Experimental determination of the thermodynamic parameters affecting the adsorption behaviour and dispersion effectiveness of PCE superplasticizers

For adsorption of three different allylether-based PCE superplasticizers on CaCO₃ surface, the thermodynamic parameters ΔH, ΔS and ΔG were determined experimentally. The GIBBS standard free energy of adsorption ΔG_0ads, the standard enthalpy of adsorption ΔH_0ads and the standard entropy of adsorption ΔS_0ads applying to an unoccupied CaCO₃ surface were obtained via a linear regression of ln K (equilibrium constant) versus 1 / T (VAN'T HOFF plot). Additionally, the thermodynamic parameters characteristic for a CaCO₃ surface loaded already with polymer (isosteric conditions) were determined using a modified CLAUSIUS-CLAPEYRON equation.For all PCE molecules, negative ΔG values were found, indicating that adsorption of these polymers is energetically favourable and a spontaneous process. Adsorption of PCEs possessing short side chains is mainly instigated by electrostatic attraction and a release of enthalpy. Contrary to this, adsorption of PCEs with long side chains occurs because of a huge gain in entropy. The gain in entropy results from the release of counter ions attached to the carboxylate groups of the polymer backbone and of water molecules and ions adsorbed on the CaCO₃ surface. With increased surface loading, however, ΔG_isosteric decreases and adsorption ceases when ΔG becomes 0. The presence of Ca²⁺ ions in the pore solution strongly impacts PCE adsorption, due to complexation of carboxylate groups and a reduced anionic charge amount of the molecule. In the presence of Ca²⁺, adsorption of allylether-based PCEs is almost exclusively driven by a gain in entropy. Consequently, PCEs should produce a strong entropic effect upon adsorption to be effective cement dispersants. Molecular architecture, anionic charge density and molecular weight as well as the type of anchor groups present in a superplasticizer determine whether enthalpy or entropy is the dominant force for superplasticizer adsorption.     

Multi-method approach to study influence of superplasticizers on cement suspensions

Superplasticizers are widely used in concrete processing to increase the rheological properties of hardening pastes. In this study, different techniques (rheology, adsorption, atomic force microscopy—AFM, and ζ-potential) are used to characterize the impact of polycarboxylate-ether based superplasticizer (PCE) on particle suspensions. Results obtained with two cements and two inert powders (MgO and calcite) show that superplasticizer efficiency is strongly influenced by polymer architecture and by the ionic species present in solution. Additionally, experiments performed with AFM and ζ-potential contributed to characterize dispersion forces exerted by superplasticizers at the solid–liquid interface. The application of plateau AFM-tips coated with platinum reveals that dispersion forces depends on the presence of ions in solution, and that multilayer formation occurs with certain superplasticizer types. A further conclusion includes the idea that the PCE has a lubricating effect between adjacent particles and PCE increases surface wettability.     

Basics of analytical methods used for the investigation of interaction mechanism between cements and superplasticizers

In order to better understand the interactions between cements and superplasticizers (high-range water reducers), various analytical methods have traditionally been applied, but some of these methods can introduce significant errors. In this paper the fundamental interaction mechanisms are reviewed and appropriate methods and sample preparation conditions are explained in detail. For superplasticizer adsorption measurements, analysis by size exclusion chromatography is useful in order to avoid the effect of other organic compounds released from cement by the action of the superplasticizer. For BET specific surface area measurements it is important to avoid pre-drying at over 60 °C, as otherwise ettringite can decompose and strongly increase the surface area. In order to evaluate pore solution chemistry correctly the aqueous phase must be extracted without dilution. In addition, the fluidity performance of many cements in superplasticized concretes can be changed simply by passage through a storage silo, for reasons which are not yet understood.     

Experimental determination of the effective anionic charge density of polycarboxylate superplasticizers in cement pore solution

