Evaluation and Selection of De-Icing Salt Based on Multi-Factor

materials

Article Evaluation and Selection of De-Icing Salt Based on Multi-Factor

Guoju Ke 1, Jun Zhang 1,* and Bo Tian 2 1 Department of Civil Engineering, Tsinghua University, Beijing 100084, China; [email protected] 2 Research Institute of Highway Ministry of Transport, Beijing 100088, China; [email protected] * Correspondence: [email protected]

Received: 18 February 2019; Accepted: 14 March 2019; Published: 19 March 2019

Abstract: De-icing salts can greatly ease trafﬁc congestion but introduce corrosion of concrete and damage to plant growth. The decision of which de-icing salt to use becomes a crucial issue. In this study, several representative de-icing salts were investigated, and the effects of de-icing ability, salt freezing corrosion on concrete, and plant growth were comprehensively tested. Finally, the decision of de-icing salt was made based on analytic hierarchy process (AHP). Results show that de-icing salts achieving the best de-icing effect are not the same at different concentrations. De-icing salts of 3% concentration have the greatest corrosion to concrete. Notably, magnesium chloride and calcium magnesium acetate have the least impact on plants among all studied de-icing salts. Using AHP, ethylene glycol and calcium magnesium acetate are selected as optimal items under different priorities.

Keywords: de-icing ability; concrete peeling amount; plant drying rate; analytic hierarchy process; optimal item

1. Introduction Frozen and slippery roads cause traffic congestion and accidents and endanger the safety of people and vehicles. Statistics show that the probability of traffic accidents in ice and snow is 4–5 times that in normal weather. The use of de-icing salts can reduce accidents caused by snow and ice by 88.3% [1]. De-icing salts can ease traffic congestion but introduce problems that cannot be ignored, such as corrosion of roads and bridge concrete, impact on plant growth, and damage to groundwater and soil. Under the action of freezing and thawing, the damage of concrete and the severity of some salt corrosion are approximately ten times those of ordinary corrosion. The service life of roads affected by de-icing salt may be decreased by more than 50% [2,3]. Under the action of de-icing salt, the salt concentration of groundwater becomes high, which causes physiological drought of plants; sodium ions cause abnormal absorption of plant nutrient elements; the accumulation of toxins in plants destroys their normal metabolism, which harms the growth of street trees, isolation belts, and lawns; these phenomena make de-icing salts a “green killer” [4–8]. In the spring of 2005, the use of de-icing salt in the urban area of Beijing resulted in the death of 11,000 pavement trees, 1.5 million shrubs, 200,000 m2 of lawns, and economic losses of more than 30 million Yuan [9]. De-icing salts can also cause groundwater pollution [10–12], thereby affecting the life of water and plants [13] and even harming human health. In the 1970s, China began to use de-icing salts. On 6 February, 2006, the amount of de-icing salt used in Beijing reached 0.73 million tons due to special weather. At present, finding a suitable de-icing salt is crucial and urgent because of the high emphasis on resource conservation and environmental protection.

Materials 2019, 12, 912; doi:10.3390/ma12060912 www.mdpi.com/journal/materials Materials 2019, 12, 912 2 of 12

The analytic hierarchy process (AHP) was ﬁrst proposed by Professor Saaty in the University of Pittsburgh in the 1970s [14]. This decision-making method is simple and practical; a model of decomposition, judgment, and synthesis based on hierarchical analysis solves multi-objective and multi-criteria problems [15]. In this study, we use the AHP to comprehensively consider the ability of de-icing salt to melt snow, the corrosion of concrete, and the inﬂuence on plants under three factors and obtain the optimal items among the alternative de-icing salts.

2. Materials and Methods

2.1. Materials De-icing salts have many types. According to composition, de-icing salt can be divided into inorganic, organic, and mixed types. De-icing salt can also be classiﬁed into chlorine salt type (e.g., sodium chloride, calcium chloride, and magnesium chloride), non-chlorinated salt type (e.g., organic or inorganic alcohol and ammonia), and mixed type (e.g., mixed chlorine and non-chlorinated salts). This study selects eight kinds of conventional de-icing salts in literature, namely, sodium chloride, magnesium chloride, calcium chloride, calcium magnesium acetate, urea, sodium formate, potassium acetate, and ethylene glycol [16–18], and one type that is commonly used in Beijing (Ji-suo II) (Table1). The ﬁrst eight are chemically pure reagents, and Ji-suo II was purchased from a Beijing company.

Table 1. De-icing salt category.

No 1 2 3 4 5 6 7 8 9 Calcium Sodium Ethylene Sodium Magnesium Calcium Urea magnesium Potassium Ji-suo II Name chloride glycol formate chloride chloride (U) acetate acetate (PA) (JS) (SC) (EG) (SF) (MC) (CC) (CMA)

2.2. Methods

2.2.1. Melting Test of De-Icing Salts In accordance with the provisions of the “Snow Melting Agent” (GB/T 23851-2017) [18], 25 mL of de-icing salt solution was prepared with concentrations of 18% and 29% and poured into a 50 mL beaker. Then, the beaker was placed in a low-temperature test chamber ((−10 ± 1) ◦C) to freeze for 12 h until use. In ten plastic boxes of the same diameter and height, 100 mL of water was added. Then, they were placed in a low-temperature incubator at (−10 ± 1) ◦C for icing. After icing, each plastic box with ice cubes was removed. The water and ice on the outer wall of the box were wiped prior to weighing. Thereafter, the spare de-icing salt was poured into the box with ice cubes, and the box was placed into the low-temperature incubator again. After 30 min, the box was removed, the liquid was poured out, and the quality of the remaining ice and the box was weighed and recorded. After weighing, the liquid was poured back into the box with ice cubes, which was placed in a cryostat. The boxes were removed at 60, 90, 120, and 150 min, separately, and the remaining ice and boxes were weighed after pouring the liquid therein.

