CEMENT ADDITIVE FOR RETARDATION OF CEMENT HYDRATION, CEMENT MIXTURES INCLUDING SAME, AND METHODS OF FORMING AND USING SAME

Abstract

Cement mixtures comprising a hydration retarder comprising algae, cement additives comprising algae and/or derivatives thereof, and methods of forming and using the mixtures and additives are disclosed. The algae can be treated to form desired functional groups to tune retardation properties of the hydration retarder.

Claims

1. A cement mixture comprising: about 0.01 wt % to about 10.0 wt % hydration retarder by weight of the cement, the hydration retarder comprising algae.

2. The cement mixture of claim 1, comprising about 0.3 wt % to about 3.0 wt % of algae by weight of the cement.

3. The cement mixture of claim 1, wherein the algae comprise one or more of chlorella, spirulina, and chlorophyta.

4. The cement mixture of claim 1, wherein the algae are not living.

5. The cement mixture of claim 1, wherein the algae are living.

6. The cement mixture of claim 1, wherein the algae comprise microalgae.

7. The cement mixture of claim 1, wherein the cement comprises hydraulic cement.

8. The cement mixture of claim 1, wherein the cement comprises portland cement.

9. The cement mixture of claim 1, wherein the cement comprises one or more of basic oxygen furnace (BOF) or ground granulated blast furnace slag cement, alkali-activated cement, sulfoaluminate cement, magnesium phosphate cement, carbonated cement, calcium aluminate cements

10. The cement mixture of claim 1, wherein the algae comprise one or more of a carboxylic acid functional group and a hydroxyl functional group.

11. The cement mixture of claim 1, wherein the algae comprise the carboxylic acid and the hydroxyl functional group.

12. The cement mixture of claim 10, wherein the algae comprise algae treated with an oxidant to increase a number of one or more of the carboxylic acid functional groups and hydroxyl functional groups.

13. A cement additive for retardation of cement hydration, the cement additive comprising: algae.

14. The cement additive of claim 13, wherein the algae have been ground to an average size between about 0.1 ?m and about 1 mm or between about 100 ?m and about 250 ?m.

15. The cement additive of claim 13, wherein the algae has passed through a sieve having a size of about 125 ?m or less.

16. The cement additive of claim 13, wherein the algae has been modified to add one or more functional groups selected from the group consisting of a carboxylic acid group and a hydroxyl functional group.

17. A method of forming a hydration retarder for cement, the method comprising the steps of: providing algae, and optionally exposing the algae to an oxidant to increase an amount of one or more of a carboxyl group and a hydroxyl group on a surface of the algae.

18. The method of claim 17, comprising the step of exposing the algae to the oxidant.

19. (canceled)

20. (canceled)

21. The method of claim 17, further comprising a step of grinding the algae to form ground algae.

22. The method of claim 21, further comprising a step of passing the ground algae through a sieve.

23. (canceled)

24. (canceled)

25. (canceled)

26. (canceled)

27. (canceled)

28. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0016] A more complete understanding of exemplary embodiments of the present disclosure can be derived by referring to the detailed description and claims when considered in connection with the following illustrative figures.

[0017] FIG. 1 illustrates rate of heat evolution and total heat for the cement pastes with 0.0% (control), 0.3%, 0.5%, 1.0% and 3.0% of chlorella (with respect to the weight of cement).

[0018] FIG. 2 illustrates FTIR spectrum of the raw chlorella showing the presence of various functional groups.

[0019] FIG. 3 schematically illustrates changes in the COOH and OH functional groups caused by the H.sub.2O.sub.2 and heat-treatment of algae.

[0020] FIG. 4 illustrates FTIR spectra of the raw chlorella and chlorella treated at 300? C. and with H.sub.2O.sub.2 solution.

[0021] FIG. 5 illustrates morphology of the (a) raw, and (b) H.sub.2O.sub.2 and (c) heat-treated treated chlorella.

[0022] FIG. 6 illustrates rate of heat evolution and total heat for the control cement paste (i.e., without addition of algae) and those with addition of the chlorella (raw, and H.sub.2O.sub.2 and heat-treated).

