International Concrete Abstracts Portal

On October 28-31, 2018, the Chinese Ceramic Society and the China Academy of Building Research (CABR), Beijing China, in association with ACI, sponsored the Twelfth International Conference on Superplasticizers and other Chemical Admixtures in Concrete in Beijing China. More than 80 papers from all over the world were received and peer reviewed. A total of 36 refereed papers were accepted for publication in the proceedings of the conference. The proceedings were published by the ACI as SP 329. Also, 54 additional papers were presented at the conference, and were published in the Supplementary Papers Volume. The organizers of the conference were the Chinese Ceramic Society, Beijing and the Committee for the Organization of International Conferences (formerly CANMET/ACI Conferences).

Admixtures for concrete offer a wide range of water reducers that are mostly based on polycarboxylate polymers (PCE) bearing polyethylene oxide nonionic side chains. It is possible to increase PCE effectiveness mixing them with a special workability retention agent (WRA). We focused on ester-based PCE, synthesized from methoxypolyethylene glycol (Mpeg) and acrylic or methacrylic acid, and allylic-type PCE, synthesized from TPeg (polyethyleneglycol isopentenyl ether) or Ipeg (polyethylenglycol methallyl ether) and acrylic or methacrylic acid. By modifyng the composition of the backbone, the kind of repeating units and the molecular weight of polyethylene oxide side chains of PCE it is possible to optimize the properties of the mixture of PCE and WRA. The structures of the most interesting polymers we prepared are described in terms of their conformations (worms or stars). WRA is a particular acrylic polymer that can hydrolyze in alkaline environment; to work properly it must have a certain molecular weight and a low polydispersibility index. If it is used alone it is an ineffective water reducer. To investigate the reason behind this behavior slump test and rheological measured were carried out on mortar.

The water-reducing typed PCE (KZJ-1) has a short branched chain and a long main chain structure, while the structure of early strength typed PCE (KZJ-2 and KZJ-3) with a long branched chain and a short main chain. A detailed investigation on the chemical structure of different typed PCEs was characterized by GPC and particle charge density test. The effects of superplasticizer on dispersion, retarding stability and strength development are studied at the same time by paste and concrete. The superplasticizer was placed into different concentration to test the surface tension. The amount of adsorption and hydration heat were carried by TOC and TAM AIR. The molecular chain of KZJ-2 and KZJ-3 stretches more than KZJ-1 in the alkaline environment of concrete. The KZJ-2 and KZJ-3 can accelerate cement early hydration, shorten the induction period significantly, promote the hydration of C3A, and accelerate the formation of AFt.

In cement paste with low water-powder ratio, non-adsorbed superplasticizer molecules increase the fluidity of paste, but their function is not investigated in detail. This study investigates the influence of non-adsorbed superplasticizer on the fluidity of cement paste using several superplasticizers having different molecular structures. The paste consisted of belite-rich low-heat Portland cement, ultra-fine silica particle, and polycarboxylate-based superplasticizer solution. First, a superplasticizer having moderate amount of functional groups was added to the paste, and adsorption was saturated. Subsequently, another superplasticizer was added. The influence of subsequent addition was related to the molecular structure of another superplasticizer. Another superplasticizer having few functional groups decreased adsorption of the first superplasticizer but the fluidity did not change. Another superplasticizer having moderate functional groups did not change adsorption but increased the fluidity. Another superplasticizer having many functional groups increased adsorption but decreased the fluidity.

The durability and mechanical properties of cement-based materials reinforced by nano-SiO₂ particles strongly depend on the dispersion characteristics of the nano-SiO₂ particles in the cement pore solution. This study presents a simple, inexpensive and environmentally friendly method to improve the distribution of colloidal nano-SiO₂ particles. Amphoteric polycarboxylate superplasticizer containing a cationic moiety in its backbone, was synthetized under microwave irradiation and introduced onto the surface of colloidal nano-SiO₂ particles being negatively charged using electrostatic self-assembly technique. The above method guarantees the formation of a core-shell structure nano-SiO₂ particle, wherein the amphoteric polycarboxylate superplasticizer shell exhibits enhanced steric hindrance repulsion and electrostatic repulsion. Moreover, the proposed method application guarantees a higher dispersion stability of colloidal nano-SiO₂ in the saturated calcium hydroxide solution, which makes it favorable for the production of cement-based materials.