Construction and Building Materials 31 (2012) 129–134

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Construction and Building Materials

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Performance of natural rubber latex modified concretein acidic and sulfated environments

Bala Muhammad ⇑, Mohammad IsmailFaculty of Civil Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Johor, Malaysia

a r t i c l e i n f o

Article history:Received 2 June 2011Received in revised form 26 December 2011Accepted 26 December 2011Available online 25 January 2012

Keywords:ConcreteLatex modified concreteCompressive strengthWater absorptionSodium sulfateSulfuric acid

0950-0618/$ - see front matter � 2012 Elsevier Ltd. Adoi:10.1016/j.conbuildmat.2011.12.099

⇑ Corresponding author. Tel.: +60 172687860.E-mail address: engrbmuhammad@yahoo.com (B.

a b s t r a c t

Deterioration of concrete due to chemical aggression is a serious menace to the two major properties ofconcrete; strength and durability. Hence, precautionary measures towards curtailing chemical attack onconcrete could be of great importance. This paper reports experimental findings regarding performanceof natural rubber latex (NRL) modified concrete in acidic as well as sulfated environments. Normal andmodified concretes were developed and subjected to two simulated aggressive curing mediums; 5% sul-furic acid (H2SO4) and 2.5% sodium sulfate (Na2SO4). Latex/water ratio was varied from 0% to 20%. Con-crete phases were studied through SEM. In addition, capacities in moisture ingress, being the maingateway to chemical attack on concrete was also investigated through water absorption test. Results haveshown that inclusion of appropriate quantity of latex into concrete plays a significant role in curbingattack from H2SO4 and Na2SO4. For instance, considering Na2SO4 alone, strength gain in the modified con-crete was 86.2% higher than the corresponding value in normal concrete within a period of 84 days. How-ever, physical observations revealed a high volume change associated with latex in the modifiedspecimens subjected to H2SO4 which suggests attack by acidic agents on hydrocarbon substances.

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1. Introduction

Generally, concrete is expected to effectively serve its intendedpurpose throughout its designed life span. However, its perfor-mance depends much on the immediate surrounding environmentor rather exposure conditions [1–4]. One of the major environmen-tal factors threatening effective performance of concrete is the at-tack from chemical agents such as acids and sulfates [5–8]. Indeed,chemical reaction as a typical mechanism of deterioration in con-crete normally occurs between a reactive substance (i.e. alumi-nates) and an aggressive substance (i.e. sulfate ions) alreadypresent within the concrete or around the environment where con-crete is serving [9]. Typical repercussions include; weakening ofthe binder, leaching, crack formations and eventual dilapidations.

Failure of normal concrete (NC) to perform satisfactorily inaggressive environments during its service life has been attributedto the particulate orientation of its compositional matrix whichnormally allows for the intrusion of both moisture and aggressiveagents. Indeed, concrete matrix consists of voids resulting fromeither incomplete consolidation of fresh mix or from evaporationof mixing water that has not been used for hydration of cement[10]. Morphological observations on hardened cement productsare reported to entertain scattered voids which apparentlytransform the matrix into a honeycomb-like network of hardened

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Muhammad).

regions and pores [11]. Eventually, chemical and of course generalmoisture ingress was observed to be primarily based on the extentof these voids. Indeed, presence of moisture within the serviceenvironment of concrete has been regarded as the main initiatorand promoter of chemical aggression on concrete. According toDhir and Newlands [12], chemical attack on concrete is only possi-ble in the presence of a transporting agent, usually moisture.

Several efforts have so far been made in order to protect con-crete against chemical attack. Ramakrishnan [13] reported thatever since the introduction of cement concrete, attempts have beenmade to address issues to do with its weaknesses including that ofpoor chemical resistance. One such attempt was the modificationof concrete through inclusion of polymeric substances such as elas-tomeric latexes [14]. In fact, one of the most impressive character-istics of this modification is its ability to check moisture ingress,thereby improving its impermeability and consequently savingthe concrete from undue deterioration due to aggressive attack.It is believed that the latex film lining the inherent capillary pores,voids and micro cracks does an excellent job in subsiding fluid flowin modified products [15].