[0023] FIG. 7 presents SEM images of (a) control cement paste and pastes with (b) raw and (c) H.sub.2O.sub.2-treated chlorella when cured for 28 days.

[0024] FIG. 8 illustrates XRD patterns of control cement paste and pastes with raw and H.sub.2O.sub.2-treated chlorella when cured for 28 days.

[0025] FIG. 9 illustrates compressive strength of control cement paste, and pastes with rawand H.sub.2O.sub.2-treated chlorella when cured for 28 days.

[0026] It will be appreciated that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of illustrated embodiments of the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE DISCLOSURE

[0027] Although certain embodiments and examples are disclosed below, it will be understood that the invention extends beyond the specifically disclosed embodiments and/or uses thereof and obvious modifications and equivalents thereof. Thus, it is intended that the scope of the invention disclosed should not be limited by the particular disclosed embodiments described below.

[0028] The present disclosure generally relates to cement mixtures, to additives for cement mixtures, and to methods of forming and using the mixtures and additives. In addition, the disclosure relates to concrete and mortar including or formed using an additive and/or mixture as disclosed herein. As set forth in detail below, the additive(s) can be or include algae, such as algae described herein, which can provide desired retardation of cement hydration.

[0029] In this disclosure, any two numbers of a variable can constitute a workable range of the variable, and any ranges indicated may include or exclude the endpoints. Additionally, any values of variables indicated (regardless of whether they are indicated with about or not) may refer to precise values or approximate values and include equivalents, and may refer to average, median, representative, majority, etc. in some embodiments. Further, in this disclosure, the terms including, constituted by and having can refer independently to typically or broadly comprising, comprising, consisting essentially of, or consisting of in some embodiments. In accordance with aspects of the disclosure, any defined meanings of terms do not necessarily exclude ordinary and customary meanings of the terms.

[0030] Algae, a class of photosynthetic organisms, can be generally divided into microalgae and macroalgae. While macroalgae or seaweeds are multicellular plants that grow up to 60 meters in length, microalgae are photosynthetic unicellular microorganisms. Algal biomass is regarded as a sustainable feedstock due to its wide distribution, high environmental tolerance, rapid growth rate, and its high capacity for CO.sub.2 fixation. Due to high nutrient uptake, algae usually dominate in high nutrient environments, at a growth rate that can double the algal biomass in periods as short as 3.5 hours. Microalgae can be cultivated in brackish water that is usually nonarable and thus does not compromise the cultivation of crops. Growth of algae could also take nutrients from organic effluent while simultaneously decontaminating it.

[0031] Algae, as high-efficient photosynthetic microorganisms, exhibit great environmental benefit by converting CO.sub.2 together with sunlight and water into cells. For example, 1 gram of dry algal biomass was found to utilize 1.8 grams of CO.sub.2.

[0032] Substantial ongoing research has been leading to the popularization of algae-based products that can be used in the pharmaceutical, cosmetic, food, agricultural and biofuel industries. Additionally, algae have also been adopted to decontaminate wastewater via a bio-adsorption process. Such adsorption capability is attributed to the presence of functional groups, including COOH, OH, RCOO, HPO.sub.4.sup.2, and NH.sub.2, from polysaccharides and lipids in the algae biomass.

[0033] The use of algae in cements has been limited. While one barrier could be the algae-induced biodeterioration on cements and geopolymers, the deterioration is mainly caused by the organism's physical activity (e.g., growth) and should be of no concern when their dead biomass is used. As an example application, the extract from marine brown algae acted as viscosity-enhancing admixture for cements via potentially forming a transient gel-like network of alginate chains and cement particles. Algae-derived biopolymers were used to mitigate shrinkage for cement pastes. Algae-dosed concrete samples also exhibited increased/comparable strength and enhanced durability in terms of chloride diffusion. Additionally, thermally-treated algae (or biochar) were shown to tailor physical properties, including strength and water absorption, and to promote carbonate formation under CO.sub.2 curing and to enhance the sequestering of CO.sub.2.