2.2.2. Salt Freezing Corrosion Test of Concrete under the Action of De-Icing Salt Conventional pavement cement concrete contains cement of 360 kg·m−3, sand of 626 kg·m−3, stone of 1270 kg·m−3, water of 144 kg·m−3, and water reducing agent of 3.6 kg·m−3. The test mold was made of PVC (Polyvinyl chloride) pipe with an inner diameter of 200 mm and a height of 80 mm. The fresh concrete was loaded into the test mold and vibrated, and wood was used to wipe the molded surface slurry. The resultant was placed into the curing room for standard curing, as shown in Figure1. The test box used in the test was made of stainless steel, and a non-metallic mat as placed on the Materials 2019, 12, 912 3 of 12

Materials 2019,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 3 of 13 bottom of the container. The mat did not absorb water, did not deform when immersed in water, and ondid the not bottom affect the of solutionthe container. during The the test.mat Afterdid not the absorb test specimen water, reachesdid not thedeform required when age, immersed the molded in water,surface and was did immersed not affect in athe certain solution concentration during the oftest. de-icing After saltthe test solution, specimen and thereaches immersion the required depth age,was 4–6the mm,molded as shown surface in was Figure immersed1. in a certain concentration of de-icing salt solution, and the immersionThe salt-freezing depth was equipment4–6 mm, as used shown DYD-1 in Figure concrete 1. single-sided salt freezing equipment, as shown in FigureThe 2salt-freezing. The intelligent equipment computer used control DYD-1 system concrete can controlsingle-sided the limit salt temperaturefreezing equipment, in the tank as shown(testable in rangeFigure from 2. The 20 intelligent◦C to −20 computer◦C). The contro freeze-thawl system systemcan control was the operated limit temperature as follows: in each the tankfreeze–thaw (testable cycle range of from the concrete 20 °C to test −20 block °C). The was freeze-thaw completed withinsystem 12 was h, whichoperated starts as follows: from cooling each freeze–thawdown to −20 cycle◦C for of 4 the h and concrete then holding test block for 3was h and completed heating towithin 20 ◦C 12 for h, 4 which h and thenstarts holding from cooling for 1 h. downEach cycle to −20 was °C repeatedfor 4 h and ten then times, holding and peelingfor 3 h and amount heatin ofg concrete to 20 °C isfor collected 4 h and then and weightedholding for while 1 h. Eachreplacing cycle the was new repeated salt solution. ten times, and peeling amount of concrete is collected and weighted while replacing the new salt solution.

Figure 1. Preparation of specimen.

Figure 2. DYD-1DYD-1 Concrete salt freezing machine.

2.2.3. Effect of de-icing De-Icing salt Salt on on plant Plant growth Growth In flowerpots ﬂowerpots of of same same sizes sizes (43 (43 cm cm × ×19 19cm cm × 14× cm),14 cm), the same the same grass grass species species were were planted. planted. The selectedThe selected grass grass species species were were mainly mainly easy easy to operat to operate,e, have have a certain a certain representativeness, representativeness, and and have have a shorta short test test period. period. When When the the growth growth was was prospe prosperousrous and and roughly roughly the the same, 25 mLml of of certain concentrations of of No. No. 1 1to to No. No. 9 9de-icing de-icing salts salts and and No. No. 10 10water water was was sprayed. sprayed. After After spraying, spraying, the growththe growth was was observed. observed. The The yellowing yellowing of ofthe the plant plant tip tip after after spraying spraying various various de-icing de-icing salts salts was recorded, as shown in Figure3 3.. DryDry dayday refersrefers toto thethe numbernumber ofof daysdays whenwhen thethe tiptip ofof thethe plantplant beginsbegins to turn yellow; the dry rate refers toto thethe proportionproportion ofof plantsplants thatthat turnturn yellow.yellow.

