Thermal and Mechanical Properties of Recyclable Composites Prepared from Bio-Olefins and Industrial Waste

Abstract

Ordinary Portland Cement (OPC) production consumes tremendous amounts of fresh water and energy and releases vast quantities of CO₂ into the atmosphere. Not only would an alternative to OPC whose production requires no water, releases little CO₂, and consumes less energy represent a transformative advance in the pursuit of industrial decarbonization, but the greater availability of safe drinking water would lead to significantly improved public health, particularly among vulnerable populations most at risk from contaminated water supply. For any OPC alternative to be adopted on any meaningful scale, however, its structural capabilities must meet or exceed those of OPC. An inverse vulcanization of brown grease, sunflower oil, and elemental sulfur (5:5:90 weight ratio) was successfully modified to afford the high-sulfur-content material SunBG₉₀ in quantities > 1 kg, as was necessary for standardized ASTM and ISO testing. Water absorption (ASTM C140) and thermal conductivity (ISO 8302) values for SunBG₉₀ (<1 wt% and 0.126 W·m⁻¹·K⁻¹, respectively) were 84% and 94% lower than those for OPC, respectively, suggesting that SunBG₉₀ would be more resistant against freeze-thaw and thermal stress damage than OPC. Consequently, not only does SunBG₉₀ represent a more environmentally friendly material than OPC, but its superior thermomechanical properties suggest that it could be a more environmentally robust material on its own merits, particularly for outdoor structural applications involving significant exposure to water and seasonal or day/night temperature swings.

1. Introduction

Concrete serves critical functions in nearly all aspects of human civilization. The composites that comprise concrete are produced by the mixing of a cementitious binder, most commonly ordinary Portland cement (OPC), aggregates, and water. In the US alone, the annual concrete production exceeds 500 million tons, with the concomitant consumption of nearly 90 million tons of OPC and 250 billion liters of water [1,2,3,4]. This demand is driven by the facts that concretes (i) can be poured and shaped, unlike natural stone, and (ii) have vastly superior mechanical properties compared to those of bricks and other shapeable masonries [2,5,6,7].

Unfortunately, concrete production has a tremendously harmful impact on the environment. For example, nearly 1 ton of CO₂ is released directly into the atmosphere for each ton of OPC produced and, furthermore, OPC production requires temperatures > 1400 °C, with an electricity demand that indirectly releases even more CO₂ into the atmosphere. Additionally, concrete production can only use fresh water, and this reduces the availability of potable water for human consumption [2]. Concretes cannot be recycled and reused in the same applications as initially poured concretes due to having significantly worse mechanical strengths. As a result, vast quantities of concretes end up in landfills. Thus, there is an urgent need for concrete alternatives that (i) produce less CO₂, or even none at all, (ii) do not consume water, and (iii) can be recycled and reused without any loss in their mechanical properties.

Given the enormous carbon footprint, energy demand, and water consumption required for the production of OPC, the development of alternatives to OPC is an area of intense interest in the scientific community. In particular, if an OPC alternative could be produced from biomatter without the need for any inorganic carbonates, then it would enable the production of concretes that could formally remove CO₂ from the atmosphere (i.e., so-called “carbon-negative” materials) [8]. A critical constraint in the design of every OPC alternative is that it must match or exceed the mechanical properties of OPC itself and of OPC-derived concretes. Most biomolecules and biopolymers, by nature, must not impair the microscopic or macroscopic flexibility and motility of an organism, and thus, nearly all streams of unmodified biomatter exhibit significantly worse compressive strengths than OPC. If covalent bonds that crosslink individual biomolecules or biopolymer chains could be introduced into biomatter, then this process could yield materials with mechanical strength properties comparable to or superior to OPC.

Inverse vulcanization, mechanistically similar to Goodyear’s vulcanization of rubber (Scheme 1a) [9] is a process developed by Pyun in 2013 that crosslinks olefinic molecules with chains of polymeric sulfur and affords high-sulfur-content materials (HSMs, Scheme 1b) that exhibit remarkable thermomechanical properties [10,11,12,13,14]. Notably, the syntheses of HSMs by inverse vulcanization do not require water, or any other solvent, and require lower temperatures than other C–S bond-forming processes reported for the synthesis of HSMs [15,16,17,18,19,20,21,22,23]. Furthermore, their relatively high sulfur contents (typically > 50 wt%) prevent any water penetration into, or uptake by, HSMs due to the inherent hydrophobicity of sulfur [24,25,26,27,28,29]. A water-repellent alternative to OPC, furthermore, could preclude entirely some of the degradation/damage pathways that afflict OPC-based concretes [30,31,32,33,34,35].

