The main aim of this article, carried out in relation to ash–cement mortars, is to determine the effect of complex additives of polyfunctional modifiers, including, in addition to superplasticizers, air-entraining and water-retaining additives, at different values of water–cement ratios. With the use of experimental–statistical models, the complex effect on the adhesive strength of cement–ash mortars of water–cement and ash–cement ratios, as well as complex additives of polyfunctional modifiers, including air-entraining and water-retaining additives, is considered. The extreme nature of the water–cement and ash–cement ratios on the adhesive strength of ash–cement mortars are established. Their optimal values are in the ranges of 0.7–0.75 and 0.35–0.4, respectively. The addition of a naphthalene-formaldehyde superplasticizer makes it possible to increase the adhesive strength of mortars by up to 40%. A positive effect is achieved along with the addition of a superplasticizer by introducing optimal amounts of air-entraining and water-retaining additives into the mortar mixtures. Quantitative parameters of mortar compositions that positively affect adhesive strength are established. The influence on the adhesive strength of the fly ash was also investigated, as well as on the binder–sand ratio. In addition, a positive effect on the adhesive strength of modified cement–ash mortars was experimentally shown by increasing the specific surface area of fly ash by regrinding it and increasing the cement–sand ratio.