Retarding Admixtures Research Articles

Arriving the optimum retarder dosage level of sugar on the setting time, compressive strength, and microstructure property of Portland pozzolana cement

AbstractCement and concrete are the most crucial and dominating engineering materials in the construction sector. Cement serves as an excellent binder for concrete and when it is treated under unusual conditions either to satisfy accelerating or retarding requirements by the construction industries, admixtures role comes into the effect. The construction industry has been searching for retarding admixtures and the optimal dosage level, particularly for ready mix concrete applications. Generally the retarding admixtures such as organic retarders (lignosulphonates, hydroxycarboxylic acids and their salts, phosphonates, sugars) or inorganic retarders (phosphonates, borates) were generally used to reduce the speed of the reaction between cement and water by altering the growth of the hydration products and/or limiting the rate of water penetration to the cement particles. The research was still in its infancy stage in terms of determining not just the appropriate dosage amount, but also the ideal retarder materials for the aforementioned uses. In considering this industrial need, an attempt was made on utilizing the usage of sugar in the concrete to study the retarding effect without affecting the strength properties of cement. The sugars usage levels were at 5, 10, 15, and 20% of the usage of cement to investigate consistency, setting time, compressive strength and micro structure properties at the curing age of 7, 14 and 28 days on the mortar specimens. The conclusions of the study reported that setting time was extended with increase of the dosage levels of sugar. However in considering the strength property, less than 5% of sugar dosage level can be better for the application as retarding agent in the industry applications.

Assessing the interactions of retarding admixtures and fine materials in long-term flowability of cement pastes

The incorporation of supplementary fine materials into mortars and concretes is a common strategy employed to enhance both their rheological and mechanical properties. While organic molecule-based admixtures are primarily utilized to regulate the setting time in pre-mixed concretes and mortars, their effects become more intricate in the presence of fine materials. This study delves into the interplay between flowability and setting times of cement pastes with 50 wt% of fines (basalt filler, fly ash and quartz filler) in place of cement and three organic admixtures at 0.5 % and 1.0 % (wt% of cement). The results unveil compelling insights: the incorporation of retarding additives proves notably advantageous, markedly decreasing viscosity and prolonging the initial stage of paste formation by as much as 305 h. Nevertheless, the efficacy of these additives diminishes significantly in the presence of fine particles, especially when utilizing basalt filler. This prompts an in-depth exploration into the intricate interactions transpiring between the fine materials and the setting retardants. The implications of these interactions underscore their pivotal role in refining the application of retarding additives in scenarios involving fine materials.

Effect of retardant admixtures type and their using method on the behavior of concrete

Construction sites may be exposed to crisis conditions during the casting process, resulting in delays of several hours and causing destruction of ready-mix concrete. This study suggests an experimental analysis of the possibility of using a specific additional dose of retardant admixtures, which may be used to ready-mix concrete before the initial setting of the concrete occurs. The effect of this additional dose on concrete characteristics in terms of workability, setting time, and compressive strength is also being studied. To conduct this investigation, three types of retardant admixtures from three branded companies were used. In addition, a penetration resistance experiment was conducted on the concrete to determine its setting time. The setting time of concrete was measured at different period intervals depending on when the additional dose of the retardant admixtures was added from the start of the concrete mixing. The results showed that concrete maintained proper workability for a period of more than 5 hours after using the additional dose of retarding admixtures. The additional dose of retarding admixtures not only delayed the concrete setting but also improved the compressive strength of the concrete. This implies that the use of an additional dose of retardant admixtures specifically tailored for ready-mix concrete is an effective option to avoid the return of ready-mixed fresh concrete.

Chemical Composition and Hydraulic Properties of Incinerated Wastepaper Sludge

The hydraulic properties of ash from incinerated wastepaper sludge were investigated. It is shown that the phase composition of wastepaper sludge after heat treatment is similar to the classical fired carbonate-clay mixes. The necessity of using the combination of retarding admixtures with different action mechanism is shown. The strength in the initial hardening period is ensured by the formation of AFm-phases and ettringite, and subsequently by increasing the role of CSH.

