Effect of the superplasticizer addition time on the fresh properties of 3D printed limestone calcined clay cement (LC3) concrete

Although 3D concrete printing (3DCP) is considered one of the most significant innovations in the construction sector, its large cement consumption remains an environmental concern. In response, limestone calcined clay cements (LC³) have emerged as a promising alternative to Portland cement (PC) due to their potential for reducing environmental impact. While previous studies have shown the influence of addition timing of superplasticizer (SP) on the fresh performance of PC, its impact on LC³, particularly in the context of 3DCP, remains less understood. This study assessed the effect of the SP addition time (either directly with the mixing water or with a 10-min delay) on the rheology and printing-related properties of LC³ and PC. Rotational rheometry and 3DP-related tests were conducted over two hours, besides hydration assessment through thermogravimetric analysis (TGA). The results showed that adding SP directly to LC³ led to significantly higher yield stress (1.2-2.5 times) and viscosity (14-59% higher) values compared to delayed SP addition within the same time frame. However, LC³ exhibited slower rates of yield stress and viscosity increase over time in contrast to PC (with Athix structuration rate approximately 92% lower for LC³). These tendencies influenced printability performance. While direct SP addition reduced PC's open time from 100 to 30 minutes, it had no impact on LC³'s open time, which remained at 80 minutes. TGA analysis yielded additional insights, highlighting that hydrating LC³ samples required a greater free water content to achieve the same yield stress as PC and this difference was attributed to the presence of calcined clay. These findings make contributions to the technical and scientific communities by enhancing comprehension of LC³ cement fresh behavior and solidifying 3DCP as a disruptive construction method.