3. Experimental results and discussion

#### 3.1 Basic characterization of H-Cement

Chemical composition of main constituents of H-Cement (HC) and Portland cement (PC) clinker are listed in Tables 1 and 2. Table 3 confirms that H-Cement shows different chemical and mineral compositions compared to PC. Both hydrated cements differ in typical characteristic values as stated in Table 4. The results suggest that hybrid cement can be used prospectively in concrete for the same purposes as the blended cements of lower-strength classes, e.g., CEM III–V, probably CEM II/B-S according to STN EN 197-1, preferably in massive constructions due to low hydration heat and aggressively exposed media because of low Portland clinker (and therefore C3A) content.

H-Cement consists essentially in a blend of materials containing 20–30% wt. Portland clinker and 70–80% alkaline cement, in turn a combination of fly ash, granulated blast furnace slag and alkaline sulphates.


#### Table 1.

produced by Lafarge Zementwerke GmbH, Mannersdorf (Austria) according to STN EN 197-1, both as reference, were chosen for the investigation. H-Cement was

Mortars with cement-to-sand ratio of 1:3 by weight adjusted on the constant

40 40 160 mm. All specimens were kept for 5-year exposure in 5% wt. sodium sulphate (Na2SO4) solution and water as reference medium after 28-day water curing (BC) at (20 1)°C. The tests of chemical resistance were conducted by own methodology of "partially accelerated test" based on keeping the mortars in strongly over-concentrated aggressive solution for a sufficient long time. The aggressive storage was specified by the following way: each 1 cm<sup>2</sup> of the exposed area of prism must be in permanent contact with at least of 10 cm<sup>3</sup> of 5% wt. Na2SO4 [33802.8 mg aggressive SO42 per 1 litre solution]. The sulphate solution and reference water were refreshed every 30 days within 90 days of testing, every 45 days between 90 and 365 days and every 60 days up to 5 years of exposure,

Consistency, density and fresh air content of the mortars were determined according to relevant STN EN Standards. The mortars were continuously tested for length changes, dynamic modules of elasticity (DME) and periodically flexural and compressive strength. After destructive tests the microstructure and pore structure were identified by X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) techniques. The ground material was sieved through a 0.063 mm mesh to receive the powder suitable for testing. For the X-ray diffraction, the Philips diffractometer was used in a 2Θ range of 5–65°. CuKα radiation and Ni filter were applied. Thermal analysis was performed on the Netzsch apparatus STA 449 F3 Jupiter in air at heating rate 10°C/min. Basic parameters of the pore

structure were identified by MIP using the high-pressure porosimeter

Tap water from the water supply system was used as the mixing water.

Quantachrome PoreMaster 60 GT using small mortar fragments for testing. The JEOL 7500F was used to study microstructure by scanning electron microscopy. Chemical composition, with special emphasis on the bound SO3 content, was esti-

Two types of binders (PC and H-Cement as HC) were chosen to prepare concrete based on steel slag. The Sika® ViscoCrete®-225 powder superplasticizer was used to improve the consistency of the fresh concrete mixture. It provides water reduction, excellent fluidity and cohesion, together with a self-compacting effect.

Two mixes designated as HC concrete and PC concrete were used within the

• HC concrete contained 0/8 mm steel slag fraction as filler, 380 kg of H-Cement, 241 kg of water, Sika superplasticizer in the amount of 0.5% of the cement weight and a retardant additive (Retardal 540) in the amount of 0.4%

consistency of 140 1 mm were prepared as prismatic specimens of size

tested as the verified sample.

Compressive Strength of Concrete

respectively.

2.6.3 Items of investigation

mated by STN EN 196-2 [19].

scope of the experimental research.

of the cement weight.

130

2.7 Concrete based on H-Cement and steel slag

2.6.2 Specimens, casting and curing

Chemical composition of fly ash (FA) and blast furnace slag (GBFS).


Table 2.

Chemical and mineralogical composition of Portland clinker.

