Chemical Admixture

Chemical admixture such as superplasticizers are used to reduce the water to cement ratio and to control the setting time, while maintaining the flow ability of cement pastes.

From: Eco-Efficient Repair and Rehabilitation of Concrete Infrastructures, 2018

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Chemistry of chemical admixtures

G. Gelardi, ... R.J. Flatt, in Science and Technology of Concrete Admixtures, 2016

Abstract

Chemical admixtures are nowadays very important for concrete design. This chapter presents an overview of the chemical structures of different organic chemical admixtures, ranging from small organic compounds to large polymers having a certain polydispersity, and of both natural and synthetic origin. The choice is guided by the fact that this is where the real added value of molecular structure comes into play in terms of design of new or modified chemical admixtures. Such admixtures offer the greatest possibility to chemists to modify properties and target improved performance by specific exploitation of structure/property relationships. The overview gives a basis for better understanding of the working mechanisms of these admixtures.

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Hydro Power

S. Jianxia, in Comprehensive Renewable Energy, 2012

6.14.5.1.3 Chemical admixtures

Chemical admixtures are usually added as liquids or powders in relatively small quantities and may be used to modify the properties during the plastic or hardened state of concrete. Chemical admixtures can be divided into five types: accelerating, retarding, water reducing/plasticizing, air entraining, and waterproofers.

Chemical admixtures should be used judiciously because the addition of wrong quantities can affect the long-term performance of concrete in many ways. For example, the use of calcium chloride as an accelerator can lead to reinforcement corrosion; overdosage of air-entraining admixtures can lead to reductions in strength, which in turn could lead to structural problems; and overdosage of plasticizers may lead to segregation or bleeding.

With the development of the chemical industry, new kinds of admixtures have started to come into use. They include self-curing admixture, shrinkage reducing/compensating admixture, corrosion inhibitors, alkali-silica reactivity inhibitors, and so on.

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Conclusions and outlook on the future of concrete admixtures

R.J. Flatt, in Science and Technology of Concrete Admixtures, 2016

Abstract

Chemical admixtures are an essential component of modern concrete. Although added in small amounts, they profoundly modify important properties of this material, like spices modify food in cooking. This analogy is developed here to better communicate the most significant concepts that govern the performance of these admixtures. To a large extent, the working mechanisms of concrete admixtures involve the modification of interfacial properties, either solid–liquid or liquid–vapor. The implications of this are summarized in this chapter, with a view on understanding both the opportunities and the limitations of using chemical admixtures. It is also emphasized that the growing use of combinations of these admixtures raises new challenges in terms of mastering the additional interactions, in particular competitive adsorption.

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Production and placement of self-compacting concrete

Alireza Kashani, Tuan Ngo, in Self-Compacting Concrete: Materials, Properties and Applications, 2020

3.1.1 Chemical admixtures

Chemical admixtures such as high-range water reducers or superplasticizers, viscosity modifying agents, shrinkage reducing and air-entraining admixtures, accelerators, retardants and mineral admixtures have been used in SCC (Şahmaran et al., 2006; Huang et al., 2018; Yu et al., 2014). Among the chemical admixtures, superplasticizers are essential ingredients for controlling the flowability or rheology of SCC. Polycarboxylate-based (PC) superplasticizers are the most common water reducing agents in SCC. Many different commercial grades of PC with different functionalities and polymer structures have been tailor-designed for SCC. The chemical structure of PC influences the workability retention and rheological performance of SCC (Felekoğlu and Sarikahya, 2008). Superplasticizers can also change the rheology of SCC at varying temperatures (Schmidt et al., 2014). The quantity of chemical admixtures is often very low (< 1 wt% of the total mass of concrete), but the increased performance at such a low concentration is phenomenal. Chemical admixtures are the most expensive ingredients for self-compacting concrete. Therefore, finding the minimum required chemical admixture for achieving specific properties is a common practice for concrete suppliers to produce cost-effective SCC. Small quantities of admixtures are often added to the concrete mixer manually whereas other ingredients such as cement and aggregates are normally transferred to the mixer via a conveyer belt. Chemical admixtures can also be dissolved in water and then pumped to the concrete mixer which can result in a more homogenous distribution of admixtures in concrete and a better performance. However, it is recommended that some admixtures are not to be added simultaneously. For instance, it is beneficial to add an accelerator before the addition of a superplasticizer to enhance strength development. Also, some admixtures are not effective when they are used together. For instance, using a calcium chloride-based accelerator in a concrete mixture, which has a sulphonate-based plasticizer and an air-entraining admixture, can increase autogenous shrinkage (Shanahan et al., 2016). It must be noted that the concrete mixture becomes saturated with admixtures at a certain ratio, meaning that adding more amounts of admixtures will not further enhance the properties of concrete. For instance, the adsorption isotherm of most superplasticizers on cement particles will be equilibrated at a certain concentration. Hence, no further adsorption (and plasticizing effect) will occur by increasing the concentration of superplasticizers (Yoshioka et al., 2002). The plasticising effect of water-reducing admixtures can be affected by the presence of supplementary cementitious materials (SCM) such as slag, resulting in a lower adsorption rate (Alonso et al., 2013). Also, the maximum benefits from the chemical admixtures for SCC are achieved by well-diluting and well-distributing them within the mixture. Sufficient mixing time and the dilution of admixtures within the water before mixing with concrete concrete mixture can help to achieve better performance at lower concentrations, thereby reducing costs associated with the use of expensive admixtures. Moreover, highly agitated mixing after adding the chemical admixture can result in excessive air entrainment, which can adversely affect the surface finish of SCC by the creation of bugholes during placement.

