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. 2024 Feb 20;12(9):3585–3594. doi: 10.1021/acssuschemeng.3c06592

Algal Biochar-Metal Nanocomposite Particles Tailor the Hydration Kinetics and Compressive Strength of Portland Cement Paste

Xu Chen †,‡, Danielle N Beatty §, Mohammad G Matar †, Huanchun Cai ‡, Wil V Srubar III †,§,*
PMCID: PMC10916760  PMID: 38456189

Abstract

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Biochar can improve the mechanical properties of portland cement paste and concrete. In this work, we produced algal biochar-zinc (biochar-Zn) and algal biochar-calcium (biochar-Ca) nanocomposite particles and studied their effect on the hydration kinetics and compressive strength of cement paste. Results show that 3 wt % biochar-Zn delayed peak heat evolution during cement hydration from 8.3 to 10.0 h, while 3 wt % addition of biochar-Ca induced a minor acceleration of peak heat from 8.3 to 8.2 h. Both biochar-Zn and biochar-Ca nanocomposite particles increased the compressive strength of cement paste at 28 days by 22.6 and 17.0%, respectively. Data substantiate that retardation or minor acceleration of the reaction kinetics was due exclusively to the presence of Zn and Ca phases, respectively, while the enhanced strength was attributed to a nucleation effect induced by such phases and the internal curing effect of biochar.

Keywords: algae, biochar, cement, admixture, hydration, compressive strength

Short abstract

Algal biochar-metal nanocomposite tailor early hydration and strength of cements.

1. Introduction

Biochar, which can be considered a carbon-negative material depending on the processing conditions,1,2 is produced from organic biomass feedstocks via limited-oxygen combustion.3 Biochar possesses a high cation-exchange capacity and a large surface area. Consequently, it has attracted significant attention in environmental, agricultural, and energy-related applications.3,4 For instance, biochar has been used to fertilize crops5,6 and to immobilize heavy metals and organic pollutants.3,7,8 Biochar has also been used as an additive to improve the compressive strength9−11 and reduce carbon dioxide (CO2) emissions of portland cement paste.12

Algal biomass is a sustainable and widely available feedstock that can be used for the production of biochar.13 Algae can be classified as microalgae (i.e., photosynthetic unicellular microorganisms) or macroalgae (e.g., seaweed).14,15 Using sunlight and either fresh water or seawater, algae convert CO2 into organic biomass (e.g., proteins, carbohydrates, lipids). One gram of dry algal biomass requires approximately 1.8 g of CO2 to produce.16 Algae cultures have also been applied as a biological water and wastewater treatment process.17 Microalgae can be cultivated in brackish water and on nonarable land, thereby circumventing land-use competition for the cultivation of other crops.16 While the production cost of algae is estimated between $472 and $1137 per ton, it could be further lowered with the use of more economical sources of nutrients and CO2 sources.18,19 High-value byproducts, such as bio-oil and biosyngas, are produced from the lipids in algal biomass. The remaining low-value organic biomass can be incinerated in oxygen-limited environments to produce biochar.20

Previous research studies have demonstrated that biochar can be doped with metals and prepared into nanocomposite particles.21 Biochar-metal oxide or -metal hydroxide nanocomposite particles can be synthesized by treating low-value organic biomass feedstocks with metal salts before or after pyrolysis.21 The resulting nanocomposite particles can be engineered to exhibit multiple functionalities, including (1) enhanced decontamination of organic compounds by doping with TiO2,22,23 (2) enhanced decontamination of phosphorus by doping with CaO and MgO,22,23 and (3) enhanced magnetism by doping with Fe3O4.24

Despite previous research on the effects of raw algae25,26 and untreated biochar on cement paste,9−12 the effects of algal biochar-metal nanoparticles on the fresh- and hardened-state properties of cement paste have not yet been investigated. Compared to untreated biochar, biochar-metal nanocomposite particles offer multiple potential benefits. Metal nanoparticles can accelerate setting and increase the strength of portland and geopolymer cement pastes at low concentrations (<1%).27−31 The high surface area of nanoparticles provides nucleation sites and facilitates the formation of calcium silicate hydrate (C–S–H), the primary product of cement hydration.30 Other additives, such as ZnO nanoparticles, can retard portland cement hydration,32 an observation that was attributed to the formation of an amorphous layer of zinc hydroxide (Zn(OH)2) on the cement phases and thus inhibited growth of early age C–S–H.33 On top of the benefits provided by biochar itself, the performance of the cementitious materials could be optimized by further tailoring the hydration kinetics at early ages.