Presently, many effective polymeric latex systems for cementconcrete have been developed and are already in use for variousapplications in the construction industry [16–18]. Neelamegamet al. [19], reported remarkable reduction in water absorption ofNRL-modified mortar when compared with the normal mix. It fact,according to the report, the high impact of the latex made on thetotal water absorbed by the modified mortar was mainly due to

Table 2Chemical analysis of the latex.

Property Value

Total solid content 61.54%Dry rubber content 60.09%Non rubber contents 1.45%Volatile fatty acid 0.018%Alkalinity 0.25%pH 10.07Mechanical stability time 1227 sSpecie Multiple

130 B. Muhammad, M. Ismail / Construction and Building Materials 31 (2012) 129–134

reduced porosity in the modified phase. In another development,inclusion of styrene butadiene rubber (SBR) into light weightaggregate concrete (LWAC) has shown significant reduction inthe water content of the concrete, and its corrosion resistancewas observed to increase as the latex provides blockage to chlorideions penetration, which suggests the use of such SBR–LWAC instructures exposed to marine environments [20]. In fact, latexesand the general polymeric substances have been successfully usedin aggressive environments especially industries due to high dura-bility characteristics exhibited by these substances [21].

The present research therefore seeks to explore performance ofNRL-modified concrete (MC) in both acidic and sulfated environ-ments by assessing optimum quantity of latex for maximum waterexclusion followed by evaluation of initial and residual compres-sive strengths after subjection to chemically simulated environ-ments. In addition, possibilities of surface leaching and mass losswere also observed and concisely discussed. Indeed, morphologicalobservations aimed at identifying physical changes between thenormal and modified phases were also involved. Ability of NRL toguard against chemical attack and moisture ingress on concretecould save concrete from undue deterioration during its service lifeparticularly in acidic soils, factory sewage, agricultural domains,coastal belts and saline clays.

2. Experimental details

2.1. Materials and material mix-proportions

Ordinary Portland cement was used throughout. Chemical composition andphysical properties of the cement are shown in Table 1. Crushed granite stones ofmaximum nominal size 10 mm and naturally occurring river-washed quartz sandwere employed as coarse and fine aggregates respectively. The fine aggregate passesASTM sieve No. 4 (4.75 mm) and it has a fineness modulus of 2.40. Concentrated la-tex treated with 0.2% low ammonia–tetramethythiuram disulfide/zinc oxide (LA-TZ)was used as the modifier. Chemical analysis of the latex is presented in Table 2. Thelatex was supplied by ‘Sime Darby Research Center’, Segamat, Malaysia.

Cement content and w/c ratio are 425 kg/m3 and 0.54 respectively. Latex/waterpercentages; 0–20%, 0–10% and 0–5% were considered for water absorption, acidand sulfate resistances respectively. While the highest range was considered forthe purpose of identifying optimum content for maximum moisture tightness, upto 10% was adopted for acid resistance test as against 5% for sulfate resistance sothat possible acid attack on the latex is fully motivated hence clearly recorded.However, in order to avoid excess water, the mixing water was reduced by 52%of the volume of latex meant for each batch [22].

2.2. Specimen preparations and curing regimes

Batching and mixing were conducted in accordance with BS 1881-125:1986. Inthe process of mixing the concrete however, latex is thoroughly dispersed into themixing water by gentle stirring prior to discharge into the mixer. Mixing wascarried out in a pan mixer conforming to BS 1881-125:1986 and casting was

Table 1Chemical composition and physical properties of thecement.

