Accelerators are one of the most popular kinds of chemical admixtures. Like water reducers, retarders and plasticizers, when added to a concrete batch either immediately before or during mixing.
To kick the set time of a batch of concrete into high gear, hit the accelerator.
Like water reducers, retarders and plasticizers, accelerators are one of the most popular kinds of chemical admixtures, added to a concrete batch either immediately before or during mixing. Accelerators make concrete set faster, also known as increasing the rate of hydration. At the same time, they promote strength development so it happens earlier in the set time of a slab.
If a contractor is using an accelerator, the odds are good that the weather is wintry. Accelerators counteract the influence of cold weather, which slows down the curing and setting process.
But accelerators aren't just for cold weather. A contractor can use one anytime a curing process needs a kick. The admixture may allow a concrete worker to remove forms earlier, get onto a concrete surface earlier for finishing, and sometimes even put loads on it earlier, such as when diverting foot traffic to do patching.
'Decorative guys will use accelerators in summer. It helps control the process when stamping,' says Terry Collins, concrete construction engineer with the Portland Cement Association.
If an accelerator is added to concrete on one half of a pour, a contractor can start stamping on that side, work his way to the other side, and enjoy a relatively consistent level of workability throughout. Decorative contractors have been using retarding admixtures for several years to achieve this effect, says Gabriel Ojeda, president of concrete admixture manufacturer Fritz-Pak Corp., and accelerators can pull off more or less the same trick.
Because accelerators cut set time, they can reduce labor costs, Ojeda says. They can also hasten the time an indoor remodel job, such as a cast-in-place countertop, takes to set, Ojeda says. 'You don't want people to have to wait seven, 10, 15 days without being able to use their kitchen,' he says.
And while they do cost a little extra, accelerators may still be a better bargain than, say, natural gas. 'If you don't accelerate, and you have a short set time, the other option is heat,' Ojeda says. 'Heat is now more expensive because of energy consumption. It may be cheaper to use accelerators than to heat a building.'
Calcium chloride pros and cons
A number of chemicals qualify as accelerators, but the most common is calcium chloride. It's cheap, plentiful, and readily available from huge chemical companies.
However, while calcium chloride may be the cheap favorite for concrete in general, it is not necessarily the best option for decorative concrete.
Calcium chloride slightly increases workability and reduces the water required to achieve a given slump in a mix, according to a report from the Federal Highway Administration. It reduces initial and final setting times, and it improves compressive and flexural strengths of concrete at early ages.
Guidelines published by the Portland Cement Association list colored concrete among the jobs in which calcium chloride 'should be used with caution.' The guidelines also state that slabs intended to receive dry-shake metallic finishes should not take calcium chloride or admixtures that contain soluble chlorides, and neither should most slabs poured in hot weather.
Decorative concrete contractors are going to be discouraged from using calcium chloride, says Collins of the PCA. It can inhibit the ability of acid stain to react with cement in the concrete. And it increases the potential for efflorescence. These aren't significant problems on generic concrete slabs, but on decorative jobs, they can be distressing.
Most people see a little bit of white powder on the sidewalk, they sweep it off and forget about it,' Collins says. 'But they see a little bit of white powder on decorative concrete and they tend to believe the world is ending, there's something wrong with it.'
Excessive amounts of calcium chloride may cause rapid stiffening and shrinkage while drying, creating cracks in the cured surface. Calcium chloride may promote corrosion in steel reinforcements and increase the potential for scaling.
Perhaps most troubling for decorative contractors, calcium chloride may darken their slab. The chemical is hydroscopic. Just as table salt gets hard absorbing water from the air, calcium chloride literally liquefies. 'If you put a pile of it on a table and come back in the morning, it will be all water,' Ojeda says.
That's essentially how calcium chloride can make concrete darker, Ojeda says. Say a colored slab with calcium chloride is half in the shade, half in the sun. Rain or moisture will linger longer on the shady portion, get sucked into the slab by the calcium chloride, and make that part darker. The change won't be significant, he acknowledges, but 'it's still enough to make a difference in color between the shaded area and sunny area.'
Calcium chloride's potential to oxidize might also change the tint of colors and pigments that are based on iron oxide, he says.
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Contractors can take steps to minimize cracking caused by calcium chloride's shrinkage, says Tim Reason, president of Chargar Corp., a manufacturer of concrete and masonry products. 'You may try to keep the surface wet and damp during the curing process. That might help a little bit.'