The objectives of this study were to produce algae-derived biochar-metal nanocomposite particles and to elucidate their effects on the fresh- and hardened-state properties of portland cement paste. Zinc (Zn) and calcium (Ca) were used to produce algal biochar-metal nanocomposite particles. The effects of those nanocomposite particles on the kinetics of cement hydration, mineralogy, and compressive strength of the cement pastes were studied by using isothermal calorimetry, X-ray diffraction (XRD), and mechanical testing, respectively.

2. Materials and Experimental Methods

2.1. Materials

Commercially available Type I/II portland cement (Quikrete) that complies with ASTM C150 was used in this study. Chlorella pellets from Earth Circle Organics (Las Vegas, NV) were ground with a mortar and pestle to pass through a sieve with a 125 μm opening. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) were both obtained from Sigma-Aldrich.

2.2. Preparation and Characterization of Biochar-Metal Nanocomposite Particles

2.2.1. Preparation

Biochar-Ca and biochar-Zn nanocomposite particles were synthesized by adapting a previously reported protocol.34 First, 20 g of Chlorella powder was added to 100 g of a solution containing 18.3 g of Zn(NO3)2·6H2O (5.0 g of ZnO) or 21.1 g of Ca(NO3)2·4H2O (5.0 g of CaO). These treated biomass mixtures were constantly stirred for 24 h at room temperature (∼21 °C) and then dried for 24 h at 80 °C.

Metal-treated biomass was pyrolyzed using a Carbolite tube furnace under a 60 mL/min flow of N2 gas. Using a heating/cooling protocol comparable to previously synthesized biochar-metal nanocomposite particles,22,34 the temperature was increased from room temperature (around 21 °C) to 500 °C at a rate of 5 °C/min, held for 1 h, and then cooled to room temperature at a rate of 5 °C/min. Untreated algae, Zn(NO3)2·6H2O, and Ca(NO3)2·4H2O were also calcined (i.e., heat-treated) under the same temperature and N2-gas flow regime to prepare additives for experimental control mixtures that contained nontreated biochar, Zn, and Ca (see Table 1). The weight of each additive (i.e., 3.0 g per 100 g of cement) was kept consistent so that any effects from their differences in compositions could be elucidated.

Table 1. Sample Nomenclature and Mixture Formulation.
sample name cement (g) untreated biochar (g) calcined Zn(NO3)2·6H2O (g) calcined Ca(NO3)2·4H2O (g) calcined biochar-Zn (g) calcined biochar-Ca (g) water (mL)
control 100 0 0 0 0 0 40
biochar 100 3.0 0 0 0 0 40
Zn 100 0 3.0 0 0 0 40
biochar-Zn 100 0 0 0 3.0 0 40
Ca 100 0 0 3.0 0 0 40
biochar-Ca 100 0 0 0 0 3.0 40
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2.2.2. Morphological and Chemical Characterization

The untreated algal biochar and biochar-metal nanocomposite particles were sputter-coated with platinum and examined using a Hitachi SU3500 scanning electron microscope (SEM) instrument operated in a secondary electron imaging mode with a voltage of 10 kV. Energy dispersive X-ray spectroscopy (EDS) was carried out using an Oxford Instruments Ultim Max EDS detector installed on the same SEM system. EDS was collected on the same specimen under an accelerating voltage of 20 kV and a working distance of 15 mm.

2.3. Preparation and Characterization of Cement Pastes

2.3.1. Preparation

First, cement powder was hand-mixed with untreated algal biochar, calcined Zn(NO3)2·6H2O, calcined Ca(NO3)2·4H2O, or Zn or Ca biochar-metal nanocomposite particles using the proportions listed in Table 1. These intermixed powders were then mixed with water at a water-to-cement ratio (w/c) of 0.40 using a Caframo Ultra Speed BDC6015 overhead stirrer. In accordance with a previously published protocol,35 the samples were mixed at 140 rpm for 30 s and then at 285 rpm for 2.5 min. The edges of the mixing cup were thoroughly scraped between the two mixing speeds.

2.3.2. Characterization

Reaction kinetics of fresh cement pastes were measured at 21 °C using a Thermometric TAM Air 8-Channel Isothermal Conduction Calorimeter. Siliceous sand (∼14 g) was used as the reference material. Freshly mixed cement paste (∼14 g) was weighed, placed into an ampule, and inserted into the other side of the same channel. Heat evolution and total heat were measured. Data were normalized by the total mass of cement paste.

