  Citation: Gonzalez-Panicello, L.; Garcia-Lodeiro, I.; Puertas, F.; Palacios, M. Influence of Accelerating Admixtures on the Reactivity of Synthetic Aluminosilicate Glasses. Materials 2022, 15, 818. https:// doi.org/10.3390/ma15030818 Academic Editor: Milena Pavlíková Received: 17 December 2021 Accepted: 17 January 2022 Published: 21 January 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Influence of Accelerating Admixtures on the Reactivity of Synthetic Aluminosilicate Glasses Laura Gonzalez-Panicello, Ines Garcia-Lodeiro , Francisca Puertas and Marta Palacios * Eduardo Torroja Institute for Construction Science (IETcc-CSIC), 28033 Madrid, Spain; laura.gonzalez@ietcc.csic.es(L.G.-P.); iglodeiro@ietcc.csic.es (I.G.-L.); puertasf@ietcc.csic.es (F.P.) * Correspondence: marta.palacios@ietcc.csic.es Abstract: This research aims at gaining a further understanding of the impact of accelerating ad- mixtures on the reactivity of supplementary cementitious materials (SCMs), which are widely used as a clinker replacement in blended cements. This was done on synthetic glasses with controlled composition and structure that mimic two types of real SCMs (slag and calcium-rich fly ash). The effects of DEIPA, TIPA, NaSCN and Na 2 S 2 O 3 on the glass dissolution, hydration kinetics and reaction products were investigated. The obtained results concluded that the pH of the NaOH solution and the composition of the synthetic glass play a key role on the effect of the admixtures. In 0.1 M NaOH (pH = 13.0), all the studied admixtures inhibited the dissolution of slag-like glasses while they enhanced the dissolution of Ca-rich fly ash-like glasses, being Na 2 S 2 O 3 the admixture that led to the highest increase of the dissolution rate of the Ca-rich fly ash-type glasses. In 1 M NaOH solutions (pH = 13.8), only the alkali admixtures (NaSCN and Na 2 S 2 O 3 ) enhanced the degree of reaction of both glasses. In slag-type glasses pastes mixed with 1 M NaOH, the addition of 2% Na 2 S 2 O 3 induced the highest increase of their reactivity as inferred by the total heat release and the amount of bound water. This is related to the formation of a high amount of S(II)-AFm, in addition to C-A-S-H, that would increase the aluminium undersaturation of the pore solution and consequently the further dissolution of the glass. Keywords: synthetic glasses; reaction kinetics; dissolution; accelerating admixtures; reaction products 1. Introduction Clinker replacement by supplementary cementitious materials (SCMs) is currently the most efficient and feasible strategy to decrease the carbon footprint of Portland cement and concrete. Blended cements generally have high long-term strength and durability, however, the substitution of clinker by SCMs has a detrimental impact on the early strength of concrete due to the slow reactivity of the SCMs, which finally limits the level of clinker replacement. Current global levels of clinker substitution are around 30%. Chemical admixtures such as superplasticizers and accelerators have been identified to play a key role to reach higher clinker replacements while preserving the overall mechanical perfor- mance [1,2]. Accelerating and strength enhancing admixtures have been widely used to increase the early reactivity and strength of cement and concrete. Their working mechanisms have been extensively investigated in pure Portland cements but barely studied on blended cements. CaCl 2 is the most effective accelerator of C 3 S and Portland cement. In C 3 S pastes, Juenger et al. [3] concluded that CaCl 2 enabled the formation of a more permeable C-S-H layer around the cement grains that led to a faster diffusion of water and ions and a higher rate of hydration during the early diffusion-controlled period [3]. Despite its great accelerating effect, the dosage of CaCl 2 is limited to minimize the risk of steel corrosion in reinforced concrete. Alkalis such as Na 2 SO 4 and NaOH accelerate the reaction kinetics of alite and cement during the early stages [4,5], however, at equal degree of hydration, the addition Materials 2022, 15, 818. https://doi.org/10.3390/ma15030818 https://www.mdpi.com/journal/materials