Low-carbon 3D printed mortar using hemihydrate phosphogypsum: Printability, pore coarsening, and mechanical performance

To reduce cement consumption and material-related carbon emissions in 3D printed mortar, hemihydrate phosphogypsum (HPG) was used to replace ordinary Portland cement at levels of 0–60%. The effects of HPG on printability, mechanical performance, water and carbonation resistance, dimensional stability, microstructure, pore structure, and carbon efficiency were systematically investigated. Increasing HPG content shortened the setting time, reduced the initial flowability, and narrowed the effective printability window. Buildability first improved and then declined, with the highest tan θ value of 11.94 obtained at 30% HPG. 10% replacement promoted early-age strength, whereas excessive HPG markedly reduced later-age mechanical performance. At 60% HPG, the 28 d compressive, flexural, and interlayer splitting tensile strengths decreased by up to 59.1%, 36.8%, and 30.2%, respectively. Increasing HPG content also increased water absorption from 7.9% to 11.6%, reduced the softening coefficient from 0.86 to 0.48, and markedly weakened carbonation resistance at replacement levels above 30%. Moderate HPG incorporation improved dimensional stability, with the 30% mixture showing 14 d autogenous and drying length change coefficients of 36 and 286 με, respectively, whereas the 60% mixture exhibited continuous autogenous expansion and the highest drying deformation. Microstructural analyses showed that excessive HPG increased gypsum-related phases, weakened matrix continuity, and promoted pore coarsening, with porosity increasing from 21.27% to 27.64% and the median pore diameter from 0.39 to 2.14 μm. The 30% mixture reduced total carbon emissions by 32.6% and achieved the lowest carbon emission intensity of 26.40 kg CO 2-eq /(m 3 ·MPa). Overall, 30% HPG was identified as the preferred replacement level, balancing engineering performance and carbon efficiency.