Fresh cement paste samples were also cast into 12.5 mm (diameter) × 25 mm (length) cylinders. Samples were sealed and stored under ambient conditions (approximately 30% humidity and 21 °C temperature). Fracture surfaces of hardened pastes were sputter-coated with platinum and examined under SEM using the same conditions as the biochar-metal nanocomposite particles described above (see Section 2.2.2). Pastes cured for 1 year were also imaged to qualitatively assess the evolution of the early age microstructure (see the Supporting Information). The remaining samples were used for compressive strength testing.

Prior to XRD testing, the hydration of samples was stopped at 4 days after which 1 g of paste was ground to pass through a 125 μm sieve, soaked in 50 mL of isopropanol for 15 min to halt cement hydration,36,37 and further dried and stored under a moderate vacuum condition. A Bruker D8 Advance XRD instrument was used to characterize the mineralogy of the pastes after 7 days of curing. Ground paste powders were mixed with ethanol and cast on a Si crystal zero-background plate, a protocol that would largely minimize any preferred orientation compared to the diffraction of powder specimens. The Cu Kα X-ray radiation (wavelength 1.5406 Å) was used to scan from 5 to 65° 2θ with a step size of 0.02° and a dwell time of 2 s per step. The resulting patterns were analyzed with a Bruker DIFFRAC.EVA software that was equipped with the International Center for Diffraction Data (ICDD) PDF-4 AXIOM 2019 database.38

Compressive strength was measured after 7 and 28 days of curing according to a modified ASTM C39/C39 M standard39 originally designed for concrete. An Instron Universal Testing Machine with a 48.9 kN capacity was operated at a loading rate of 0.25 ± 0.05 MPa/s. Six cylindrical specimens were tested for each formulation.

3. Results and Discussion

3.1. Characterization of Biochar-Metal Nanocomposite Particles

3.1.1. Morphology and Chemical Composition

The morphology and chemical composition of the biochar, biochar-Zn, and biochar-Ca nanocomposite particles were examined using SEM-EDS. As shown in Figure 1, the biochar-Zn and biochar-Ca nanocomposite particles are smaller in size (Figure 1c,e) compared to the untreated biochar (Figure 1a). This observation is consistent with the altered surface area and porosity shown in a similar metal-biochar system examined by Li and colleagues.40 The metal-free biochar particles exhibit a relatively smooth surface (Figure 1a), which is most evident at higher magnification (Figure 1b). In contrast, the biochar-Zn (Figure 1d) and biochar-Ca (Figure 1f) nanocomposite particles contain embedded and superficial inclusions. These inclusions are consistent with previously reported literature regarding similar biochar-metal nanocomposite particles.21

Figure 1.

Figure 1

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Morphology of (a, b) biochar, (c, d) biochar-Zn, and (e, f) biochar-Ca nanocomposite particles at both low (left) and high (right) magnification. Circled areas are either smooth (biochar) or embedded and superficial inclusions (biochar-Zn and biochar-Ca). Scale bar is 100 μm in panels (a, c, e) and 3 μm in panels (b, d, f).

Nanoscale inclusions in biochar-Zn and biochar-Ca particles (Figure 1d,f) and the absence of such inclusions in the biochar (Figure 1b) indicate that these embedded and superficial particulates contain Zn/Ca metals. EDS characterization of the biochar-Zn and biochar-Ca nanocomposite particles (Figure 2) confirmed the presence of these metals. EDS maps of Zn (Figure 2a) and Ca (Figure 2c) are shown in the insets of Figure 2b,d, respectively. Point scans were conducted for each nanocomposite particle (Figure 2a,c) at two different locations on particles that exhibited distinctive surface morphologies. While one surface location was relatively smoother than the other, both locations for the biochar-Zn (Figure 2b) exhibited an evident Zn signal. Conversely, the biochar-Ca EDS spectra (Figure 2d) exhibited comparable Ca intensity regardless of the smoothness of the biochar surface (denoted by the square and circle in Figure 2c).

Figure 2.

Figure 2

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SEM micrographs of (a) biochar-Zn and (c) biochar-Ca nanocomposite particles that show both smooth and rough morphologies and corresponding EDS spectra (b,d) and EDS maps (shown in inserts) of (b) Zn and (d) Ca.

