Approaches to achieve fluidity retention in low-carbon calcined clay blended cements. (15th August 2021)

• Record Type: Journal Article
• Authors: Li, Ran; Lei, Lei; Sui, Tongbo; Plank, Johann
• Is Part Of: Journal of cleaner production. Volume 311(2021)
• Publication Date: 2021-08-15
• Subjects: Low carbon cement -- Calcined clay -- Polycarboxylate (PCE) -- Superplasticizer -- Fluidity retention -- LDH nanocomposite; Factory and trade waste -- Management -- Periodicals; Manufactures -- Environmental aspects -- Periodicals; Déchets industriels -- Gestion -- Périodiques; Usines -- Aspect de l'environnement -- Périodiques; 628.5
• DOI: 10.1016/j.jclepro.2021.127770
• Languages: English
• ISSNs: 0959-6526

Abstract

Abstract: High meta kaolin content (e.g. 50 wt%) present in a calcined clay blended into a composite cement is positive with respect to early strength development, but significantly decreases the dispersing effectiveness of PCE superplasticizers. Moreover, it has been observed that for such cements, slump retention is much more difficult to achieve than in OPC or other composite cements. In this study, several approaches to achieve extended workability times in mortars prepared from composite cements holding 20–40 wt % of a calcined clay were investigated. First, it was found that the slump retaining performance of a common industrial ready-mix type HPEG PCE rapidly decreased when the portion of calcined clay was increased in the blended cement. Furthermore, a combination of the ready-mix HPEG PCE and a retarder (sodium gluconate) which is commonly applied in ready-mix concrete also could not much improve fluidity retention, thus highlighting the difficulty of slump retention for such cements. To solve this problem, a new admixture formulation is introduced based on a combination of a precast type HPEG PCE and a novel PCE-LDH nanocomposite. This approach to improve slump retention was tested on composite cements holding 20–40 wt % of a calcined clay high in meta kaolin content. Mortar tests revealed that the high water-reducing (precast) type HPEG PCE and the PCE-LDH nanocomposite work synergistically and can achieve a significant improvement in fluidity retention of such calcined clay blended cements. A mechanistic investigation revealed that the PCE which was intercalated in between the [Ca2 Al(OH)6 ] + main layers of the PCE-LDH nanocomposite is released gradually from the mixed metal hydroxide via anionic exchange with sulfate anions present in the pore solution. This way, an extended workability time was achieved in the mortar. Graphical abstract: Image 1 Highlights: Slump retention of meta kaolin rich calcined clay blended composite cements studied Calcined clay increases PCE dosages and strongly reduces slump retention Ready-mix PCE alone or in combination with gluconate cannot provide slump retention A novel PCE-LDH nanocomposite achieved workability retention in CC blended cements PCE gradually released from PCE-LDH via exchange with SO4 2− produces slump retention