Natural hydraulic lime (NHL), as an inorganic cementitious material with both hydraulic and air-hardening properties, mainly consists of dicalcium silicate, calcium hydroxide and calcium carbonate. It has moderate strength, good breathability and low soluble salt content, and its inherent cementitious properties are significantly superior to traditional lime in the restoration of cultural relics and ancient buildings. However, there are still considerable challenges in the large-scale application of NHL in the restoration of ancient buildings, and the performance requirements for NHL in different restoration projects of ancient buildings may also vary. Due to the typical phase composition of NHL, the pozzolanic effect of silica fume can significantly affect the hydration process in NHL. However, the mechanism of silica fume action around typical performance development is not clear, which is obviously insufficient to explain the relationship between the physical and mechanical properties of NHL under the action of silica fume and the phase composition and microstructure evolution, as well as the macro and micro development influence mechanism. In this work, the physical-mechanical properties of NHL under the action of silica fume and its phase composition and microstructure were studied, and the corresponding relationship between phase composition, pore structure and performance was explored.

The experiment utilized NHL (NHL2 level), whose main mineral composition was dicalcium silicate (β-C₂S), calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaCO₃). Silica fume (SF) was used, which was composed of micro-nano amorphous silica spherical particles. The designed water-binder ratio was 0.55, and the binder-sand ratio was 1:3. The dosage of polycarboxylate superplasticizer was 0.4% (in mass, the same below). SF was used to replace 0%, 5%, 10% and 15% of the NHL mass respectively to prepare paste and mortar specimens. The specimens were covered with isolation film to prevent excessive evaporation of free water and were cured in a standard curing chamber for 72 h before demolding. After that, the specimens were cured under conditions of (20±2) ℃ and RH of 65%±5% for 7, 14 and 28 days.

The setting time, fluidity, compressive and flexural strength, drying shrinkage and mass change were tested in accordance with relevant standards. The phase composition was analyzed by XRD (Ultima IV, Rigaku) and thermogravimetry (STA 449 F3, NETZSCH). The microstructure and pore structure were analyzed by SEM (JSM-6700F) and MIP (Autopore 500) respectively. Before testing, the samples need to be treated with isopropanol to interrupt hydration, and then dried to a constant mass in a vacuum drying oven at 40 ℃.

SF has a significant promoting effect on the setting of NHL paste, the initial and final setting times of NS15 paste were shortened by 0.96 h and 0.60 h respectively, while the fluidity of its slurry and mortar decreased by 48.3% and 68.1% respectively. The mechanical strength of NS mortar with SF was significantly increased compared to NHL mortar. The compressive strength of NS5, NS10 and NS15 mortar at 28 days reached 4.62, 8.51 MPa and 10.92 MPa respectively. The drying shrinkage rate and mass loss rate of NS mortar with SF gradually increased, and the shrinkage amount exceeded that of NHL mortar after 7 days. At 28 days, the drying shrinkage reached about 1400 μm·m^-1, which was significantly greater than that of NHL mortar. The pozzolanic reaction caused free water to transform into chemical bound water, and the evaporation of free water in pores increased the number of pores available for the transmission of external CO₂ into the interior, promoting the occurrence of carbonation in the system and resulting in an increase in mass.

SF has a high specific surface area and may act as a nucleation site to promote the hydration of C₂S and the formation of C-S-H gel and Ca(OH)₂ in NS. The pozzolanic reaction caused by SF in NS promotes the generation of a large amount of C-S-H gel while consuming Ca(OH)₂. SF will further react with the Ca(OH)₂ generated by hydration through "secondary hydration (pozzolanic reaction)" to form C-S-H gel. In addition, SF has a significant promoting effect on the carbonation of NS system. These effects significantly influence the pore structure composition in the hardened NS slurry. SF can refine the pore size and increase the pore density, thereby promoting a significant increase in mechanical strength. Conclusions SF can significantly shorten the setting period of NHL paste, but it will cause a significant decrease in its fluidity. The compressive strength of NS mortar with SF addition is significantly increased, and the 28-day compressive strength of NS15 mortar reaches 10.92 MPa, which is 3.97 times higher than that of NHL mortar. SF shows a change pattern of first inhibiting and then promoting the drying shrinkage of NHL mortar. The hydration of C₂S to form C-S-H gel and Ca(OH)₂ is the basic hydration process in NHL. The combined effects of SF acting as a nucleation site to promote the hydration of C₂S, the pozzolanic reaction between SF and Ca(OH)₂ in NHL, and the secondary hydration of SF and Ca(OH)₂ formed by the C₂S hydration, and result in a significant increase in the C-S-H and a significant decrease in the content of Ca(OH)₂ in NS system. The content of CaCO₃ generated by carbonation in NS with SF addition increases significantly. Pore structure of NHL is mainly composed of capillary pores and macro pores, with a total porosity of 51.31% at 28 days. The proportion of gel pores and transition pores in NS significantly increases, and with the increase of SF dosage, the most probable pore diameter shifts towards smaller pores, and the average pore diameter also decreases significantly. The filling effect of SF and the generation of more C-S-H and CaCO₃ due to the hydration and carbonation of NHL can significantly refine the pore size and densify the pore structure.