IntroductionIn application, different types of concrete admixtures are usually mixed to solve different problems. The varying adsorption capacities of organic molecules or polymer additives on particle surface of different mineral phase result in “competitive” or “preferential” behaviors, leading to intricate interference or synergistic effects among the additives. Competitive adsorption occurs among retarders, viscosity modifying admixtures (VMA), and superplasticizers, especially when the surface coverage is relatively high. The investigation of competitive adsorption relies on the quantification of each type of admixture based on the special characteristic signal of each admixture (e.g. signal by light scattering or gel permeation chromatography due to the clear difference of solution size between VMA, polycarboxylate superplasticizer (PCE), and retarder molecules; signal of phosphorus in some P-containing retarders). Labeling of PCE molecule by chromophores is an effective strategy. However, the low signal intensity of the labeled PCE in the few former reports limited the universality. Herein the paper reported the synthesis of a labeled PCE by the grafting of naphthylamine onto backbone as chromophore. The mixture of naphthalene-ring-labeled PCE and conventional PCE could be quantified based on UV absorption spectra and total organic carbon (TOC) method. The effect of adding two PCEs with different side chain lengths on the fluidity of cement paste was studied.MethodsJiangsu Helin P·II 52.5 Portland cement with density of 3.06 g/cm³ and BET specific surface area of 0.83 m²/g was used. The labeled PCE (PCE-A) was prepared by the simultaneously grafting naphthylamine (99%, Aladdin reagent) and polyetheramine (monofunctional, primary amine, with a terminal methyl group, from Zhejiang Lukean Chemical Co., Ltd) to polyacrylic acid (weight-averaged molecular weight M_w 2000) in the presence of 1% (in mass) H₂SO₄ under vacuum at 130 ℃. The molar ratio of chain unit as confirmed by ¹HNMR spectra was acrylic acid/naphthylamine/polyetheramine = 4.73/0.70/1.00. PCE-B (poly(methacrylic acid) grafted with poly(ethylene glycol) monomethyl ether (MPEG, M_w 1000), molar ratio of methacrylic acid to MPEG was 3/1) was supplied by Jiangsu Sobute New Materials Co., Ltd.The UV absorption spectra was recorded between 200-400 nm. All the cement pastes were prepared with water to cement ratio of 0.18. The flow spread of cement paste (GB/T 8077—2012) with the addition of only PCE-A, only PCE-B, and both PCEs was measured, respectively. The total adsorption amount of PCE was measured by TOC method. The adsorption amount of PCE-B when both PCEs were added, was calculated based on UV absorption spectra. The standard curve (UV absorbance at 305 nm against TOC concentration) was obtained by the supernatant of cement paste when only PCE-B was added. The amount of ettringite (AFt) was calculated based on total heat absorbed between (40-105 ℃) in differential scanning calorimetry (DSC) as compared with synthetic AFt. The morphology of AFt was investigated by scanning electron microscope (SEM).Results and discussionThe main UV absorption band of PCE-A (naphthene ring) located between 250 nm and 320 nm, which enabled the differentiation with the background from the pore solution. The quantification by the UV method was confirmed by the supernatant of cement paste with only PCE-B. An excellent linear relationship (correlation coefficient of over 0.99) was found between absorbance at 295, 305 nm and TOC concentration.When only one type of PCE was added, the flow spread of cement paste achieved a maximum value at ~6 mg/g, then slight decrease could be observed at higher dosage. The maximum flow spread of cement paste with PCE-A (lower adsorption affinity, but higher saturated adsorption amount) is much larger than PCE-B, indicated a stronger steric hindrance, due to the longer side chain. When both PCE-A and PCE-B were added, at PCE-A of 2 mg/g and 4 mg/g, the flow spread of cement paste underwent a gradual increase and then decrease (especially after PCE-B of 4 mg/g) with the increase of dosage of PCE-B. At PCE-A of 6 mg/g, lower flow spread would be found at higher dosage of PCE-B. Regardless of PCE-A dosage, the adsorption amount of PCE-A would be lower at higher PCE-B dosage. The total adsorption amount always became lower at higher dosage of PCE-B, except for the condition of PCE-A 2 mg/g and PCE-B lower than 4 mg/g. The behind reason was, PCE-B could occupy the adsorption site for PCE-A. At fixed PCE-A dosage, the total surface coverage always increased with the addition of PCE-B. However, at high PCE-A dosage, the replacement of surface-adsorbed PCE-A would reduce the steric hindrance.The increase of PCE dosage induced lower amount of AFt with finer morphology. The size of AFt particle with PCE-B is smaller than PCE-A, due to the even higher adsorption affinity for the inhibition of particle growth. When both PCEs were added, high amount of PCE-B would promote the transformation of AFt particles from rod-like to fine needle-like. The aspect ratio of AFt increased from ~10.6 (PCE-B 2 mg/g) to ~14.1 (PCE-B 6 mg/g). The increase of aspect ratio would result in worse particle packing and therefore lower flow spread.ConclusionsA PCE labeled with naphthalene ring (UV chromophore) was prepared based on the grafting reaction of naphthylamine, which enabled the quantification of solution concentration in cement paste by absorbance at ~300 nm and therefore the investigation of competitive adsorption with another PCE. At extremely low water to cement ratio (0.18), as the dosage of short side chain PCE increases, the adsorption amount gradually increases, while the adsorption amount of long side chain PCE gradually decreases. At high dosage of PCE with long side chain, with the increase of dosage of short side chain PCE, part of the sites that could have been occupied by long side chain PCE are occupied by short side chain PCE. In addition, the morphology of ettringite (AFt) changes from rod-like to finer needle-shaped, showing a larger aspect ratio, which will deteriorate the packing behavior. The fluidity of the cement paste is therefore reduced.