Effect of soapstock in the composition of modified additive for improving strength characteristics of concrete structures

The article presents the results of research on the influence of soapstock used in the composition of two-component modified additive. Standard tests of beam samples for flexural and compressive strength, and standard cubic samples for water absorption and frost resistance were performed. Tests were performed for samples with different contents of soapstock (Sp): 5, 7.5, 10 and 12.5 % by weight of cement, microsilica and phosphogypsum. The strength measurements of the beam samples showed that the maximum effect concerning the increase in material strength was achieved at 5% of the soapstock content. However, it should be noted that the subsequent decrease in strength with increasing concentration of soapstock is not significant, up to Sp=10% does not exceed 1%. Thus, the optimal concentration of soapstock, at which the maximum effect on the strength of the material will be achieved, is 5-10%. The obtained curve of dependence of water absorption change on the soapstock concentration showed the optimal gradient of water absorption, which corresponds to Sp=10%. With further increase of soapstock, the decrease of water absorption index is not significant. Tests on frost resistance showed that the maximum resistance to cyclic freezing is observed in samples with Sp=10%, further increase reduces frost resistance. The regularity in the increase of frost resistance with increasing concentration of soapstock is logical because with each increase in concentration, the hydrophobization of the material increases. However, if the hydrophobicity of samples with Sp=12.5%, although not significantly, still increases about Sp=10%, the frost resistance decreases.