Research ArticleEffects of Chemical Admixtures on the Working and MechanicalProperties of Ordinary Dry-Mixed Mortar
Shu-Chun Zhou,1,2,3 Heng-Lin Lv ,1,2,3 Ning Li,2 and Jie Zhang2
1State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology,Xuzhou 221116, China2School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 221116, China3Jiangsu Collaborative Innovation Center for Building Energy Saving and Construction Technology, Xuzhou 221116, China
Correspondence should be addressed to Heng-Lin Lv; [email protected]
Received 17 July 2018; Accepted 3 January 2019; Published 22 January 2019
*e effects of hydroxypropyl methyl cellulose ether, starch ether, bentonite, and redispersion emulsoid powder on the workingand mechanical properties of fresh dry-mixed mortar were studied. *e results show that hydroxypropyl methyl cellulose etherhas the greatest impact on the consistency and water retention of ordinary dry-mixed mortar and that redispersion emulsoidpowder reduces the water action and starch ether has essentially no effect on water retention. It also shows that the time of mortarcondensation when mixed with hydroxypropyl methyl cellulose ether is the longest, followed by redispersion emulsoid powderand bentonite. Starch ether can slightly, but not obviously, extend the setting time of cement mortar. Hydroxypropyl methylcellulose ether has the greatest impact on the mechanical properties of ordinary dry-mixed mortar, followed by redispersionemulsoid powder, starch ether, and bentonite. As the water retention increases, the setting time of the mortar also increases. *euse of water as a thickening material has a retarding effect on the mortar, increases the water-retention rate, and increases theretarding effect. Moreover, increasing the content of the chemical admixtures decreases the strength of cement mortar.
1. Introduction
*e difference between the use of dry mortar and concrete asa structural material is not only the necessary strength butalso the need for water retention, bonding, water proofing,crack resistance, impact resistance, anti-freeze-thaw, high-temperature endurance, thermal insulation, and other re-quirements. *us, many types of admixtures, such ashydroxypropyl methyl cellulose ether, starch ether, ben-tonite, and redispersible latex powder, are added to drymortar to improve its properties.
*e effect of admixtures on dry mortar is a topic ofinterest for researchers [1–4]. Research results have in-dicated that the water-retention ability of dry mortar wassignificantly improved by the addition of hydroxypropylmethyl cellulose ether [2, 3]. Moreover, the consistency alsoincreased [4, 5]. As starch ether was mixed into the drymortar, the vertical degree decreased [6]. *e bonding
strength of dry mortar was enhanced substantially by thepowder [7, 8]. Some additional admixtures can improve theworking performance and mechanical behavior of drymortar to other degrees [9, 10]. However, most of thecurrent studies have focused on single chemical admixtures;thus, a comprehensive comparative study of the effects ofvarious admixtures on the properties of dry-mixed mortar isneeded. As the effects of ordinary dry mortar admixtures onthe performance of mortar are determined, engineers andtechnicians can add chemical additives according to theprocess requirements.
*is paper studies the influence of four types of ad-mixtures that are hydroxypropyl methyl cellulose ether,starch ether, bentonite, and redispersible latex powder onthe properties of dry-mixed mortar and analyzes their ap-plicability in ordinary dry-mixed mortar and the relation-ship between the performances. It aims to improve the workand mechanical properties of dry-mixed mortar which is
HindawiAdvances in Materials Science and EngineeringVolume 2019, Article ID 5978089, 10 pageshttps://doi.org/10.1155/2019/5978089
used as wall plastering, ground mortar, and special mortar.*e investigation refers to Chinese standard JGJ70-2009,and the main tests performed include the consistency test,water-retention test, setting time test, and cubical com-pressive strength test.
2. Experimental Program
2.1. Materials. Ordinary Portland cement of 42.5 R gradeand class II fly ash were utilized for the experiment, andnatural river sand with a fineness modulus of 2.43 was usedas the fine aggregate. Detailed parameters of the cement,sand, and fly ash are presented in Tables 1–3.
