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In this study,the effects of five different ionic and nonionic surfactants on the physical properties and microstructures of a metakaolin-based geopolymer (MKG) were investigated.It is the first comprehensive comparative study of the effects of sodium lauryl sulfonate (SLS),alkyl polyglycoside (APG),benzalkonium chloride (BAC),sucrose fatty acid esters (SE),and stearic acid (STA) on MKG.Viscosity,densities,apparent water absorption,and compressive strength were measured,and pore structures,micro-defects,and gels observed through scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP).In the MKG slurry,a high affinity of surfactants to liquid-air interfaces increased viscosity and promoted the generation of bubbles.Based on both the ionic types and molecular configurations of the surfactants,stronger adsorption ofa surfactant on the surface of the metakaolin resulted in better dispersion of metakaolin particles and a denser microstructure of the MKG.The surfactants with weaker adsorption (SLS and APG) caused higher porosity,a larger pore size,and more micro-defects,while those with stronger adsorption (BAC,SE,and STA) led to relatively lower porosity and denser microstructures.Density,water absorption,and compressive strength were closely related to the total intrusion porosity of the MKG.The mechanisms underlying surfactant adsorption to the surface of metakaolin are proposed.