Reason also downplays the concern that calcium chloride may promote corrosion of reinforcing steel, noting that, even if it occurred, the corrosion would take a long time to become a real problem. 'The corrosion issue is something they've been saying for a hundred years, but I don't know if they've definitely proved it,' he says. 'It may. But how long is it going to take? It's not like it's going to deteriorate in a year or so.'
Alternatives
In any case, contractors who want to accelerate their concrete aren't forced to use calcium chloride. There are alternatives. 'Although calcium chloride is an effective and economical accelerator, its corrosion-related problem limited its use and forced engineers to look for other options, mainly nonchloride accelerating admixtures,' states the FHA report.
Sodium thiocyanate, triethanolamine, and calcium formate, nitrate and nitrite are among the 'nonchlorides' that have been successfully used to accelerate concrete set times.
There are four of the most common:
Accelerators are typically added at the manufacturer level, and with many manufacturers, the exact chemical makeup will be closely held, Ojeda notes. 'We use calcium formate as a base, but we add other materials to boost performance of the formulation. That's proprietary.'
Again, none of these alternative admixtures costs as little as calcium chloride does. 'Calcium chloride is the least expensive of all the materials,' Ojeda says. 'That's the main reason people would not use a nonchloride accelerator.'
Of course, decorative concrete contractors also have other techniques for accelerating concrete besides adding an accelerator. Using cement that promotes high early strength will work. So will adding a water reducer, curing at higher temperatures, and lowering the water-cement ratio by putting more cement into the concrete.
'It's very common for decorative concrete guys to just increase the cement content by 100 pounds and just call it a day,' Collins says. 'That will have the same effect.'
The antimicrobial property is the most important assessment factor for antimicrobial concrete, that varies with the addition of different types of antimicrobial agents, as summarized in Table 1 . Antimicrobial concrete, with the addition of diverse antimicrobial agents against microorganisms involving in microbial induced corrosion, especially in sewer systems, have been extensively studied in the literature. Nickel and tungsten have been known to protect concrete from microbial corrosion owing to their antimicrobial effect towards causative bacteria, i.e., Thiobacillus thiooxidans (T. thiooxidans). Negishi et al. [41] found that the cell growth of A. thiooxidans, including strain NB 1'3 (isolated from corroded concrete in Fukuyama, Japan) was strongly inhibited by 20 μΜ sodium tungstate, and completely inhibited by 50 μΜ sodium tungstate. Similarly, Sugio et al. [42] reported that cell growth of an iron-oxidizing bacterium, Acidithiobacillus ferroxidans (A. ferroxidans), was strongly inhibited by 0.05 mM and completely inhibited by 0.2 mM of sodium tungstate. In the study of Maeda et al. [40], concrete containing 0.1% metal nickel and concrete with 5 mM nickel sulfate were found to completely inhibit the cell growth of strain NB 1'3 of T. thiooxidans isolated from corroded concrete. Moreover, Kim et al. [61] conducted an investigation to evaluate the antibiosis of antimicrobial ingredients (Ni and W) of antimicrobial watertight admixture mixed in mortar and concrete on Thiobacillus novellus (T. novellus). Broth Microdilution MIC test indicated that T. novellus could not survive in the area where the admixture is dropped. As reflected in Table 1, the total colony test numerically shows that T. novellus in culture solution with mortar added with the admixture were disappeared after 24 h. The biochemical corrosion simulation test also indicated that the number of T. novellus was much lower in the case of mortar mixed with the admixture than plain mortar specimens. The results suggested that the addition of antimicrobial watertight admixture in cement mortar and concrete suppressed the growth of T. novellus. Furthermore, Southerland et al. [66] found that tungsten used alone is able to inhibit growth of T. novellus, whereas molybdenum, ammonium molybdate or a mixture of ammonium molybdate and tungstate activates growth of the same bacteria. Likewise, it is reported that molybdenum activates growth of T. novellus but inhibits growth of T. thiooxidans, indicating SOB of the same genus Thiobacillus have different growth inhibitory mechanism. It is noteworthy that the antimicrobial property of antimicrobial agent Ni and W is not only largely dependent on their contents, but greatly affected by pH. It is generally recognized that nickel compounds are suitable for neutral environment, while tungsten compounds are more effective in acidic environment [23], [43]. Maeda et al. [40] observed that the amount of nickel contained in the strain NB 1'3 cells treated without nickel, treated with 10 mM nickel sulfate at pH 3.0 and treated with 10 mM nickel sulfate at pH 7.0 was 1.7, 35 and 160 nmol nickel per mg protein, respectively. The results indicated that nickel is able to bind to strain NB 1'3 cells, and much more nickel binds to the cells at neutral pH than at acidic pH demonstrated that nickel ions have a better inhibitory effect towards the microbe in neutral environment than in acid environment [40]. The findings of Negishi et al. [41] and Sugio et al. [42], as detailed in Table 1, demonstrated that the antimicrobial property of tungsten is more effective in acidic environment than in neutral environment.