[0034] This disclosure relates to using algae as a hydration of hydraulic cement (or cement mixtures), such as portland cement. The algae can possess functional groups, such as COOH and OH, that exhibit potential for retarding the hydration. While some biopolymers (e.g., cactus mucilage) have exhibited retardation effects on cement hydration, the alginate-based biopolymer was found to accelerate the hydration of calcium alumina cements, because the COOH groups absorb calcium ions and serve as nucleation sites for the formation of calcium aluminate hydrate phase. The presence of COOH groups from the raw algae biomass can affect the hydration of hydraulic (e.g., portland) cements.

SPECIFIC EXAMPLES

[0035] In the examples below; chlorella (one of the most studied microalgae) was studied to determine the algae's effect on the hydration of hydraulic (e.g., portland) cement and to understand the mechanisms associated with such effects: however, unless otherwise noted, the disclosure is not limited to such examples or algae. As set forth in more detail below, algae biomass was found to substantially retard the hydration of portland cement. To potentially promote the algae's application as a retarder, their effect on the microstructure and strength of the cement pastes were examined.

[0036] We have demonstrated for the first time that algae tailor the hydration kinetics of portland cements and elucidated the mechanisms. Specifically, the addition of chlorella (a widely cultivated type of algae) was found to substantially retard the cement hydration, as indicated by an 82.4% delay in the main peak of heat evolution measured by isothermal calorimetry. We further mechanistically elucidated that such retardation is caused by the COOH and OH functional groups, by having (1) confirmed their presence in the algae via FTIR, (2) demonstrated an enhanced retardation with increasing groups of COOH and OH, and (3) demonstrated an elimination of retardation by removing these groups in the algae through H.sub.2O.sub.2 and heat-treatment, respectively. Furthermore, the effects of algae on the morphology, crystal structures, and compressive strength of the cement pastes were found to be negligible, suggesting the algae's role as a potential retarder for cementitious materials.

[0037] Commercially available Type I/II portland cement (QuikreteR) 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 of a 125-?m opening, before adding into the cement pastes. Hydrogen peroxide (H.sub.2O.sub.2, 30%, Fisher Scientific, Waltham, MA) was also obtained and diluted for treating the chlorella algae before adding into the selected cement pastes.

[0038] In one treatment, the ground chlorella powders were heated using a Carbolite tube furnace under a 50 ml/min flow of N.sub.2 gas. The temperature was increased from room temperature (around 20? C.) at 10? C./min to 300? C., a temperature that exhibited a maximum rate of thermal decomposition for a chlorella sample based on thermogravimetric analysis. Upon holding at 300? C. for 1 hour, the sample was then cooled down at 20? C./min to room temperature (around 20? C.).

[0039] In the other treatment, 10 grams of the ground chlorella powder were soaked in 200 ml of 10% H.sub.2O.sub.2 solution (which was diluted from a 30 wt % stock solution with de-ionized water) and kept stirring for 24 hours. The mixture was then centrifuged with a Thermo Scientific (Sorvall Legend X1R) at 5000 rpm for 30 minutes. Once the liquid was poured out, the solid residue was then mixed homogeneously with a total of 200 ml of de-ionized water (same amount as the initial 10 wt % H.sub.2O.sub.2 solution) and centrifuged for 30 minutes, a washing process that was repeated for a total of 3 times.

[0040] Each cement-paste sample, at a water to cement ratio of 0:4, was mixed with a Caframo Ultra Speed BDC6015 overhead stirrer at 140 rpm for 30 seconds and then at 285 rpm for 2.5 minutes, the same mixing protocol we adopted in an earlier study. In between the two-speeded mixing, materials on the edges of the mixing cup were scraped. In samples with addition of algae, either raw or H.sub.2O.sub.2/heat treated, the algae were intermixed with cement particles before further mixing with water. Immediately following mixing, the cement pastes were cured, at ambient conditions (approximately 30% humidity and 20? C. temperature) for 2 days and then in a sealed condition at ?20? C. until further testing/processing. To stop the hydration for selected characterization tests, 1.0 gram of the paste samples, upon being ground to pass through a 125-?m sieve, was soaked in 50 ml of isopropanol for 15 mins.

