Enhancing mechanical performance of one-part sodium sulfate-activated slag mortars using calcium-rich materials

One-part alkali-activated slag (AAS) binders offer a simpler preparation process and are suitable for on-site casting, providing a practical alternative to traditional two-part systems. In this study, sodium sulfate was chosen as the activator over traditional high-alkalinity options like sodium hydroxide and sodium silicate due to its lower alkalinity, which reduces both environmental impact and safety concerns. While sodium sulfate is an environmentally friendly choice, it tends to reduce early-age strength. To overcome this challenge, various calcium-rich materials were incorporated into the one-part AAS system to enhance its performance. Mixtures were prepared using sodium sulfate at 2 % and 4 %, with partial replacements of granulated blast furnace slag by Portland cement (10 %, 20 %), hydrated lime (10 %, 20 %), and quicklime (5 %, 10 %). Compressive strength and ultrasonic pulse velocity tests were conducted at 7, 28, and 90 days. The microstructure was analyzed using scanning electron microscopy and Fourier transform infrared spectroscopy, while the thermal properties were investigated using thermogravimetric analysis. The challenges associated with low-alkalinity activators were effectively addressed by incorporating calcium-rich materials in measured proportions. Samples with 2 % and 4 % sodium sulfate had very low 7-day strength (3.4 MPa and 3.9 MPa). Even the lowest levels of calcium-rich replacements increased early-age compressive strength to between 12.6 MPa and 16.9 MPa. At a 2 % sodium sulfate dosage, the addition of 5 % quicklime resulted in approximately an 86 % increase in final compressive strength. This improvement in strength was supported by microstructural analysis, which revealed that the incorporation of calcium-rich materials enhanced the hydration process, promoting the reaction of unreacted particles and the formation of additional hydration products.