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Impact of chemical admixtures on cement hydration

D. Marchon, R.J. Flatt, in Science and Technology of Concrete Admixtures, 2016

12.2.2 Inhibition of the dissolution of anhydrous phases

Chemical admixtures may act on dissolution of anhydrous phases through adsorption, reducing the rate of ion release into solution. By electric conductivity measurements, Comparet (2004) observed that PCE superplasticizers delay the increase of conductivity coming from the dissolution of the anhydrous C3S and the free growth of C–S–H. He concluded that PCEs strongly slow down C3S dissolution with a probable additional effect on growth of hydrates, even blocking it for the polymers with the highest charge density. This is confirmed by Nicoleau (2004) and Pourchet et al. (2007), who showed that latexes composed of a core copolymer of styrene and butadiene with carboxylate groups on the surface slow down strongly the silicate dissolution after adsorption (Figure 12.4). This conclusion was reached on the basis of measuring the concentration of calcium and silicon by inductively coupled plasma optical emission spectrometry (ICP-OES) measurements during the dissolution of C3S in a very diluted lime-saturated solution containing the latexes.

Figure 12.4. Dissolution of 1.5 mg of C3S in 200 mL of 11 mmol/L lime-saturated solution in the presence of 0.4 g of two carboxylated latexes, POLYAC1 and POLYAC2. These latexes are composed of a core copolymer of styrene and butadiene with carboxylate groups on the surface.

Adapted from Nicoleau (2004) with permission.

It is worth noting that, in both cases, the experiments were performed in lime-saturated and very diluted systems with a high admixture dosage, which are not representative of the real conditions in early-age hydration of cement paste. However, the results show clearly that carboxylate additives can strongly decrease the dissolution rate of C3S as well as the primary C–S–H nucleation, which leads us to the next hypothesis. However, we must first also mention the fact that admixtures may slow down dissolution by hindering the opening of etch pits (Suraneni and Flatt, 2015b). Such a situation would probably shift the main hydration peak backwards, in a way compatible with retardation observed with PCEs. The possible impact of admixtures on the opening of etch pits has also been emphasized recently considering that a large fraction of etch pits may open during the course of hydration and that the frequency of their opening could be reduced by chemical admixtures (Nicoleau and Bertolim, 2015).

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Sulfoaluminate cement-based concrete

Kedsarin Pimraksa, Prinya Chindaprasirt, in Eco-Efficient Repair and Rehabilitation of Concrete Infrastructures, 2018