Performance evaluation of heavy oil fly ash as a retarder of Portland cement hydration

Retarding admixtures have the ability minimize the adverse effects of concreting under hot weather conditions. Liquid admixtures are generally used for this purpose. However, these have limited shelf-life and therefore there is a need to develop solid retarding admixtures. This paper reports results of a study conducted to assess the performance of solid retarders for retardation of cement. The examined materials included heavy oil fly ash (HOFA) (a byproduct of oil combustion) and chemical retarders (sodium gluconate, sucrose and zinc oxide). The ability of up to 3% HOFA, by weight of cement, to extend the setting time was compared with other chemical retarders. The performance of the selected retarders was evaluated by measuring heat evolution, workability/slump retention, drying shrinkage, and compressive strength. The experimental results indicated that HOFA, sodium gluconate, sucrose and zinc oxide can effectively extend the initial and final setting time of cement. Further, the calorimetric data demonstrated that cement hydration was temporary hindered by the addition of HOFA and the selected chemical retarders. The 28-day compressive strength of concrete with the selected retarders, including HOFA, was similar to that of OPC concrete, and it ranged from 48.4 to 52.6 MPa. However, the rate of early-age drying shrinkage strain increased due to the addition of the selected retarding agents which may necessitate precautionary measures. Finally, it can be concluded that the retarding efficiency of HOFA was generally comparable to that of the selected chemical admixtures.

Impact of aging on the hydration of tricalcium aluminate (C3A)/gypsum blends and the effectiveness of retarding admixtures

Abstract In the production of concrete from cement powder and water, setting behavior of the slurry is determined by the formation of ettringite (Ca6Al2(OH)12 · (SO4)3 · 26 H2O) from tricalcium aluminate (Ca9Al6O18, abbreviated C3A) and gypsum (CaSO4 · 2 H2O). Due to the high reaction potential of cement and water, premature hydration can occur after unintentional exposure to moisture. Model binary mixtures of C3A and gypsum stored at 90% relative humidity and 35 °C produced ample amounts of ettringite, which subsequently reacted with atmospheric CO2 to CaCO3, Al(OH)3 and gypsum. Investigated were the two main polymorphs of tricalcium aluminate encountered in cement, pure, cubic C3A and orthorhombic C3A in which calcium is partially substituted by sodium or potassium. Alkali substituted C3A converted to ettringite faster and more completely than pure C3A. Ettringite from prehydration caused a seeding effect, which promotes crystal growth and accelerates bulk hydration of the C3A/gypsum mixtures. Set retarders commonly applied in cement were dissolved in the mixing water prior to hydration to investigate their ability to counteract this acceleration. Sodium gluconate merely delayed the crystal growth but does not prolong the hydration process overall. Potassium pyrophosphate retarded much more effectively by suppressing the seeding effect via removal of calcium ions from the hydration reaction.

Development of a concrete set retarder utilizing electric arc furnace dust

Retarding admixtures are used to increase the setting time of concrete, especially under hot weather conditions. They provide longer time for concrete to be transported, placed and consolidated. Several commercial retarders are used to extend the setting time of concrete. However, they have a shorter shelf-life as they are commonly supplied in a liquid form. Consequently, there is a need to develop a solid (powder) set retarder that can be efficient in retarding the setting time of concrete and at the same time have a longer shelf-life. This paper presents the results of a study conducted to assess the possibility of developing a set retarder utilizing electric arc furnace dust (EAFD), a by-product of steel industry. The retardation efficiency of up to 6% EAFD, by weight of cement, was evaluated in comparison with other two commercial retarders. The effect of EAFD addition on the heat of hydration, slump retention, compressive strength, and drying shrinkage was evaluated. The experimental data indicated that EAFD is effective in increasing the initial and final setting time. The calorimetry data indicated that the hydration of cement was delayed due to the addition of EAFD. The 28-day compressive strength of concrete was not adversely affected by EAFD or the commercial retarders, and it was in the range of 46.5 MPa to 50.7 MPa. However, the drying shrinkage strain increased by 24 to 37%. It can be concluded that the performance of the solid retarder developed using EAFD was comparable to that of the commercial retarders. The use of EAFD, an industrial byproduct, as a retarder will result in technical, economic and environmental benefits.

Effects of Retarding Admixture, Superplasticizer and Supplementary Cementitious Material on the Rheology and Mechanical Properties of High Strength Calcium Sulfoaluminate Cement Paste

In this paper, the effects of retarding admixture, polycarboxylate (PCE) superplasticizer and supplementary cementitious material (SCM) on the rheology of high strength calcium sulfoaluminate (CSA) cement paste are investigated. At the water to binder ratio of 0.3, one PCE, two retarding admixtures, and three SCMs were used in the test. The test results show that much more PCE is needed for paste with citric acid than that with sodium borate to achieve a similar initial flowability. The reference paste and pastes mixed with sodium borate or fly ash exhibit shear thickening; pastes mixed with citric acid or silica fume exhibit shear thinning. The addition of citric acid increases the maximum shear stress of pastes, while the maximum shear stress can be significantly reduced by silica fume. Both the retarding admixtures and SCMs will strongly reduce the early-age compressive strength of the cement paste. At 28 days of age, CSA cement paste incorporating retarding admixtures has comparable strength to the reference paste. Silica fume has less negative effect on the compressive strength of the CSA cement paste due to the significant filling effect.

