Investigation of the Segregation Behavior of Different Mortar Constituents with TGA/SDTA

Introduction 

The mineral constituents of mortar mixtures can be divided into two main groups - “reactive binders” and “inert fillers”. By mineral binders, we usually mean cement, e.g. Portland cement or aluminous (high-alumina) cement, anhydrite and/or hemi-hydrate (calcium sulfate) that hydrate when mixed with water. The hydrates form a solid matrix that binds the mortar together. Quartz or carbonate-rich sand and/or their finely crushed powder are used as fillers. Other possible mortar constituents are latent hydraulic binders, glasses, lightweight fillers and a large number of organic additives such as cellulose ethers and redispersible powders.

Fresh mortar is made by adding water and mixing well. The particular working consistency of the mortar depends on the application. When used as an adhesive for wall tiles, the shear strength and viscosity should be on the high side. For self-leveling flooring, the mass may segregate to a certain degree. The mortar must remain homogeneous, i.e. the coarse mineral constituents should not settle, and the water with the dissolved and dispersed fine components should not rise to the surface too much. This article describes how the segregation behavior of freshly made mortar was quantitatively analyzed using TGA/SDTA. A simple tile adhesive recipe was used as an example.

Sample Preparation

The tile adhesive consists of 40% Portland cement (CEM I 52.5 R, JCF, Wildegg), 59.6% quartz sand (0.1-0.3 mm, Zimmerli Mineralwerke, Zürich) and 0.4% methylhydroxyethyl cellulose (MHEC 15000 PFF from Aqualon). of the dry mixture were stirred with 23 g of water and transferred to a concrete slab with a suitable tool. After five minutes, a 50x50 mm earthenware tile was placed on the mortar and a load of 2 kg applied. This compressed the mortar layer to 1.6 mm. After setting, the tile was broken off and samples of the mortar were taken sequentially along a vertical profile from the tile surface to the concrete slab. The weight of each sample compared with the total weight of all the samples allowed the distance along the 1.6 mm long profile to be approximately estimated.

Summary and conclusions  

To investigate the segregation behavior in the fresh mortar state, several samples were taken along a depth profile of the mortar after setting. The composition of the samples was determined quantitatively using TGA/SDTA, which allowed the heterogeneous distribution curve of each mortar constituent to be determined over the entire length of the profile. Hydrates (e.g. portlandite) and carbonates can be stoichiometrically quantified via each weight loss step of their dehydration/carbonate decomposition. In principle organic additives such as methylhydroxyethyl cellulose (MHEC) can also be quantified via the weight loss step of their exothermal combustion reaction. The relative amounts of organic additives are frequently so small that the corresponding weight loss steps are overlaid by dehydration reactions of different hydrates (C-S-H phases?).

In the case described, however, the relatively strong exothermic decomposition of the small amount of 0.4% MHEC produces a clear SDTA signal. Using a calibration curve set up beforehand, the MHEC content can be determined with sufficient accuracy even for low MHEC contents using the SDTA peak area. Quartz fillers and cements are thermally stable in the temperature range of the measurements and together make up the ignition residue. The quartz content can also be quantified using the endothermic transition from - to -quartz at 573 °C by means of a calibration curve. The cement content is obtained from the difference between the ignition residue and the quartz content. In addition the inhomogeneous distribution of substances such as the enrichment of MHEC toward the tile and concrete slab can be clearly shown using the depth profiles for different mortar constituents obtained from TGA/SDTA measurements.

All in all one can say that TGA/SDTA is a very good method for the analysis of mortar. If the mortar systems are not too complex, all the important constituents can be quantitatively or semi-quantitatively determined. In addition the inhomogeneous enrichment of substances can be detected, which in turn yields information on different fractionization processes in the fresh mortar state (sedimentation, surface activity and migration with the pore water) and also after setting (different local degrees of hydration and carbonate formation).

^{Investigation of the segregation behavior of different mortar constituents with TGA/SDTA | Thermal Analysis Application No. UC 133 | Application published in METTLER TOLEDO Thermal Analysis UserCom 13}