Composition (%)

ChemicalSilicon dioxide (SiO2) 20.1Aluminium oxide (Al2O3) 4.9Ferric oxide (Fe2O3) 2.4Calcium oxide (CaO) 65Sulphur oxide (SO3) 2.3Magnesium oxide (MgO) 3.1Insoluble residue 1.9Loss on ignition 2Lime saturated factor 0.85

Property Value

PhysicalSurface area (Blair’s) m2/kg 290Initial setting time (min) 105Final setting time (min) 190Soundness (mm) 8.7

conducted in accordance with BS 1881-108:1983. Standard steel cube molds;100 mm were employed for all specimens. However, 75 mm diameter cored speci-mens are used as water absorption samples. In fact, coring was diligently carriedout in order to avoid visible cracks. Fig. 1 presents coring and cored samples forwater absorption assessment.

Two curing mediums containing 5% H2SO4 and 2.5% Na2SO4 each are used asaggressive environments. In order to understand what really happens when con-crete is ‘cast-in-situ’; where concrete is introduced to the service area during its ini-tial age, specimens for H2SO4 were placed into the aggressive curing environmentimmediately after removal from moulds. However, specimens for Na2SO4 as wellas water absorption test were given one month treatment in ordinary water at23 �C and 80 ± 5% RH before subjection to the aggressive medium and coringrespectively. In fact, in the case of specimens for Na2SO4, 72 h air drying at20 ± 3 �C and 80 ± 5% RH was entertained before immersion into the diluted curingmedium.

Although, air-curing in addition to the moist-curing is necessary for latex film todevelop in the modified specimens, these were given similar moist-curing treat-ment with the NC. Uniform curing treatment was considered important so thatMC do not absorb higher content of the simulated aggressive moisture when im-mersed into the curing medium. This could obviously render results from the twocategories of concrete non-comparative.

In the case of morphological observations, samples were cured for 6 months un-der laboratory atmosphere; 20 ± 3 �C and 80 ± 5% RH. Latex-film was obtained bydrying few drops of latex in an oven at 85 �C. The latex-film was also stored undersimilar laboratory atmosphere until day for testing. Conditioning specimen for SEMobservations is necessary in order to expel moisture so that a clear realization ofmicrostructural matrix is achieved.

2.3. Testing program

2.3.1. Scanning electron microscopyJEOL Scanning Electron Microscope JSM 6390 LV was used. Morphologies were

obtained at a current and working distance of 15 kV and 9 mm respectively. Spec-imens were coated with 10 nm platinum in an ‘Auto Fine Coater’ before positioningagainst electron gun. Platinum coating was carried out at 20 mA for about 60 s.

2.3.2. Water absorption testMeasurement of water absorption was conducted in accordance with BS 1881-

122:1983; ‘Method for Determination of Water Absorption’. The cores were kept inan oven for 72 h at 105 ± 5 �C followed by subsequent cooling for 24 h in dry air-tight vessels. At the end of drying and cooling processes as described in the stan-dard specimens were immersed in water for 30 ± 0.5 min at 20 ± 1 �C and thenweighed. Average water absorption of three cored specimens expressed as a per-centage of dry samples is considered as the water absorbed in each particular batch.

2.3.3. Chemical resistanceIn order to determine the level of destructions caused by the two chemically

aggressive environments; H2SO4 and Na2SO4, specimens were removed from thecuring mediums at the end of each observation period followed by compressivestrength test. The entire compressive strength tests were in conformity with BS12390-3:2002. An average compressive strength of three cubes is considered asthe strength value of a particular batch. Indeed, during the process of compressiontests, it was observed that there was no significant aggressive destruction from thechemical agents to cause an abrupt termination in the tests.

3. Results and discussion

3.1. Morphologies

Fig. 2 presents morphologies of normal mix (NM), modifiedmixes (MM-10% and MM-20%) as well as latex-film. NM was ob-served to entertain voids or inter particle spacing including gaps

Fig. 1. (a) Coring and (b) cored samples for water absorption test.