Furthermore, Kong et al. [62], [65] conducted an investigation to evaluate the impact of adding five bactericides in concrete towards the selected bacteria (as listed in Table 1), and to study their applicability for controlling and preventing microbial corrosion of concrete. They reported that concrete with sodium bromide and zinc oxide exhibited excellent antimicrobial property towards the tested bacteria, especially Bacteroidetes, as the number of microbial populations decreased substantially. However, the antimicrobial effect of concrete with a dispersion of sodium tungstate on microbes is worst, as reflected by the lowest bactericidal rate (21.95%), it even promotes the growth and reproduction of Proteobacteria. They also observed the dead and live microorganisms within biofilm with confocal scanning laser microscopy (CLSM), as seen in Fig. 3 . The number of live cells within the biofilm all decreased to a certain degree, indicating all the tested bactericides have a certain sterilizing effect. Similarly, Bao [67] obtained that the surface roughness of the control mortars and mortars with sodium tungstate and sodium bromide was 46.65, 14.3 and 9.02 μm after a 3-month immersion in intensified sewage, respectively. Therefore, they concluded that the addition of sodium tungstate and sodium bromide could effectively inhibit the growth and reproduction of microorganisms attached to the surface of cement mortar. In addition, Sun et al. [44] studied the bactericidal effect of FNA on microbes in sewer biofilms of two concrete coupons. They observed that, as for the intact corrosion biofilm, H2S uptake rates (SUR) were reduced markedly 15 days after FNA spray and viable bacterial cells severely decreased by over 80% within 39 h (detailed in Table 1), suggesting that biofilm cells were killed by the treatment. As for a suspended solution of corrosion biofilms scraped from the concrete coupon, ATP level and ratio of viable bacterial cells were also severely decreased by the treatment, as clearly seen in Fig. 4 , demonstrating that FNA strongly deactivates bacteria of acidic corrosion biofilm [44].
Zeolite containing metal ions has been investigated a lot to be used in concrete due to its excellent antimicrobial property. For example, Haile et al. [28] evaluated the antimicrobial characteristics of mortar specimens coated with silver bearing zeolite with A. thiooxidans. They observed that biomass concentration of A.thiooxidans dry cell weight (DCW) of control specimens (236 mg TSS/L and 181 mg TSS/L) was as much as 2-fold higher compared to the mortars coated with silver-loaded zeolite (125 mg TSS/L and 80 mg TSS/L). The reduced microbial numbers evidenced that the mortar specimens coated silver bearing zeolite exerted antimicrobial characteristic on A.thiooxidans and inhibited bacterial growth. They also found that bacteria were not affected in the nutrient solution indicated that the antimicrobial characteristics of zeolitic coatings were only apparent on solid surface particles [28]. Moreover, Haile et al. [30] discovered that no biomass growth was observed upon exposure of the bacterium to silver-loaded zeolite coated concrete specimens, and there was no oxygen uptake measured, meaning no viability of A. thiooxidans cells for the silver-loaded zeolite coated concrete specimens. The research results confirmed that zeolite containing 5 wt% Ag is inhibitory to planktonic and biofilm of A. thiooxidans [30]. Similarly, De Muynck et al. [69] observed that mortar specimens with silver-copper zeolites (zeolites contain 3.5% silver and 6.5% copper) obtained a 12-fold decrease of ATP content after 24 h, while inhibition of antimicrobial fibers on bactericidal activity was limited, indicating biocidal effect towards SOB was limited in the case of antimicrobial fibers and that of antimicrobial zeolites was much better. Moreover, De Muynck et al. [25] investigated the antimicrobial effectiveness of silver copper zeolites against E. coli, Listeria monocytogenes, Salmonella enterica or S. aureus in a quantitative way. A clear decrease in the total ATP content was observed for mortar specimens containing silver copper zeolites, indicating the occurrence of antimicrobial activity by the presence of silver and copper ions. Furthermore, they concluded that the concentration of silver copper zeolites is required to be more than 3% so as to obtain a bactericidal effect on mortar surfaces [25]. In the experiment of Haile et al. [70], cellular ATP in concrete contained 2.6 wt% silver-loaded chabasite declined to zero with a corresponding DCW value of 35 mg, indicating there was no growth after bacteria were exposed to 2.6 wt% silver-loaded chabasite, whereas the biomass was 51 mg DCW and cellular ATP was 0.21 mg for concrete coated 18 wt% silver-loaded chabasite. The