Materials 2019, 12, 912 4 of 12 Materials 2019, 12, x FOR PEER REVIEW 4 of 13

FigureFigure 3. Process 3. Process of plant of plant tip tipyellowing. yellowing. 3. Results and Analysis 3. Results and analysis 3.1. De-Icing Ability 3.1. De-icing ability 3.1.1. De-Icing Ability Test under 18% Concentration of De-Icing Salt 3.1.1. De-icing ability test under 18% concentration of de-icing salt Table2 shows that the cumulative amount of de-icing of the nine de-icing salts increases with the increaseTable 2 inshows de-icing that time,the cumulative De-icing accumulation amount of de-i referscing toof the amountnine de-icing of ice salts that increases can be melted with bythe de-icingincrease saltin de-icing over a certaintime, De-icing period accumulation of time. The maximumrefers to the value amount is reached of ice that at 120can min,be melted and theby amountde-icing ofsalt de-icing over a decreases certain period with the of increasetime. Th ine de-icingmaximum time value after is 120 reache min.d Theat 120 reason min, is and that the concentrationamount of de-icing of de-icing decreases salt affectswith the the increase level of in freezing de-icing point. time Aafter high 120 concentration min. The reason of de-icing is that saltthe correspondsconcentration to of a de-icing low freezing salt affects point the and level strong of freezing melting point. effect ofA high de-icing concentration salt. By contrast, of de-icing a small salt concentrationcorresponds to of a de-icinglow freezing salt indicatespoint and a highstrong freezing melting point. effect Therefore,of de-icing the salt. ideal By de-icingcontrast, effecta small is notconcentration achieved whenof de-icing the temperature salt indicates is low. a high “Snow freezing Melting point. Agent” Therefore, (GB/T the 23851-2017) ideal de-icing [18] requireseffect is thatnot achieved the ability when to melt the snowtemperature and ice is is low. not less“Snow than Melting 90% of Agent” the capacity (GB/T of 23851-2017) sodium chloride. [18] requires Thus, potassiumthat the ability acetate to melt does snow not meet and the ice requirements is not less than and 90% is unsuitable of the capacity as a de-icing of sodium salt. chloride. Thus, potassium acetate does not meet the requirements and is unsuitable as a de-icing salt. Table 2. De-icing ability test (18% concentration). Table 2. De-icing ability test (18% concentration). De-Icing Accumulation/g Baseline Percentage No De-Icing Salt 30 minDe-icing 60 min accumulation/g 90 min 120 min 150 min Baseline(30 min)/%percentage No De-icing salt 1 Sodium chloride30 min 8.3 60 min 9.2 90 min 11.3 120 min 11.5 150 min 11.6 (30min)/% 100.0 2 Ethylene glycol 8.7 10.3 12.0 12.2 12.2 104.8 13 Sodium chloride Urea 8.3 7.6 9.2 8.8 11.3 9.6 11.5 9.7 11.6 9.9 100.0 91.6 24 CalciumEthylene magnesium glycol acetate 8.7 7.9 10.3 9.0 12.0 9.8 12.2 10.4 12.2 10.5 104.8 95.2 5 Sodium formate 7.8 8.9 10.1 10.2 10.3 94.0 36 MagnesiumUrea chloride 7.6 8.08.8 9.39.6 10.09.7 10.49.9 10.6 91.6 96.4 7Calcium Calcium magnesium chloride 8.5 9.7 10.1 10.3 10.4 102.4 4 7.9 9.0 9.8 10.4 10.5 95.2 8 Potassiumacetate acetate 5.8 6.6 7.5 7.7 6.8 69.9 9 Ji-suo II 9.7 11.1 10.9 7.8 −0.7 116.9 5 Sodium formate 7.8 8.9 10.1 10.2 10.3 94.0 3.1.2.6 De-IcingMagnesium Ability chloride Test under 8.0 29% Concentration 9.3 10.0 of De-Icing 10.4 Salt 10.6 96.4 7 Calcium chloride 8.5 9.7 10.1 10.3 10.4 102.4 In accordance with the standard “Snow Melting Agent” (GB/T 23851-2017), eight different 8 Potassium acetate 5.8 6.6 7.5 7.7 6.8 69.9 de-icing salts other than potassium acetate are selected to determine the ability under 29% concentration for9 comparing Ji-suo the effectsII of de-icing9.7 salt11.1 at different10.9 concentrations.7.8 -0.7 116.9 As shown in Table3, all the de-icing salts reach the maximum amount of de-icing at 60 min, but3.1.2. the De-icing amount ability decreases test under at 90 min.29% concentration This result is dueof de-icing to the factsalt that the concentration of de-icing salt affectsIn accordance the level with of freezing the standard point. A“Snow high concentrationMelting Agent” of de-icing(GB/T 23851-2017), salt implies eight a low different freezing pointde-icing and salts strong other melting than effect potassium of de-icing acetate salt. Onare theselected contrary, to adetermine small concentration the ability of de-icingunder 29% salt concentration for comparing the effects of de-icing salt at different concentrations.

Materials 2019, 12, 912 5 of 12 corresponds to a high freezing point. Therefore, the ideal de-icing effect is not achieved when the temperature is low. At 29% concentration, the sodium formate solution has crystallized after 12 h of freezing in the low-temperature test chamber at −10 ◦C.

Table 3. De-icing ability test (29% concentration).

De-Icing Accumulation/g No De-Icing Salt 30 min 60 min 90 min 1 Sodium chloride 15.75 18.71 17.13 2 Ethylene glycol 10.69 13.62 8.9 3 Urea 7.91 10.75 7.05 4 Calcium magnesium acetate 9.31 12.55 8.73 5 Sodium formate — — — 6 Magnesium chloride 9.07 11.28 4.1 7 Calcium chloride 14.28 17.45 15.51 8 Ji-suo II 10.78 8.39 4.15

3.2. Salt Freezing Corrosion of Concrete under the Action of De-Icing Salt From Table4, the following results are obtained:

(1) Under 3% concentration of de-icing salt, the peeling amount of concrete surface increases with the increase in the number of cycles. Under the action of ﬁve de-icing salts, namely, sodium chloride, ethylene glycol, urea, sodium formate, and calcium chloride, the peeling amount of concrete is high. Among the ﬁve de-icing salts, ethylene glycol, sodium formate, and urea have higher salt freezing corrosion on concrete than sodium chloride. By contrast, magnesium chloride and calcium magnesium acetate have less salt corrosion damage to concrete. (2) Under 10% concentration of de-icing salt, the peeling amount of concrete surface increases with the increase in the number of freeze-thaw cycles. Among the eight de-icing salts, the peeling amount of concrete with sodium chloride, ethylene glycol, urea, and sodium formate is more serious. Ethylene glycol and sodium formate are more corrosive to concrete than sodium chloride. On the contrary, calcium magnesium acetate, calcium chloride, and magnesium chloride have less salt corrosion damage to concrete. (3) Under 20% concentration of de-icing salt, the peeling amount of concrete surface is extremely small. In particular, urea, calcium magnesium acetate, and magnesium chloride have not peeled off the surface of concrete after 30 freeze-thaw cycles.

