Investigation of the cellulose ethers effect on the Portland cement hydration by thermal analysis

Abstract

Cellulose ethers (CE) are introduced in almost all cement-based dry mortars in order to retain water in mortar mass avoiding losing it too quickly by substrate absorption or water evaporation. In this way the workability of the fresh material, the adherence to the substrate and internal-strength characteristics of mortar, render or tile adhesive are improved. One of the side effects of cellulose ethers is the Portland cement hydration delaying. The influence of six commercial cellulose ethers, hydroxyethylmethyl cellulose (HEMC) type, on the hydration of Portland cement CEM I 42.5 R, was followed by thermal analysis (TG and DTA curves). Three of these cellulose ethers are unmodified, and have different viscosities, while three of them have the same viscosity but differ in the degree of modification (unmodified, one with medium modification and one with high modification). The interest of dry mortars producers for the effects of these cellulose ethers, is generated by the wide offer available on the market and by the absence of systematic data on the effect of different viscosities and degrees of modification on dry mortars properties. In order to quantify the effect of the CE on the cement hydration, the surface area of the endothermic effect corresponding to the dehydration of portlandite (Ca(OH)2), formed after 1, 3, and 7 days of hydration, was defined. It was noted that the proportion of Ca(OH)2 in samples containing CE after 1 day was 30–40 % lower than in reference sample. After 3 and 7 days of hydration the proportion of Ca(OH)2 in samples containing CE approaches that of reference sample (10–20 % less). For the same period of hydration, the different viscosity, and different degree of modification of cellulose ethers cause variations in narrow limits of the proportion of Ca(OH)2, and the degree of cement hydration, respectively.