Investigation into the Role of Different Accelerators in Enhancing Concrete Strength Parameters by IRJET Journal

Investigation into the Role of Different Accelerators in Enhancing Concrete Strength Parameters

Akshay Gupta1 , Dr. Ravindra Gautam2 , Suraj Mishra3

1M. Tech Scholar, Dept. of Civil Engineering, Technocrats Institute of Technology – Excellence

2Professor, Dept. of Civil Engineering, Technocrats Institute of Technology – Excellence

3Assistant Professor, Dept. of Civil Engineering, Technocrats Institute of Technology – Excellence

Abstract - Concrete, being the most widely used construction material, often requires modification of its fresh and hardened properties to meet specific construction demands, particularly where rapid setting and early strength development are essential. Among various chemical admixtures, accelerators play a significant role in enhancing the rate of hydration, thereby improving the early-age strength of concrete. The present experimental study was undertaken to investigate the effect of different accelerators on the strength parameters of concrete, with special reference to M20 and M30 grade mixes. In this work, three commonly used accelerating admixtures, namely Calcium Nitrite, Sodium Nitrite, and Potassium Carbonate, were incorporated in concrete mixes, and their influence on compressive strength and flexural strength was examined at different curing ages of 1, 3, 7, 14, and 28 days. The primary objective was to identify the most effective accelerator for enhancing strength characteristics and to compare the performance of accelerators in normal-strength (M20) and medium-strength (M30) concretes. The experimental findings revealed that Calcium Nitrite emerged as the most effective accelerator, yielding maximum percentage gains in both compressive and flexural strength parameters across all curing ages for both grades of concrete. In terms of compressive strength, M20 grade concrete showed a remarkable 30% increase at three days of curing, while M30 grade concrete recorded a maximum gain of more than 20% at seven days when Calcium Nitrite was used. However, at 28 days, the variation in compressive strength between accelerated and non-accelerated mixes was marginal for all three admixtures, indicating that the primary benefit of accelerators lies in early-age strength enhancement. Flexural strength analysis further supported these trends. The highest improvement was observed with Calcium Nitrite, showing a 14% gain at one day in M20 and a similar gain at three days in M30 grade concrete. In contrast, Potassium Carbonate exhibited negligible improvement in flexural strength, while Sodium Nitrite showed moderate effectiveness, particularly in the early curing stages. The comparative performance demonstrated that accelerators had a more pronounced effect on M20 mixes than on M30, especially during the initial curing period. From the overall investigation, it can be concluded that Calcium Nitrite is the most reliable and efficient accelerator for achieving rapid early-age strength in concrete, making it highly suitable for applications where early formwork removal, accelerated construction schedules, or cold-weather concreting is required. The study also highlights that while accelerators significantly influence the initial strength development, their impact at later ages is relatively minor. These findings can serve as a useful reference for the construction industry in selecting appropriate admixtures for practical applications involving accelerated strength gain.

Key Words: Concrete,Accelerators,CalciumNitrite,SodiumNitrite,PotassiumCarbonate,CompressiveStrength,Flexural Strength,Early-ageStrengthDevelopment

1. INTRODUCTION

Cement properties can be modified either by varying the proportions of its fundamental constituents or by partially substituting cementitious or aggregate phases. In recent years, increasing attention has been given to achieving high early strength and reducing construction time, particularly in infrastructure projects. Rapid-setting concrete mixtures are especially beneficial for highway rigid pavement slab replacements, as they enable early opening to traffic. However, such practicesalsoposechallenges,primarilyduetotheelevatedheatofhydration,whichcanincreasethelikelihoodofcracking. Since a minimum strength must be attained before the pavement can be opened to vehicles, the use of accelerating admixtureshasbecomecommontomeetstrengthrequirementswithinlimitedtimeframes.Itisnoteworthythatwhilethere areprescribedminimumstrengththresholdsforsuchapplications,nomaximumstrengthlimitationsexistforpavementslab replacementworks.Acceleratorsachievefasterstrengthgainbyenhancingthehydrationprocess,buttheiruseoftenresults inhigherconcretetemperaturesandgreaterthermalgradients.Suchconditionsarisebecausethesemixestypicallyemploy high cement content, heated mixing water, and sun-warmed aggregates, producing what are often termed as “hot mixes.” These mixes exhibit shorter workability periods and an increased risk of early-age cracking, as the fresh concrete temperature can approach 100°F (≈38°C) even before the accelerator is introduced. The central objective of the present studyistoexaminetheeffectsofdifferentaccelerators PotassiumCarbonate,SodiumNitrite,andCalciumNitrite onthe