results indicated that antibacterial characteristics of concrete specimens coated with 2.6 wt% is superior to the specimens with 18 wt% silver-loaded chabasite. The results of the experiment conducted by Xu and Meng [64] indicated that the content of E. coli in concrete incorporating silver-bearing zeolite and polypropylene fiber was reduced compared to the control samples, demonstrating that silver-bearing zeolite and polypropylene fiber play a bactericidal role and reduce the breeding of E. coli. Likewise, Li [32] discovered that concrete specimens added with 0.5% silver-loading zeolite and polypropylene fiber had the most pronounced bactericidal effect towards E. coli, as reflected by the greatest OD value (the greater the OD value, the lower the bacterial concentration of the concrete samples) according to antibacterial test results. While antimicrobial effect of concrete specimens incorporated with fly ash and mineral powder was not evident.
Researchers have paid much attention to the effect of antimicrobial nanoparticles on antimicrobial property of concrete. Singh et al. [47] admixed ZnO nano powder into cement composite and evaluated the antimicrobial effect of the formed cement-ZnO composites against two bacterial strains E. coli, Bacillus subtilis and fungal strain Aspergillus niger. As shown in Fig. 5 , the antibacterial and antifungal effects of cement-ZnO composite increased as the ZnO concentration increased in the range of 0,5, 10, 15 wt%. Moreover, it was also noted that both antibacterial and antifungal activities of cement-ZnO composite was enhanced under sunlight compared to dark condition. In addition, Wang et al. [48] conducted a research to study the antimicrobial effect of high-performance concrete (HPC) added with nano ZnO against E. coli and S. aureus. The results showed that the antibacterial rate of the two groups of antibacterial concrete against E. coli reached 100%, however the antibacterial rate against S. aureus was 54.61% and 99.12%, respectively. Through SEM observations, it is found that nano ZnO and its resulting compounds precipitation adhered to surface of cement hydrate, thus inhibited the growth of bacteria, accounting for the significant antibacterial effect of HPC [48]. Sikora et al. [54] conducted a series of tests to evaluate the antimicrobial effect of four metal oxide nanoparticles (Al2O3, CuO, Fe3O4, ZnO) used in cement-based composites. They discovered that all the studied nanoparticles inhibited microbial growth, and the growth kinetics showed that the highest inhibitory effect on E. coli ATCC TM and E. coli MG was Fe3O4 nanoparticles, ZnO nanoparticles, respectively. The biofilm formation assay indicated that the tested nanoparticles were able to reduce the formation of bacterial biofilms, E. coli ATCC TM biofilms were inhibited by all nano-oxides, ZnO nanoparticles significantly affected the formation of P. aeruginosa and S. aureus biofilms. However, the viability of P. aeruginosa cells in sample with Al2O3 was significantly higher compared to the control sample. Similarly, Dyshlyuk et al. [71] evaluated antibacterial and fungicidal properties of ZnO, TiO2 and SiO2 nanoparticle solution by interaction with eight types of microorganisms commonly causing bio-damage to buildings and concrete structures. They found that ZnO nanoparticles of 2'7 nm in size with a suspension concentration of 0.01'0.25% displayed the most noticeable antimicrobial properties against the tested strains, decreasing microorganisms by 2'3 orders of magnitude. They also revealed that ZnO nanoparticles interacted specifically to a microorganism type, leading to a decrease in the number of Bacillus subtilis B bacterium by 2 orders of magnitude, and that of fungi of Penicillium ochrochloron F 920 by 3 orders of magnitude. However, TiO2 and SiO2 nanoparticles exhibited a low antimicrobial activity. Nano-TiO2, with its excellent photocatalytic effect, has aroused much interest of many researchers in the aspect of microorganism inactivation. For instance, Ganji et al. [50] investigated the antimicrobial performance of cement samples containing 1,5 and 10 wt% nano-TiO2 against E. coli under UV irradiation. They found that bacterial inactivity enhanced as the amount of TiO2 nanoparticles in cement samples increased, however, the inactivation effect was not obvious even the amount of TiO2 nanoparticles further increase to 10 wt%. Therefore, 5 wt% TiO2 is proposed to be the most proper content in cement samples for inactivation of E. coli taking into account both the photocatalytic inactivation and cost. Linkous et al. [72] employed nano-TiO2 in concrete to inhibit the attachment and growth of oedogonium. They discovered that concrete containing 10 wt% TiO2 nanoparticles obtained a 66% reduction in the growth of oedogonium.
