Characteristics of inorganic insulation foam modified by lithium tetraborate in high humidity conditions

To overcome the bottleneck of poor water resistance that limits the engineering application of water glass-based inorganic insulation foam in high-humidity external wall scenarios, this work developed a high-performance inorganic insulation foam modified by lithium tetraborate. The influences of lithium tetraborate on the chemical structure, physical properties and water resistance of the material under humid environments were systematically investigated. Mechanism analysis demonstrates that the improved water resistance arises from three synergistic effects. First, boron atoms substitute silicon in the Si-O-Si framework, which elevates the matrix electronegativity and enhances the binding capacity toward alkali metal cations. This effectively suppresses the ion exchange reaction between water molecules and Na⁺ in the network, which is the primary cause of material corrosion. Second, the incorporation of lithium tetraborate promotes dehydration polymerization of silicate tetrahedra during sintering, significantly reducing the number of hydrophilic -OH groups and weakening the hydrolysis path of the silica network. Third, excess lithium ions react with silicate to form a water-insoluble lithium silicate film on the pore wall, serving as a physical barrier to further block water molecule invasion. Benefiting from these effects, the softening coefficient of the modified samples increases by up to 63.46%, and the leaching rates of Na and Si decrease by 90.59% and 61.13%, respectively, indicating excellent resistance to water erosion. Although lithium tetraborate addition reduces material porosity and slightly increases thermal conductivity, the sample with 3 wt.% lithium tetraborate delivers an apparent density of 118 kg/m 3 , a thermal conductivity of 0.056 W/(m·K) and a compressive strength of 0.48 MPa, which satisfies the basic performance criteria for external wall insulation materials.