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compressiveandflexuralstrengthofconcretemixes.Itisanticipatedthattheinclusionofthese admixtureswillsignificantly improve early-age strength, while theirinfluence on later-agestrength may be less pronounced. The acceleratedhydration process is also expected to increase the internal temperature of concrete, generating higher thermal strains. When the concreteelementisrestrained,thesestrainsmaytransformintostressesthatelevatetheriskofcracking.Previousresearch has also indicated that admixtures such as Calcium Chloride (CaCl₂), though effective in producing high early strength and allowingearlyformwork removal,tendtoincreasevolumetricchangesunderbothmoistcuringanddryingconditions.The use of CaCl₂ accelerates hydration and consequently promotes autogenous shrinkage. Elevated temperatures during hydrationfurthercontributetothermalvolumechanges.Alongsidethese,chemicalshrinkage,plasticshrinkage,anddrying shrinkage are additional causes of volume instability in concrete. Chemical shrinkage occurs due to hydration and selfdesiccation,leadingtoautogenousshrinkage,whiledryingshrinkageisaninevitableprocessthatcanonlybeminimizedor delayedthroughadequateandcontinuouscuring.

2. PREVIOUS RESEARCH

Wan et al. (2024) investigated the influence of calcium nitrite accelerator on the early-age strength development and microstructuralcharacteristicsofcementpasteexposedtofrozencuringconditions.Theirstudyconfirmedthattheaddition ofcalciumnitrite(1–1.5%byweightofcement)notonlyimprovedhydrationkineticsbutalsoenhancedfrostresistanceby densifying the internal structure of the hardened paste. The compressive strength and ultrasonic pulse velocity of accelerated mixes were consistently higher than those of the control samples at 1, 3, 7, and 28 days. The results demonstratedthatcalciumnitritesignificantlyimprovedearlyfrostresistanceandprovidedameasurableincreaseinearly strengthparameters,makingitasuitableadmixtureforconcretescastincoldclimates.

Choi et al. (2023) explored the performance of nitrite/nitrate-based accelerators as admixtures for cold-weather repair materials and rapid-strength concretes. Their experimental program revealed that these accelerators markedly improved veryearlycompressivestrength(withinhoursandupto3days),enablingearlierformworkremovalandrapidconstruction progress.However,theyalsoreporteda trade-off,ashigherdosagesincreasedthe riskofshrinkageand early-agecracking due to higher internal stresses. The study concluded that nitrite/nitrate accelerators are highly effective for strength enhancement at low temperatures, but dosage optimization is critical to balance early strength gain with durability and deformationcontrol.

Wang et al. (2022) presentedacomprehensivereviewontheuseofacceleratorsinconventionalconcrete,focusingontheir mechanisms, applications, and long-term effects. The paper systematically categorized accelerators into nitrates, nitrites, alkali salts, calcium-based compounds, and alkali-free admixtures, analyzing their impact on cement hydration, setting behavior, and strength development. It was highlighted that accelerators are most effective in improving early-age compressive and flexural strength, but their influence on long-term strength depends strongly on the chemical family, dosage, cement type, and curing regime. The reviewconcludedthat calcium nitriteand sodium nitriteare amongthe most reliableadmixturesforrapidstrengthdevelopment,thoughexcessivedosagescanleadtodurabilityconcerns.

Yoneyama et al. (2021) studiedthehydrationmechanismsandmechanicalpropertiesofnitrite/nitrate-basedaccelerators in cementitious systems at low temperatures. Their results showed that these admixtures accelerate the formation of ettringite and nitrite/nitrate-AFm phases during early hydration, leading to a substantial improvement in compressive strength at 1 and 3 days. They also noted that the effect on long-term strength beyond 28 days was relatively small, with someriskofdimensionalinstabilitywhendosages exceededrecommendedlimits.Thestudyprovidedvaluablemechanistic insights linking hydrate formation to improved early-age strength, thereby confirming the suitability of nitrite/nitrate admixturesincold-weatherconcreting.