14.4.5 Role of chemical admixtures

Chemical admixtures are normally used to reduce the limitations of cement hydration, with examples being: water reducer, superplasticizer, retarder, accelerator, shrinkage preventer, segregation reducer, and heat evolution reducer. Interactions between chemical admixtures and cement phases are very important and need to be examined carefully in order to obtain the desired properties and the most durable construction materials. In general, chemical admixtures affect cement particles differently depending on the type of cementitious materials and type of admixture and content. Surface adsorption takes place when organic admixtures meet cement particles due to electrostatic forces between charged particles and ionic groups of admixture molecule (e.g., SO3, COO) (Jolicoeur and Simard, 1998). Organic admixtures, e.g., lignosulfonates contains hydrophobic, polar and ionic groups acting very usefully for changing surface chemistry of cement particles. Due to very fast setting of CSA cement thus hindering the workability, chemical admixtures such as retarder are required in order to modify the rheology of cement, mortar and concrete slurries in maintaining high early strength as desired by the construction design. For example, organic chemical admixtures such as carboxylic retarders can adhere to the precipitated hydration products, thus hampering further growth on their surfaces. Zajac et al. (2016) studied the effect of retarders on the early hydration of CSAs. This paper studied the use of different retarders (sodium gluconate, tartrate, and borax) and investigated the developed hydration products. Used CSA clinker consisted of 24.8 wt% of C4A3Ŝ, 52.4 wt% of C2S, 6.6 wt% of C4AF, 2.1 wt% of C2F, 1.2 wt% of CA, 1.9 wt% of CaCO3, 0.5 wt% of K2SO4, 1.5 wt% of C2KŜ, and 2.2 wt% of anhydrite. Two cement samples were prepared as ground clinker, and ground clinker with 10 wt% of anhydrite. To study the hydration and pore solution, a w/c ratio of 2.0 and 2 wt% of each retarder were used. For microstructural analysis, the w/c ratio was 0.5. Without retarder, both samples showed very fast dissolution rate with a huge release of heat during the induction period. CSA cement with the presence of additional anhydrite showed the highest heat release. However, the cumulative heat releases of samples with the presences of sodium gluconate and borax at early hydration (<10 h) were the largest and smallest, respectively, for ground clinker without additional anhydrite. For ground clinkers with additional anhydrite, and tartrate and sodium gluconate, their cumulative heat releases were the highest and about the same values. The cumulative heat release of samples with the presence of gluconate at 100 hours was the smallest. In 30 minuets, 5 wt% of ettringite (AFt) can be formed in samples without additional anhydrite and retarder and the formations were retarded to form 5.7, 4.6, and 0 wt% and of AFt at 8 hours, with the use of tartrate, gluconate and borax, respectively. Thus, borax was the most powerful retarder, according to the heat of hydration experiment. The amount of AFt found at 168 hours from the sample with the presence of gluconate was smallest. Cement clinker with additional anhydrite contained 4.9 wt% of AFt in 30 minutes. When tartrate, gluconate, and borax were used, AFt formations were 3.8, 3.9, and 0.0wt%, respectively at 8 hours. It was suggested that tartrate and gluconate retarded the dissolution of C4A3Ŝ and precipitation of AFt via the adsorption of negatively charged tartrate and gluconate onto AFt surfaces, thus inhibiting its growth while borax acted as pH reducer to retard the dissolution of C4A3Ŝ phase.

