Influence of Selected Admixtures on the Microstructure of Renovation Plaster Mortars

K e y w o r d s : Microstructure of the renovation plaster mortars; Mortars admixture; Porosity; Renovation plaster mortars. 3/2018 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 79 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 3/2018 d o i : 1 0 . 2 1 3 0 7 / A C E E 2 0 1 8 0 4 0 W . B r a c h a c z e k admixtures are found in the batched water together with aeration admixtures [9]. When using aeration admixtures together with hydrophobizing admixtures, there may be problems with maintaining the proper level of aeration of the mixture and the porosity of the hardened mortar resulting from the lack of compatibility. The problem of the effect of hydrophobizing admixtures on the porosity of renovation plaster mortars is scarcely described in the literature and requires further indepth studies. The paper presents the results of research on the influence of selected chemical admixtures on the microstructure of mortars. The original mix design of porous plaster mortars modified with aeration and hydrophobizing admixtures was used for testing as a porous plaster model. On this basis, a general dependence of the influence of selected components on the microstructure of renovation plaster mortars was developed. 2. SUBJECT OF THE STUDY AND RESEARCH METHODOLOGY USED The purpose of the study was getting to know the influence of aeration and hydrophobizing admixtures on the porosity and pore size distribution in renovation plaster mortars. 2.1. Investigation of the influence of aeration admixtures on the plaster mortar microstructure The influence of aeration admixtures on pore size distribution was investigated. The following aerating admixtures were selected: aerator 1 – based on sodium α-olefin sulfonate (C14-16) and aerator 2-based on sodium lauryl sulfate (SLS) (Fig.1). Six plaster mortar mix designs with the compositions detailed in Table 1, were selected for the tests. Samples were made of plaster mortars and marked with symbols S1 to S6. The required amount of water needed to generate air bubbles by the aeration admixture was determined on the basis of preliminary tests. The optimum water / cement ratio (w/c) in fresh mortar, in the aspect of aeration of fresh mortar using aeration admixture, was set at 1.6. 80 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 3/2018 Figure 1. Examples of aeration admixtures used in renovation plasters Figure 2. An example of a hydrophobizing admixture Table 1. The composition of dry mixtures of plaster mortars Components Symbols and compositions of dry mixtures, (% by weight) S 1 S 2 S 3 S 4 S 5 S 6 Portland cement CEM I52.5 R 15 15 15 15 15 15 Quartz sand 0.0 ÷ 1.0 mm 73.66 73.36 73.15 72.34 74.69 73.84 Limestone powder 45 μm 5 5 5 5 5 5 Hydrated lime Ca (OH)2 5 5 5 5 5 5 Pearlite 1 1 1.5 2 0.5 HEMC (Hydroxyethyl methyl cellulose) 0.1 0.1 0.1 0.1 0.1 0.13 PVAc (Poly vinyl acetate) 0.2 0.5 0.2 0.5 0.2 0.5 Aerator 1 0.04 0.04 0.05 0.06 0.01 0.03 Aerator 2 0.005 0.01 0.005 0.01 – – Water* 24 23 25 26 24 23 w/c 1.6 1.53 1.67 1.73 1.6 1.53 * water is given with respect to the weight of dry ingredients necessary to achieve a consistency of 17 ± 1 cm I N F L U E N C E O F S E L E C T E D ADM I X T U R E S ON TH E M I C ROS T RU C T UR E O F R ENO VAT I O N P L A S T E R MOR TA R S An automatic laboratory mixer was used to mix mortars. Mixing time was determined experimentally based on the density changes of fresh mortar. The optimum mixing time was determined to be 3 minutes at 62 rpm/140 rpm. Based on such mortar, the samples were prepared in the shape of cylinder with the height of 20 mm and the diameter 100 mm. The conditions of these samples curing and the method of preparation were in accordance with PN-EN 1015-11:2001 [13]. The porosity measurement was carried out in accordance with the methodology described in WTA Merkblatt 2-09-04/D Sanierputzsysteme [14]. The pore volume distribution against their effective radii was determined using Carlo-Erba 4000 mercury porosimeter. Differential pore volume distribution allows to specify which range of dimensions is the most common for these pores; furthermore, it explains whether the pore distribution is monoor polymodal. The pore volume for the equivalent diameter from 7.5 μm to 1.8 nm was obtained by determining the volume of mercury which, when gradually increasing the pressure, fills the pores of increasingly lower diameter. The pore diameter of up to 