[0041] Reaction kinetics of the cement pastes with and without addition of algae was monitored at 21? C. Each freshly mixed paste (?14 g) was weighed in a glass ampoule and placed into a chamber of a Thermometric TAM Air 8-Channel Isothermal Conduction calorimeter. Siliceous sand (?14 g) was used as the reference material. While the heat generated from the hydration was monitored, the heat evolution and total heat were normalized by weight of the cement paste.

[0042] The morphology of the algae particles was examined using a Hitachi SU3500 SEM instrument. The microscope was operated between 10 and 15 kV in secondary electron imaging mode. Prior to imaging, samples were coated with a gold film of ?10 nm under a vacuum condition (<0.15 mb).

[0043] The nanostructures of the synthetic gels with and without sucrose were examined through an attenuated total reflectance (ATR) Fourier-transform infrared (FTIR) instrument (ThermoScientific Nicolet iS20 FTIR). Each spectrum was an average of 32 measurements scanned from 2000 to 600 cm.sup.?1 at a resolution of 4 cm.sup.?1.

[0044] After having understood the effects of algae on the hydration kinetics of cement pastes, their effects on the hardened properties, including the morphology, crystal structures and strength, were examined.

[0045] The fracture surfaces of the hardened cement pastes with and without addition of algae at 28-day curing were coated with gold film and examined under SEM, in the same condition for algae as described above (see above).

[0046] A Bruker D8 Advance XRD instrument was used to characterize the crystal structures of the cement pastes. Powders of each sample upon stopping reaction at 28-days curing were mixed with ethanol, and a thin layer of the paste was casted on a Si crystal zero-background plate. Each sample was scanned using Cu Ka X-ray radiation (wavelength 1.5406 ?) from 5 to 60? 20 with a step size of 0.02? and a dwell time of 2 seconds per step. Crystalline phases were identified using Bruker DIFFRAC.EVA software and the International Center for 20 Diffraction Data (ICDD) PDF-4 AXIOM 2019 database.

[0047] The compressive strength was measured for the cement pastes with and without algae at both 7 and 28 days of curing. Five cylindrical samples per test group were used in accordance with ASTM C39/C39M, a standard designed for concrete cylindrical samples. For our cement paste samples, we adopted smaller sized samples, specifically 12.5 mm in diameter and 25 mm in length. Samples were tested using an Instron Universal Testing Machine with 48.9 kN capacity at a loading rate of 0.25?0.05 MPa/s.

[0048] FIG. 1 shows the curves of heat evolution and total heat of the cement pastes with the raw ground chlorella powders, up to 3.0% by weight of cement. While the heat evolution for the 3.0% mixture remains low throughout the first 45 hours, the curves of all other mixtures exhibit a dormant period followed by a main reaction peak that is composed of acceleration and deceleration periods.

[0049] The right shift of the main reaction peak reveals a substantially-retarded hydration by the chlorella. Specifically, as summarized in Table 1, the peak time increases from 8.5 hours (control cement paste) to 11.0 hours (0.5% chlorella) and 15.5 hours (1.0% chlorella), respectively. A much further delay to 77.5 hours (not shown in FIG. 1) was observed for the 3.0%-chlorella dosed paste. Correspondingly, the total heat was much decreased, for example, from a 40-h heat of 163 J/g for the control paste to 1.7 J/g for the paste with addition of 3.0% chlorella.

TABLE-US-00001 TABLE 1 The peak time of the heat evolution and the total heat at 40 hours for cement pastes with and without addition of algae. Peak time % Heat at Chlorella compared 40 hours Cement pastes wt % of cement hours to control J/g No additive 0.0 (control) 8.5 NA 163 Raw algae 0.3 9.9 16.5 162 0.5 11.0 29.4 161 1.0 15.5 82.4 155 3.0 77.5 811.8 1.7 Treated algae 0.5 (H.sub.2O.sub.2 treated) 12.8 50.6 162 0.5 (heat treated) 8.5 0 162

[0050] To understand the retardation mechanisms of the chlorella, its chemical structure was examined via FTIR. As labeled in FIG. 2, various functional groups were observed, including C?O in COOH (1725 cm.sup.?1), C?O stretch in ketone and carbonyl acid (1646 cm.sup.?1), OCH.sub.3 (1454 cm.sup.?1), OH in acid (1396 cm.sup.?1), COH (1059 cm.sup.?1) and CC(700 cm.sup.?1), the assignments of which were based on an earlier study.

