The calcium leaching behavior of cement paste and silica fume modified calcium hydroxide paste, exposed to hydrochloric acid solution, is reported in this paper. The kinetic of degradation was assessed by the changes of pH of hydrochloric acid solution with time. The changes of compressive strength of specimens in hydrochloric acid with time were tested. Hydration products of leached specimens were also analyzed by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric (TG), and atomic force microscope (AFM). Tests results show that there is a dynamic equilibrium in the supply and consumption of calcium hydroxide in hydrochloric acid solution, which govern the stability of hydration products such as calcium silicate hydrate (C-S-H). The decrease of compressive strength indicates that C-S-H are decomposed due to the lower concentration of calcium hydroxide in the pore solution than the equilibrium concentration of the hydration products. Furthermore, the hydration of unhydrated clinker delayed the decomposition of C-S-H in hydrochloric acid solution due to the increase of calcium hydroxide in pore solution of cementitious materials.

The agglomeration of nano‐CaCO₃ (NC) is the largest bottleneck in applications in cementitious materials. If nano‐CaCO₃ modifies the surface of micron‐scale limestone powder (LS), then it will form nano‐CaCO₃/limestone composite particles (NC/LS). It is known that micron‐scale limestone is easily dispersed, and the “dispersion” of NC is governed by that of LS. Therefore, the dispersion of nano‐CaCO₃ can be improved by the NC/LS in cementitious materials. In this work, the preparation of NC/LS was carried out in a three‐necked flask using the Ca(OH)₂‐H₂O‐CO₂ reaction system. The morphology of NC/LS was observed by a field emission scanning electron microscope (FE‐SEM). The effects of NC/LS on the hydration products and pore structure of cementitious materials are proposed. 5% NC/LS was added into cement paste and mortar, and the mechanical properties of the specimens were measured at a certain age. Differential scanning calorimetry (DSC), thermal gravimetric analysis (TG), and backscattered electron imaging (BSE) were conducted on the specimens to investigate the hydration products and pore structure. The properties of specimens with NC/LS were compared to that of control specimens (without NC/LS). The results revealed that NC/LS reduced the porosity and improved the mechanical properties of the cementitious materials.

The technology of 3D printing concrete has undergone rapid development in the last few years due to its lower environmental impact than that of conventional concrete. To investigate the fresh and the hardened behavior as well as the microstructure of 3D printing cementitious materials (3DPC) containing nano-CaCO₃ (NC), four replacement ratios of NC to binder from 1 to 4% are investigated. 3DPC without NC was used as a control specimen. The workability, such as fluidity, extrudability, printability limit, and deformation under self-weight, and the flexural and compressive strength of 3DPC are tested. The strength development of 3DPC is compared with that of the cast specimens. The hydration products and the microstructure of specimens were also investigated by derivative thermogravimetry (DTG), thermogravimetry analysis (TGA), scanning electron microscopy (SEM), and backscattered electron (BSE) imaging. The results indicate that the fluidity, extrudability, printability limit, and deformation under self-weight demonstrate a decreasing tendency to increase the NC content. This is attributed to the large specific surface area of NC, which increases the mixtures’ consistency. Adding 2% of NC exhibited about 7.2, 39.1, and 22.5% higher compressive strength than that of the control mixture at 7, 28, and 90 days. The enhancement of strength of 3DPC with NC can be ascribed to the filler effects and the seeding effects of NC. Furthermore, NC refines the pore structure and improves the microstructure of 3DPC due to its filler effects and accelerating effects.

This paper focuses on inspecting the influences of anti-foaming agent (AFA) on the performance of 3D printing cementitious materials (3DPC). The mini-slump, spreading diameter, yield stress, and strength of 3DPC were evaluated. Additionally, the air-void content, air-void morphology, and air-void size distribution of mortar with and without 0.05% AFA were assessed through image analysis. The mechanical performance and air-void structure of 3D printed samples were also investigated and compared to that of conventionally mould cast samples. Test results show that an optimal AFA content enables 3DPC to achieve favorable workability and mechanical performance. The addition of AFA exhibits lower air-void content in 3DPC than that of the sample without the AFA addition. This reduction in air-void content is further strengthened by the results of strength analysis. Electron microscope analysis shows that the use of AFA results in the suppressed formation of large air-voids during the process of fresh 3DPC. Moreover, the air-void morphology substantially influenced the mechanical performance of hardened 3DPC.