Influence of curing on pore properties and strength of alkali activated mortars

The paper investigates the effect of wet/dry, wet and dry curing on the pore properties and strength of an alkali activated cementitious (AACM) mortar. The pore characteristics were determined from the cumulative and differential pore volume curves obtained by mercury intrusion porosimetry. AACM mortars possess a bimodal pore size distribution while the control PC mortar is unimodal. AACM mortars have a lower porosity, higher capillary pore volume, lower gel pore volume and lower critical and threshold pore diameters than the PC mortar which indicate greater durability potential of AACMs. Wet/dry curing is optimum for AACM mortars while wet curing is optimum for the PC mortar. Shrinkage and retarding admixtures improve the strength and pore structure of the AACMs.

Influence of set retarding admixtures on calcium sulfoaluminate cement hydration and property development

Little published data is available to guide specification calcium sulfoaluminate cement (CSA) mixtures with citric acid retarder dosage rates capable of achieving adequate field working times, nor to understand the effect of retarder dosage on hydration and property development, representing a significant barrier to widespread CSA concrete use. Thus, this study investigated the use of citric acid with two commercially-available CSA cements, tracking the effects of dosage on phase development, hydration, setting, and compressive strengths. Key findings were that: citric acid successfully retarded initial set past 120min for both cements despite significantly different chemistries; increasing cement anhydrite content reduced retarder effectiveness and altered hardened binder microstructure, reducing compressive strengths; greater retarder dosages did not negatively affect cumulative hydration, nor strengths; and the time at which the maximum rate of heat evolution occurred correlated with final setting, a relationship useful for predicting field mixture behavior based on laboratory testing.

Adsorption kinetics of retarding admixtures on cement with time controlled addition

Retarders are widely used in construction materials to control the setting and hardening. The present study focuses on changes in the rheological behavior, the admixture adsorption and the pore solution composition with a controlled addition of sodium gluconate and ethylenediamine tetra(methylene phosphonic acid) (EDTMP) in cement.The adsorption of the sodium gluconate was influenced by time of addition of dosage. Delayed addition induced a depot effect which was reduced by the ongoing hydration whereas the adsorption of EDTMP was not affected by time of dosage. Nearly the complete admixture concentration was directly adsorbed on clinker phases or first hydration products.For sodium gluconate, an interaction with ferrite containing clinker phases and a calcium complexing mechanism were suggested. In contrast, EDTMP seemed to have no significant influence on the calcium concentration. With regard to the sodium concentration a fast adsorption of sodium gluconate was determined. Furthermore, a competitive adsorption between dissolved sulfate and retarder molecules or a complex formation mechanism is supposed.

Effect of Gum Arabic Karroo as a water-reducing admixture in cement mortar

The aim of this study was to develop Gum Acacia Karroo (GAK) as set retarding-water reducing admixture in cement mortars. Retarding admixtures are used to counter effect the accelerated hydration of cement at elevated temperatures by slowing down the retarding process especially during the day when concreting work is done. However most retarding admixtures available in the market are expensive, thereby making them out of reach for small consumers of concrete in Africa are expensive and not readily available. GAK, which contains soluble sugars, was investigated as a set-retarding water reducing-admixture. Setting time was measured in cement pastes with different dosages of GAK and a commercial retarding agent (Tard CE). Compressive strength, bleeding and flow test were investigated on cement mortars with the control being cement mortar without admixture. GAK was found to increase final setting time by 6h above control. Compressive strength increased when water cement ratio was reduced from 0.5 to 0.4. Thermogravimetric analysis revealed increased dosage of GAK reduced hydration rate.

Effect of Chloride‐Based Accelerator in the Presence of Water‐Reducing and Retarding Admixture on Autogenous Shrinkage

Rapid repair concrete mixtures commonly used for full‐depth concrete pavement repair sections can use large dosages of accelerating admixtures to increase strength gain rates and decrease the time to traffic opening. Most often, these mixtures also contain water‐reducing and retarding admixtures (WRRAs) to allow for the use of a low water–cementitious material ratio in order to meet strength requirements. The use of large dosages of accelerating admixtures in combination with retarding admixtures could have significant side effects on concrete. Autogenous shrinkage of low water–cementitious concrete can contribute to high tensile stresses and cracking problems. The effect of calcium chloride‐based accelerating admixture dosage, when used with WRRAs, on autogenous shrinkage was measured. It was found that the inclusion of calcium chloride‐containing accelerating admixtures has a nonlinear effect on the pore size distribution and consequently a nonlinear increase on the autogenous shrinkage.