The specific anionic charge density of polycarboxylate superplasticizers can be determined experimentally by titration with a cationic polyelectrolyte. In this study, the anionic charge densities of several polycarboxylates based on methacrylate ester chemistry were measured in aqueous solution at pH 7 and 12.6, resp., and in cement pore solution. The anionic charge of the polycarboxylates increases with increasing pH value as a result of deprotonation of the carboxylate groups in the polymer backbone. Addition of Ca²⁺ ions generally causes a decrease of the anionic charge density. The reduction in anionic charge varies and depends on the architecture of the polycarboxylate. The effect results from the binding of calcium ions by the carboxylate groups, both through complexation and counter-ion condensation. Consequently, the effective anionic charge density of polycarboxylates in cement pore solution can differ significantly from the charge density which is calculated based on the chemical composition. Generally the -COO⁻ functionality may coordinate Ca²⁺ as a monodentate or bidentate ligand. The type of coordination depends on the steric accessibility of the carboxyl group. In PC molecules possessing high side chain density, the -COO⁻ group is shielded by the side chains and coordinates as bidentate ligand, producing a neutral Ca²⁺-PC complex. Accordingly, this type of PC shows almost no anionic charge anymore in cement pore solution. In PCs possessing high amount of -COO⁻, Ca²⁺ is coordinated monodentate, resulting in an anionic complex. Consequently, this type of PC shows significant anionic character in pore solution. Its adsorption behaviour is determined by a gain in enthalpy which derives from the electrostatic attraction between the PC and the surface of cement. This way, by utilizing the relatively simple method of charge titration, it is possible to assess the electrostatic attraction which, besides entropy gains, is the driving force behind the adsorption of polycarboxylates on the cement surface and thus determines their effectiveness as dispersing agent. The findings are generally applicable to other anionic admixtures used in cement.     

聚醚型聚羧酸减水剂的合成和表征及对水泥的微观作用机制

本文以聚醚、丙烯酸、丙烯酸羟乙酯为原料,通过自由基聚合法合成了聚羧酸减水剂JS-PCE和BT-PCE。通过正交优化所得的JS-PCE合成条件为:聚合温度25 ℃,抗坏血酸-巯基丙酸混液、丙烯酸滴加时间分别为3 h、2.5 h,酸醚物质的量比为4.25∶1,引发剂加量为聚醚质量的1.10%。添加JS-PCE的水泥净浆流动度达230 mm,表现出较好的流动性。采用界面化学及电化学等方法探究了减水剂对水泥的微观作用机制,结果表明聚羧酸减水剂在水泥颗粒表面存在饱和吸附量,其与水泥颗粒存在强吸附作用,因此产生分散和减水作用。流变行为分析得出JS-PCE和BT-PCE的最佳折固掺量分别为0.3%、0.4%。     

Adsorption of superplasticizer admixtures on alkali-activated slag pastes

Alkali-activated slag (AAS) binders are obtained by a manufacturing process less energy-intensive than ordinary Portland cement (OPC) and involves lower greenhouse gasses emission. These alkaline cements allow the production of high mechanical strength and durable concretes. In the present work, the adsorption of different superplasticizer admixtures (naphthalene-based, melamine-based and a vinyl copolymer) on the slag particles in AAS pastes using alkaline solutions with different pH values have been studied in detail. The effect of the superplasticizers on the yield stress and plastic viscosity of the AAS and OPC pastes have been also evaluated.The results obtained allowed us to conclude that the adsorption of the superplasticizers on AAS pastes is independent of the pH of the alkaline solutions used and lower than on OPC pastes. However, the effect of the admixtures on the rheological parameters depends directly on the type and dosage of the superplasticizer as well as of the binder used and, in the case of the AAS, on the pH of the alkaline activator solution. In 11.7-pH NaOH-AAS pastes the dosages of the superplasticizers required to attain similar reduction in the yield stress are ten-fold lower than for Portland cement. In this case the superplasticizers studied show a fluidizing effect considerably higher in 11.7-pH NaOH-AAS pastes than in OPC pastes. In 13.6-pH NaOH-AAS pastes, the only admixture observed to affect the rheological parameters is the naphthalene-based admixture due to its higher chemical stability in such extremely alkaline media.     

The effect of superplasticizers and air-entraining agents on the Zeta potential of cement particles

The effect of superplasticizers and air-entraining agents on the Zeta Potential (ZP) of a Portland cement was studied by microelectrophoresis. The purpose of the experiments was to investigate the adsorption mechanism of electronegative polymers and surface-active agents, and the interaction between these when added in combination. As a preliminary experiment the microelectrophoresis technique was investigated in order to obtain the most precise and reproducible method of measuring Zeta Potentials. The results show that the superplasticizer type with the longer polymer chain and thereby the largest amount of electronegative charges per chain gives the highest negative Zeta Potential. This is interpreted as indicating that this type of superplasticizer has a higher dispersing capability. Data from dosage of air-entraining agents show that a part of the air-entraining effect is due to the fact that the molecules are adsorbed on the cement surface, thereby making it hydrophobic. The results of combined use of superplasticizers and air-entraining agents show that only a slightly smaller ZP is obtained than when a superplasticizer is dosed alone, which however still indicates an interaction. Finally the ZP has been determined of cement alone.     