3.2. Heat Evolution of Cement Pastes with Nanocomposite Particles

Figure 3 shows the heat evolution and total cumulative heat of fresh cement pastes without and with biochar-Zn and biochar-Ca nanocomposite particles at a dosage of 3 wt % cement. Results from the cement paste formulations dosed with 3 wt % untreated biochar and the calcined Zn and Ca additives (without biochar) are also presented.

Figure 3.

Figure 3

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Rate of heat evolution and total cumulative heat for cement pastes with the addition of biochar-Zn, biochar-Ca, calcined Zn (Zn(NO3)2·6H2O), and calcined Ca (Ca(NO3)2·4H2O) additives. Heat evolution and total cumulative heat are represented by solid and dashed lines, respectively.

Like the control cement paste without any additives, all mixtures exhibit an induction period, followed by a main reaction peak composed of acceleration and deceleration periods. The time-to-peak heat for all mixtures was within 10 h, as shown in Table 2. The accumulated total heat at 10 and 24 h is also summarized for comparison.

Table 2. Peak Heat Evolution and Total Cumulative Heat of Cement Pastes at 10 and 24 h.

    peak time
 10 h heat (J/g)
 24 h heat (J/g)
cement pastes peak heat flow (W/g) hour % change J/g % change J/g % change
control 2.86 × 10–3 8.3   61.7   145.7  
biochar 2.81 × 10–3 8.9 7.2 57.9 –6.2 144.4 –0.9
Zn 3.17 × 10–3 7.8 –6.0 65.7 6.5 147.1 1.0
biochar-Zn 3.12 × 10–3 10.0 20.5 46.1 –25.3 146.5 0.6
Ca 3.15 × 10–3 5.89 –29.0 89.2 44.6 138.1 –5.2
biochar-Ca 2.91 × 10–3 8.2 –1.2 61.9 0.3 145.4 –0.2
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The addition of 3 wt % untreated biochar induced a moderate delay in the time-to-peak heat from 8.3 to 8.9 h. This small delay is likely due to residual –COOH and –OH upon the pyrolysis of the algae, as indicated in the Fourier transform infrared spectroscopy (FTIR) results (Figure S3). These functional groups, as commonly seen in cement retarders, were confirmed in our earlier study to be the origin of the retardation effects of raw algae.25 These functional groups and the retardation effect are eliminated upon heat treatment.25 A comparison of 24 h cumulative heat indicates no significant difference between the control formulation and the mixture containing 3 wt % untreated biochar (Table 2).

The addition of the biochar-Zn nanocomposite particles induced a significant right shift of the main reaction peak, which indicated substantial retardation of portland cement hydration. As summarized in Table 2, the time-to-peak heat increased from 8.3 h for the control formulation to 10.0 h for pastes containing biochar-Zn. Correspondingly, the total cumulative heat decreased 25.3% from a 10 h heat of 61.7 J/g for the control formulation to 46.1 J/g for the biochar-Zn formulation. Such retardation is more substantial than that caused by the biochar alone, which suggests a synergistic effect that aligns with previous research that has shown that Zn-based nanoparticles alone can delay cement hydration.32,33,41 Interestingly, there was no loss in cumulative heat at later ages (24 h) despite the reduction in early age (10 h) cumulative heat (see Table 2), a phenomenon that suggests such retardation would not affect the later development of strength.

In contrast to the biochar-Zn-induced retardation, the time-to-peak heat was slightly accelerated via the calcined Zn additive, comparable to a similar acceleration via 1.0 wt % ZnO (particle size not reported) as reported in an earlier study.42 A lack of a retardation effect observed herein, however, as observed in other studies, is likely due to the much larger particle sizes of the calcined Zn (∼100 μm; see Figure S1) compared to the biochar-Zn (Figure 1) additives. More specifically, while the calcined Zn under the current conditions would be in the form of ZnO,43 a compound which possesses extremely low solubility under cement-relevant pH of 12–13.44 While ZnO has been reported to retard the cement hydration,32,33 no ZnO additives were reported to exhibit the crystal-like morphology with well-defined smooth surfaces (see Figure S1a). Such a morphology, together with the large particle size of this calcined Zn, could have resulted in little dissolution and thus, likely, little retardation during cement hydration compared to other Zn nanoparticles used in other studies. Highly efficient retardation is normally induced by ZnO with much smaller nanoscale sizes.41,45,46