혼화재 및 혼화제의 조절에 의한 서중 콘크리트의 효과적 관리

본 연구는 슬럼프 감소, 연행 공기 부족, 건조수축 균열, 강도감소 등 굳지 않은 콘크리트와 굳은 콘크리트의 성질에 대한 타설시 고온의 악영향에 대한 원인분석을 실시하고 이를 경감시키기 위한 감수제, 공기연행제, 지연제 등의 화학적 혼화제와 플라이 애쉬, 고로 슬래그 등의 무기질 혼화재의 다양한 적용이 검토되었다. 즉, 서중 콘크리트의 품질향상을 위한 다양한 콘크리트 배합이 설계되고 각 배합에서의 굳지 않은 콘크리트 및 굳은 콘크리트 에 대한 영향이 분석되고 몇 가지의 최적 배합이 도출되었다. The undesirable effects of elevated external temperatures at placement on the properties of the fresh and hardened concrete are discussed briefly, and the possible use of the mineral admixtures to mitigate them and the association with water-reducing and retarding admixtures in terms of the mix design which are critical for minimizing slump loss and entrained air loss are examined in this study. To investigate the effects of such the mineral and chemical admixtures on the fresh and hardened properties of concrete exposed to high temperature, a series of concrete mixtures subjected to the high temperature were carried out and then fresh and hardened properties of the mixtures were analyzed and evaluated. Based on the results, new guide lines concerning the appropriate admixtures for hot weather are suggested.

Characteristics of Cementitious Paste for use in Deep Borehole Disposal of Spent Fuel and High Level Wasteforms

ABSTRACTDeep borehole disposal (or DBD) is now seen as a viable alternative to the (comparatively shallow) geologically repository concept for disposal of high level waste and spent nuclear fuel. Based on existing oil and geothermal well technologies, we report details of investigations into cementitious grouts as sealing/support matrices (SSMs) for waste disposal scenarios in the DBD process where temperatures at the waste package surface do not exceed ∼190ºC. Grouts based on Class G oil well cements, partially replaced with silica flour, are being developed, and the use of retarding admixtures is being investigated experimentally. Sodium gluconate appears to provide sufficient retardation and setting characteristics to be considered for this application and also provides an increase in grout fluidity. The quantity of sodium gluconate required in the grout to ensure fluidity for 4 hours at 90, 120 and 140°C is 0.05, 0.25 and 0.25 % by weight of cement respectively. A phosphonate admixture only appears to provide desirable retardation properties at 90°C. The presence of either retarder does not affect the composition of the hardened cement paste over 14 days curing and the phases formed are durable under conditions of high temperature and pressure.

Use of calorimetry and other methods in the studies of water reducers and set retarders interaction with hydrating cement paste

The calorimetric method was used as a main tool in the studies of the effect of superplasticizing and set retarding admixtures on cement hydration. The experiments were carried out using two types of cements (neat Portland cement CEM I 42.5R and blended Portland cement type CEM II/B-M (S-V) 42.5N), at varying w/c ratio 0.5 and 0.3 respectively. The hampering of cement hydration process at early age in the presence of these admixtures was generally observed. This is in practice equivalent to the set retardation. The dissolution, as well as the precipitation of hydration products were retarded too, as it has been proved by the DTA/TG, conductometric measurements and the evaluation of so-called chemical shrinkage. However in case of superplasticizers, after a prolonged “dormant” period the process is continued; the reactions occur at fairly good rate giving substantial heat effects – it can be concluded from the total heat evolved values after standard 41h. At longer time a significant amounts of products are formed as well, but it seems that the crystallization of calcium oxide is disturbed; simultaneously the amount and composition of calcium silicate hydrates alters towards the “more amorphous” product. This could be observed under SEM. The set retarders give strong delaying effect, particularly at low amount of water and in the pastes produced from Portland cement. This can be due to the formation of insoluble layer of calcium and sugar or calcium and phosphorus containing compounds surrounding the hydrating grains; the formation of this “barrier” can be distinguished as a small peak on the calorimetric curve and it is poorly removable on further hydration. The shrinkage and conductivity measurements seem to prove not only the change of kinetics of heat evolution/hydration but also the modification of the process and resulting products.

