THE INTERACTIONS BETWEEN ANTI-FOAMING AND SUPERPLASTICIZING ADMIXTURES AND THEIR CONSEQUENCE FOR PROPERTIES OF SELF-COMPACTING MORTAR AND CONCRETE

The first of the above tests allows assessing the homogeneity of the ready admixture during its storage. Uniformity is one of the basic features of mortar and concrete admixtures, which is subjected to quality control in accordance with Table 1 in EN 934-1. The test itself consists of placing the prepared admixtures in the oven at 40°C for seven days. Increased temperature causes the acceleration of ageing processes. After one week, the visual stability of the domes is assessed.

The effectiveness of anti-foaming admixture (Table 1) depending on its type and SP type (Table 2) was investigated in tests of flow and air-entrainment of mortar according to EN 1015-3:2000/A2:2007 [21] and EN 1015-7:2000 [22], respectively. Self-compacting mortars were composited with river sand 0–2 mm.

PCP1 is synthesized with long side chains. PCP2 has a rapid absorption to the cement particles and covers less surface, which ensures a large surface of cement particles to react with water and then accelerates the cement hydration. The process of mixing of the mortars started with dry ingredients. Then the PCP were added:

The mixing of mortars components was carried out in accordance with the procedure of PN EN 480-1 [23].

The cement CEM II/B-S was used because previous author’s research result indicated that most problems with adequate air-content of self-compacting mortar and concrete are in case of cement with mineral additives, like fly ash or granulated blast furnace.

The properties of the mortars were described by density, tested in accordance with EN 1015-10:2001 [24] standard, compression and flexural strength tested according to EN 1015-11:2001 + A1:2007 [25]. To assess AFA influence on the time relating to workability loss of the mortar, the assessment of mortar flow was measured after 10 and 60 minutes, counting from the time of components mixing. All tests of hardened mortars were carried out after 28 days on 4×4×16 cm mortar blocks curing in 20°C water.

In next step of the research, self-compacting concrete (Table 3) was made with CEM II/B-S 42.5 R, limestone (LS), 2/8 mm and 8/16 mm fraction gravel aggregate, 0/2 mm fraction sand, tap water, admixtures: PCP1, PCP2, and the most effective in action AFA and compatible with PCP1, indicated on the basis of mortars research result (two-step of the investigation, point 2.2).

The properties of the self-compacting concrete mixture were investigated using the following methods: self-compatibility test in accordance with the procedure [26], density test – in accordance with EN 12350-6:2000 [27], air content – the pressure method according to EN 12350-7:2001 [28].

The compressive strength of concrete was investigated according to EN 12390-3 [30]. The microstructure and porosity of hardened concrete P1, P2, P1A6b, after 28 days curing in water, were investigated using the following methods: porosity structure parameters are investi-gated according to PN-EN 480-11 [29], the microstructure of concrete according to SEM (Scanning Electron Microscopy). All tests of hardened concretes were carried out after 28 days on 15×15×15 cm concrete blocks curing in 20°C water.

Table 4 presents the results of ageing tests on the tested compatibility of admixtures. The lack of compatibility between the defoamer and the liquefaction admixture is manifested at this stage of the research by the lack of homogeneity of the liquid sample, e.g. sediment precipitation, delamination of two phases, turbidity. On the basis of research results presented in Table 4, it was concluded that only PCP1 is compatible with all types of anti-foaming admixture, while PCP 2 with AFA 6 only. Compatible admixture systems were selected for further testing.

In Figs. 1–5, test results on fresh mortars are presented. It was observed, that different type of PCP produced a radically different volume of air in self-compacting mortars. In case of normally consolidated mortars, 4% of air volume was observed. The mortar with air-entraining PCP1 contained even about 12% of air volume.

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Figure 5.

According to research results in Figs 3–5, AFA type 6 is most effective in decreasing the air-content and increasing mortar flow diameter without its segregation (Figs. 1 and 2). The type of AFA affects the time of flow mortar (plastic viscosity). Test results of mortars showed that AFA based on polyether-modified silicone (type 6), reduce the air-content, plastic viscosity and adhesion of fresh mortar most effectively.

In Figs. 3–5, tests results of the fresh mixture containing anti-foaming admixture mixing technology are presented. In one case, the admixture was introduced immediately after PCP addition. In other cases, admixture was introduced 10 minutes after starting the process of mixing and PCP addition.

The presented tests results prove that the time of introduction of anti-foaming admixture is not essential for the effectiveness of decreasing of mortar’s air-entrainment. This conclusion is very important because the application of anti-foaming admixtures may be used in order to: prevent the excessive air-entrainment of the mixture (introducing anti-foaming admixture with air-entrained PCP) or decrease existing air content (produced by air-entrained SP) in the fresh mixture. However, the type of AFA influences on air-content and consistency of mortar, independence when AFA was introduced into mortars (Figs. 6 and 7). Thus, the antifoaming admixture causes the increase of the flow diameter of the mortar and lowers the plastic viscosity, regardless of the time of its introduction. Anti-foaming admixture allows better self-compacting of the mortar. Research results in Fig. 8 indicate that the actions of different ad-mixtures disturb the relationship between diameter flow and air content in mortar.

Figure 6.

Figure 7.

Figure 8.