*e admixtures selected for this investigation werehydroxypropyl methyl cellulose ether (60YT10000) producedby Shandong Teng Chemical Co., Ltd., starch ether producedby Longhu Technology (Beijing) Co., Ltd., redispersionemulsoid powder, and bentonite. *e parameters of theadmixtures satisfy the requirements of Chinese standards JG/T l64-2004, GB/T 29594-2013, and GB/T 20973-2007.
2.2. Test Design. In this experiment, different admixtureswere utilized to investigate and analyze the properties ofcommon dry-mixed mortar. *e mix is shown in Table 4.
2.2.1. Consistency Test. In this test, the consistency of thefresh mortar is measured by using a mortar consistencymeter according to the Chinese standard JGJ70-2009, asshown in Figure 1.
2.2.2. Water-Retention Test. *ewater-retention of the freshmortar is measured by using a water-retention test in-strument according to the Chinese standard JGJ70-2009, asshown in Figure 2.
*e test procedure is illustrated as follows. (a) Weigh thebottom impervious sheet and the dry proof mass and 15medium-speed qualitative filter papers. (b) Put the mortarmixture into the test mold after being inserted with a spatulafor several times. As themortar is slightly higher than the edgeof the test mold, the excess surface should be removed at 45°,and the mortar is smoothed at a relatively flat angle in theopposite direction of the surface of the test mold. (c)Wipe outthe mortar on the side of the test mold, and weigh the testmold, the bottom impervious sheet, and themortar. (d) Coverthe surface of the mortar with a metal filter, place 15 pieces offilter paper on the surface of the filter, and then, cover thesurface of the filter paper with an upper impervious sheet, andpress the upper impervious sheet with a weight of 2 kg. (e)After being placed for 2 minutes, the heavy object and theupper impervious sheet should be removed and the filterpaper (excluding the filter) is taken out and weighed quickly.(f) Calculate the water content of the mortar according to themix ratio of the mortar and the amount of water added.
2.2.3. Setting Time Test. *e setting time of the fresh mortaris measured by using a setting time tester according to theChinese standard JGJ70-2009, as shown in Figure 3.
*e test procedure is illustrated as follows. (a) Put theprepared mortar into the container and shock compaction,and then, put it in the test condition of (20± 2)°C. (b) Placethe container on the pressure gauge disc and adjust themeasuring instrument. (c) Test the penetration resistancevalue with a penetration test needle with the cross-sectionarea of 30mm2 to contact the surface of the mortar, andthen, press the needle into the mortar vertically to a depth of25mm within 10 seconds slowly and uniformly. *erefore,record the meter value Np every time as the penetration ismade. (d) Under the test condition of (20± 2)°C, the actualpenetration resistance value should be measured at 2 h aftermolding, and then measured every half hour. As the pen-etration resistance value reaches 0.3MPa, it should bemeasured every 15min until the penetration resistance valuereaches 0.7MPa.
Penetration resistance strength fp can be achievedaccording to the following formula:
fp �Np
Ap, (1)
where fp is the penetration resistance strength with the unitof MPa, Np is the static pressure at penetration depths up to25mm with the unit of N, and Ap is the cross-sectional areaof the test needle with the value of 30mm2.
width× height) are made for the compressive strength test,as shown in Figure 4. *e 1.3 times of the average value ofthree specimens was taken as the compressive strength of themortar (accurate to 0.1Mpa). When the maximum orminimum of the three measured values has a difference fromthe intermediate value exceeding 15% of the intermediatevalue, the maximum value and the minimum value arerounded off together and the intermediate value is taken asthe pressure strength. If the difference between the twomeasured values and the intermediate value both exceed15% of the intermediate value, the test results are invalid.
Compressive strength fm,cu can be achieved according tothe following formula:
fm,cu �Nu
A, (2)
where fm,cu is the compressive strength of the mortar cubespecimen with the unit of MPa, Nu is the ultimate load withthe unit of N, and A is the area of the cube specimen with theunit of mm2.
