Synergistic effect of EVA copolymer and sodium desulfurization ash on the printing performance of high volume blast furnace slag mixtures

Abstract

Considering the long-term development of digital construction, a cost-effective and environmental printing material was designed with blast furnace slag as main cementitious material and sodium desulfurization ash as activator. However, weak alkaline environment lead to the insufficient 3D printability (e.g. yield stress, viscosity). Ethylene/vinyl acetate copolymer (EVA) was used to make up this defect, and it was found that there was a synergistic effect between EVA and sodium desulfurization ash (NS), which is beneficial to the improvement of the printing performance. The synergistic effect of EVA and NS was investigated in terms of fresh, hardened and microstructure properties. Experimental results show that under varying resting times and shear rates, the addition of 0.6−1.6% (i.e.by mass of binder) EVA could increase the yield stress and apparent viscosity of HVGS. Moreover, it was also found that the addition of EVA could help improve the polymerization degree of printing materials and increase the compressive strength of 28d by about 22%. To verify the practical value of the printing material, an industrial product was printed, and the maximum number of continuous printing layers can be up to 30 layers.

Introduction

In order to solve the labor shortage in the traditional construction industry and improve construction quality and productivity, more and more studies have begun to focus on digital construction [1], [2], [3], [4]. In 2000, 3D printing technology began to develop in the field of construction [5]. Until now, though more and more groups have paid their attention to the development of 3D printing, studies on 3D printing cementitious material especially 3D printing geopolymer pastes have still been little. If digital construction is to replace traditional construction methods, it is necessary to try to design a cheap thixotropic matrial that can be extruded smoothly from the nozzle and undergo the load of subsequently deposited layers.

Most of the 3D printing paste designs developed so far involve of the adoption of Portland cement as the main composition for widespread utilization in the digital construction [6], [7]. However, with more and more attentions attained to the protection of the environment, it’s necessary to find alternative materials, due to the high CO₂ emission in cement production [8], [9]. Geopolymer has the potential to disruptively change traditional building materials by combined application of various industrial wastes and by-products to produce free-form construction components [10], [11]. However, the application of these environmental materials to digital construction has put forward higher requirements for material properties. One of the major problems to be addressed in the implementation is the adjustment and control of the geopolymer properties. Since little admixture can be directly applied to geopolymer, they will fail or become less effective under high PH environment. Another issue to be faced in their practice is that general two-part mixes present problems in terms of field application and viscosity of the alkaline activator [12], [13]. The main method handling this issue is the utilization of one-part geopolymers [14], [15], [16], which will limit its large-scale application in construction for its relatively high cost. Therefore, on the basis of ensuring its printability, activating geopolymer at low PH and price is one of the main goals of the present studies.

Studies have shown that industrial by-product(sodium desulfurization ash), whose main composition is sodium sulfate, can react with calcium hydroxide produced by the hydration of Portland cement, thus improving the pH of pore solution and the hydration of geopolymer materials [17], [18], [19]. Biranchi Panda et al has developed a high volume fly ash mixture with sodium desulfurization ash as activator. However, the limited activation effect of NS restricts the mechanical strength and printability of printing material, so it is necessary to find an admixture to improve its properties. Currently, there is still little research in this area. In this study, EVA is added in to solve the problem. Past works on EVA shows that the addition of EVA could form polymer membranes that seal partial pore walls and reduce the water/cement ratio of paste at a given consistency [20], [21]. Other studies have also shown that polymeric particles can adsorb onto the hydrating C₃S particles, thus hindering the growth of hydrated crystals and the dissolution of C₃S [22]. At present, EVA is mainly used as a water redispersible powder added to mortar and is thought to delay cement hydration. From this point of view, the addition of EVA will hinder the hydration of GGBS and reduce the buildability of mixtures. However, different from other studies, EVA is used to assist activation in this study. It was found that the synergistic effect of EVA and NS (whose main composition is sodium sulfate) had good effect on the performance of 3D printing pastes. This study focuses on the influence of EVA on high volume Slag mixes (HVGS) activated by NS. These mixes not only provide a practical formulation to replace conventional mortar, but also reduces the cost of 3D printing and contributes to the promotion of digital construction in buildings.

The main research goal of this paper is to study the influence of EVA on the rheological properties (e.g. yield stress, viscosity) of HVGS and improve the insufficient printability caused by low reactivity of raw materials via the synergistic effect of EVA and NS. Moreover, the compressive strength of the printing mixes was measured to evaluate the mechanical performance of the printing mixes. Finally, the reaction mechanisms of printing mixes were characterized via isothermal calorimetry, and verified by micro-structural analysis performed by fourier transform infrared spectroscopy (FTIR) and thermogravimetric(TG) analysis.