Both Zn and biochar-Zn caused increases in peak heat compared with the control formulation. Higher intensities of peak heat indicate rapid growth of C–S–H,47 suggesting that Zn and biochar-Zn could have enhanced early age nucleation of C–S–H. While the nanoscale morphology of Zn and biochar-Zn (as circled in Figure S1b for Zn and Figure 1d for biochar-Zn) provides some nucleation sites, ZnO could have further enhanced the nucleation/growth of the C–S–H hydrates since it alters the C–S–H to grow away from the cement grains.48 Additionally, any dissolved Zn ions (if any) would bind with calcium ions to form some potential calcium-bearing nucleates in the solution that ultimately enhance the peak intensity.33 As a result, regardless of any retardation/acceleration in peak time, the peak intensity is increased in the presence of Zn or biochar-Zn.

Unlike the biochar-Zn pastes, the biochar-Ca pastes exhibit a trivial acceleration (peak time of 8.2 h) (Table 2) and a similar rate of heat evolution compared to the control formulation (Figure 3). The minor acceleration induced by the biochar-Ca pastes is more evident when comparing the heat flow of the cement paste sample containing biochar alone (Table 2 and Figure 3). This result indicates that the incorporation of Ca in the biochar counteracted the biochar-induced retardation of portland cement hydration, as observed in the biochar-Zn sample. Such an acceleration effect induced by the Ca-containing biochar-Ca nanocomposite particles is consistent with the observation that the calcined Ca additive alone induced substantial acceleration, likely due to the liberation of some Ca2+ into solution, which enhances nucleation of C–S–H. The pastes containing the calcined Ca additive exhibited a significant acceleration of heat evolution and higher cumulative heat at early ages (Figure 3 and Table 2), for example, by 44.6% at 10 h from 61.7 to 89.2 J/g. Such Ca-induced acceleration is attributable to the nucleation effect and the capability of calcium salts to flocculate C–S–H colloids, thereby resulting in faster rate of seeding, nucleation, and growth of hydration products.49

3.3. Characterization of Cement Pastes with Nanocomposite Particles

3.3.1. Morphology

In general, cement pastes without and with biochar-metal nanocomposite particles exhibit similar morphologies (Figure 4). Under low magnification, the control paste and pastes dosed with biochar, biochar-Zn, and biochar-Ca all exhibit dense microstructures, suggesting that the biochar-based additives did not substantially affect the porosity of the cement pastes. At higher magnification, all samples contain particles attributable to unreacted cement (dashed yellow arrow) and gel-like phases attributable to C–S–H (solid white arrow), as expected.37

Figure 4.

Figure 4

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SEM images of cement pastes after 4 days of curing: (a, b) control, and those with the addition of (c, d) biochar, (e, f) biochar-Zn, and (g, h) biochar-Ca nanocomposite particles at both low (left) and high (right) magnifications. Dashed yellow and solid white arrows denote the unreacted cement and hydrated gel phases (i.e., C–S–H), respectively. Circled areas exhibit needle-like morphology, suggesting the presence of ettringite.

Some needle-like phases were evident in biochar and biochar-Zn samples (circled in Figure 4). Given that this needle-like morphology did not resemble any structural features that were observed in the biochar or biochar-Zn particles (as circled in Figure 1), these phases are attributable to ettringite, a needle-like calcium sulfate crystal commonly observed in cementitious materials,50 as also observed from the XRD patterns below (Figure 5). The fiber-like slenderness and size of these features (up to ∼2 μm thickness) are comparable to ettringite identified in an earlier study.51 The formation, shape, and size of ettringite could have been affected by the presence of ions (e.g., Zn), the presence of organic functional groups from the biochar, and their corresponding alteration in the chemistry of the surrounding environment (e.g., alkali adsorption). The phenomenon has been summarized in an earlier work that describes the effect of additives on ettringite and whether the additives adsorb on the end or the side surfaces of the crystals, which could exhibit as a six-sided needle, a short rod, or spheres.52 The amount of ettringite is usually high at relatively early hydration of ordinary portland cements and then decreases as the hydration continues.53 The presence of ettringite in the biochar and biochar-Zn samples (Figure 4) suggests that 4 days was not sufficient for ettringite to react and convert into other hydration products. The presence of the fibrous ettringite in the biochar and biochar-Zn cement pastes could suggest that such conversion was delayed, somewhat consistent with the retardation of the biochar and biochar-Zn mixtures as discussed above. However, after a year of curing, no ettringite-like structures were observed (Figure S2).