Table 4. Peeling amount after 30 freeze–thaw cycles under different concentrations of de-icing salt.

Concrete Peeling Amount (kg/m2) No De-Icing Salt 3% 10% 20% 1 Sodium chloride 0.46 0.40 0.04 2 Ethylene glycol 0.52 0.44 0.12 3 Urea 0.57 0.42 0.00 4 Calcium magnesium acetate 0.28 0.19 0.00 5 Sodium formate 0.53 0.35 0.06 6 Magnesium chloride 0.14 0.20 0.00 7 Calcium chloride 0.41 0.23 0.10 8 Ji-suo II 0.49 0.38 0.05

The peeling amount of concrete is compared after 30 freeze-thaw cycles under 3%, 10%, and 20% concentrations of de-icing salt. When the concentration of de-icing salt is 3%, the surface of concrete is highly exfoliated. When the concentration of de-icing salt is 20%, the surface of concrete barely peels off after 30 freeze-thaw cycles. Materials 2019, 12, x FOR PEER REVIEW 6 of 13 Materials 2019, 12, x FOR PEER REVIEW 6 of 13 5 Sodium formate 0.53 0.35 0.06 56 MagnesiumSodium formate chloride 0.530.14 0.350.20 0.060.00 Materials 201967 , 12, 912 MagnesiumCalcium chloride chloride 0.140.41 0.200.23 0.000.10 6 of 12 78 CalciumJi-suo chloride II 0.410.49 0.230.38 0.100.05 8 Ji-suo II 0.49 0.38 0.05 3.3. Effect of De-Icing Salt on Plant Growth 3.3. Effect of De-Icing Salt on Plant Growth 3.3. Effect of De-Icing Salt on Plant Growth AsAs shown shown in Figuresin Figures4 and 4 5and, the 5, plants the plants sprayed sprayed with waterwith water are prosperous are prosperous and show and noshow dryness no ondryness theAs seventh shownon the day inseventh Figures after sprayingday 4 andafter 5, ofspraying the the plants solution. of thesprayed solution. The wi dryth Thewater rate dry of are plantsrate prosperous of sprayedplants andsprayed with show calciumwith no magnesiumdrynesscalcium magnesiumon acetate the seventh is 6%,acetate whichday is after 6%, is the sprayingwhich lowest is theof group thelowe solution. ofst plantsgroup The inof theplants dry experimental rate in theof plantsexperimental group. sprayed This group. with result indicatescalciumThis result that magnesium indicates calcium magnesiumthatacetate calcium is 6%, acetatemagnesium which has is the acetate leastlowe hasst damage group the least toof plantsplants damage amongin tothe plants experimental the nineamong de-icing the group. nine salts. TheThisde-icing dry result rate salts. ofindicates plants The dry sprayedthat rate calcium of with plants magnesium sodium sprayed chloride acetate with is hassodium 22%, the thatleast chloride of damage plants is 22%,to with plants that magnesium amongof plants the chloride, withnine calciumde-icingmagnesium chloride, salts. chloride, The and dry Ji-suo calcium rate II of is chloride, lessplants than sprayedand 22%, Ji-suo and with II that issodium less of plants than chloride 22%, with and urea,is 22%, that ethylene ofthat plants of glycol,plants with sodiumurea,with formate,magnesiumethylene and glycol, potassium chloride, sodium calcium acetate formate, ischloride, more and than potassiumand 22%. Ji-suo In acetateII other is less words, is thanmore these22%, than de-icingand 22%. that In saltsof other plants are words, more with harmfultheseurea, toethylene plantsde-icing than saltsglycol, sodium are sodiummore chloride. harmful formate, to plantsand potassium than sodium acetate chloride. is more than 22%. In other words, these de-icing salts are more harmful to plants than sodium chloride. 5 CMA 5 CMA EG MC 4 EG MC 4 3 U SF CC 3 U SF CC PA JS 2 PA JS

Dry time (days) 2 SC Dry time (days) 1 SC 1 0 0 012345678910 012345678910No. No. Figure 4. Plant tip yellowing days under 3% concentration of de-icing salt. Figure 4. Figure 4.Plant Plant tip tip yellowing yellowing daysdays under 3% concentration concentration of of de-icing de-icing salt. salt.