Our group [36,37,38] and others [18,20,39,40,41,42,43,44] have extensively studied multiple classes of bio-olefins as the organic substrates in inverse vulcanization reactions to produce HSMs with commercially viable thermomechanical properties. Triglycerides and free fatty acids [45,46] are of particular interest to us as components of OPC alternatives given that many of them naturally contain C=C bonds and thus do not require chemical functionalization. If plant-based triglycerides are the only organic substrates in an inverse vulcanization reaction, then the resulting HSM could formally achieve a negative carbon footprint.

A potential issue for the manufacture of HSMs is that when scale-up to kilogram quantities is undertaken, there is the potential for temperature spikes resulting from autoacceleration (the Trommsdorf effect) [47,48]. The previous reports indicate that a possible remedy to temperature control issues is to use rapid mechanical stirring, rather than the magnetic stirring used in small bench top-scale reactions, and to monitor internal reaction temperature rather than external heating bath temperatures as is often done for small-scale reactions.

We have previously reported SunBG₉₀ [49], an HSM produced from the inverse vulcanization of a mixture of brown grease and sunflower, and provided data that showed that its compressive strength and flexural strength values were more than 100% greater than the respective values in OPC, and significantly higher than in other analogous plant oil- or terpene-derived HSMs [50,51]. Based on these findings, we sought to scale up the synthesis of SunBG₉₀ significantly to enable more rigorous mechanical strength tests based on ASTM- and ISO-certified methods. Herein we report the synthesis of SunBG₉₀ to prepare multikilogram-scale prototypes and the shaping of SunBG₉₀ into tiles and bricks suitable for ASTM/ISO analyses. Our tests demonstrate that SunBG₉₀ is superior to OPC in multiple aspects, with 84% lower water absorption, 94% lower thermal conductivity, and an abrasion resistance between those of marble and granite.

2. Materials and Methods

2.1. Materials and Precautions

Small batches (<100 g) of SunBG₉₀ can be prepared as previously reported [49]. Caution must be taken during this process, as heating elemental sulfur with organics can result in the formation of H₂S gas. H₂S is toxic, foul-smelling, and corrosive. The preparation of SunBG₉₀ in the large batches that were needed to prepare multi-kilogram prototypes such as the brick used for the ISO 8302 test involved a reaction of sunflower oil, brown grease, and sulfur in a 1:1:18 mass ratio, according to the following procedure, in order to achieve material identical to that resulting from the previously reported small batches. Sulfur was first melted in an oil bath at 160 °C with rapid mechanical stirring. The internal temperature was then increased to 180 °C over 30 min. Once the temperature had stabilized, brown grease was slowly added to the sulfur while stirring and monitoring the temperature. No increase in temperature beyond the typical ±3 °C was observed. After the addition of brown grease was complete, the sunflower oil was added dropwise to the brown grease–sulfur mixture in an analogous manner, again with minimal (±3 °C) temperature variation. The reaction mixture was subsequently stirred for an additional 24 h at 180 °C. After this time a homogeneous reddish-black solution was observed. Upon cooling to room temperature, the material solidified to a dark solid identified herein as SunBG₉₀ in quantitative yield.

2.2. Testing According to ASTM C140

Specimens were immersed in deionized water (temperature range: 15–27 °C) for 24 to 28 h such that the top surfaces of the specimens were at least 150 mm below the surface of the water. Specimens were separated from each other and from the bottom of the tank by at least 3 mm. Samples were weighed before immersion, while suspended by a metal wire and completely submerged in water, and after removal from water. After saturation, the specimens were dried in a ventilated oven at 110 ± 5 °C for not less than 24 h, and the weights of the dried specimens were recorded. The utilized test describes the quantity of water a unit can maintain when saturated, indicating a concrete mix’s level of compaction or the volume of voids within a specimen. The primary metric used to measure water absorption is the water absorption rate, typically measured as a percentage of the dry weight of the tile.

$$Absorption, \%=\frac{w_s-w_d}{w_d}\times 100$$

$$Density \left(D\right),\frac{lb}{f{t}^3}=\frac{w_d}{w_s-w_i}\times 62.4$$

2.3. Testing According to ASTM C1353

A specimen was abraded using rotary rubbing action under controlled pressure conditions and abrasive action. The test specimen was mounted on a turntable platform and turned on a vertical axis against the sliding rotation of two abrading wheels. One abrading wheel rubbed the specimen outward toward the periphery and the other rubbed it inward toward the center while a vacuum system removed wear debris generated during the test. The resulting abrasion marks formed a crossed arc pattern over an area of approximately 30 cm². Abrasion resistance was evaluated by determining the loss of volume due to abrasion and calculating an index of abrasion resistance. This test method allowed the quantification of the abrasion resistance of dimension stone for relation to end-use performance, which is used to comparatively rank material performance.