Figure 5.

Figure 5

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XRD diffractograms of biochar, biochar-Zn, and biochar-Ca cement pastes overlaid onto the control cement paste XRD diffractogram for comparison. Hydration of these samples was stopped at 7 days using the solvent extraction method as described in the text. Cs = calcium silicate hydrate (PDF 00-014-0035); Ch = portlandite (PDF 00-004-0733); C3 = Alite (PDF 00-055-0740); C2 = Larnite (PDF 00-033-0302); B = Brownmillerite (PDF 04-014-6640); E = ettringite (PDF 04-022-3982); Cc = calcium carbonate (PDF 00-066-0867).

3.3.2. Mineralogy of Cement Pastes

Figure 5 shows XRD diffractograms for the control, biochar, biochar-Zn, and biochar-Ca cement pastes. The diffraction peaks were assigned to various cementitious phases, including tricalcium silicate (C3S), dicalcium silicate (C2S), and Brownmillerite phases such as tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) from the unreacted cement as well as C–S–H and calcium hydroxide as the reaction products. Assignments here further confirm our earlier interpretation that unreacted cement and hydration products coexist, as observed in the SEM micrographs (Figure 4). The addition of the biochar or biochar-metal nanocomposite particles did not result in peak shifting for any phase, an observation that usually indicates that the lattice spacing of the identified crystal structures is not altered.35

The diffraction patterns reveal that peaks of unreacted cement phases exhibit a relatively higher intensity in the control sample than in the other samples. For a more direct, semiquantitative comparison, the XRD patterns were normalized so that the peak ∼ 18° (2θ), which is assigned to calcium hydroxide, exhibited equal or similar intensity. Considering any preferred orientation would have been eliminated/reduced during the data collection (see the Materials and Experimental Methods) and types of crystalline phases are similar among the tested samples, such a normalization makes possible a qualitative comparison of phases between specimens. After this normalization, it was revealed that the peak intensities of C2S and C3S are, in fact, higher in the control compared to each of the other samples. This difference suggests that the addition of the biochar and biochar nanocomposite particles enhanced the overall degree of reaction.

The enhanced degree of reaction is expected for several reasons. First, the biochar could serve as an internal curing agent. More specifically, highly porous biochar particles serve as internal water reservoirs that absorb water upon initial mixing. This water is slowly released as cement hydration continues, thereby promoting hydration and improving strength development, especially in low w/c ratio cementitious materials.11 Biochar54 and rice husk ash have previously been reported to exhibit internal curing characteristics.55 Second, the presence of nanoparticles, as observed in SEM (Figures 1 and 2), could have provided nucleation sites to enhance cement hydration as observed in the marginal increases in peak heat shown in Figure 3 and Table 2, yielding more C–S–H at early ages.27,29 The XRD results alone were insufficient to determine whether one or more strength-enhancing mechanisms could be active.

3.3.3. Compressive Strength

The 7- and 28-day compressive strengths of the cement pastes without and with the addition of biochar and biochar-metal nanocomposite particles are shown in Figure 6. For each cement paste, an increase of strength from 7 to 28 days was observed, as expected, due to continued cement hydration.53 Biochar-Zn and biochar-Ca exhibited increased compressive strength compared to that of the control, especially at 7 days. More specifically, the biochar-Zn and biochar-Ca samples exhibited a 7-day compressive strength of 23.4 ± 1.4 and 22.2 ± 1.4 MPa, respectively, which were statistically validated to be 24.4 and 17.9% higher than the control (18.8 ± 1.2 MPa). Although not statistically significant, the mean values of the 28-day compressive strength for biochar-Zn and biochar-Ca were observed to be 22.6 and 17.0% higher, respectively, compared to the control (21.4 ± 4.3 MPa).

Figure 6.

Figure 6

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7- and 28-day compressive strength of cement pastes. Error bars represent ±1 standard deviation.