3. Results and Discussion
3.1. Consistency Test. Different admixtures were mixed intothe fresh dry mortar, and the influence on the consistency ofthe ordinary dry mortar was analyzed. *e test results areshown in Figure 5.
When the water content of the dry mortar was 19% andthe cellulose ether content was 0.03%, the consistency of themortar was very small and the fluidity was extremely poor, as
2 Advances in Materials Science and Engineering
shown in Figure 5(a). *is characteristic is mainly due to thefact that cellulose ether is insoluble in water and only swellswhen a substituent is introduced into the molecular chain todestroy the hydrogen bonds. After the cellulose ether ex-pands and the solution enters, it becomes water soluble andforms a highly viscous slurry suspension. As shown inFigure 5(b), an increase in the redispersible latex powdercontent improves the consistency and flowability of themortar, indicating that the redispersible latex powder has a
water-reducing effect. *is effect is the result of the redis-persible latex powder increasing the gas content of themortar and thus lubricating the fresh mortar. Latex powdercan provide protection from water when the colloid isdispersed and can improve the viscosity of the slurry and thecohesion of the construction mortar, thereby improving theworkability. Starch ether has the most obvious effect on theearly consistency of the mortar, as shown in Figure 5(c).Starch ether improves the consistency increase and fluidity
Table 2: Main properties of sand.
Mud content (%) Clay lump (%) Fineness modulus Apparent density (kg/m3) Loose density (kg/m3) Porosity (%)1.05 0.15 2.43 2630 1500 44Note.Mud content is the particle content of natural sand with nominal particle size less than 0.075mm. Clay lump: the nominal particle size in natural sand isgreater than 1.25mm and the lump is cleaned with water, and the content of particles is smaller than 630 microns after hand pinching.
Table 3: Main properties of class II fly ash.
Performance index National standard Test results ConclusionFineness (45 μm sieve) (%) ≤25 4.2 QualifiedWater content (%) ≤1.0 0.2 QualifiedMobility ratio (%) ≤105 86 QualifiedStability – – Qualified
Table 4: Mix of ordinary dry-mixed mortar (percentage of powder).
3 d 28 d 3 d 28 d6.9 8.6 36.2 47.3 28.4 2.45 3.26 Qualified
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and prolongs the stirring time. Bentonite is the additive thatis added in the largest amount. It is a rock composed ofmontmorillonite, and it is highly water absorbent and highlyexpandable after absorbing water. Bentonite can be dis-persed into a gel-like and suspended state in an aqueousmedium. Such a solution has a high degree of viscosity and a
high adsorption capacity for water.*erefore, the thickeningeffect of bentonite is very good. When the bentonite contentreaches 3%, the consistency of the mortar is very small andthe fluidity is very poor, as shown in Figure 5(d). In sum-mary, the cellulose ether content is the most importantfactor in the consistency of common dry mortars, followed
(a) (b)
Figure 2: Water-retention test instrument.
Figure 3: Setting time tester.
(a) (b)
Figure 4: *e compressive strength test.
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by the starch ether and bentonite content, while redispersionemulsoid powders have a water-reducing eect [10].
3.2. Water-Retention Test. Dierent admixtures were mixedinto fresh dry mortar, and the in�uence on the water-retention rate of the ordinary dry mortar was analyzed.�e test results are shown in Figure 6.
�e standard for the construction mortar performancetest method JGJ/T70-2009 stipulates that the mortar water-retention rate can be calculated according to the followingformula:
W � 1−m4−m2
α ×(m3−m1)[ ] × 100, (3)
where m1 is the weight of the impervious sheet and the dryproof mass,m2 is the weight of 15 medium-speed qualitative�lter papers, m3 is the weight of the test mold, the bottomimpervious sheet, and the mortar, and m4 is the weight ofthe weight of 15 medium-speed qualitative �lter papers inthe wet state, and α is the moisture content of mortar.