Tomita (2020) carriedoutanin-depthexperimentalstudyontheeffectsofcalciumnitrateandcalciumnitriteaccelerators on the hydration process and mechanical performance of cementitious materials. The investigation revealed that the accelerators significantly accelerated the hydration of alite, leading to rapid strength gain within the first seven days of curing. Microstructural analysis confirmed the increased formation of ettringite and AFm phases in the accelerated mixes, whichcorrelatedwiththeobservedstrengthimprovement.Thestudyconcludedthatcalcium-basedacceleratorsarehighly effective for early-age strength development, making them particularly suitable for shotcrete and precast applications requiringrapidhardening.

Kim et al. (2019) examinedtherelationshipbetweenhydrateformationandcompressivestrengthwhenusingnitrite-based accelerators in concrete mixes under low-temperature conditions. Their study demonstrated that accelerated hydration promoted the early formation of C-S-H and monosulfate phases, which directly contributed to the higher compressive

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strengthobservedinthefirstsevendays.However,theyalsonotedthatthelong-termstrengthgainwasnotassignificant,as the influence of accelerators diminished after the 28-day curing period. The findings confirmed that nitrite-based accelerators are particularly effective in situations where rapid early strength is required, though their contribution to ultimatestrengthislimited.

Malikyar et al. (2018) investigated the use of calcium-nitrite accelerators in mortars and concrete under both pre-curing andsteam-curingconditions.Theirresultsindicatedthatanoptimumdosageofaround4%significantlyimprovedveryearly compressivestrength,especiallywithin1–3daysofcuring.Steamcuringcombinedwithcalciumnitriteadditionwasfound to be particularly effective, producing higher early-age strength without compromising 28-day strength. The study concludedthatcalciumnitriteacceleratorsarebeneficialforprecastapplicationsandcold-weatherconcretingwhererapid strengthgainisessential.

3. RESEARCH METHODOLOGY FRAMEWORK

Step 1: Material Selection & Characterization

 Cement:OPC43GradeconformingtoIS8112.

 FineAggregate:Naturalriversand(ZoneIII,IS383–1970).

 CoarseAggregate:Crushedstone,20mmnominalsize(IS383–1970).

 Accelerators:PotassiumCarbonate,SodiumNitrite,CalciumNitrite(1.5%byweightofcement).

 Physicalpropertiesofmaterialsrecorded(Tables2&3).

Step 2: Mix Design

 MixproportiondesignedforM30gradeconcreteasperIS10262.

 Controlmix(withoutaccelerator)preparedforbaselinecomparison.

 Testmixespreparedwitheachacceleratoraddedatfixeddosage.

Step 3: Specimen Preparation

 Cubespecimens(150×150×150mm)→forcompressivestrength.

 Beamspecimens(100×100×500mm)→forflexuralstrength.

 Castingcarriedoutusingstandardprocedures.

 Specimensdemouldedafter24hoursandwatercured.

Step 4: Testing Procedure

 CompressiveStrengthTest:ConductedoncubesasperIS516usingCTM.

 FlexuralStrengthTest:Conductedonbeamsusingtwo-pointloadingasperIS516.

 Testsperformedatcuringagesof1,3,7,14,and28days.

Step 5: Data Recording & Analysis

 Strengthvaluesofacceleratedmixesrecordedandcomparedwithcontrolmix.

 Percentagegaininstrengthcalculatedforeachaccelerator.

 Graphicalanalysisconductedtostudyearly-ageandlong-termstrengthtrends.

 Comparativeevaluationofaccelerators(PotassiumCarbonate,SodiumNitrite,CalciumNitrite) intermsofefficiency forM20andM30grades.

