL/min were the conditions established for the thermogravimetric test.