As shown in Figure 6(b), many microbubbles are formeddue to the incorporation of cellulose ether into the cementmortar. �ese bubbles act like ball bearings, improving theworkability of the fresh mortar. In hardening the mortar, the
bubbles are retained to form pores that are independent ofone another and act to block the pores to reduce the waterabsorption of the mortar. �erefore, the water retention ofmortar increases with increasing hydroxypropyl methylcellulose ether content. In addition, cellulose ethers havegood water-retention capacity due to intermolecular forces(Van derWaals forces). When the content of cellulose ethersis 0.02%, the water-retention rate reaches 88%; when thecontent is 0.03%, the water-retention rate is 92%. �esewater-retention values exceed the national standard (≥88%).
As the content of redispersible latex powder increases,the water-retention rate of the mortar also increases becausethe added powder has a dispersing eect and results insubstantial air entrainment in the mortar mixture. �ere-fore, its water reduction eect is strong. However, redis-persible latex powder has a limited role in improving thewater retention of mortar and enhancing its cohesiveness.
�e water-retention eect of starch ether was not ob-vious. When the blending content increased from 0% to0.02%, the water-retention rate of the mortar increased andthen decreased with further increases in the blendingamount. When the amount reached 0.06%, the water-retention rate of the mortar essentially did not change.
When the amount of bentonite is greater than 2%, thewater-retention rate of themortar reaches the national standard
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(≥88%). As the bentonite content increases, the water-retentionrate of themortar slowly increases.When the bentonite contentwas 6%, the water-retention rate of the mortar was 91.3%.
In summary, cellulose ether is the most important factorin the water retention of ordinary dry-mixed mortars, fol-lowed by redispersion emulsoid powder. Bentonite plays animportant role in water retention, but starch ethers havevirtually no water-retention eect.
3.3. Setting Time Test. �e setting time periods of the ex-perimental specimens are illustrated in Figures 7 and 8.
Dierent admixtures were mixed into the fresh ordinarydry mortar, and the eect on the setting time was analyzed.�e test results show that when no admixture is incorporated,the setting time of themortar increases slightly with increasingwater consumption. As the admixture was incorporated, thesetting time of mortar increased. Since the addition of cel-lulose ether has a strong eect on the hydration of cement, thecoagulation time is the longest when cellulose ether is
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Figure 6: In�uence of admixture on the mortar water-retention rate. (a) Hydroxypropyl methyl cellulose ethers. (b) Redispersion emulsoidpowder. (c) Starch ethers. (d) Bentonite.
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Setti
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h)
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Figure 7: In�uence of water amount on ordinary dry mortarsetting time.
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incorporated, followed by redispersion emulsoid powder andbentonite. �e incorporation of starch ether slightly, but notobviously, prolongs the setting time of the mortar.
3.4.CompressiveStrength. �e 28-day compressive strengthsof the experimental specimens are shown in Figure 9.
Dierent admixtures were mixed into the fresh drymortar, and the in�uence of the admixtures on the com-pressive strength was analyzed. �e test results show thatthe addition of cellulose ether not only introduces a largenumber of bubbles but also in�uences the hydration of thecement. Cellulose ether has the greatest impact on thecompressive strength of dry-mixed mortar, and the com-pressive strength of dry-mixed mortar containing celluloseether decreases the most in 28 days. �e reason thatredispersion emulsoid powder aects the compressivestrength of mortar is that excessive addition leads to theintroduction of excessive bubbles, resulting in a decreasingtrend in the compressive strength. �e next greatest impacton the compressive strength is due to starch ethers, fol-lowed by bentonite. �is result is closely related to the roleof bentonite in cement mortar. �e volume expansion ofbentonite after water absorption can compensate for thedrying shrinkage problem of cement stone and can reducethe occurrence of shrinkage cracks. �e colloidal particlesformed by clay particles can enhance the adhesion of ce-ment paste, which �lls in the pores and acts as a solid toincrease the strength.
3.5. Relationship between the Water-Retention Rate andCondensation Time. �e relationship between setting timeand the change in water-retention rate is shown in Figure 10.