The enhanced strength of biochar-Zn and biochar-Ca samples could be due to enhanced C–S–H nucleation effects induced by the Zn and Ca nanoscale particles compared to those of biochar alone (Figures 1 and 2). Enhanced nucleation could explain the enhanced degree of cement hydration (Figure 5), which further increases compressive strength.53 The enhanced compressive strength achieved by Zn samples was likely due to the nanoscale morphology of the Zn additive (Figure S1b), given that the Zn additive alone increased the 7- and 28-day compressive strength of the control formulation by 51.6 and 46.8%, respectively (Figure 6). In contrast, the Ca additive did not exhibit a nanoscale morphology akin to that of Zn (Figure S1d). However, unlike the less soluble Zn, the Ca additive can release Ca2+ ions in an alkaline environment that could have led to enhanced nucleation of C–S–H49 and, thus, resulted in enhanced compressive strength (Figure 6).

In addition to nucleation effects, internal curing effects from the biochar11 also contributed to increases in compressive strength. It is noted that no further porosity or other experiments were carried out to investigate the influence of the internal curing, which has been noted as an effect of biochar on cementitious materials.11 Strength decreases were likely observed because additional water was not added to account for the potential water absorption of the biochar. The biochar sample (Figure 6) exhibited a compressive strength 40.1% lower than the control at 7 days but a compressive strength comparable to the control after 28 days of curing. This delayed effect is characteristic of cement pastes and concretes containing internal curing agents, given that the internal curing agents supply additional water only when the ongoing cement hydration process has consumed readily available water.56 It is evident in Figure 6 that the samples containing the biochar-metal nanocomposites (i.e., biochar-Zn, biochar-Ca) also exhibited increases in compressive strength due to the concurrent effects of both Zn- and Ca-induced C–S–H nucleation and internal curing imparted by the biochar.

4. Significance

We have elucidated that algal biochar-metal nanocomposite particles tailor the hydration kinetics and compressive strength of cement pastes. For cement hydration kinetics, the addition of 3 wt % biochar-Zn nanocomposite particles delayed the main peak time of heat evolution by 20.5%, while the addition of 3 wt % biochar-Ca nanocomposite particles exhibited a slight (if any) acceleration on the main peak of the heat evolution. For the mechanical properties, it was found that biochar-Zn and biochar-Ca nanocomposite particles evidently enhanced the compressive strength (see Figure 6), indicating that these nanocomposites possess the potential to customize the performance of the cement-based materials.

In addition to serving as an alternative to chemical admixtures or clay-type additives, algal biochar nanocomposite particles exhibit great advantages in reducing carbon emissions and economic cost. While some carbon dioxide has been fixed by raw algal biomass, the highly porous structure of the biochar could help adsorb extra carbon dioxide,57 as quantitatively depicted according to the formula below

4. 1

where Ccarbon (kg CO2e/kg) is the embodied carbon dioxide from the algal biochar-metal nanocomposites, Calgal (kg CO2e/kg) is the embodied carbon dioxide from the raw algal biomass, which approximates around −1.83 kg CO2e/kg,58n1 is the weight percent (%) of algal biochar that can be yielded from the algae biomass, which was found to be 40.5% (see Figure S4 and Table S1), a value that aligns with the reported range from approximately 31–52%6,59 using the fast pyrolysis method adopted in this study, n2 is the remaining weight percent (%) of the sequestrated carbon in the biochar versus that in the original algae prior to the pyrolysis, which is about 85%.60 Additionally, Cadsorb (kg CO2e/kg) is the adsorption weight of carbon dioxide by a unit weight of algal biochar, which is about −0.462 kg CO2e/kg.61 It is known that 1 m3 of concrete typically consumes ∼450 kg cement,62 and the carbon dioxide content of a Type I Portland cement is ∼0.95 kg CO2e/kg.63 Based on eq 1, if 3 wt % algal biochar-metal nanocomposite particles are added to cement, it reduces the carbon dioxide emissions of cement by 10.9–17.3%. Furthermore, an even higher percent of such reduction could be achieved, considering a higher dosage of such nanocomposites could be introduced potentially at a minimal sacrifice to the strength.

In addition to the reduction in carbon emissions, the algal-based nanocomposites exhibit advantages in terms of economic cost. The cost of algal biochar nanocomposites could be estimated based on that of a similar biochar-metal composite (around USD $207.36–$813.49 per ton, being composed of the algal-metal biomass production costs at $150–$732 per ton, algal-metal biomass dewatering and harvesting costs at $9.52–$33.65 per ton, and algal-metal biomass pyrolysis costs at ∼$47.84 per ton).64,65 The algal biochar-metal nanocomposite particles are more economically preferred than the common chemical admixtures. For instance, as compared to the typical increase of ∼$11 for each 1 m3 concrete by the polycarboxylic ether-based superplasticizer,62 these nanocomposites at the same dosages (i.e., 1.26 wt % of cement as the reported upper boundary) would only induce an increase by $1.18–$4.61. It is noteworthy that the ultimate cost could decrease even further with potential improvements in the algal biochar production process.