B. Muhammad, M. Ismail / Construction and Building Materials 31 (2012) 129–134 131

at interfacial boundary between cement oxides and aggregate par-ticle as portrayed on the lower right portion of Fig. 2a. Indeed, thecomparatively porous textured features shown in this matrix indi-cates a typical level of porosity associated with normal mixes.

Modified mixes on the other hand, have shown more com-pacted features depending upon the latex contents included inthe mix. For instance, while MM-10% shown in Fig. 2b yielded arelatively denser texture than NM, MM-20% shown in Fig. 2c por-trayed a texture free from inter particle gaps. In fact, except forshallow depressions, modification by 20% latex/water appearedto have completely filled up voids and possibly coated all surfacesof cement oxides as well as aggregates particles.

The latex-film in its entity has demonstrated a fine-grain tex-tured membrane as shown in Fig. 2d. In its liquid state, NRL mainlyconsists of a dispersion of poly 1–4 isoprene particles. However, asthe dispersed medium which is normally water, drains away,

Fig. 2. Morphologies; (a) normal cement-sand matrix, (b) cement-sand

inter-particle spacing diminishes until coalesceness is achieved,thereby resulting into a continuous film. However, when added intoconcrete in small quantities a continuous film may not be achieved,rather a cluster of the isoprene particles may be present in capillarypores and voids. This is manifested in NM-10% where the previouslyobserved larger and deeper voids in NM appeared to become smallerand shallow without a continuous bridging across the cement andaggregate particles. Meanwhile, denser features exhibited by themodified mixes have indicated suitable qualities for raising levelof restriction to fluid flow into and within these phases. Thus, the la-tex performs its action by blocking the access of the moisture whichnormally transports the chemical agents into the concrete matrix.

Indeed, differences in microstructural units of modified mortarsdue to changes in latex content as witnessed in the present workhave been reported. For example, polymer particles in polymermodified concrete were observed to be partitioned between the

with 10% latex, (c) cement-sand with 20% latex and (d) latex-film.

132 B. Muhammad, M. Ismail / Construction and Building Materials 31 (2012) 129–134

inside of hydrates and the surface of anhydrous cement grains [23].In addition, total porosity of pore volume was reported to diminishwith increase in polymer content thereby raising impermeabilityand durability characteristics [14].

3.2. Water absorption

Fig. 3 presents water absorption results. From the results, MC-5% absorbed the least water content, above and below which morewater was absorbed. In fact, the more the latex content above thislimit the higher the water absorbed.

Since NC contains voids as depicted in its structure by the mor-phology in Fig. 2a, it follows that latex content below 5% is insuffi-cient to provide the most effective water rejection qualities. Eventhough, MC-2.5% indicated a marked improvement over NC, thiscontent failed to yield optimum result. Similarly, modifications be-yond 5% depicted poor water exclusion properties perhaps due toexcess latex over that which is sufficient for the most effectivevoids filling. Indeed, excess latex beyond that which is necessaryto fill capillary pores may prevent proper compaction of aggregatesby appearing at interface boundaries. Thus, as the latex coalesces,which is normally accompanied by volume reduction, more voidsmay arise over that witnessed in MC-2.5% and MC-5%.

In fact, another important point regarding the issue of voids for-mation in the modified systems due to withdrawal of water aroundhydrocarbon particles is in relation to the morphologies previouslydiscussed. It was observed that inclusion of latex up to 10% yieldedtraces of inter particle spaces after dry conditioning for 6 months.This suggests that prolonged air curing or heat treatment as inthe water absorption test employed in this work could cause in-crease in void contents mainly due to higher expulsion of moisture.

Meanwhile, NR latex has been reported to cause increase inwater absorption of concrete [14]. But, the quantity of latex usedin the research was 10% latex/cement ratio which represents about20% latex/water ratio. Indeed, the effect of adding 20% latex/waterratio was observed in the present research to cause more than 50%increase in the water absorption of NC. However, according thepresent work a 5% addition causes reduction in the water absorp-tion by about 6.6%.