200 μm was determined by measuring the mercury penetration when equalizing the pressure to atmospheric pressure (after degassing and filling with mercury). The results of the measurement of total porosity (%), density (g/dm3) and the volume distribution of capillaries depending on their effective radii are presented in Table 2. In Fig. 3 and 4, the curves for differential pore volume distribution and capillary volume distribution depending on their effective radii, is presented. The lowest porosity was that of S5 and S6 plaster mortars. The total porosity [%] for these mortars was 24.5% and 32.7%, respectively. The curve of the differential pore volume distribution was monomodal (Fig. 4a.). In the case of S5 plaster mortar, pores with a diameter of 0.01 to 7 μm dominated, while for S6 plaster mortar the largest pore volume occurred for diameters from 0.01 to 20 μm. Observed differences could be due to the change of aggregates grain size from 0.0–0.5 to 0.0–1.0 mm and a higher dose of aeration admixture 1. For these plaster mortars, pores created in the intergrain space, which is partially filled with hydrates produced during cement hydration, can be expected. Due to the ability to transport moisture, the determined pore range corresponded to capillary active pores, with a significant proportion of pores with a diameter below 0.01 μm, which are gel pores and impact plaster mortar durability [15, 16]. For mortars S1 to S4, the total porosity ranged from 41.6% to 58.4% (Table 2).The difference in pore volume distribution depending on their radii was polymodal. The largest pore volume was between 0.07 and 30 μm, and then the pore volume decreased slightly. The next increase in volume was corresponding to the pore size above 40 μm (Fig. 3a). It was also found that for plaster mortars with different porosity, the peaks in the graphs (Fig. 3a) were not shifted. This means that as the dose of aeration admixtures is increased, pores of all dimensions are formed in similar proportions. The polymodal pore size distribution can be explained by the use of two different aeration admixtures in the formulations: aerator 1 – based on sodium α-olefin sulfonate (C14-16) and aerator 2 – based on sodium lauryl sulfate (SLS) and pearlite, which can form pores in a wide range of diameters. In addition, it can be concluded that there was an interaction of aeration admixtures with pearlite. The study was conducted based on S5 recipe, Table 1. The differences, which stem from changing the amount of the ingredients, were regulated up to 100% with quartz sand. C I V I L E N G I N E E R I N G 3 /2018 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 81 Table 2. Porosity Pc (%), pore volume as a function of radii, as total porosity percentage (%), density (kg/dm3) of the studied plasters Symbol Total porosity (% vol) Radius of capillaries (μm) Density (kg/dm3) <0.01 0.01÷0.1 0.1–1.0 1.0–10 10-100 >100 Percentage of pore volume (%) S1 41.6 – 4.5 15.6 32.3 34.8 12.8 1.35 S2 45.4 – 3.7 13.4 27.9 34.5 20.5 1.31 S3 52.1 – 3.2 11.9 28.9 38.7 17.3 1.11 S4 58.4 – 6.2 11.6 27.8 38.5 15.9 1.03 S5 24.5 2.3 12.7 69.7 12.7 2.3 0.3 1.55 S6 32.7 3.0 9.7 14.0 66.9 5.6 0.8 1.45 ce W . B r a c h a c z e k With the increase of the amount of aeration admixtures, the porosity of the hardened mortar grew unevenly (Fig. 5a). For aeration admixture – 1, with lower amounts of this admixture ranging from 0.0 to 0.02%, the porosity increased slower than for the amounts from 0.04 to 0.07%. It reached a maximum pore volume of 48% with 0.07% of admixture 1. A further increase in the amount of this admixture caused a decrease in porosity. For admixture 2, an increase in porosity occurred already with a content of 0.01%, with the porosity reaching 23%, close to the maximum (Fig. 5a). It was found that the porosity of hardened mortars can be influenced by admixture properties, the most important of which include 82 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 3/2018 Figure 5. The influence of the amount of aeration admixtures 1 and 2 on the porosity Pc (%) (Fig. 5a), pore volume distribution depending on the effective radii (Fig. 5b) Figure 3. The curves for differential pore volume distribution (Fig. 3a) and capillary volume distribution depending on their effective radii for samples S1–S4 (Fig. 3b) Figure 4. The curves for differential pore volume distribution (Fig. 4a) and capillary volume distribution depending on their effective radii for plasters of S5–S6 mortars (Fig. 4b) a b