3.2.2. Properties of Hardened Concrete

• Compressive strength

The compressive strength was determined, after 2, 7, 28 and 90 days of curing, on 10 cm cubic samples. For concretes with w/c = 0.35, strength tests were also performed after 1 day of curing. The results are presented in Figures 5 and 6.

The highest compressive strength, at w/c = 0.35 and 0.60, was obtained by concrete made of Portland multicomponent cement C(30S-10LL). The lowest compressive strength was achieved with C(30V-10LL) on Portland ash and lime cement, despite the fact that it contained more Portland clinker compared to the other two CEM VI cement concretes. Limestone and siliceous fly ash show a synergistic effect only with early strength (2 days) and only with 10% addition of limestone, which confirms the results obtained by De Werdt et al. [40–42]. The results obtained from strength tests of concretes made of slag-calcareous cements provide a reason for the prospective wider use of limestone in cement composition.

Decreasing the water-cement ratio (w/c) from 0.60 to 0.35 resulted in a significant increase in the compressive strength of concretes made of all cements tested (Figure 7). This increase is particularly visible in the initial period of concrete hardening, i.e., until the 7th day. It can be noted that the lowest results were obtained for concrete using cement C(30V-10LL), however, the compressive strength after 28 days was nearly 70 Mpa and was almost twice as high as the strength obtained at w/c = 0.60. To sum up, it should be stated that a low w/c ratio is a very effective factor in shaping the strength characteristics of concrete made of cements with low Portland clinker content (Figure 7).

**Figure 5.** Compressive strength of concrete (w/c = 0.60).

**Figure 6.** Compressive strength of concrete (w/c = 0.35).

**Figure 7.** The impact of reduced w/c ratio on the compressive strength of concretes made of tested ternary cements.

• Water absorption and water penetration under pressure

All tested concretes, at the same w/c, show similar water absorption. For concretes with w/c = 0.60 the absorption varies between 6.6% to 7.0%, while for concretes with reduced w/c = 0.35 it is much lower and ranges from 3.3% to 4.1% (Figure 8). Extension of the curing time to 90 days resulted in a slight decrease in absorption of the tested concretes, most noticeable for concrete made with C(35S-20V) cement.

(**b**)

**Figure 8.** Water absorption of concrete after: (**a**) 28 days of curing, (**b**) 90 days of curing.

The results of the study on the depth of water penetration under pressure (Figure 9) show that concretes with w/c = 0.35 are characterized by very high tightness, especially after 90 days of curing. The depth of water penetration under pressure at w/c = 0.35 was maximum 15 mm for concrete with cement C(35S-20V) after 28 days of curing. Concretes with w/c = 0.60 showed water penetration depth after 28 days of hardening at the level from 15.3 mm to 43.7 mm and from 7.7 mm to 14.7 mm for concrete curing for 90 days (Figure 9).

**Figure 9.** Water penetration depth of concrete after: (**a**) 28 days of curing, (**b**) 90 days of curing

Omitting the influence of the w/c ratio, the differences in the depth of water penetration inside the concrete matrices primarily result from the different activity of the main components of cements used. The most active component, apart from Portland clinker, is ground granulated blast furnace slag, whereas fly ash is a component with pozzolanic activity (ability to react in the presence of moisture, with Ca(OH)2 from the hydration of silicate phases of Portland clinker). The impact of this reaction on the properties of mortar (concrete) is earliest visible about 28 days and later (Figure 9b). The addition of limestone improves the porosity of the cement-ash/slag system. Limestone, as a soft component, is ground into very fine grains, which fill the voids between cement and ash/slag grains. It results in increased early strength (after 2 days) in relation to the cement included only fly ash. After a longer period of time (28 days and later), cements containing granular blast furnace slag in the composition with ash (S, V) or limestone (S, LL) have higher strength and tightness.

• Carbonation susceptibility

The type of cement used was assessed for its susceptibility to carbonation (Figure 10). The test was carried out using an elevated CO2 concentration 4%, the test duration was 70 days (accelerated method). Analyzing the results obtained for concretes at w/c = 0.60, it can be seen that the highest depth of carbonation is characterized by concrete with Portland multicomponent cement C(30V-10LL), after 28 days of hardening the depth of carbonation reaches 29.7 mm, and after 90 days it is 18.6 mm. Reducing the water–cement ratio to the level w/c = 0.35 very effectively lowered the depth of concrete carbonation (Figure 10). A significant decrease in the depth of carbonation linked with the extension of curing period should be associated with the activity of the cement components used, mainly ground granulated blast furnace slag (hydraulic activity) and fly ash (pozzolanic activity). Additional amounts of products formed later (after 90 days of curing) from the course of reaction between cement hydration products and active mineral additives, settle in the pores of hardening cement slurry and hinder the permeation and penetration of aggressive ions [21,23].

(**a**)

**Figure 10.** Carbonation depth of concrete after: (**a**) 28 days of curing, (**b**) 90 days of curing.

• Chloride ions permeation

The permeability limitation of the concrete matrix is confirmed by the results of chloride ion permeation (Figure 11). Extending the curing period to 90 days or decreasing the water and cement ratio to w/c = 0.35 results in a significant decrease in the permeation of chloride ions corresponding to low or very low permeation class according to ASTM C 1202-05 [37] for both test dates. The differences in chloride ion permeability between concrete samples can be explained in the same way as was described in the water penetration analysis, as this feature is strictly related to tightness of concrete.

**Figure 11.** Chloride ions permeation of concrete after: (**a**) 28 days of curing, (**b**) 90 days of curing.

• Freeze–thaw resistance

An important feature of concrete, used in areas with minus temperatures, is its resistance to such environmental impact. Resistance of concrete to cyclic freezing and unfreezing was determined by the ordinary method (150 cycles of freezing at −18 ◦C and unfreezing of concrete at 18 ◦C, duration time of 1 cycle was 6 h (3 h of freezing and 3 h of thawing)) for concretes with coefficient w/c = 0.35. The test was performed after 28 and 90 days of concrete curing. The result of the test is positive if the decrease of compressive strength is less than 20% and the loss of mass is not more than 5% of weight. The test of concrete surface resistance to frost (56 cycles) in the presence of NaCl de-icing salt was also performed. Concrete was evaluated after 28 days of curing. The test results are presented in Table 8 and Figures 12 and 13.

**Table 8.** Results of frost-resistance tests.


The type of applied cement affects the durability of concrete under cyclic freezing and unfreezing conditions, especially when de-icing agents are used. The worst results were obtained for concrete made of Portland multicomponent cement C(30V-10LL) (Figures 12 and 13; Table 8). The decrease in compressive strength after the frost resistance test using the normal method reached 19.2% for concrete subjected to alternating temperatures after 28 days of curing and 15.1% for concrete subjected to the test after 90 days of curing. For comparison, concretes made of other cements were characterized by strength decreases at a much lower level of 7.6–7.7% for 28-day samples and 0.2–1.5% for 90-day samples. Concrete samples after the frost-resistance test did not show significant changes in mass.

When analyzing the results of the surface resistance of concrete to frost in the presence of de-icing salt (Table 8, Figure 13), it is clear that only concrete made of Portland multicomponent cement C(30S-10LL) can be classified as resistant. However, remaining concretes, especially concrete made of Portland multicomponent cement C(30V-10LL), show considerable scaling.

**Figure 12.** Frost resistance of concretes tested by the standard method [38].

**Figure 13.** Frost resistance of concretes in the presence of de-icing salt in relation to resistance category according to EN 13877-2:2013-08 [43].
