Effect of silica fume and cement on properties of non-autoclaved aerated concrete and microanalysis

LUO Jiebo; PENG Xiaotong; LIU Zhaolei; WANG Zhongqing; ZHANG Xiuzhi

doi:10.13732/j.issn.1008-5548.2025.02.014

Abstract：Objective An innovative non-autoclaved aerated concrete(NAAC)is developed to achieve an optimal balance between thermal insulation and mechanical properties,addressing the demand for energy efficiency and environmental sustainability in construc-tion.Traditionally,the production of aerated concrete relies on autoclaving to enhance material strength,which is highly energy-consuming and has stringent requirements on equipment.To overcome these limitations,the study focuses on the non-autoclaved process,systematically optimizingraw materials ratios to enhance material performance.By refining silica fume dos-ing and cement grade,the optimal test ratios for strength enhancement are determined without significantly increasing thermal conductivity. Methods This study used a single-factor test method to optimize the composition in aerated concrete.By designing tests with varying silica fume substitution rates,the dry density and compressive strength of each specimen were analyzed.Based on these results,the cement grade was varied to determine the optimal silica fume admixture and cement grade.Firstly,a small amount of silica fume was used to address the insufficient strength of traditional autoclaved aerated concrete while ensuring minimal impact on thermal conductivity.The effects of silica fume content on material performance were assessed to determine the optimal substitution rate.Subsequently,the effects of different cement grades on NAAC performance were explored through additional tests.Finally,the microstructure of the optimal NAAC was analyzed using a scanning electron microscope(SEM). Results and Discussion Optimizing silica fume content and cement grade proved effective in improving the strength of NAAC,while maintaining the ideal thermal conductivity for self-insulated wall panels.Increasing silica fume content improved the dry density and compressive strength of the material.At a silica fume substitution rate of 2％,the dry density was the smallest,while at a substitution rate of 10％,the compressive strength was the highest.A 4％substitution rate was proved to strike the best bal-ance:the dry density increased minimally by 0.92％,and compressive strength increased significantly by4.17％,with an increase of only 7％in thermal conductivity compared to the benchmark group.These findings demonstrated that the material is both lightweight and high-strength.The addition of silica fume,which had a lower density and higher reactive silica content com-pared to fly ash,resulted in lighter wall panels.Changing the cement grade also influenced material properties.Specimens made with P.O52.5 silicate cement exhibited higher compressive strength and lower dry density than those made with P.O42.5 silicate cement.However,the thermal conductivity of the P.O52.5 specimens increased by 5.43％at the same mix ratio.Microstructural analysis revealed that silica fume's pore structure did not significantly change thermal conductivity,and the fibres could be pulled out from the matrix,enhancing compressive strength.Moreover,the use of silica fume and P.O52.5 cement promoted the distribution of the acicular hydrated calcium silicate(C-S-H)and hexagonal flake or plate calcium hydrox-ide(CH)along the inner side of the pore walls.These hydration products intertwined to form a mesh-like structure,which enhanced the overall strength and stability of the material. Conclusion The optimal silica fume content for non-autoclaved self-insulating aerated concrete is determined to be 4％(mass fraction).Compared to fly ash,silica fume's lower density and higher reactive silica content effectively reduce the weight of the wall panel materials when used as an admixture.As self-insulated wall panels are mainly self-supporting,their compressive strength requirements are relatively low.Specimens prepared with P.O52.5 silicate cement demonstrate superior performance in engineering practice,achieving a dry density of 798 kg/m3,a compressive strength of 3.9 MPa,and a thermal conductivity coeffi-cient of 0.095 53 W/(m·K).These values meet the A2.5 B06 grade specifications for aerated concrete,ensuring both perfor-mance and compliance.