Chemical admixture such as superplasticizers are used to reduce the water to cement ratio and to control the setting time, while maintaining the flow ability of cement pastes. SP molecules can stick to cement particle surfaces that change the cement surface charges in order to repel each other. Ma et al. (2014) studied the compatibility between polycarboxylate (PC) superplasticizer and belite-rich SAC, by investigating the setting time and hydration properties. The used cement consisted of 25.4 wt% of C4A3Ŝ, 56.2 wt% of C2S, 6.6 wt% of C3A, and 11.8 wt% of C4AF, which was made of raw meal containing limestone, FA, and FGD–gypsum and which was sintered at 1320°C. Cement clinker was mixed with 10 wt% of gypsum, with varying amounts of PC from 0.025%–0.25% (42.83% of active phase). The water to solid ratio was 0.26. Heats of hydration during the induction period for PC-added samples was not significantly different, but total heat releases of samples without PC and with 0.25% PC were relatively different in showing the retardation of hydration. With less than 0.075% PC, total heat releases were not significantly different. Due to the negatively charged PC, it could effectively adsorb onto positively charged cement particles, prolonging initial setting times from 18 minutes (without PC) to 38 minutes (with 0.25% PC). The compressive strengths at 28 days curing of samples without PC and with 0.25% PC were 61.4 MPa and 66.8 MPa, respectively. The maximum strength was at 0.075% PC addition, relating to the largest amount of AFt formation. In addition. Furthermore, the most advantageous effect of PC was a modification of particle–particle interaction, changing from macropores to micropores over time. Another work investigating the addition of polycarboxylic superplasticizer (25 wt% of active matter) in the range of 0%–0.4% in high SAC was reported by Garcia-Mate et al. (2012). The used CSA cement consisted of 72.3 wt% of C4A3Ŝ, 14.5 wt% of C2S, 6.8 wt% of calcium titanate, and 2.5 wt% of C4AF, 1.6 wt% of MgO, 1.4 wt% of C2MS2, and 0.9 wt% of calcium sulfate (where M stands for MgO). Gypsum additions were observed at 10, 20, and 30 wt%. Cement pastes were prepared using w/c ratios of 0.4 to 0.5 with the higher content of gypsum. Water to cement ratios of 0.5 and 0.6 were varied, in case of mortar preparation. The rheology of cement pastes showed shear thinning behavior for every amount of gypsum addition (without PC) and became Newtonian fluids when PC was added. At only 0.1 wt%, viscosity was reduced significantly. At 0.4 wt% addition, the viscosity became overdeflocculated showing an increase in the viscosity. The optimum PC content for 10, 20, and 30 wt% of gypsum was 0.2%, 0.15%, and 0.15%, respectively. An increase in gypsum content did not affect the rheology of cement pastes. The ettringite amount was the largest (47.6%) at 7 days in 30% gypsum added sample without PC addition while AFt amounts of 40.6% and 39.4 for samples with w/c ratios of 0.4 and 0.5, respectively at the same content of PC (0.15%) could be obtained. In samples with 10 and 20 wt% gypsum additions, AFt amounts were lower than that with 30% addition at 7 days curing. Without PC, cement paste with 30 wt% of gypsum showed the lowest open porosity, whereas the lowest open porosity could be obtained in paste with 10 wt% of gypsum with the presence of PC. Cement mortars were prepared in order to investigate strength development. At 7 days, mortars with 10 wt% of gypsum and w/c ratio of 0.5 attained the highest compressive strength (50 MPa). The higher w/c ratio degraded the strength in all cases. Significantly, mortar without PC obtained slightly higher strength than that with PC. In addition, compressive strength of CSA mortar was comparable to that of OPC mortar (50.6 MPa) at 7 days curing. The difference in strength development of those two research papers (Garcia-Mate et al., 2012; Ma et al., 2014) was caused by the water to cement ratio: the lower the w/c ratio, the higher compressive strength—although containing a lower content of ye’elimite phase. In addition, the active phase of polycarboxylate superplasticizer significantly played a role in the different hydration properties.

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The development of alkali-activated mixtures for soil stabilisation

P. Sargent, in Handbook of Alkali-Activated Cements, Mortars and Concretes, 2015

21.1 Introduction

Chemical (admixture) stabilisation introduces cementitious materials to soft problematic soils, with a view to improving their engineering properties including strength and durability. Portland cement (CEM-I; BS EN: 197–1) and lime have long been utilised as binders; the former is considered more favourable in providing rapid strength enhancements (Rogers et al., 2000; Hossain, 2010; Jegandan et al., 2010). The presence of soil water and calcium silicates/aluminates within the binders react to form hydration products including calcium silica hydroxide (C-S-H) and calcium aluminate hydroxide (C-A-H) gels.

Negative environmental and financial issues are associated with utilising CEM-I and lime as binders; specifically high energy consumption, financial cost, greenhouse gas and carbon emissions. The continued use of these binders is not sustainable. Hence, there is a need to identify more environmentally and financially sustainable replacement binders. These binders should provide engineering performances that are either comparable or surpass those of CEM-I and lime within similar curing times.

A popular route for selecting new binders has been to recycle industrial byproducts (IBPs), preferably those which are alumino-silicate based (i.e., pozzolanic). The introduction of alternative alkali activators such as sodium hydroxide to these IBPs, can increase the rate at which the mechanical properties of stabilised soils are improved by increasing soil pH, thereby allowing pozzolanic reactions and cementitious bonding to occur (Palomo et al., 1999).

The primary aim of this chapter is to present the most up-to-date literature on the development and use of alkali-activated cements as binders for chemical soil stabilisation. The first few sections address the basic mechanisms of chemical soil stabilisation, traditionally used binders and this technique’s applicability in geotechnics. Findings from recent research on sustainability in terms of environmental and financial costs will then be presented, outlining the need for new sustainable binders. The final sections will then present findings from recent laboratory-based research on the development of new alkali-activated cement mixtures, in addition to possible future trends in this research area.

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Formulation of commercial products

S. Mantellato, ... R.J. Flatt, in Science and Technology of Concrete Admixtures, 2016

15.1 Introduction

Commercial chemical admixtures are classified according to their main action in concrete, as listed in EN 206-1 (ASTM C494, 2013). Most admixtures are usually supplied as low-density aqueous solutions in a range of concentrations by mass between 15% and 40%. Additionally, they can also be present in powder, such as in ready-mix mortars.

