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Ordinary Portland cement (OPC) is by far the largest cementitious material used in concrete and mortar. In parallel, specialty cements, such as sulfoaluminate cement (SAC) are attracting increasing attention, not only due to their superior technical properties such as fast early strength development, lower shrinkage, high frost-resistance and corrosion-resistance, but also due to the much lower environmental impacts of the manufacturing process of SAC, including lower energy consumption and lower CO2 emission etc.
Superplasticizers, particularly polycarboxylate superplasticizers (PCE) have become one of the indispensable components in the formulations of modern concrete. PCE is a group of comb-like polyelectrolyte, usually with poly (acrylic acid-co-acrylic ester/ether) as backbone and polyethylene oxide (PEO) as side chains [13]. When PCE is mixed into fresh cement paste (fcp), deprotonation of carboxyl groups in alkaline condition leads the backbone of PCE molecules negatively charged and the PCE molecules are adsorbed onto the surface of various mineral phases in fcp, such as the surface of cement grains, hydration products like AFt etc., mainly due to the electrostatic interaction between the mineral surfaces and PCE molecules. The adsorbed PCEs provide electrostatic repulsion and/or steric hindrance between the mineral surfaces. In this way, the cement grains are well dispersed in fcps and the fluidity of fcps is greatly increased by the addition of PCE.
Substantial progress has been made in the last decades in PCE technology and PCE science. However, most of the studies are targeted to OPC systems. With increasing consumption of SAC, tailor-made PCE superplasticizers for SAC system are in urgent need. It is well known that OPC is mainly composed of four major clinker phases and additional sulfate carriers such like gypsum, anhydrate and hemihydrate, while the major composition of SAC is calcium sulfoaluminate, sulfate phases and C2S etc.. Thus, the charging features of the surfaces of OPC grains and SAC grains must be different and as a result the adsorption behavior of a given PCE accordingly varies in the two cement systems. In addition, the hydration kinetics as well as the hydration products of the two types of cement are entirely different. The setting time of OPC paste is usually several hours while that of SAC is usually from several minutes to several tens of minutes. The major hydration product of OPC is C-S-H while that of SAC is AFt crystals. It has been recognized that the initially formed AFt crystal phase adsorbed a large amount of PCE molecules due to its highly positive charging feature and huge specific surface area. Thus, the adsorption of PCE in SAC pastes could be much more than that in OPC pastes due to the vast early formation of AFt crystals in SAC pasts. Therefore, the PCE superplasticizers developed for OPC systems usually do not work as well in SAC systems, expressed by the high dosage of PCE, lower initial fluidity of fcps and hard control of the fluidity loss over time.
Taking all these into consideration, in this paper, we synthesized two typical PCEs with varied charge density and investigated the fluidizing effect of the two PCEs in OPC as well as in SAC systems. The PCEs were prepared by copolymerizing acrylic acid (AA) and α-methallyl-ω-hydroxy poly (ethylene glycol) (HPEG) monomers via radical polymerization. By varying the monomer ratio of AA to HPEG, the charge density of the obtained PCEs could be tuned. The working mechanism of the PCEs were studied by means of adsorption measurement, BET surface area measurement of the particulate matters in hydrating cement pastes, XRD, calorimetry etc. The objective of this paper is to lay down a fundamental principle for designing a PCE superplasticizer for different cement systems.
Two PCEs with acid to ether monomer ratio of 4 and 22, marked as PCE4 and PCE22 respectively were prepared via radical polymerization in order to study their impacts on fluidity and early hydration in Portland cement (OPC) and Sulfoaluminate cement (SAC) systems. Dynamic light scattering measurement reveals that hydrodynamic radius of PCE22 is more sensitive to Ca2+ from being crosslinked. Fluidity tests show that PCE22 exhibits lower initial fluidizing effect than the lower charged PCE4 in OPC paste because of the inter-polymer crosslinking by Ca2+ bridging effect while the opposite comparison is observed in SAC paste due to the lower [Ca2+] in SAC paste than in OPC paste. PCE22 is found to significantly depress the early formation of AFt crystal in SAC paste, indicated by the results of XRD, TGA, BET and calorimetric tests. Thus, PCE22 behaves better in fluidity retention than PCE4 in SAC paste.
(Quoted from Cement and Concrete Research Volume 115, January 2019, Pages 43-58)
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