Development of End-Result Specifications for Concrete in Virginia

A study described the development of the Virginia Department of Transportation's (VDOT) end-result specifications (ERSs) for hydraulic cement concrete. The work had two phases. In Phase 1, a special provision was developed and applied to two pilot projects. In Phase 2, the special provision was applied to more pilot projects from districts across Virginia. Pay adjustments were not applied for the pilot projects; VDOT's current specifications were applied for acceptance and rejection. The ERSs have three parts. The first part introduces process control measures. The contractor is responsible for the concrete design and is required to provide a quality control plan. The plan addresses all elements that affect quality, including mixture designs, aggregate sources, ingredients, tests and testing frequency, fresh and hardened concrete properties, and control charts. The second part covers the mixture design approval by VDOT. The third part covers project acceptance, which includes pay adjustments depending on test results. The pilot projects are continuing. However, some of the features of the ERS are already included in the supplemental specifications, such as using combined cementitious material and using curing boxes with continuously recording thermometers. Or they are permitted if requested by the contractor and approved by the project engineer, such as using combined aggregates, mixing gravel and crushed stone, combining retarding admixtures and water-reducing admixtures, and exceeding the 300 revolution limit or the time limit of 1.5 h from the addition of water.

Impact of Extremely Hot Weather and Mixing Method on Changes in Properties of Ready Mixed Concrete during Delivery

Riyadh is located in central Saudi Arabia. Its extremely hot, dry environment was used to investigate concrete compressive strength, slump, and temperature changes during ready mixed concrete (RMC) delivery. Representing three plants employing a truck-mixing method and three plants employing a central-mixing method in their operations, testing was performed using a total of 80 delivery trucks. Differences in RMC property changes that were not significant were indicated by study results during delivery in both Riyadh's extremely hot, dry summertime environmental conditions and the midwestern United States' much milder summer conditions. The study showed that during summertime delivery, concrete slump lost 37% of its initial value and concrete temperature increased by an average of 1.1 degrees Celsius (2.0 degrees Fahrenheit). There was a slightly higher compressive strength at the site than at the plant. Long travel times did not significantly affect compressive strength. Effective control of adverse hot weather effects on concrete production and delivery, even under the region's extremely hot prevailing weather conditions, was shown possible by controlling concrete temperature, avoiding delivery during noon hours, and using water-reducing and retarding admixtures, per ACI 305R recommendations. In this study, there was no observation of the anticipated increased slump loss rate problem associated with hot weather use of water-reducing and retarding admixtures.

Effect of Temperature on Oscillatory Shear Behavior of Portland Cement Paste Incorporating Chemical Admixtures

The construction of reinforced concrete structures in hot weather dictates fast placement of fresh concrete to prevent hot weather concreting difficulties such as rapid loss of workability, pumping problems, acceleration of cement hydration, fast evaporation of mixing water, and forming of cold joints. In this process, the selection of adequate chemical admixtures is of paramount importance. In the current study, the viscoelastic properties of portland cement pastes with a water–cement ratio (w∕c) of 0.35 and 0.50 were investigated at different temperatures in the range of 20–45°C through oscillatory rheological tests conducted using an advanced shear-stress/shear-strain controlled rheometer. The influence of water reducing and retarding admixtures, melamine-based, polycarboxylate-based, and a new generation of polycarboxylate-based high-range water reducing admixtures on the oscillatory rheological properties of cement paste at various temperatures was also examined. The results indicate that the viscoel...

Rapid-Hardening Concrete Mixes

Results of experimental investigation of the properties of rapid-hardening concrete are presented. Three types of cements that can achieve 14MPa compressive strength and 2.4MPa modulus of rupture after 3h at 22°C and remain workable for 10to15min were selected based on an extensive literature review. The variables in this study were (1) cement type, (2) cement content, (3) water-cement ratio, (4) use of admixtures, and (5) temperature. The measured parameters were (1) strength gain with time, (2) slump loss with time, (3) workability, (4) plastic and drying shrinkage, and (5) relationship between compressive strength and modulus of rupture. Test results showed that for those selected cements it is possible to achieve a workable concrete having 14MPa compressive strength and 2.4MPa modulus of rupture after 3h. The strength gain in flexure was more rapid than the strength gain in compression. The concrete had good flowable characteristics with vibration, and the use of retarding admixtures increased workability time up to 25min. Plastic and drying shrinkage test results showed that rapid-hardening cement is more susceptible to plastic shrinkage than ASTM Type I cement, and care should be taken that exposed surfaces are protected by curing to avoid water loss.