FigureFigure 5. 5.Dry Dry rate rate of of plants plants atat seventhseventh day unde underr 3% 3% concentration concentration of of de-icing de-icing salt. salt. Figure 5. Dry rate of plants at seventh day under 3% concentration of de-icing salt. AsAs shown shown in Figuresin Figures6 and 6 7and, the 7, plants the plants sprayed sprayed with 10%with sodium10% sodium chloride chloride and potassium and potassium acetate haveacetate theAs earliesthave shown the occurrence inearliest Figures occurrence time6 and of 7, yellowing time the ofplants yellowi phenomenon. sprayedng phenomenon. with Therefore, 10% sodium Therefore, the twochloride the de-icing two and de-icing saltspotassium have salts the fastestacetatehave damagethe have fastest the to earliestdamage plants. occurrence Theto plants. tip of theThetime planttip of yellowiof sprayedthe plantng phenomenon. with sprayed calcium with Therefore, magnesiumcalcium magnesiumthe acetatetwo de-icing isacetate the salts last is to showhavethe thelast the yellowingto fastest show damagethe phenomenon yellowing to plants. phenomenon (i.e., The on tip the of (i.e., fifththe planton day). the sprayed Thisfifth findingday). with This showscalcium finding that magnesium shows calcium that magnesiumacetate calcium is acetate,themagnesium last although to show acetate, hasthe certainalthoughyellowing damage has phenomenon certain to plants, damage (i.e., has toon theplants, the least fifth has damageday). the leastThis to damagefinding plants comparedshowsto plants that compared withcalcium other de-icingmagnesiumwith other salts. de-icing acetate, salts. although has certain damage to plants, has the least damage to plants compared withAfter other 7 daysde-icing of spraying salts. with de-icing salt, all plants are dry. The dry rate represents the damage of the final de-icing salt to the plant. As shown in Figure7, the plants sprayed with 10% concentration of calcium magnesium acetate, magnesium chloride, and calcium chloride have lower dry rate than those sprayed with sodium chloride. Therefore, the three de-icing salts are more harmful to plants than sodium chloride. Ethylene glycol, urea, sodium formate, and potassium acetate are also slightly more harmful to plants than sodium chloride. MaterialsMaterials2019, 201912, 912, 12, x FOR PEER REVIEW 7 of 137 of 12 Materials 2019, 12, x FOR PEER REVIEW 7 of 13

5 CMA 5 CMA MC 4 MC ) 4 ) ays EG U

ays 3 3 EG U 2 SF CC JS 2 SF CC JS

Drytime (d SC PA

Drytime (d 1 1 SC PA 0 0 012345678910 012345678910 No. No. Figure 6. Plant tip yellowing days under 10% concentration of de-icing salt. FigureFigure 6. Plant 6. Plant tip tip yellowing yellowing daysdays under 10% 10% concentration concentration of de-icing of de-icing salt.salt.

50 PA 50 SF PA EG SF 40 EG 40 U U JS SC JS 30 SC CC 30 CC

y Rate Rate y (%) 20 r

y Rate Rate y (%) 20 D r D 10 10 0 0 012345678910 012345678910 No. No. FigureFigure 7. Dry 7. Dry rate rate of of plants plants at at seventh seventh day unde underr 10% 10% concentration concentration of de-icing of de-icing salt. salt. Figure 7. Dry rate of plants at seventh day under 10% concentration of de-icing salt. FigureAfter8 shows 7 days that of whenspraying the with concentration de-icing salt, of de-icingall plants salt are is dry. 10%, The the dry dry rate rate represents of plant is the greater After 7 days of spraying with de-icing salt, all plants are dry. The dry rate represents the thandamage that under of the 3% final concentration. de-icing salt This to the result plant. indicates As shown that in high Figure concentration 7, the plants ofsprayed de-icing with salt 10% is more damage of the final de-icing salt to the plant. As shown in Figure 7, the plants sprayed with 10% harmfulconcentration to plants of than calcium low magnesium concentration acetate, of de-icing magnesium salt. chloride, Moreover, and calcium 3% and chloride 10% concentrations have lower of concentration of calcium magnesium acetate, magnesium chloride, and calcium chloride have lower de-icingdry saltrate showthan those a consistent sprayed trend with ofsodium damage chloride to plants.. Therefore, The effects the three of ethylene de-icing glycol, salts are urea, more sodium dry rate than those sprayed with sodium chloride. Therefore, the three de-icing salts are more harmful to plants than sodium chloride. Ethylene glycol, urea, sodium formate, and potassium formate,harmful and to Ji-suo plants II onthan the sodium dry rate chloride. of plants Ethyle atne seventh glycol, dayurea, are sodium greater formate, than the and effect potassium of sodium acetate are also slightly more harmful to plants than sodium chloride. chloride.acetate The are effects also slightly of calcium more magnesiumharmful to plants acetate, than magnesium sodium chloride. chloride, and calcium chloride on the Figure 8 shows that when the concentration of de-icing salt is 10%, the dry rate of plant is rate are lessFigure than 8 theshows effect that of when sodium the chloride.concentration The dryof de-icing rate of salt plants is 10%, at the the seventh dry rate day of isplant the is lowest greater than that under 3% concentration. This result indicates that high concentration of de-icing undergreater the actions than that of under calcium 3% acetateconcentration. and magnesium. This result indicates Therefore, that thehigh effect concentration of calcium of de-icing magnesium salt is more harmful to plants than low concentration of de-icing salt. Moreover, 3% and 10% saltMaterials is more 2019, 12 harmful, x FOR PEER to REVIEWplants than low concentration of de-icing salt. Moreover, 3% and8 10%of 13 acetateconcentrations on plant growth of de-icing is the salt weakest. show a consistent trend of damage to plants. The effects of ethylene concentrations of de-icing salt show a consistent trend of damage to plants. The effects of ethylene glycol, urea, sodium formate, and Ji-suo II on the dry rate of plants at seventh day are greater than glycol, urea, sodium formate, and Ji-suo II on the dry rate of plants at seventh day are greater than the effect of sodium chloride. 100The effects of calcium magnesium acetate, magnesium chloride, and the effect of sodium chloride. The effects of calcium magnesiumPA acetate, magnesium chloride, and calcium chloride on the rate are less than the effect ofSF sodium chloride. The dry rate of plants at the calcium chloride on the rate are less than the effect of sodium chloride. The dry rate of plants at the seventh day is the lowest under80 the actionsEG of calcium acetate and magnesium. Therefore, the effect seventh day is the lowest under the actions of calcium acetate and magnesium. Therefore, the effect of calcium magnesium acetate on plant growth U is the weakest. of calcium magnesium acetate on60 plant SCgrowth is the weakest. JS CC y Rate (%) y Rate