2.4. Testing According to ISO 8302

The measurement of the thermal conductivity of the material under study was carried out with the Thermal Conductivity Test Tool λ-Meter EP500e according to ISO 8302, which describes the use of guarded hot plate apparatus that offers measurement accuracy < 1.0% in the range λ = 10–60 mW/(m·K) (mostly < 0.7%) and reproducibility < 0.5% (mostly < 0.2%). This meter features a cooling/heating unit with 12 high-performance air-cooled Peltier modules. The material was placed in the center of the sample holder plate of the equipment, which closed automatically, applying a pressure of 500 Pa and thus ensuring complete contact with the temperature plates. A guard of insulating material had to be placed around the sample. The equipment had means of visualization and controls through touch screens.

2.5. Testing According to Mohs Hardness Test

The Mohs hardness scale was used to measure the scratch resistance of a material. The idea behind the Mohs hardness scale is quite simple: the hardest material scratches the softest material. The Mohs hardness scale is based on a scale from 1 to 10, with diamond—which has a value of 10—being the hardest material. Materials are tested against each other; if one scratches another, it has a higher value on the Mohs hardness scale.

3. Results and Discussion

3.1. Rationale and Design

One of the principal challenges associated with the use of bio-olefins in plastics, concretes, or other structural materials is that most bio-olefins are major components of foodstuffs and contain significant caloric value to both humans as well as animals. Using an edible bio-olefin to prepare an inedible plastic, for example, trades a reduction in CO₂ released for an increase in food insecurity. Brown grease, on the other hand, is a coproduct of animal rendering processes that is not suitable for consumption and is frequently disposed of by incineration, releasing CO₂ into the atmosphere. The inedibility of brown grease is caused in part by the high levels of free fatty acids it contains, which can exceed 30% by weight. By using brown grease as the bio-olefin feedstock in HSM syntheses via inverse vulcanization, there can be a significant decrease in the carbon footprint of animal-derived foodstuff production.

Inverse vulcanization reactions cannot proceed unless the bio-olefin is sufficiently miscible in molten elemental sulfur. To improve the miscibility of brown grease, our protocols dictated that we combine it with a plant-based triglyceride in a 1:1 ratio. Although triglycerides contain significant caloric value, obtaining them from plant-based sources maximizes the amount of CO₂ removed from the atmosphere. Conversely, sulfur is produced by petroleum refining, which is a net CO₂ producer, but millions of tons of elemental sulfur go unused every year and are instead discarded as solid waste [10,44,49,52,53,54,55,56,57]. Using inverse vulcanization to crosslink the bio-olefins thus has the additional beneficial environmental effect of actively reducing the amount of solid waste that has already accumulated in landfills.

Our group previously demonstrated that SunBG₉₀—an HSM prepared via the combination of 5 wt% brown grease, 5 wt% sunflower oil, and 90 wt% sulfur [49]—exhibited a compressive strength value (35.9 MPa,

Table 1. Compressive and flexural strength values for SunBG₉₀ and comparison to other materials.

Materials	Compressive Strength (MPa *)	Flexural Strength (MPa *)
SunBG90	35.9	7.7
OPC	17	3.7
Clay Bricks	10	3.55
Oak Wood	11	56.5
Marble	12.5	3.4
Limestone	10	2.8
SunS	17.9	ND
CanBG90	32.0	6.5

* Numerical values represent maximum values per material.

) that was more than double the compressive strength values of OPC (minimum of 17 MPa required for building foundations), clay bricks (10 MPa), oak wood (11 MPa), marble (12.5 MPa), and limestone (10 MPa). As evidence that brown grease does contribute to mechanical strength, rather than simply diluting the contribution of sunflower oil, it was found that the compressive strengths of CanBG₉₀ (identical to SunBG₉₀ but with canola oil in place of sunflower oil) [52] and SunBG₉₀ were more than double that of the analogous HSM comprising only sunflower oil, SunS₉₀ (17.9 MPa). The inclusion of sunflower oil with brown grease was, however, previously reported to be required for the effective mixing of brown grease and sulfur to form a homogeneous material [52].

In the current work, it was also of interest to evaluate the durability of SunBG₉₀ after exposure to low temperature and temperature changes. SunBGS₉₀ was thus evaluated after 4 d at −25 °C followed by a rapid return to room temperature. Following this thermal strain, cylinders of SunBG₉₀ showed no changes in their compressive strengths compared to the cylinders that were allowed to stand at room temperature prior to testing.