3.3. Resistance to sulfuric acid

Results of compressive strength after aggressive treatment byH2SO4 are presented in Fig. 4. Generally, strength in both NC andMC are low among all immersion period since specimens wereintroduced into the aggressive medium immediately after demoul-ding as already explained under specimen preparations. While NCentertained gradual increase in strength until 56 days where itdrops and later pickups at 84 days, MC-1.5% continue to experiencereductions until 84 days before it shows signs of strength develop-ment. The other modifications; MC-5% and MC-10% continue todepreciate throughout the 84 days observation period.

0 2 4

MC-0%

MC-2.5%

MC-5%

MC-7.5%

MC-10%

MC-15%

MC-20%

Water A

Lat

ex/w

ater

Con

tent

(%

)

Fig. 3. Effect of latex conten

Even though, concrete is expected to progressively continue todevelop in compressive strength especially during the first onemonth, in this case, only the NC increases by a very small amount.MCs on the other hand, not only failed to increase in strength butsuffer minute reductions. Obviously, lack of appreciable progress instrength particularly in the first one month could be the result ofcounter destructive effect caused by H2SO4. At 28 days old for in-stance, the NC should have reached the designed concrete strength,but the impact of attack coming from the H2SO4 clearly hinderedthis normalcy. Thus, the main consequence of introducing freshconcrete into aggressive environment is the initiation of attackprior to the development of normal strength. In this type of situa-tion, concrete may never achieve its designed strength.

Comparatively, NC develops strength at a rate higher than MC,perhaps due to lack of favorable condition for the formation of la-tex-films. In general, strength development in polymer modifiedconcrete is known to be a combine action of cement hydrationand latex-film formations [13,14]. Therefore, as long as there ismoisture around the latex molecules not only its expected contri-bution will cease to exist but its presence in between the binderparticles may further reduce the binding capacity thereby affectingthe overall strength.

However, where the latex in the modified matrix is relativelysmall, hydration process may easily drain the water around the la-tex molecules since the moisture around the molecules should beproportional to its contents. This might serve as an opportunityfor the latex particles to effectively block the passage of externalmoisture and possibility contribute to the strength also. To this re-spect, MC-1.5% is of the outmost advantage and this could be thekey factor towards its strength increase after 56 days of immersion.Indeed, during the third month, strength development in MC-1.5%was 33.7% as against 15.1% in NC. Meanwhile, positive strengthdevelopment in the other modifications; MC-5% and MC-10%may be registered when the moisture surrounding the larger con-tents of latex in these mixes are drained through cement hydrationor air curing. In fact, the little increase in MC-5% during the finalcuring period could be a testimony to this hypothesis.

Another factor which might contribute to the low performanceof modified phases especially MC-5% and MC-10% could be attackon the hydrocarbon particles by the H2SO4. In fact, at the end ofthe last two curing regimes a layer of latex was found on the sur-face of specimens. Close observations revealed that the peripherallatex especially at the top horizontal surface has been transformedinto jelly like form with substantial increase in volume. This wasbelieved to be caused by the action of the H2SO4. Previous reportsregarding acidic substances coming into contact with polymer sub-stances suggested that harm can occur on the polymeric Si–O–Albonds with consequence strength weakening through ejection oftetrahedral alumina from the aluminosilicate [24,25].

Therefore, while the NC suffers from the usual acidic attack oncement paste only, the MC receives double fold; conversion ofcompounds of the cement to calcium sulfate, and ejection of tetra-hedral alumina from the aluminosilicate framework. Thus, as the

6.1

6.0

5.7

6.9

8.3

9.5

10.4

6 8 10 12

bsorption (%)

t on water absorption.