r 40 D 20

012345678910 No.

FigureFigure 8. Dry 8. Dry rate rate of plantsof plants at at seventh seventh dayday under di differentfferent concentrations concentrations of de-icing of de-icing salt. salt.

4. Comprehensive evaluation of de-icing salt by analytic hierarchy process

4.1. Establishment of a Hierarchical Analysis Structure According to the Chinese standard “Snow Melting Agent” (GB/T 23851-2017), the ability of de-icing is the most important indicator in the reference standard. Damage to plants and salt damage to concrete are the indicators that can be specified depending on needs. In determining the alternative de-icing salt, the de-icing salt with a de-icing ability lower than 90% of that of sodium chloride is removed, and the unknown component of Ji-suo II is removed. The remaining seven de-icing salts are selected and sorted by combining test indicators. Following previous analysis, the de-icing ability of 18% concentration of de-icing salt in 30 min is used as the de-icing ability index. The peeling amount of concrete with 3% concentration of de-icing salt under 30 freeze–thaw cycles is the corrosion index of concrete. The dry rate at seventh day of 3% concentration of de-icing salt sprayed on plants is used as an indicator of the influence on plants. The hierarchical analysis structure is constructed and shown in Figure 9.

Figure 9. Hierarchical analysis structure of preferred choice of de-icing salt.

4.2. Establishment of Judgment Matrixes After the hierarchical analysis model is established, the elements of each layer are compared by two to construct a comparison judgment matrix. The judgment matrix is constructed by the commonly used a 1–9 scale method. The size of the judgment value represents the importance of the corresponding column element compared with the row element. A large judgment value indicates high importance of the column element, as shown in Table 5.

Materials 2019, 12, x FOR PEER REVIEW 8 of 13

100 PA SF 80 EG U 60 SC JS CC y Rate (%) y Rate

r 40 D 20

0 012345678910 No. Materials 2019, 12, 912 8 of 12 Figure 8. Dry rate of plants at seventh day under different concentrations of de-icing salt.

4. Comprehensive4. Comprehensive Evaluation evaluation of of De-Icingde-icing salt Salt by by analytic Analytic hierarchy Hierarchy process Process

4.1.4.1. Establishment Establishment of of a Hierarchicala Hierarchical Analysis Analysis StructureStructure AccordingAccording to to the the Chinese Chinese standard standard “Snow“Snow MeltingMelting Agent” (GB/T 23851-2017), 23851-2017), the the ability ability of of de-icingde-icing is theis the most most important important indicator indicator inin thethe referencereference standard. Damage Damage to to plants plants and and salt salt damage damage to concreteto concrete are theare indicatorsthe indicators that canthat be can speciﬁed be specified depending depending on needs. on Inneeds. determining In determining the alternative the de-icingalternative salt, de-icing the de-icing salt, the salt de-icing with a de-icingsalt with abilitya de-icing lower ability than lower 90% ofthan that 90% of of sodium that of chloride sodium is removed,chloride and is removed, the unknown and componentthe unknown of component Ji-suo II is removed. of Ji-suo TheII is remainingremoved. The seven remaining de-icing saltsseven are selectedde-icing and salts sorted are selected by combining and sorted test by indicators. combining test indicators. FollowingFollowing previous previous analysis, analysis, the the de-icing de-icing ability ability of of 18% 18% concentration concentration ofof de-icingde-icing salt in 30 min is usedis used as the as de-icing the de-icing ability ability index. index. The peeling The peeling amount amount of concrete of concrete with3% with concentration 3% concentration of de-icing of saltde-icing under salt 30 freeze–thaw under 30 freeze–thaw cycles is thecycles corrosion is the co indexrrosion of index concrete. of concrete. The dry The rate dry at rate seventh at seventh day of 3%day concentration of 3% concentration of de-icing of de-icing salt sprayed salt sprayed on plants on isplants used is as used an indicatoras an indicator of the of inﬂuence the influence on plants. on Theplants. hierarchical The hierarchical analysis structureanalysis structure is constructed is constructed and shown and inshown Figure in 9Figure. 9.

Figure 9. Hierarchical analysis structure of preferred choice of de-icing salt.

