**Edyta Spychał \* and Przemysław Czapik**

Faculty of Civil Engineering and Architecture, Kielce University of Technology, 25-314 Kielce, Poland; p.czapik@tu.kielce.pl

**\*** Correspondence: espychal@tu.kielce.pl

Received: 20 November 2020; Accepted: 19 December 2020; Published: 20 December 2020

**Abstract:** In this article, the effect of hydroxyethyl methyl cellulose (HEMC), which is a polymeric viscosity modifying admixture on the mineral based composites setting processes, was studied. Previous studies available in the literature included the evaluation of the influence of this admixture on the hydration processes of cement or lime pastes. In this paper, the analysis of this issue was extended to include cement-lime composites. The composition of the pastes and mortars differed in the type of binder (the tests were carried out on cement-based and cement-lime-based materials, in which the cement was replaced in 50% with hydrated lime), as well as the amount and viscosity of the admixture. The study of mortars setting processes and hardening processes using the ultrasonic method was supplemented in the work with calorimetric measurements and phases analysis by the X-ray diffraction method. Finally, it was found that the HEMC reduces the rate of a hydration reaction in cement and cement-lime pastes. The amount of admixture used has a greater influence on the changes taking place during the setting process than the admixture viscosity or the type of binder.

**Keywords:** cement; hydrated lime; HEMC; mortar; paste; hydration; setting process; ultrasonic technique