Besides above, researchers have also investigated antimicrobial effects of antimicrobial concrete towards some other commonly microbes threatening concrete. For example, Umar et al. [36] evaluated the antimicrobial activity of four types of semi-circular modified cement composite specimens using Serratia marcescens collected from seashore and then isolated from microbe samples. The results showed that cement composites admixed with sodium nitrite-based inhibitor performed better with the least percentage increment of total viable count at the end of 144 h as compared to the cement composite with styrene acrylate copolymer, with acrylic polymer, and cement composite without any admixture, respectively. This can infer that cement composite with sodium nitrite-based inhibitor exhibited noticeably improved ability to suppress the growth of Serratia marcescens in marine environment. NORGANIX [73] is able to endue concrete with powerful antimicrobial property to eliminate Salmonella, Listeria, E. coli, Clostridium, and mold spores not just on the surface but deep within the concrete. Moreover, antimicrobial concrete with NORGANIX can prevent microbes from re-entering concrete from any directions because NORGANIX will hydrate with the unused Portland cement within the concrete to generate new cement, thereby sealing the capillary system. Paiva et al. [20] determined the antimicrobial efficiency of BioSealed for ConcreteTM, a hydro-silicate catalyst in a colloidal liquid base, to prevent Salmonella spp. attached on concrete bricks in food industry. They found that concrete bricks treated with BioSealed for ConcreteTM after inoculation, before and after inoculation had immediate bactericidal effects towards the tested five strains of Salmonella in contrast with bricks not treated with BioSealed for ConcreteTM and bricks treated with BioSealed for ConcreteTM before inoculation, as observed by significantly lower viable counts of Salmonella.
Yamanaka at al. [38] studied the inhibitory effects of formats on the growth of bacteria causing concrete corrosion in sewerage systems. They found that the growth of SOB isolated from corroded concrete were completely inhibited by 10 mM calcium formate for 18 days, while the growth of acidophilic iron-oxidizing bacteria was inhibited by 10 mM calcium formate during 34 days. This finding shows that even the same antimicrobial agent has different inhibitory effect on different microbes. In addition, they also observed that the formation of ATP in bacterial cells was ceased after the addition of calcium formate into concrete test pieces. Erbektas et al. [57] evaluated the antimicrobial efficacy of silane quaternary ammonium chloride (SQA) aqueous salt solution against planktonic Halothiobacillus Neapolitanus and A.thiooxidans. They found that the antimicrobial efficacy directly related to bacterial population and activity, and indirectly depends on pH. Furthermore, antimicrobial effectiveness occurs when the pH is greater than 4. In the research undertaken by Do et al. [59], cement mortars with isothiazoline/cabamate exhibited a good antifungal effect against Aspergillus niger, while mortars with nitrofuran did not show inhibitory effect even the content of nitrofuran was up to 5 wt%. Moreover, the antifungal effect of cement mortar containing isothiazoline/cabamate on Aspergillus niger enhanced almost linearly as the content increases (0%,0.3%,0.5%,1%,2% and 5% by mass to cement). According to [74], researchers of former Soviet Union tested mortar samples with alkyl nitro-bromide (A ' B) stored for 6 years. The results indicated that the microbial retention rate on the surface of mortar specimens was merely 0.6% and 0.1% when the content of A ' B is 0.025 wt% and 0.05 wt%, respectively, after 5 h of irradiation, confirming the strong and long-lasting antimicrobial ability of A ' B.