4.2. Establishment ofFigure Judgment 9. Hierarchical Matrixes analysis structure of preferred choice of de-icing salt.

4.2.After Establishment the hierarchical of Judgment analysis Matrixes model is established, the elements of each layer are compared by two to construct a comparison judgment matrix. The judgment matrix is constructed by the After the hierarchical analysis model is established, the elements of each layer are compared by commonly used a 1–9 scale method. The size of the judgment value represents the importance of the two to construct a comparison judgment matrix. The judgment matrix is constructed by the corresponding column element compared with the row element. A large judgment value indicates commonly used a 1–9 scale method. The size of the judgment value represents the importance of the high importance of the column element, as shown in Table5. corresponding column element compared with the row element. A large judgment value indicates The values of 2, 4, 6, and 8 indicate that the inﬂuence of the i-th factor relative to the j-th factor is high importance of the column element, as shown in Table 5. between the above-mentioned two adjacent levels, aij = 1/aji.

Table 5. Signiﬁcance of scale.

Importance of ai Relative to aj Signiﬁcance The Same Slightly Stronger Stronger Evidently Strong Absolutely Strong

Scale aij 1 3 5 7 9

Criteria layer: When using different de-icing salts, the weight of de-icing ability, the salt freezing corrosion of concrete, and the inﬂuence on plant growth should be considered. The weight of de-icing salt should also be considered to satisfy the three criteria. The priority considered between the three criteria is different in dissimilar situations. When the ability of de-icing is the most important consideration, the inﬂuence on plant is second, and the corrosion of concrete is the least important, the judgment matrix A-B (a) is as shown in Table6a. When the ability of de-icing is as important as the impact on plant, followed by the corrosion of concrete, a different judgment matrix A-B (b) is obtained as shown in Table6b. Materials 2019, 12, 912 9 of 12

Table 6. Judgment Matrix A-B.

(a) A B1 B2 B3 W11 B1 1 3 2 0.539 B2 1/3 1 1/2 0.164 B3 1/2 2 1 0.297 (b) A B1 B2 B3 W12 B1 1 3 1 0.429 B2 1/3 1 1/3 0.142 B3 1 3 1 0.429

λmax = 3.0055, CI = 0.003, RI = 0.58, CR = 0.006 < 0.10, Satisfactory consistency. λmax = 2.9933, CI = −0.003, RI = 0.58, CR = −0.006 < 0.10, Satisfactory consistency.

Scheme layer: In this optimization, sodium chloride is used as the reference de-icing salt. The de-icing amount, peeling amount, and drying rate are 100. The corresponding indexes of other de-icing salts are converted on the basis of the test results, as shown in Table6. The values of the three indicators of the seven de-icing salts are equally divided into nine equal parts, and the importance degree is calibrated. As shown in Table7, a large amount of de-icing indicates an excellent performance; the maximum interval value is 9, and the minimum interval value is 1. Moreover, a small amount of peeling implies an excellent performance; the minimum interval value is marked as 9, and the maximum interval value is marked as 1. Furthermore, a small rate of dryness rate corresponds to an excellent performance; the minimum interval value is marked as 9, and the maximum interval value is marked as 1. After the pair wise comparison, the judgment matrix is constructed (e.g., the C1 calibration value is 6, the C2 calibration value is 9, C12 is 1/4, and C21 is 4). From the analysis, the scheme layer judgment matrices B1-C, B2-C, and B3-C are obtained, as shown in Tables8–10.

Table 7. Relative value calibration.

B1/Relative Value B2/Relative Value B3/Relative Value No De-Icing Salt Calibration Calibration Calibration C1 Sodium chloride 100.0/6 100.0/3 100.0/5 C2 Ethylene glycol 104.8/9 113.3/2 150.0/1 C3 Urea 91.6/1 124.8/1 122.7/3 C4 Calcium magnesium acetate 95.2/3 60.8/7 22.7/9 C5 Sodium formate 94.0/2 116.1/1 163.6/1 C6 Magnesium chloride 96.4/4 31.5/9 50.0/8 C7 Calcium chloride 102.4/8 89.5/4 79.5/6

Table 8. Judgment Matrix B1-C.

B1 C1 C2 C3 C4 C5 C6 C7 W2 C1 1 1/4 6 4 5 3 1/3 0.150 C2 4 1 9 7 8 6 2 0.381 C3 1/6 1/9 1 1/3 1/2 1/4 1/8 0.026 C4 1/4 1/7 3 1 2 1/2 1/6 0.054 C5 1/5 1/8 2 1/2 1 1/3 1/7 0.037 C6 1/3 1/6 4 2 3 1 1/5 0.079 C7 3 1/2 8 6 7 5 1 0.273

λmax = 7.3089, CI = 0.051, RI =1.32, CR = 0.039 < 0.10, Satisfactory consistency. Materials 2019, 12, 912 10 of 12

Table 9. Judgment Matrix B2-C.

B2 C1 C2 C3 C4 C5 C6 C7 W3 C1 1 2 3 1/5 3 1/7 1/2 0.076 C2 1/2 1 2 1/6 2 1/8 1/3 0.050 C3 1/3 1/2 1 1/7 1 1/9 1/4 0.032 C4 5 6 7 1 7 1/3 4 0.258 C5 1/3 1/2 1 1/7 1 1/9 1/4 0.032 C6 7 8 9 3 9 1 6 0.440 C7 2 3 4 1/4 4 1/6 1 0.111

λmax = 7.2715, CI = 0.045, RI =1.32, CR = 0.034 < 0.10, Satisfactory consistency.

Table 10. Judgment Matrix B3-C.