As the water-retention rate of the mortar increases, thesetting time of the mortar also increases. �e water-retaining thickening material has a retarding eect onthe mortar. �e greater the water-retention rate of themortar, the more obvious the retarding eect.�e period oftime between the mixing of the dry-mixed mortar and theconstruction application is called the storage time. Con-sistency is a parameter that characterizes the constructionperformance of mortar. �e consistency of mortar is re-duced to varying degrees as the storage time increases. Inthe cement particles, the mineral components C3A and C3Sreact with water to form ettringite and C-S-H gels, therebyconsuming a large amount of free water, reducing the�uidity of the cement slurry, and decreasing the consis-tency of the mortar. Simultaneously, the nucleation of thecement hydration product increases, which causes theagglomeration of cement particles, leading to a decrease inthe �uidity of the cement slurry and a decrease in theconsistency of the mortar [10]. A large number of testsshowed that increasing the storage time reduces the con-sistency and �uidity of the mortar and deteriorates themortar compactness, which aects the hardening prop-erties of the mortar. �e addition of water-retaining andthickening materials reduces the amount of free water inthe dry-mixed mortar after the water is added, which re-sults in a slower rate of hydration of the mineral com-ponents C3A and C3S, thus slowing the mortar.
3.6. Relationship between the Water-Retention Rate andCompressive Strength of Concrete. �e relationships betweenthe 28-day compressive strength of the specimens and thewater-retention rate are illustrated in Figure 11.
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Setti
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Without admixture0.03% methyl celluloseether0.06% methyl celluloseether
As shown in Figure 11, the 28-day compressive strengthof the mortar decreases with increasing mortar water re-tention. When the mortar is admixed with additives, theamount of soft polymer in the pores of the mortar increases,and these �exible polymers do not provide rigid supportwhen the composite matrix is pressed, resulting in a decreasein the strength of the mortar. Some additives with methylgroups (such as cellulose ether and redispersion emulsoidpowder) have a certain amount of bleed air. As the additivecontent increases, the bleed air eect gradually increases, thecompactness of the mortar gradually decreases, and thestrength decreases.�e air-entraining eect of the admixturecan improve the workability of the mortar, but it reduces itsstrength. Usually, for every 1% increase in gas content, the28-day strength will decrease by 3% to 5%. However, thestrength of the mortar does not steadily decrease with theaddition of the admixture; there are some �uctuations,mainly because the bubbles caused by admixture in-corporation have a negative eect on the development ofmortar strength. However, these bubbles can improve theworkability of mortar and reduce the water-cement ratio,resulting in an increase in strength. �erefore, as the ad-mixture content increases, the strength of the mortar showsa decreasing trend.
4. Conclusion
Cellulose ether has the most obvious eects on the con-sistency, water-retention rate, and compressive strength ofcommon dry mortar. Redispersion emulsoid powders have awater-reducing eect. Bentonite provides a degree of waterretention, and starch ether has essentially no water-retentioneect. �e addition of admixtures can prolong the settingtime of the mortar, and the longest setting time is observedwhen cellulose ether is added.
�e setting time of the mortar is prolonged as the water-retention rate increases. Water-retaining and thickeningmaterials have a retarding eect on the mortar, and themortar 28-day compressive strength decreases with in-creasing mortar water-retention rate. �e stronger thewater-retention capacity of the mortar is, the lower the 28-day compressive strength.
Data Availability
All data generated or analyzed during this study are includedin this published article.
Conflicts of Interest
�e authors declare that they have no con�icts of interest.
Acknowledgments
�e authors gratefully acknowledge the �nancial supportfrom the Fundamental Research Funds for the CentralUniversities (2017XKZD09). �e experimental work de-scribed in this paper was conducted at the Jiangsu KeyLaboratory of Environmental Impact and Structural Safetyin Civil Engineering in the China University of Mining andTechnology. Sta and students at the laboratory are greatlyacknowledged for their help during the testing.
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Figure 11: Change of the 28-day compressive strength of mortar with the increase of water-retention rate. (a)With cellulose ether added. (b)With starch ethers added. (c) With bentonite added. (d) With redispersion emulsoid powder added.
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