[0051] Among the above functional groups, the carboxyl (COOH) and hydroxyl (OH) groups have been found to retard the cement hydration. The addition of glycolic acid that possesses carboxyl and hydroxyl groups has shown to substantially retard the hydration of cement. For instance, addition of 0.2% glycolic acid extended the induction period from 1.5 to 8 hours for a cement paste (water/cement ratio of 0:35).

[0052] Mechanistically, the carboxyl and hydroxyl groups retard cement hydration via adsorption onto the cement phases, especially for hydroxylated C.sub.3S. For instance, these functional groups from glycolic acid or calcium glycolate were hypothesized to interact with surrounding water molecules and form hydrogen bonds, which then form a stable hydrogen bond network covering the surfaces of the cement phase. Such hypothesis was supported by the experimental observation that calcium glycolate adsorbed on the calcium hydroxide surfaces. It was further supported by the simulation that showed the hydroxyl groups of calcium glycolate form a strong hydrogen bond with the calcium hydroxide and C.sub.3S surfaces, a phenomenon that rejects other alternative mechanisms, such as that hydroxy carboxylic acids chelate calcium and adsorb on hydration products.

[0053] Such adsorption is confirmed by further nanostructural evidence. Based on a series of comprehensive 2D .sup.13C-.sup.1H and .sup.29Si-.sup.1H NMR tests, the COH and the CH.sub.2OH functional groups from sucrose molecules were found to adsorb at silanol and silicate hydration products via hydrogen-bonding interactions. In another study by the same research team, carboxylic acids, as degradation products of glucose under alkaline solution, were found to non-selectively adsorb on hydrated silicates and aluminates (i.e., the hydration products of cement phases).

[0054] To further validate if the retardation effects of chorella demonstrated above are due to the presence of the OH and COOH functional groups, the chlorella was treated to potentially increase or reduce the amount of these groups, prior to mixing with the cement pastes.

[0055] One treatment was to heat the chlorella under N.sub.2 gas, a common process for producing biochar. The other was to soak it in H.sub.2O.sub.2 solution. To examine any changes of the OH and COOH groups in the algae, we review the chemical (and physical) reactions that potentially occur during treatments and then present the experimental characterization of the associated structural changes.

Potential Chemical Reactions during Treatments

[0056] During heat treatment, algae go through a series of chemical (and physical) reactions. Under 150? C., the main change is the evaporation of free water. As temperature further increases, the proteins, lipids and carbohydrates from algae are decomposed. Taking lignocellulose and algae as an example, their pyrolysis between 300 and 400? C. generate CO.sub.2, a reaction that has been attributed to the cracking and reforming of C?O and COOH functional groups, as schematically depicted in FIG. 3. More specifically, for an algae sample, the CO.sub.2 emission has been attributed to the decomposition of carbohydrates between 180? C. and 270? C. and protein from 320? C. to 450? C.

[0057] The H.sub.2O.sub.2 treatment, though only reported to oxidize the biochar samples so far, is hypothesized to also react with the raw algae biomass. In a biochar sample, H.sub.2O.sub.2 solutions (up to 30 wt %) increased the FTIR peak of COOH (1700 cm.sup.?1) but decreased that of C?C (1585 cm.sup.?1), a change that occurs more substantially as H.sub.2O.sub.2 is more concentrated. This is consistent with an increase of COOH groups in a peanut hull-based biochar upon H.sub.2O.sub.2 treatment. Additionally, such treatment increased the CH peak (775 cm.sup.?1), likely caused by the conversion of the C?C ring structure.

[0058] In FIG. 4, the FTIR spectra of both heat- and H.sub.2O.sub.2-treated chlorella, together with the raw chlorella, are presented. The two most substantial changes upon the treatments occur at peaks of 1729 cm.sup.?1 and 1062 cm.sup.?1, which are attributed to the COOH and OH groups, respectively. Specifically, while the H.sub.2O.sub.2 treatment enhanced both these peaks, the heat treatment decreased or even eliminated both of them. Such changes indicated that the COOH and OH groups were increased with the H.sub.2O.sub.2 treatment but substantially decreased with the heat treatment, an observation that aligns with the potential reactions discussed above.