B3 C1 C2 C3 C4 C5 C6 C7 W4 C1 1 5 3 1/5 5 1/4 1/2 0.103 C2 1/5 1 1/3 1/9 1 1/8 1/6 0.027 C3 1/3 3 1 1/7 3 1/6 1/4 0.055 C4 5 9 7 1 9 2 4 0.376 C5 1/5 1 1/3 1/9 1 1/8 1/6 0.027 C6 4 8 6 1/2 8 1 3 0.268 C7 2 6 4 1/4 6 1/3 1 0.144

λmax = 7.3276, CI =0.0546, RI = 1.32, CR = 0.041 < 0.10, Satisfactory consistency.

4.3. Consistency Check and Hierarchical Ordering Constructing a judgment matrix mathematically solves complex problems and simplifies the analysis of the problem. The method of consistency check usually first calculates the maximum eigen value λmax and the weight eigenvector W of the judgment matrix and then determines whether the judgment thinking is consistent using CI = (λmax − n)/(n − 1) [13,14]. When the value is 0, the judgment matrix has complete consistency. The consistency of the judgment matrix can be determined by the average random consistency index RI of the judgment matrix and the judgment matrix of first to ninth order. The values of RI are as shown in Table 11.

Table 11. Average random consistency indicator RI value.

Matrix Order 1 2 3 4 5 6 7 8 9 RI 0 0 0.58 0.9 1.12 1.24 1.32 1.41 1.45

The random consistency ratio is used to measure whether the judgment matrix has satisfactory consistency. This ratio is the ratio of the consistency index of the judgment matrix to the same-order average random consistency index. When CR = (CI)/(RI) < 0.10, the judgment matrix has satisfactory consistency; otherwise, the judgment matrix is adjusted until it has satisfactory consistency.

4.4. Hierarchical Total Ordering In accordance with the judgment matrix of the criterion and scheme layers, the results are combined to obtain a hierarchical integrated ranking, as shown in Table 12. Considering the different weights of the three indicators, different A-B judgment matrices and weight values and hierarchical comprehensive sorting conclusions are obtained. Materials 2019, 12, 912 11 of 12

Table 12. Hierarchical comprehensive sorting.

No. B1(0.539/0.429) B2(0.164/0.142) B3(0.297/0.429) Wtotal1/Wtotal2 C1 0.150 0.076 0.103 0.124/0.119 C2 0.381 0.050 0.027 0.222/0.182 C3 0.026 0.032 0.055 0.036/0.039 C4 0.054 0.258 0.376 0.183/0.221 C5 0.037 0.032 0.027 0.033/0.032 C6 0.079 0.440 0.268 0.194/0.211 C7 0.273 0.111 0.144 0.208/0.195

4.5. Decision Making After the total weight value of each de-icing program for the total target is obtained, it is divided into three levels according to the total weight value of de-icing salt. The evaluation type with a weight value greater than or equal to 0.2 is excellent, that is, grade A. The evaluation type with a weight value greater than or equal to the evaluation type of 0.1 and less than 0.2 is good, that is, grade B. The evaluation type of a weight value less than 0.1 is qualiﬁed, that is, grade C. When the ability of de-icing is the most important consideration, the impact on plants is second, and the corrosion of concrete is the third, the total weight values of each de-icing salt are 0.124, 0.222, 0.036, 0.183, 0.033, 0.194, and 0.208. The de-icing salts of grade A are ethylene glycol and calcium chloride; those of grade B are sodium chloride, calcium magnesium acetate, and magnesium chloride; the rest is grade C with ethylene glycol as the preferable item. When the ability of de-icing and the impact on plants are important concerns, the total weight values of each de-icing salt are 0.119, 0.182, 0.039, 0.221, 0.032, 0.211, and 0.195. The de-icing salts of grade A are calcium magnesium acetate and magnesium chloride; those of grade B are sodium chloride, ethylene glycol, and calcium chloride; the rest is grade C with calcium magnesium acetate as the preferable item.

5. Conclusions (1) At 18% concentration of de-icing salt, the de-icing accumulation of all de-icing salts increases as de-icing time progresses. The maximum value is obtained at 120 min, and then the amount of de-icing decreases. At 29% concentration, the amount of de-icing reaches the maximum at 60 min and decreases at 90 min. (2) When the concentration of de-icing salt is 3%, the surface of concrete is highly exfoliated. When the concentration of de-icing salt is 20%, the surface of concrete is barely peeled off after 30 freeze-thaw cycles. (3) At 10% concentration, the dry rate of plants is greater than 3%. Moreover, 3% and 10% concentrations of de-icing salt show a consistent trend of damage to plants. (4) Seven de-icing salts are tested under de-icing at 18% concentration, salt peeling at 3% concentration, and plant dry rate at 3% concentration. A hierarchical analysis model is established on the basis of the priority of the three criteria, and a comparative judgment matrix is constructed. A de-icing salt that meets the requirements is selected. The preferred de-icing salts differ for dissimilar priorities of the three criteria.

Author Contributions: G.K. and B.T. designed the experiments; G.K. performed the experiments, analyzed the results, and structured the manuscript; J.Z. and B.T. contributed with the discussions of experimental results. All authors contributed in the review of the manuscript. Funding: This research was funded by [National Natural Science Foundation of China] grant number [51308264]. Conﬂicts of Interest: The authors declare no conﬂict of interest. Materials 2019, 12, 912 12 of 12

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