Impact of zeta potential of early cement hydration phases on superplasticizer adsorption

The zeta potential of early hydration products of cement was found to be a key factor for superplasticizer adsorption. A highly positive zeta potential results in a strong superplasticizer adsorption whereas a negative zeta potential does not allow adsorption. Synthetic ettringite precipitated from solution shows a highly positive zeta potential, hence it adsorbs great amounts of negatively charged superplasticizer. Monosulfate (AF_m) has a less positive zeta potential. Therefore, it adsorbs smaller amounts of superplasticizers. For syngenite, portlandite and gypsum, the zeta potential is around zero or negative. These phases do not adsorb superplasticizers. Consequently, a hydrating cement grain is best represented by a mosaic structure, with superplasticizer molecules mainly adsorbed on ettringite and some on monosulfate and C-S-H nucleated at surface.     

Adsorption characteristics of superplasticizers on cement component minerals

Adsorption characteristics of various superplasticizers on portland cement component minerals were investigated. Adsorption isotherms of various types of superplasticizers and ζ-potentials of cement component minerals at the maximum adsorption of the superplasticizers were measured. The value of the adsorption isotherm was calculated from the amount of the superplasticizer adsorbed on a cement component mineral in an equilibrated solution. The maximum amounts of adsorption and the adsorption isotherms varied with types of component mineral and superplasticizer. For all types of superplasticizers, a larger amount of superplasticizer was adsorbed on C₃A and C₄AF than C₃S and C₂S. However, the equilibrated concentration of each superplasticizer at the maximum adsorption was not influenced by types of superplasticizer. Without superplasticizer, C₃S and C₂S had negative ζ-potential. On the contrary, C₃A and C₄AF had positive ζ-potential. Therefore, accelerated coagulation of cement particles might occur due to their electrostatic potentials that are opposite each other. However, all component minerals of cement had negative ζ-potential when they were mixed with any superplasticizer. Fluidity of fresh cement paste is improved due to electrostatic repulsion acting between particles.     

Effects of polycarboxylate-type superplasticizer on fluidity and hydration behavior of cement paste

Polycarboxylate (PC)-type superplasticizers were synthesized with different average molecular weight of polyethylene oxide (PEO) graft groups, the molar ratios of graft group to carboxylic group, and then the chemical structure, polymerization condition, and physical and chemical properties were analyzed. In order to evaluate the effects of PC-type superplasticizers in cement paste, the adsorption, the initial plasticity and slump retention characteristics and the degree of hydration reaction were investigated. As the average molecular weight of graft group decreased and molar ratio increased, the conversion rate to copolymers, the adsorption amount of PC-type superplasticizer on cement particles improved; on the other hand, the hydration reaction was delayed.     

Relating the molecular structure of comb-type superplasticizers to the compression rheology of MgO suspensions

We have investigated the effect of superplasticizers on the rheological properties of concentrated MgO suspensions. The comb-type anionic polymers with grafted polyethylene oxide chains adsorb onto the MgO surface and infer a steric repulsion where the range scales with the length of the PEO side chains. Consolidation experiments, where the volume fraction gradient of particle networks has been determined in response to a centrifugal force field, offer a simple, yet accurate, way of investigating flocculated, partly stabilized and stable suspensions under compression. The compression rheology behaviour could be related to the estimated thickness of the adsorbed superplasticizers and a scaling analysis was used to quantitatively assess the importance of the length of the grafted PEO-chains on the magnitude of the inter-particle bond strength.     

Influence of superplasticizers on rheological behaviour of fresh cement mortars

To assure required workability of high performance concrete (HPC), various superplasticizers are used. Only by using superplasticizers can rheological properties of HPC mix be adequately adjusted to the methods and conditions of concrete processing. Thus, the key element in efficient workability shaping is the complex knowledge how superplasticizers influence the rheological properties of fresh concrete in different technological circumstances.In the paper, the methodology and test results of an investigation into the influence of chemically different superplasticizers on the rheological properties of standard mortars are presented and discussed. The rheological parameters of mortars yield value g, and plastic viscosity h were determined using VISCOMAT PC rotational rheometer. In the research, the influence of the performance of superplasticizers was investigated taking into account following factors: chemical origin of superplasticizers (SNF/naphthalene sulfonic acid/, AP/polycarboxylate acid, PC/policarboxylate ester/), superplasticizer dosage, W/C ratio, cement type (CEM I, CEM II and CEM III), cement physical and chemical properties and temperature.The results presented in the paper show that by testing rheological parameters of mortars with rotational viscometer, it is possible to complex and precisely determine the performance of superplasticizers. On the ground of obtained results, it is possible to optimise the composition of mortars and concretes from workability point of view.     