[0059] The morphological changes of the treated versus the raw algae were further examined under SEM. In FIGS. 5 (a, b), both the raw and H.sub.2O.sub.2-treated chlorella exhibited rugged surfaces that are composed of small-sized particles/plates (roughly under 5 ?m in diameter). On the other hand, the surface becomes smooth upon the heat treatment, as shown in FIG. 5 (c). Such different morphology upon the heat treatment (i.e., lack of small-sized particles/plates) could relate to the disappearance of the OH and COOH functional groups as schematically and experimentally seen in FIG. 3 and FIG. 4. As an additional minor point, the clean and smooth morphology of the heat-treated sample suggests the treatment is gentle, consistent with an earlier examination that a 300? C. treatment did not cause porous and ruptured morphology as seen under higher temperatures.

[0060] Calorimetry curves of cement pastes with and without the chlorella additives (raw, H.sub.2O.sub.2, and heat-treated) are presented in FIG. 6. The Control and Raw curves in FIG. 6 are the same as curves of cement pastes with 0.0% (i.e., control 602) and 0.5% raw chlorella presented in FIG. 1. Compared to the sample with raw chlorella (604), the H.sub.2O.sub.2-treated chlorella (606) induced a further delay in the main reaction peak, specifically from 11.0 to 12.8 hours, as summarized in Table 1. To the contrary, the heat-treated chlorella (608) accelerated the peak position to 8.5 hours, which overlaps with the peak for the control paste (602). The 30) total heat for all these samples, however, remained approximately the same, for example, around 162 J/g at 40 hours (as summarized in Table 1). This same total heat at a relatively later age indicated that the algae-induced retardation occurs only temporally, a characteristic that is much preferred for retarders of cementitious materials.

[0061] The discrepancy in such retardation versus acceleration is likely caused by the different structural changes induced by these treatments. The H.sub.2O.sub.2 treatment increased the COOH and OH functional groups, while the heat treatment has removed these two functional groups. Considering the capability of these groups to substantially retard the cement hydration as mechanistically discussed above, the further retardation with H.sub.2O.sub.2 treatment but acceleration with heat treatment provided further direct evidence that COOH and OH groups are the origin of the algae's retardation.

[0062] We have validated the strong retardation effects of algae (i.e., chlorella as a typical type) and found out the retardation was due to the presence of the COOH and OH functional groups. Considering that algae are widely and cheaply available, their strong retardation effects could potentially be exploited in engineering practice. Here we show the structures and mechanical strength of the algae-dosed cement pastes. Since the heat-treated algae exhibited no retardation (FIG. 6), here we only focus on raw and oxidized (H.sub.2O.sub.2-treated) algae.

[0063] FIG. 7 shows the morphology of cement pastes with and without additive of algae. The control paste (control) and those with raw and H.sub.2O.sub.2-treated algae all exhibit a pretty dense microstructure. Upon a closer look, all these pastes exhibit both relatively-smooth (dashed oval) and rough (solid oval) fracture surfaces. While the rough-surface region is composed of small particles that could be calcium silicate hydrate particles, the smooth surface is likely the particles that have evolved into a bulk structure. We could not clearly identify the presence of any algae, likely due to the low dosage (0.5 wt %). Overall, the addition of the raw- and H.sub.2O.sub.2-treated chlorella does not appear to alter the microstructure of the cement pastes.

[0064] FIG. 8 shows XRD patterns for control, raw; and H.sub.2O.sub.2 treated cement pastes after 28 days of curing. Diffraction peaks occur at the same angles and with the same intensities for all three cement pastes. Search and match results also indicate the formation of the same hydrated cement phases for the control, raw, and H.sub.2O.sub.2 treated pastes. The presence of similar hydrated phases between pastes is consistent with SEM results which show no significant change in morphology with the addition of raw or treated chlorella. While addition of algae delays hydration kinetics, it does not appear to alter the crystal structures of the hydration products.