Many factors need to be considered when selecting a chemical admixture, the main one being its final application. For instance, in ready-mix concretes, the main requirement is a good slump retention until the concrete is placed. The time span can go from a few to several hours. In some circumstances, such as traffic jams or long distances from the concrete plants to the job sites, the workability should be maintained for a long time. On the other hand, precast concretes require a high water reduction, whereas the slump retention is important only in the first 30 min after mixing. Moreover, a fast hardening process is necessary in precast concretes to increase the number of deforming cycles during the working hours. This aspect is also becoming important in ready-mix applications to speed up construction processes. Readers more interested in the chemical nature of these admixtures may consult the overview proposed by Gelardi et al. (2016).

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Bio-based admixture with substances derived from bacteria for the durability of concrete

Françoise Feugeas, ... Sara Chakri, in Eco-Efficient Repair and Rehabilitation of Concrete Infrastructures, 2018

20.3 Case study: bio-based admixture from bacterial extracellular substances

20.3.1 Bio surfactant as bio admixture

Many admixtures (chemical admixtures and mineral admixtures) are used to improve the rheological properties of fresh concrete and to influence the behavior of hardened concrete (Palacios and Puertas, 2005; Şahmaran et al., 2006; Černý et al., 2006; Izaguirre et al., 2009).

Bioadmixtures attract increasing scientific interest, as a result of the increasing demand for inhibitors displaying substantially improved environmental properties (Zhor and Bremner, 1996; Plank, 2004; Riedel and Nickel, 2000; Orts et al., 2007). Bioadmixtures generally result from industrial biotechnological processes (e.g., welan gum) (Sonebi, 2006). They are extracted from fermentation processes by culturing fungi or bacteria (e.g., extracellular polysaccharide) (Roux et al., 2010), or include polymers with significant biodegradability (e.g., polyaspartic acid) (Amjad, 2011).

The application of bioadmixtures in the cement-based industry is relatively recent. Kahng used the extracellular production from Paenibacillus sphaericus as a viscosity-modifying bioadmixture for cement mortar (Kahng et al., 2001). Khayat and Yahia investigated the effects of combined bioadmixtures of welan gum and napthalene-based water reducer on the rheological properties of cement grouts (Khayat and Yahia, 1997). Lignosulfonate is a plasticizer which improves the workability of concrete (Wang et al., 2012). Patural studied the effect of cellulose ethers as bioadmixture on both water retention and rheological properties of cement-based mortars (Patural et al., 2011).

The present study concerns the development of environmentally friendly products to be incorporated into concrete (admixtures) in accordance with REACH requirements and simultaneously fulfilling two conditions: limiting biocontamination of the surface of concrete and improving the corrosion resistance of the reinforcement. The efficiency against biocontamination of cementitious surfaces has been analyzed previously (Chagnot et al., 2015). Furthermore, standardized tests have permitted the validation of this product as admixture for concrete (He et al., 2014), leading to the elaboration of concretes that are more eco-friendly and more resistant to natural environments than traditional Portland cement concrete (CEM I) (Roux et al., 2013). The work presented below focuses on the improvement of durability of concrete structures by protecting their reinforcement from corrosion.

The active ingredients of the admixture consist of bacterial exo-products. They are eco-friendly and in agreement with the criteria for establishing Environmental and Health Data Sheets (SDS).

Many bacteria are able to produce amphipathic substances, i.e., “biosurfactants,” which are potentially interesting compounds in many fields of study, due to having both hydrophilic and hydrophobic moieties in the same molecule (Shubina et al., 2015). Some of them have antimicrobial activity, such as lipopeptides (surfactin, iturin, etc.) produced by Bacillus and Pseudomonas (Lang, 2002), or amphisin compounds which possess strong antifungal activity (Nielsen et al., 2002). These biomolecules are often controlling plant pathogenic germs (zoospores) and creating ducts in the cytoplasmic membrane of the infectious germ (Raaijmakers et al., 2006). Biosurfactants also have the advantage of being efficient and stable (Bacillus subtilis (Joshi et al., 2008)) in a wide range of environmental conditions (pH, temperature, salinity). Some of them are able to maintain their surface activity at temperatures up to 90°C (Kretschmer et al., 1982) or for salinity up to five times higher than those tolerated by chemical surfactants (Donaldson and Staub, 1981). The emulsifying properties of surfactants synthesized by Serratia marcescens are also preserved at temperatures in the range of 10–120°C and pH values ranging from 2 to 12 (Pruthi and Cameotra, 1997). Moreover, biosurfactants exhibit good biodegradability and lower toxicity (Nitschke and Costa, 2007).