硅烷改性聚羧酸减水剂-Na2SO4-水泥体系分散及流变性能的研究

采用γ-甲基丙烯酰氧基丙基三甲氧基硅烷(KH570),丙烯酸(AA)以及甲基烯丙基聚氧乙烯醚(HPEG)单体,通过自由基聚合法合成了硅烷改性聚羧酸减水剂(SPC).试验研究了引入硅烷官能团后,减水剂分子成分、电荷密度以及对Na2SO4-水泥体系分散及流变性能的影响规律.研究结果表明,在SPC红外特征峰中发现了Si-O的伸缩振动峰,说明SPC成功引入硅烷官能团;电荷密度测试表明,PC比SPC具有较高的-COO-含量;通过分散性能、流变性能及吸附量测试,表明与PC相比,SPC能够通过≡Si-OH与水泥颗粒表面的-OH发生化学缩合反应,提高减水剂分子在Na2SO4-水泥体系中对水泥颗粒表面的吸附能力,降低屈服应力和塑性粘度,从而提高减水剂对水泥浆体的分散及流变性能.     

The effect of superplasticizer molecular weight on its adsorption on, and dispersion of, cement

The effect of molecular weight of a superplasticizer on the adsorption on cement was investigated by microelectrophoresis and UV-absorption techniques. The purpose of the experiments was to investigate any differences in the magnitude of the Zeta Potential (ZP) and in the amount of superplasticizer adsorbed by using sulfonated polystyrenes of different molecular weight, MW = 4,000 g/mole, MW = 16,000 g/mole, MW = 31,000 g/mole, and MW = 70,000 g/mole. The results show that the superplasticizer with the largest molecular weight gives the largest negative ZP, and would therefore be assumed to have a higher dispersing capability. UV-absorption results have shown that the polymer of MW = 16,000 g/mole is the most adsorbed of the four superplasticizers, whereas the polymers of MW = 4,000 g/mole, MW = 31,000 g/mole, and MW = 70,000 g/mole are less adsorbed, with MW = 70,000 g/mole being the least adsorbed. That the polymer which is the least adsorbed gives the highest negative ZP is explained by the diffuse double-layer theory.     

Organic admixtures and cement particles: Competitive adsorption and its macroscopic rheological consequences

In this work, we study the competitive adsorption in cement paste between a superplasticizer and a so-called viscosity agent along with the competitive adsorption between the same superplasticizer and a retarder. We develop a new protocol using dynamic light scattering, which allows for distinguishing the relative adsorption of different fractions of admixtures when introduced simultaneously in cement paste. Our results show that the result of competitive adsorption is not only a matter of admixture chemical structures or physical properties but is also very dependent on surface coverage. We identify from our results, similarly to other authors in polymer literature, three main competitive adsorption regimes. We finally show that the outcome of competitive adsorption measured at the microscopic scale can explain the measured macroscopic variations in rheology in terms of yield stress and viscosity.     

Polyelectrolyte Effects on the Rheological Properties of Concentrated Cement Suspensions

Polyelectrolyte species, known as superplasticizers, dramatically affect the rheological properties of dense cement suspensions. We have studied the influence of sulfonated naphthalene formaldehyde condensate (SNF) and carboxylated acrylic ester (CAE) grafted copolymers of varying molecular architecture on the surface (e.g., adsorption behavior and zeta potential) and rheological properties of concentrated cement suspensions of white portland cement and two model compounds, β-Ca₂SiO₄ and γ-Ca₂SiO₄. The adsorption of SNF species was strongly dependent on cement chemistry, whereas CAE species exhibited little sensitivity. The respective critical concentrations (Φ*) in suspension required to promote the transition from strongly shear thinning to Newtonian flow (flocculated → stable) behavior were determined from stress viscometry and yield stress measurements. Theoretical analysis of interparticle interactions suggested that only colloidal particles in the size range of ≤1 μm are fully stabilized by adsorbed polyelectrolyte species. Our observations provide guidelines for tailoring the molecular architecture and functionality of superplasticizers for optimal performance.     