3.2. Synthesis and Fabrication

To prepare bricks and tiles of sufficient sizes for ASTM and ISO testing, the sizes of test samples fabricated from SunBG₉₀ had to be scaled up by a factor of over one hundred compared to prior reports. Previous scale-up studies on HSMs had revealed the potential for temperature spikes resulting from autoacceleration (i.e., the Trommsdorf effect) [22,23] that could be attenuated by using rapid mechanical stirring and fastidious monitoring of internal reaction temperature as reagents were mixed. These precautions were also effective and necessary to produce large batches of SunBG₉₀. Indeed, carrying out the large-scale preparation of SunBG₉₀ without these temperature control measures had led to the generation of H₂S gas and a material with 70% lower compressive strength than had been observed for previously reported small-scale (<100 g) preparations of SunBG₉₀. The careful control of reaction temperature employed in the current report effectively produced SunBG₉₀ having identical thermal, mechanical, and morphological properties to those SunBG₉₀ samples previously reported.

Tile- and brick-shaped SunBG₉₀ substrates for water absorption and density (ASTM C140), abrasion resistance (ASTM C1353) [58,59], thermal conductivity (ISO 8302) [60,61], and Mohs hardness testing [62,63,64] were prepared by pouring the molten HSM into the appropriate silicone mold and allowing each substrate to solidify gradually at room temperature. Tiles of SunBG₉₀ for water absorption and density tests were obtained from molds with dimensions of 2 in × 2 in × 0.25 in (Figure 1). For abrasion resistance tests, the necessary tiles were obtained from larger molds with dimensions of 4 in × 4 in × 0.25 in. For tests of compressive strength, cylinders with dimensions of 10 mm × 6 mm were prepared.

3.3. Water Absorption and Density (ASTM C140)

In routine construction applications, water absorption by various OPC samples, as measured by ASTM C140, typically falls in the range of 5.3–8.3 wt% (Table 2). Water absorption values above this range can cause dangerous decreases in OPC compressive strength that may lead to structural failures [65,66,67]. Water absorption by SunBG₉₀ was dramatically lower (0.83 wt%) than that by OPC, and intermediate of the absorptivity values for marble and limestone (0.12 and 1.0 wt%, respectively). The density of SunBG₉₀, measured using ASTM C140, was 1.70 g·cm⁻³, a value that was significantly lower than that of any of the construction materials listed in Table 2 besides oak. However, the density of SunBG₉₀ did fall within the range of values considered acceptable for lightweight concretes (1.12–1.92 g·cm⁻³); thus, SunBG₉₀ could potentially function as a drop-in replacement for OPC-containing lightweight concretes.

3.4. Thermal Conductivity (ISO 8302)

The test of thermal conductivity was carried out on an HSM composite with a square geometry and dimensions of 148.3 × 148.7 mm and 40.6 mm thickness (Figure 2) that was in a dry state and showed relative parallelism and flatness on its sides. The mass of the sample was 1212 g and its bulk density was 1329 kg/m³. The relative humidity of the environment surrounding the equipment was 36% and the ambient temperature was 14 °C. Three measurements were made, with temperatures of 10 °C, 25 °C, and 40 °C. The results were obtained after 8 hours of measurement, and the polynomial y = f(t) = 0.3263(T + 123.40) was adjusted to obtain the thermal conductivity value of 126.66 mW/(m·K). The result of the thermal conductivity test of the measured sample yielded 0.126 W/m·K, a value falling within the range of values for familiar construction materials (Table 2 [68,69,70,71,72,73,74]) such as wood (0.13 W/m·K) and cellular concrete (0.14 W/m·K). These results, summarized in Table 2, show that SunBG₉₀ is a material with thermal insulation capacities, surpassing in this capacity other conventional materials such as OPC concrete (2.25 W/m·K), clay bricks (0.9 W/m·K), adobe bricks (0.85 W/m·K), and limestone (0.85 W/m·K). It has been observed that considering the value of the thermal conductivity of this material, it would work very well to cover surfaces that must be thermally insulated, since it exceeds all conventional construction materials based on cement or masonry, approaching only the value of wood.

Table 2. Water absorption, density, and thermal conductivity data.

Materials	Water Absorption (% *)	Density (g/cm3)	Thermal Conductivity (W/m·K *)
SunBG90	0.83	1.70	0.126
OPC	5.3–8.3	3.15	2.25
Clay Bricks	8.22	2.5	0.9
Oak Wood	12	0.82	0.197
Marble	0.12	2.64	2.95
Limestone	1	2.71	0.85

* Numerical values represent maximum values per material.