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4. RESULTS OUTCOMES OF THE EXPERIMENTAL WORK

Table 1: Gain in compressive strength of M-20 at different days of curing using accelerator I Potassium Carbonate

Table 2: Gain in compressive strength of M-20 at different days of curing using Accelerator II Sodium Nitrite

Table 3: Gain in compressive strength of M-20 at different days of curing using accelerator III Calcium nitrite

Table 4: Percentage increase in compressive strength of M-20 at different days of curing using all three accelerators

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Table 5: Gain in compressive strength of Mix M-30 with accelerator I Potassium Carbonate

Table 6: Gain in compressive strength of M-30 at different days of curing using Accelerator II Sodium Nitrite

Table 7: Gain in compressive strength of M-30 at different days of curing using accelerator III Calcium nitrite

Table 8: Percentage increase in compressive strength of M-30 at different days of curing using all three accelerators

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Table 9: Gain in flexural strength of M-20 at different days of curing using accelerator I Potassium Carbonate

Table 10: Gain in flexural strength of M-20 at different days of curing using Accelerator II Sodium Nitrite

Table 11: Gain in flexural strength of M-20 at different days of curing using accelerator III Calcium nitrite

Table 12: Percentage increase in flexural strength of M-20 at different days of curing using all three accelerators

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Table 13: Gain in flexural strength of M-30 at different days of curing using accelerator I Potassium Carbonate

Table 14: Gain in flexural strength of M-30 at different days of curing using accelerator II Sodium Nitrite Days Mix

Table 15: Gain in flexural strength of M-30 at different days of curing using accelerator III Calcium nitrite Days

Table 16: Percentage increase in flexural strength of M-30 at different days of curing using all three accelerators

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5. CONCLUSIONS AND OUTCOMES OF THE RESEARCH

Theresultsofthisexperimentalstudyclearlyestablishtherelative effectivenessofthethreeacceleratorsonthe strength development of concrete. Among the admixtures investigated, Calcium Nitrite emerged as the most effective accelerator, consistentlyproducingthehighestpercentagegainincompressivestrengthatallstagesofcuring,namelyat1,3,7,14,and 28 days, for both M20 and M30 grade concretes. While substantial gains were recorded at early ages, the difference in compressive strength across the differentaccelerators became marginal by the 28-daycuring period, suggesting that the role of accelerators is particularly significant in enhancing early-age strength rather than influencing long-term performance.Thistrendwasespeciallyevidentwhencomparingthetwoconcretegrades.InM20grademixes,theeffectof acceleratorswasmorepronounced,withcompressivestrengthshowingsharperincreasesat1and3dayswhencompared to M30 grade concrete. The most remarkable improvement in compressive strength for M20 grade was observed with Calcium Nitrite, where the strength gain exceeded 30% at three days of curing, highlighting its efficiency in promoting rapid strength development. In contrast, M30 grade concrete displayed its maximum compressive strength gain of over 20% at seven days of curing with the same accelerator, indicating a comparatively slower, yet still significant, strength response.Thepatternofresultsforflexuralstrengthcloselymirroredthoseofcompressivestrength.Hereagain,Calcium Nitriteprovedsuperior totheother accelerators,consistentlydeliveringthehighestimprovements across all curing ages for both M20 and M30 concretes. On the other hand, Potassium Carbonate showed only negligible improvements in flexural strength throughout the study period, thereby underlining its limited effectiveness in enhancing the mechanical performanceofconcrete.Incontrast,SodiumNitritedemonstratedamoderatebutnoticeableimprovement,particularlyat earlyages,thoughitwasstilllesseffectivethanCalciumNitrite.ThecomparativeanalysisshowedthatbothSodiumNitrite andCalciumNitriteenhancedflexuralstrengthsignificantlyattheearlycuringstagesof1,3,and7daysforbothgradesof concrete. In terms of specific values, the most substantial flexural strength gain for M20 grade concrete was more than 14%atonedayofcuringwithCalciumNitrite.Similarly,forM30gradeconcrete,themaximumflexuralstrengthgainalso exceeded 14%, recorded at three days of curing with the same accelerator. Overall, the findings emphasize that acceleratorsplayacrucialroleinimprovingtheearly-agestrengthpropertiesofconcrete,whichisparticularlybeneficial in construction scenarios where rapid setting and early load application are required. While all three accelerators contributed to some degree of strength enhancement, the comparative results leave no doubt that Calcium Nitrite offers thegreatestpotentialforpracticalapplications,especiallyincaseswhereearlystrengthisofprimaryconcern. Potassium Carbonate, by contrast, was found to be the least effective, while Sodium Nitrite provided intermediate results. These outcomes underline the importance of selecting the appropriate accelerator not only based on the required grade of concretebutalsoonthedesiredbalancebetweenearly-ageandlong-termstrengthperformance.

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