In addition to reducing carbon emissions and economic cost, the algal biochar-metal nanocomposite particles exhibit great benefits in wastewater decontamination,66−69 on top of the capability to tailor the cement properties as demonstrated in this paper. This study lays the technical foundation for preparing and using biochar-metal nanocomposite particles to tailor the properties of portland cement. While the synthesis of such particles in the current study utilized reagent-grade chemicals, future studies should focus on the facile synthesis of similar biochar-based multifunctional particles from metal-rich aqueous waste streams (e.g., polluted water). Such an approach would achieve the benefit of environmental remediation while yielding multifunctional particles that can lead to property enhancement when used in cementitious materials. Algal biomass has adsorption rates of 0.37 mg/g of Al and 0.18 mg/g of Zn per day in 1 m2,70 purifying and reducing the regulated metal pollution in wastewater discharged from coal-fired power stations. Upon adsorption of heavy metals, biomass residues could be processed into biochar-heavy metal composites as cement additives. For example, the algal biomass could adsorb a high Ca (11–26%) and Si (2–4%) content71 from cooling tower wastewater of a cement-processing industry, and the residues could be produced into an algal biochar-Ca/Si composites, a potential additive to accelerate cement hydration as demonstrated in the current study. Such a technical route would allow us to decontaminate and immobilize the heavy or regulated metals (e.g., Zn, Fe, and Al) from wastewater,72 and, at the same time, to tailor the performance of the cementitious materials.

5. Conclusions

The results from this work substantiate that algal biochar-metal nanocomposite particles can be used to tailor the early age cement hydration kinetics and mechanical properties of portland cement pastes. A 3 wt % addition of algal biochar-Zn nanocomposite particles delayed the main peak of heat evolution by >20% during cement hydration but increased the 7- and 28-day compressive strength of portland cement paste by 24.4 and 22.6%, respectively, compared to an experimental control. In contrast, a 3 wt % addition of biochar-Ca nanocomposite particles induced a minor acceleration of cement hydration and increased the 7- and 28-day compressive strength by 17.9 and 17.0% compared to the control formulation.

The effects on cement hydration kinetics were attributable to the presence of Zn and Ca nanocomposite particles in the biochar—two well-known retarders and accelerators, respectively, of ordinary portland cement hydration. Despite the Zn- and Ca-induced effects on cement hydration, the observed increases in compressive strength of cement pastes containing both Zn- and Ca-based nanocomposite particles were attributable to both Zn- and Ca-induced nucleation effects at early ages as well as internal curing effects imparted by algal biochar at later ages. Together, these combined mechanisms resulted in an improved compressive strength at 7 and 28 days compared to the experimental control.

Acknowledgments

This work was sponsored by the United States National Science Foundation (Award No. CMMI-1943554), the United States National Science Foundation Graduate Research Fellowship Program (Award Number DGE-2040434), and the United States Advanced Research Projects Agency-Energy (Award Number DE-AR0001145). This research was also supported in part by the Colorado Shared Instrumentation in Nanofabrication and Characterization (COSINC): CHR (Characterization facility), College of Engineering & Applied Science, University of Colorado Boulder, specifically for SEM/EDS characterization. The authors acknowledge the support of the staff (Tomoko Borsa) and the facility that have made this work possible. XRD experiments were conducted in the geological sciences department at the University of Colorado Boulder. Any use of trade, firm, or product names was for descriptive purposes only and did not imply endorsement by the U.S. government.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acssuschemeng.3c06592.

  • Characterization of Zn and Ca additives and morphology of cement pastes, and the calculations of embodied carbon and economic cost (PDF)

The authors declare the following competing financial interest(s): W.V.S. is a listed coinventor on a patent application (PCT/US2020/020863) filed by the University of Colorado on April 3, 2020, related to biomineralized building materials. W.V.S. is a cofounder and shareholder of Prometheus Materials and Minus Materials Inc. and a member of their scientific advisory boards. D.N.B. is a cofounder, consultant to, and shareholder of Minus Materials Inc.

Supplementary Material

sc3c06592_si_001.pdf (619KB, pdf)

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