Use of alkyl sulfonate fractions (with overall hydrocarbon chain-length range of Cg-C,) as admixtures to improve the freezing resistance of Portland cement/sand mortars was studied.Alkyl sulfonates were synthesised from hydrocarbon mixtures obtained by polyethylene waste pyrolysis. They act as air-entraining agents but do not show plasticising effect. All of the three different fractions improve freezing resistance but higher than 9% air contents are detrimental to the strength values. The best compromise is the fraction Cg-C,y in 0.02% amount that raises the freezing resistance by 22-28% as estimated from the values of the dynamic modulus of elasticity measured after 200 freezing/thawing cycles. Under these conditions, the bending and compression strength retain not less than 90% of the values measured for the reference mortar after a six-month hardening period.