0

5

10

15

20

25

7 28 56 84

Com

pres

sive

Str

engt

h (N

/mm

2 )

Curing Age (Days)

NC MC-1.5% MC-5% MC-10%

Fig. 4. Effect of sulfuric acid on compressive strength.

B. Muhammad, M. Ismail / Construction and Building Materials 31 (2012) 129–134 133

latex bulges out the aggressive agents percolates to occupy theplaces previously occupied by the hydrocarbon substances. As a re-sult, greater reduction in strength was registered in the modifiedconcrete containing higher contents of latex.

3.4. Resistance to sodium sulfate

Fig. 5 presents compressive strength results of both NC and MCsafter 84 days immersion in Na2SO4. Unlike in the previous assess-ments where initial strengths among all specimens were observedto be low due to immediate transfer of specimens from moulds tothe aggressive curing medium, in this case, initial strengths wererelatively high since specimens were cured for 28 days beforeimmersion into the Na2SO4. During the 84 days, all categories ofspecimens gain strengths at the beginning but suffer strength losstowards the end. However, NC suffers the highest strength loss. Infact, NC with second highest strength value ended up as the leaststrength among all.

Superiority of MC-1.5% over NC at the initial strength marginindicates 1.5% modification as a suitable content for increase instrength. Indeed, about 1.5% latex/water ratio was reported as anoptimum value for maximum compressive strength [26]. Inclusionof latex beyond optimum content resulted in lower strength valuesas witnessed in MC-3% and MC-5%. Meanwhile, strength develop-ment in both NC and MCs was observed to initially retard and asthe aggressive curing advances further, this was followed by agradual reduction in the overall strength.

However, since common initial strength does not exist amongthe four categories of concrete, relative percentage strength gainand loss were therefore considered as the main criterions for judg-ing the individual performance qualities. Fig. 6 presents strengthgain and loss percent in each of the categories. The strength gainand loss (fg and fs) percents were expressed as ratios (f28 � f0)/f0 � 100 and (f28 � f84)/f28 � 100 respectively, where f0, f28 and f84

20

25

30

35

40

45

0 7 28 56 84

Com

pres

sive

Str

engt

h (N

/mm

2 )

Age (Days)

NC MC-1.5%

MC-3% MC-5%

Fig. 5. Effect of sodium sulfate on compressive strength.

are the compressive strength values at 0, 28 and 84 days of immer-sion into the Na2SO4 respectively.

Considering lowest and highest strength gains against NC andMC-3% respectively, it follows that the increase in strength associ-ated with the modified concrete was 86.2% higher than the corre-sponding increase in NC. Since at this stage most of the mixingwater has been utilized through cement hydration, the latex is ex-pected to give a positive impact towards filling the voids and thusblocking the passage of the Na2SO4.

On the other hand, absence of latex particles in the capillariesand voids as in the case of NC might have rendered the matrix vul-nerable to attack through ingress of the surrounding Na2SO4 ions.Indeed, sulfates react with the hydrated aluminates in the cementto give high-sulfate tricalcium sulphoaluminate or ettringite whichnormally occupy more than twice the molecular volume of the alu-minate and its formation in hardened pastes is accompanied byexpansive forces which can exceed the tensile strength of the con-crete [27].

Furthermore, highest strength loss; 18.9% was observed againstNC and this dramatically falls to 2.86% only in the MC-5%. Anotherinteresting observation was that of decrease in strength loss withincrease in latex contents; 6.1–2.9% as indicated in Fig. 6. This de-scribes suitability of using NRL as a precautionary measure to-wards curtailing chemical attack form sulfate contaminatedenvironments particularly Na2SO4. According to Liguang et al.[28], chemical resistance of ordinary concrete could be improvedwhen blended with polymeric substances.