Working mechanism of post‐acting polycarboxylate superplasticizers containing acrylate segments

Acrylate‐containing polycarboxylate superplasticizers were synthesized by copolymerizing acrylic acid, α‐methallyl‐ω‐methoxy poly(ethylene glycol) ether, and acrylic esters [hydroxyethyl acrylate (HEA) or hydroxypropyl acrylate (HPA)]. Their dispersing effects and fluidity‐retaining capabilities were evaluated by spread tests of cement pastes. The temperature sensitivity of the performance was especially focused upon. It was found by Fourier transform infrared spectroscopy and specific anionic charge density measurements that the ester groups in the acrylate‐containing copolymers hydrolyze in alkaline conditions, and thus additional R? COO^? groups are continuously produced. The gradual production of R? COO^? leads to increasing charge density and hence increasing adsorption of the polymer on the cement surface. The hydrolysis of HEA units is slightly faster than of HPA units in the corresponding copolymers. By this mechanism, a post‐acting, or delayed, dispersing effect is realized. Such post‐acting polymers alone or formulated with normal polycarboxylate superplasticizer can be used achieve a long fluidity retention of fresh concrete, especially under elevated temperatures. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45753.     

有机助磨剂对水泥性能的影响及其有机水泥化学分析(英文)

现代水泥、混凝土中大量使用化学外加剂,特别是有机化合物和高分子聚合物化学外加剂,例如：水泥助磨剂、混凝土超塑化剂、引气剂、增稠剂等,大量有机物的加入改变了水泥水化过程、水化动力学、微观结构的发展,传统的水泥混凝土化学不再能很好地解释其微观结构与宏观性能的关系。为此,提出一个新兴的水泥混凝土化学的补充分支—有机水泥化学,在未来的水泥混凝土研究中该给予更多的重视。以有机化学外加剂—助磨剂为例,说明其对水泥水化动力学、水化产物形态以及水泥浆体的超塑化剂需求量、流变特性、强度发展等宏观性能的影响。水泥中加入微量的助磨剂,不仅改变了水泥颗粒分布,还改变了水化动力学,促进起始离子的溶解和铝酸钙（C3A）和铁铝酸钙（C4AF）的早期水化,明显地提高早期强度和28 d强度。助磨剂吸附在水泥表面改变了水泥的表面性质,其中助磨剂和Ca2＋、Fe2＋螯合起关键作用。     

Influence of Ca2+ on the performance of poly(acrylic acid)-g-poly(ethylene glycol) comb-like copolymers in cement suspensions

Poly(acrylic acid)-g-poly(ethylene glycol) (PAA-g-PEG) comb-like copolymer (PCE) is one of the best kind of superplasticizers for cement suspensions. But PCE can be considered as polyelectrolyte which is easily affected by the ions in cement suspensions. In cement system, calcium ions are the most abundant cations which would have great influence on the performance of polymer superplasticizer. Here, the effects of calcium ion on the surface charge, adsorption and dispersion in cement system with PCE as dispersant are investigated in detail. And the changes in the conformation of PCE before and after adding calcium ions are studied by methods of dynamic light scattering and transmission electron microscopy. Calcium ions increase the zeta potential of cement particles and make the molecular chains of PCE curlier, which induces the changes in adsorption and dispersion behavior. The adding of calcium ions makes the ion bridging between calcium and PCE to take place and the ion bridging makes the PCE molecules shrink to smaller size. In this paper, it tells that the molecular structure of PCE polymer could be changed based on the conclusion to adapt the cement with different content of calcium. In cement suspensions, the effect of calcium ions on the performance of PCE polymer would be weakened by decreasing the content of carboxyl groups in the molecular chain.     

Contribution of non-adsorbing polymers to cement dispersion

It has been noticed recently that at low w/c ratios (≤ 0.30), non-adsorbed polycarboxylate (PCE) polymers can contribute as well to cement dispersion. This study aimed at defining more specifically the structural requirement for such non-adsorbing polymers. For this purpose, a cement paste (w/c = 0.30) containing a conventional MPEG PCE superplasticizer was admixed with additional quantities of a polyester polymer prepared via homopolymerization of MPEG methacrylate ester macromonomer, the macromonomer used in the homopolymerization, and the polyethylene glycol contained in the macromonomer. It was found that when admixed individually, all three polymers do not adsorb on cement and cannot fluidize the paste but enhance dispersion and fluidity significantly when combined with the PCE superplasticizer. A potential explanation is that the non-adsorbing polymers act as lubricants between cement, which are particularly densely packed at low w/c ratios. The pore fluid loaded with non-adsorbed polymer exhibits superior lubrication compared to pristine cement pore solution.