Analyzing the early structural build-up of accelerated cement pastes

Abstract

Extrusion-based additive manufacturing imposes high requirements on the material stability right after the extrusion. Therefore, a thorough understanding of the chemical reactions that determine the early reduction in processability is necessary. Accelerators are especially considered here, which have a major influence on the early reaction. This study contributes to these issues by analyzing the influence of 0.1 wt% TEA (triethanolamine) and 2.0 wt% Ca(NO3)2 on the hydration of two CEM I 52.5 R. The hydration was analyzed by isothermal heat flow calorimetry and in-situ X-ray diffraction. Vicat needle penetration, a penetrometer of own design, and ultrasonic P-wave velocity development were used to monitor the early change in workability. The obtained results indicate that ettringite formation is the main factor influencing workability during the first 60 min of hydration. Afterwards, the influence of ettringite is exceeded by the formation of C–S–H. Ca(NO3)2 was shown to enhance C–S–H formation and had no significant effect on the workability during the first 60 min of hydration while rapidly decreasing workability during the induction period. TEA was shown to increase brownmillerite dissolution and ettringite formation and, consequently, promote the workability loss during the initial hydration period. The time of initial and final setting determined by the Vicat needle test depended mainly on the formation of C–S–H phases.

Working mechanism of calcium nitrate as an accelerator for Portland cement hydration

“…Regarding the silicate reaction of OPC, it has been shown that Ca(NO 3 ) 2 shortens the time until the main hydration of alite starts, and it increases the rate of alite hydration and possibly also that of belite 15–17,25 . Justnes et al 18 .…”

“…Regarding the silicate reaction of OPC, it has been shown that Ca(NO 3 ) 2 shortens the time until the main hydration of alite starts, and it increases the rate of alite hydration and possibly also that of belite. [15][16][17]25 Justnes et al 18 suggested that the dissolution of Ca(NO 3 ) 2 results in a fast supersaturation of the pore solution with respect to portlandite (CH), and that the resulting early precipitation of CH is responsible for the early onset and the increased rate of alit hydration. Nicoleau 26 argues that calcium salts allow the homogenous nucleation of C-S-H by increasing the availability of calcium in solution, and that this enhanced nucleation is responsible for the acceleration of alite hydration.…”

Impact of triethanolamine on the sulfate balance of Portland cements with mixed sulfate carriers

“…Such knowledge will help to optimize the interaction of TEA and other admixtures with cements, for example, to control the setting and hardening process, and could potentially facilitate the development of recent applications of Portland cement. In particular, a precisely timed acceleration will be a great advantage in digital fabrication of concrete [19][20][21] .…”

Reactivity of C3S and model cement in presence of Na2S2O3 and NaSCN

“…Accelerating admixtures are widely used chemicals to enhance the reactivity of cementitious materials [1]. Their addition to concrete formulations enables to shorten the hardening time of precast concrete, the production of ready-mix concrete at low temperatures and a fast setting for shotcrete.…”