It is worthwhile noting that some organic antimicrobial agents are extremely suitable to add into concrete due to their antimicrobial power to combat against diverse microbes, rather than only a single type of microbe. For example, Kong et al. [62] found that concrete added with copper phthalocyanine exhibited outstanding antimicrobial effect with high bactericidal rates towards Bacteroidetes (90.82%) and Proteobacteria (64.25%), and the bactericidal rate towards all tested microbes is as high as 82.59%. The number of live cells within the biofilm attached to concrete added with copper phthalocyanine showed a significant drop, and the content of live cells was only 12% of that attached to plain concrete. A large number of dead microbes was observed, as seen in Fig. 3 (f). Vaquero et al. [16] studied the bactericidal ability of 15 commercial bactericides blended into concrete against microbial induced corrosion by culturing microbes and evaluating the antimicrobial efficiency. Research results indicated that the multicomponent formulation PL-UV-H-2B was the sole formulation to succeed in all the evaluation process among all formulations. Concrete samples fabricated with PL-UV-H-2B, of which the actives are 30% 2-octyl-2H-isothiazol-3-one + Terbutryn and 15% 2,4,4'-trichloro-2'-hydroxy-diphenyl ether (calcium filler as a dispersive matrix), exhibited high effectiveness in antimicrobial tests against algae (Scenedesmus vaculatus and Stichococcus bacillaris), fungus (Aspergillus niger), and bacteria (S. aureus and E.coli), both before and after accelerated aging processes, as exhibited in Fig. 6 . They also paid special attention to the reasons responsible for failure of some biocide formulations and concluded that the water-soluble bactericide showed a lower retention rate in concrete and thus plays a poor role in protecting concrete in the long term [16]. Urzìet al. [45] evaluated the efficiency of three water-repellent compounds and two biocide compounds, i.e. ALGOPHASE and the new water miscible formulation ALGOPHASE pH 025/d having the same active ingredient 2,3,5,6-tetrachloro-4-methylsulfonyl-pyridine, against microbial colonization of mortars both in laboratory conditions and outdoors. They observed that the application of water-repellent alone was insufficient to prevent biofilm growth on the surface, whereas the combined application of water repellents and biocides in a single step prevent microbial growth, reflecting by complete absence of bacterial colonization, absence of algal colonization, dramatically reduced colonization by fungi on the surface of mortars (seen the representative samples T4 and T5 shown in Fig. 7 ). Single-step application of biocide and water repellent exhibits excellent performance due to biocide compound randomly distribute below, between and above the hydro-repellent film. In this way, the biocide has the ability to remove the remains of old colonization below, and stop new microbial colonization on the surface [45]. Shook and Bell [37] evaluated the antimicrobial effect of ConShield using wafers of concrete mortar incubated with a bacterial suspension of T.thiooxidans, T. thioparus, and T. denitrificans. The results indicated that the viable bacterial count of concrete wafers treated with ConShield is zero, suggesting that ConShield killed all of the tested bacteria with a complete 100% kill after 24 h. Moreover, it is reported that ConBlock MIC [75], whether integrated throughout the matrix of concrete when used as an admixture and/or directly applied to concrete as a surface treatment, it inhibits the growth of bacteria, fungi, mold, and algae. Freed et al. [56] evaluated the efficacy of concrete reinforced with fibers incorporating Microban B. The inhibition zone of concrete treated with polypropylene fibers containing Microban B towards E. coli, S. aureus, and mixed mold(fungi) was 3,4, and 2 mm, respectively, indicating fibers carrying Microban B could kill microorganisms.
Above investigations have indicated that antimicrobial agents could endow concrete with antimicrobial property to varying degrees. Antimicrobial properties of antimicrobial concrete is largely depending on respective intrinsic natures, types and contents of antimicrobial agents. However, the existing researchers paid little attention to the impact of the addition of antimicrobial agents on the microstructure of concrete. It is necessary to establish the underlying connections between different properties as well as the microstructure of concrete after adding antimicrobial agents. Moreover, high retention rate of antimicrobial agents in concrete is required in order to maintain the long-lasting inhibitory or killing effect towards microbes, while the long-term retention rate of a biocide and its influence on the other properties of concrete are poorly understood [35], [65].
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