[0065] The compressive strength of the control cement paste and those with addition of rawand H.sub.2O.sub.2-treated chlorella is shown in FIG. 9. All the pastes exhibited an increase of strength from 7 to 28 days. Compared to the control paste, the addition of algae exhibited a comparable, or even higher, strength at both 7 and 28 days, for example, from 22 MPa for control paste to 26 MPa for raw chlorella dosed paste at 7 days. The comparable or even slightly increased strength is consistent with the calorimetry and SEM results. Despite a delay of reaction at the early age, the addition of the algae exhibited similar accumulated heat at around 40 hours and later (see FIG. 6). The addition of algae does not induce any morphological changes in the cement pastes from the SEM examination (see FIG. 7). Such minimal effects on strength and microstructures indicate a great potential of chlorella, as a typical type of algae, to be promoted as a sustainable bio-based retarder for cementitious materials.

[0066] We have demonstrated and understood the retardation effects of chlorella (a typical algae) on cement hydration. A high retardation efficiency has been observed, given that 1 wt % of algae delayed the main peak of the heat evolution by 82.4%, a degree of retardation slightly higher than that seen with 0.15 wt % of sucrose (a well-known strong retarder for cements). Considering algae's wide availability and sustainability as reviewed in the introduction, they could be regarded as a sucrose's competitor for retarders. Furthermore, the addition of algae was found to not affect the microstructures and compressive strength of the cement pastes, further suggesting the potential of algae to be used as a retarder in engineering practice.

[0067] Besides serving as a retarder and other similar applications (e.g., viscosity-modifying agent), algae exhibit great potential for developing functional cementitious materials. The introduction of algae, among other microorganisms, potentially equips cementitious materials with functionality of removing heavy metal in waste water and cleaning pollutants from the air. Additionally, the algae's functional groups (e.g., COOH, OH, RCOO.sup.?, HPO.sub.4.sup.2, NH.sub.2) exhibit great potential to be functionalized with other organic/inorganic or metallic materials and functional components, as seen in fields of cementitious materials materials/biotechnologies, implicating a promising area of algae-based functional cementitious materials (including geopolymers).

[0068] In terms of potential large-scale applications, the advantage of algae-based admixtures lies in their cost effectiveness on top of the other benefits (as detailed in the introduction). Considering that algae cost between $472-$1137 per ton and 1 m.sup.3 of concrete normally consumes ?450 kg cements, an addition of 0.5 wt % algae (i.e., 2.25 kg) would cost around $1.6-$2.6, a rate that lies at the lower end for typical chemical admixtures of cementitious materials. This could be even more promising as significant (more than 50%) cost reductions of the algal production can be achieved when using lower cost CO.sub.2, nutrients and water, a condition that is likely suitable for the growth of algae due to their high environmental tolerance.

[0069] We have for the first time substantiated that algae (e.g., chlorella) retards the hydration of hydraulic (e.g., portland) cements and elucidated the mechanisms involved with this retardation. Moreover, the addition of algae was found not to affect the microstructures and strength of the cement pastes, suggesting the potential role of algae to serve as a retarder for cementitious materials.

[0070] The retardation effects of algae were confirmed via calorimetry. The heat-evolution peak for the cement hydration was substantially delayed, specifically by 29.4%, 82.4% and 811.8% with addition of 0.5%, 1.0% and 3.0% of algae, respectively.

[0071] Such retardation is caused by the COOH and OH functional groups of the algae. While COOH and OH are retarding functional groups for cementitious materials, their presence in the algae was confirmed via our FTIR tests. Furthermore, we have demonstrated an enhanced retardation by algae with enhanced COOH and OH groups but an elimination of retardation by reducing these groups in the algae, through H.sub.2O.sub.2 and heat treatment, respectively.

[0072] To ultimately promote algae as a retarder for cements in engineering practice, we have further demonstrated that the addition of algae exhibited negligible effects on the morphology, crystal structure, and mechanical strength of the cement pastes. Considering the high retarding efficiency and negligible effects on structures and strength, the low economical cost on top of high environmental benefits promises a large-scale application of algae in cementitious materials.