The results of the tests presented in Figs. 9 and 10 prove that mortars containing AFA maintain consistency longer than mortars without AFA. However, too many AFA negatives affect the mechanical properties and microstructure (Figs. 11 and 20). Moreover, the time of maintaining of consistency of mortar depends on the type of AFA (Fig. 10).

Figure 9.

Figure 10.

Figure 11.

Until laboratory tests of fresh mortars, it was indicated that AFA admixtures reduce the physical adhesion of fresh mortar. Degree of this reduction depends on the type of AFA. This conclusion needs further author’s laboratory tests, for the reasoning causes of this. How strong it depends on the type of AFA. The mortar with AFA loosens more easily from the cone to the mortar spreading test. Unfortunately, there is no standard way to test the adhesion of fresh mortars. The mechanism of adhesion has been studied for years. In order to provide an explanation for adhesion phenomena, several theories have been proposed. However, there is no standardized theory that describes all adhesive bonds in general in a comprehensive way. The bonding of an adhesive to a substrate includes numerous mechanical, physical, and chemical forces that influence each other. As it is impossible to separate these forces from each other, it can be divided into 5 different adhesion mechanisms, including mechanical, electrostatic, adsorption, chemisorption and diffusion theory [30, 31]. AFA admixtures reduce the physical adhesion of cement mortar. The physical adhesion results from the molecular contact between two materials and these two materials are held together by the “van der Waals” forces (Fig. 11). These are the weakest forces that contribute to the adhesive bonding but are quite sufficient to make strong joints. Van der Waals forces act between molecules of gaseous and liquid substances as well as between molecules in molecular crystal networks. Van der Waals forces can be divided into four main groups: dispersal forces, conformational forces, dipole forces and inductive forces.

In Figs. 12 and 13, the test results of mortars are presented. It was noticed that the anti-foaming admixtures significantly improve the compressive strength of mortars, due to reducing of air-content in the mortars. The compressive and flexural strength of mortars incorporating AFA (type 6) is like mortar strength with “not entraining” PCP2 (Figs. 12 and 13). AFA type influences on relationships between air-content and compressive strength of mortar (Fig. 13). Therefore, the compatibility tests of AFA and PCP admixtures should be extended to test the properties of the joints of the cement mortar, especially its strength.

Figure 12.

Figure 13.

In Figs. 14–16 the results of testing mixtures are presented. The analysis of the results indicates that the higher air-content as a result of PCP1 action causes a higher decrease of fresh mixture flow diameter. The introduction of AFA into the volume of SCC, apart from the air content reduction, leads to increase of its flow diameter and decreasing of flow time. Again, it has been proven that the presence of AFA, especially with a larger amount in SCC, allows for the initial workability of SCC to be maintained. However, too much AFA in the SCC volume negatively affects the compressive strength of the SCC, although the air content is reduced (Fig. 17).

Figure 14.

Figure 15.

Figure 16.

Figure 17.

Figure 18.

Figure 19.

Figure 20.

A similar effect of the influence of high dosage of AFA was noticed in the publication [14]. The compressive strength of ultra-high compressive strength concrete has firstly raised and then declined with the increasing addition of anti-foaming admixture. The maximum compressive strength of concrete at 28 days reaches 111.6 Mpa, 115.4 Mpa and 109.1 Mpa, being associated with 94.5 Mpa for the blank sample when AFA was added correspondingly. Therefore, these research results prove also that the addition of anti-foaming admixture has a significant increasing effect on strength of concrete. The reason for this may be the cracking of the microstructure (Figure 6).

Research results according to EN 480-11 are presented in Figs. 21–23. This research showed that AFA changes the size of airvoids and its volume in SCC. The air content in concrete with AFA is more than twice reduced, the value of the air void space factor is bigger when PCP and AFA are incorporated. The minimum point occurs at the dosage level of 0.50%, being in accordance with the strength and air content results mentioned above (Figs. 14–16 and 10).

Figure 21.

Figure 22.

Figure 23.

Figures 4–6 show the results of SEM (Scanning Electron Microscopy) of concrete with PCP 2 research results. It is evidently visible that the type of PCP and AFA admixtures influence significantly the SCC structure. Admixture generates different porosity, CSH structure and contact zone in SCC. The CSH phase of SCC with PCP2 is dense and homogeneous (Fig. 4). The cement paste tightly binds the aggregate. No calcium hydroxide was observed.

In Figure 5, the consequence of PCP1 action on the concrete microstructure is presented. This structure is the most amorphous, a lot of large pores are present in SCC microstructure. Moreover, a lot of calcium hydroxide (its hexagonal structure) overgrowing the air voids was observed.

In Figure 6, the microstructure of concrete with PCP1 and AFA type 6 was presented. Structure of concrete with AFA and PCP1 is less heterogeneous than the structure of SCC with PCP1 (compare Fig. 5 and 6). Moreover, no calcium hydroxide was found in SCC structure. The CSH phase is homogeneous and dense. The cement paste tightly binds the aggregate. There are no visible large air voids in Fig. 6.

Too much admixture AFA can cause micro-cracking of the concrete microstructure (Fig. 6), which reduces the compressive strength (Table 7). Results [19] also showed that lower yield stress and/or plastic viscosity and higher incorporation of AFA reduce the content of the air bubbles in hardened concrete. Higher mixing speed is more beneficial for reducing the content and average diameter of entrapped air bubble. This conclusion will be the starting point for further research by the author.