Table 2. Water absorption, density, and thermal conductivity data.

Materials	Water Absorption (% *)	Density (g/cm3)	Thermal Conductivity (W/m·K *)
SunBG90	0.83	1.70	0.126
OPC	5.3–8.3	3.15	2.25
Clay Bricks	8.22	2.5	0.9
Oak Wood	12	0.82	0.197
Marble	0.12	2.64	2.95
Limestone	1	2.71	0.85

* Numerical values represent maximum values per material.

3.5. Abrasion Resistance (ASTM C1353) and Mohs Hardness Test

Surface abrasion is a noted vulnerability in OPC, OPC-derived concretes [75,76,77], and other tile materials, and can be a major pathway by which loss of mechanical strength and structural integrity occurs [78,79,80]. An initial evaluation of SunBG₉₀ for its potential abrasion resistance was undertaken by conducting a quick evaluation of its hardness on the Mohs scale.

Table 3. Mohs hardness values.

Materials	Hmin	Hmax
SunBG90	2	2.5
Asphalt	1	2
Sulfur	1.5	2.5
Gold	2.5	3
Silver	2.5	3
Marble	3	4
Limestone	3	4

[81] indicates the known Mohs hardness values, showing that SunBG₉₀ has qualitative surface scratch resistance comparable to that of copper, gold, or silver and nearing that of marble or limestone. These preliminary tests suggested that SunBG₉₀ may be a candidate for reasonably good abrasion resistance.

When ASTM C1353 was performed on SunBG₉₀, an abrasion resistance value of 16 (I_W,

Table 4. Abrasion resistance.

Materials	Abrasion Resistance (Iw, HA)
SunBG90	16
Granite	25
Marble	10
Limestone	10

Values from ASTM C1353 Abrasion Resistance of Dimension Stone Subjected to Foot Traffic Using a Rotary Platform Abrase documentation.

) was observed. The abrasion resistance of SunBG₉₀ was intermediate to that of limestone and granite (10 and 25, respectively); this was particularly impressive given that limestone and granite are composed almost entirely of inorganic oxides and carbonates, whereas the SunBG₉₀ prepared for this work could have contained no more than 0.4 wt% of such inorganic substances.

4. Conclusions

We previously demonstrated that brown grease, sunflower oil, and elemental sulfur, in a 5:5:90 mass ratio, underwent inverse vulcanization to yield SunBG₉₀, an HSM which exhibited compressive and flexural strengths that exceeded those of OPC. In the present work, we successfully increased the scale of the inverse vulcanization reaction to prepare multikilogram batches of SunBG₉₀, thus enabling the thermomechanical properties of SunBG₉₀ to be tested according to the same ASTM and ISO standards used for OPC-based concretes.

Water absorption by SunBG₉₀ increased the sample mass by <1% when tested according to ASTM C140; this result indicated a value that was five-fold lower than the values typically measured for OPC (5.3–8.3%). The greater hydrophobicity of SunBG_90, as an alternative to OPC, would significantly diminish the likelihood of freeze-thaw and corrosion damage, which often lead to dangerous structural weaknesses in OPC-based concretes.

The surface abrasion of OPC-based concretes can likewise introduce weaknesses that ultimately result in mechanical failure. In contrast, the abrasion resistance of 16 measured for SunBG₉₀, when tested according to ASTM 1353, was intermediate to the values observed for limestone and granite (10 and 25, respectively). The value of 16 for SunBG₉₀ is all the more impressive when considering the fact that SunBG₉₀ is essentially metal-free (<0.4 wt% metals), whereas limestone contains has nearly 40 wt% metal content.

Thermal stresses, particularly those resulting from large temperature changes in the day/night cycle, can cause significant damage to materials exposed to the outdoors. For SunBG₉₀, a thermal conductivity value of 0.126 W·m⁻¹·K⁻¹ was measured using ISO 8302; this value was 94% and 87% lower than the corresponding values for OPC and clay bricks, respectively. As a result, the internal volume of SunBG₉₀ that undergoes temperature changes is significantly smaller, and thus, there will be fewer potential sites for mechanical failure.

Collectively, these results provide compelling evidence that SunBG₉₀ will be dramatically more resistant to the predominant damage mechanisms which most commonly cause structural failures in OPC-based concretes. Thus, not only does SunBG₉₀ constitute a carbon-sequestering alternative to the heavy carbon-footprint material OPC, but it is also likely to have superior long-term environmental robustness over OPC-based concretes and thereby decreases risks associated with the structural failures of concrete.