In this study, a lipopeptide biosurfactant (viscosinamide) derived from Pseudomonas fluorescens was used. It was produced on a solid medium, as previously described (Meylheuc et al., 2001). Briefly, plates of King’s B (KB) agar were densely inoculated with two loops of a suspension of Pseudomonas fluorescens 495. The inoculated plates were incubated for 4 days at 22°C. The bacterial lawns were scraped from the agar and the cells were resuspended in sterile, demineralized water and dispersed, using a vortex mixer operated at maximum speed. The supernatant containing the biosurfactant was separated from the cells by centrifuging for 30 minutes. The supernatant was then filtered through a 0.22-μm pore-size filter and stored at 4°C. The surface activity of the biosurfactant and its critical micelle dilution (CMD) were determined by surface tension measurements of the supernatants and dilutions using the Wilhelmy plate method and a tensiometer.

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Hot weather concreting

C. Ishee, in Developments in the Formulation and Reinforcement of Concrete, 2008

Use of chemical admixtures

There are many types of chemical admixtures that can provide better performance characteristics of concrete. Some of these benefits include lower water demand and extended periods of workability. Chemical admixtures can provide these benefits without any reduction in strengths comparable to concrete without chemical admixtures placed at lower temperatures. The effectiveness of the chemical admixtures is dependent upon the reactions with the cement they are to be used with. Any chemical admixture without a history of better performance in hot weather conditions should be evaluated prior to use (ACI 305R-99). Chemical admixtures should be added in accordance with the manufacturer’s technical data sheets. The dosage of most chemical admixtures is mix design specific and should be evaluated prior to use.

Some chemical admixtures are designed to allow for an extended setcontrol of the freshly mixed concrete. Most extended set-control admixtures comply with the requirements of ASTM C494 as a Type B, retarding admixture, or Type D, water-reducing and retarding admixture. These admixtures are often referred to as hydration control admixtures and benefit the concrete in that they can temporarily stop the hydration process of both the silicate and aluminate phases in the Portland cement. These extended set-control admixtures are designed to allow for longer haul times or for additional finishing times when needed.

Water-reducing admixtures are chemical admixtures designed to reduce the water:cement ratio of concrete without adversely affecting the rheological properties. Most of these water-reducing admixtures comply with the requirements of ASTM C494 as Type A, water-reducing, or Type F, high-range water-reducing mixtures. One major benefit of these materials is that they provide up to 15% of the water in a concrete mix design. Typically, water-reducing admixtures do not affect the setting time of the concrete at lower dosages, but at higher dosages can increase the setting time.

Other chemical admixtures can provide high-range, water-reducing and retarding effects on the freshly mixed concrete. Most high-range, water-reducing and retarding admixtures will comply with the requirements of ASTM C494 as a Type G and ASTM C1017 as a Type II for plasticizing and retarding admixtures. These admixtures are often referred to as super-plasticizers and can provide significant benefits for producing flowing concrete in hot weather concrete conditions (ACI 305R-99). Most super-plasticizers are synthetic water-soluble polymers such as sulfonated naphthalene formaldehyde (SNF), sulfonated naphthalene polymer (SNP), modified sugar-free lignosulfonate polymer (MLP), and most recently polycarboxylic ether polymers (PCE). Research has shown that the type of superplasticizer affects the plastic shrinkage strain in the concrete (Al-Amoudi et al. 2006). The interaction between the cement and the superp-lasticizer is crucial because there have been cases where the wrong combination resulted in faster slump loss and additional plastic shrinkage (Ravina and Soroka 2002).

Most of the concrete produced in the United States will have some form of chemical admixture added to the mix design. With hot weather conditions, admixtures are typically used to control the plastic properties of the mix without any long-term strength reductions. It is not uncommon to see multiple chemical admixtures in a mix design designed for hot weather conditions. In Florida, a combination of a high-range, water-reducing and retarding admixtures will be used with a mid-range, water-reducing admixture. The high-range admixture will be kept constant in all of the production loads of concrete and the mid-range will be adjusted for changes in temperature, humidity, and absorption of the aggregates. Because of the amounts of chemical admixtures being used, a majority of the concrete facilities have concrete mix designs for the hot weather summer conditions and a different set of mix designs for winter conditions.

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