3.5. Surface leaching and mass loss

In the case of H2SO4, physical observations revealed that a neg-ative volume change was entertained in the concrete matrixthrough slight facial leaching. In fact, close inspections on speci-mens particularly at the end of the 84 days curing period disclosesminute leaching of the cement paste, gradual disintegration of fineaggregate particles and exposure of some coarse aggregate parti-cles. This indeed correlates previous report where the action ofacids on Portland cements was observed to cause leaching of cal-cium hydroxide from the cement paste [27].

However, high positive volume change was entertained in thelatex particularly at the top face of the modified specimens. The ex-panded latex contains moisture thus behaved like swelled jelliesand these form layers on the surfaces affected as mentioned previ-ously. Attempt to completely remove the jelly-like latex threatensthe integrity of the surfaces. Eventually, the overall weight wasfound to be balanced or even slightly greater than the originalweight most likely due to the moisture present in the remaininglatex which sticks to the surfaces.

Unlike specimens in the H2SO4 environment where slight sur-face leaching was noticed, samples cured in Na2SO4 medium

5.8

8.5

10.8

9

18.9

6.1 6.0

2.9

0

4

8

12

16

20

NC MC-1.5% MC-3% MC-5%

Com

pres

sive

Str

engt

h (%

)

Concrete Type

Gain (%) Loss (%)

Fig. 6. Effect of sodium sulfate on gain/loss in compressive strength.

134 B. Muhammad, M. Ismail / Construction and Building Materials 31 (2012) 129–134

showed no sign of leaching throughout the 84 days curing period.In fact, at the end of the last batch there was no eroded residue ofcement paste, sand particles or latex at the base of the plastic con-tainer. Hence, variation in weights of specimens at the end of thecuring exercise was observed to be insignificant.

4. Conclusions

Based on the experimental parameters and investigation condi-tions employed in this work, the following conclusions weredrawn.

(1) Inclusion of NRL into concrete transforms fairly porousmicrostructural features of NC into a relatively densermatrix. In fact, SEM observations have shown reductions inthe inter particle gaps with increase in latex contents. Thisis an indication of qualities suitable for raising the level ofrestriction to fluid flow into and within the modified phases.

(2) Presence of NRL in the capillaries and voids of concreteenhances its water exclusion capacities. The improvementattained at 5% latex/water ratio was observed to be about6.6%. However, absence of the heat treatment involved inthe water absorption test might improve the moisture tight-ness, since draining of moisture around the latex moleculesseems to be responsible for additional empty spaces as thelatex forms films.

(3) Regarding H2SO4 environment, initial strength developmentin NC was observed to be higher than in MC, perhaps due tolack of favorable condition for the formation of latex-films.However, during the third curing month the rate was higherparticularly in MC-1.5%; 33.7% as against 15.1% in NC, whichsuggests effectiveness of NRL towards blocking internal con-tact between the aggressive agents and cement paste.

(4) With respect to Na2SO4, highest strength loss; 18.9% wasobserved against NC and this dramatically falls to 2.86% onlyin the MC-5%. On the other hand, considering lowest andhighest strength gains against NC and MC-3% respectively,it follows that increase in strength associated with the mod-ified concrete was 86.2% higher than the correspondingincrease in NC. Thus, absence of latex particles in the capil-laries and voids of NC might have rendered the matrix vul-nerable to attack by the surrounding Na2SO4 ions.

(5) Close inspection on specimens subjected to H2SO4 environ-ment discloses some leaching of the cement paste, minutedisintegration of fine aggregate particles and slight exposureof gravels. Indeed, this observation was noticed in both NCand MC specimens, especially on the batches cured for84 days. However, samples cured in Na2SO4 mediumshowed no sign of leaching or weight loss throughout thecuring period.

Acknowledgements

The authors gratefully acknowledge the financial backingreceived from Bayero University Kano (BUK) Nigeria, Universiti

Teknologi Malaysia (UTM) and Group of Six (G6); Husaini M.D., Sa-nusi B.K., Ali G.T., Mustapha M.K., Umar S.Y. and Hassan H.K.

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