World cement production represented approximately 4 billion tonnes in 2013 (1). About 4.2 GJ energy is required to produce 1 tonne of OPC resulting in approximately 0.8–1.0 tonne of CO₂ released into the atmosphere (2). Global annual production of cement thus accounts for about 5% of the total anthropogenic CO₂ emission (3). A second problem aside from CO₂ emissions, which is worthy of a serious consideration is connected with the negative ecological and health impact of red and brown mud ponds containing high alkaline wastewater rich in NaOH. The large environmental accidents like the one in Hungary in 2010 may occur during the storage of such waste (4). The land filled by this hazardous waste also raises other problems occurred due to the high concentrations of the heavy metals present and the variety of chemical compositions, depending on a large extent on season and weather conditions. High alkalinity can be neutralized by addition of H₂SO₄ and CO₂ with subsequent formation of either Na₂SO₄ or Na₂CO₃ (5).

The main task of the research in recent times is the development of less energy intensive and more environmentally acceptable cements with highly efficient recovery and safe disposal of wastes and industrial by-products such as fly ashes from power and heating plants, metallurgical slags - mainly granulated blast furnace slag (GBFS), highly alkaline wastewaters from red mud ponds as well as sulphuric acid from car battery recovery (6).

One of potential solutions is the production of new cement binders, like alkali-activated cements (geopolymers) and hybrid alkali cements with Portland clinker addition (7, 8). Hybrid alkali cements have various compositions of alkali-activated alumino silicates with low Portland cement clinker content (9). Despite of low clinker content, the hybrid binders can also attain useful early mechanical strengths (10).

Typical mixtures of hybrid binders are composed of Portland cement clinker, usually less than 30 of mass % with fly ash activated by Na₂SO₄ and Na₂CO₃ (11); metakaolin and clinker activated by Na₂SO₄ (12); blast furnace slag, metakaolin and clinker activated by Na₂CO₃ and K₂CO₃ (13); the mixture of fly ash, OPC and Ca(OH)₂ activated by a calcined mixture of kaolin with NaOH or KOH (14). The activators Na₂CO₃, K₂CO₃, Na₂SO₄ and K₂SO₄ are not characterized by highly alkalinity.

High-alkaline NaOH is generated at the presence of Ca(OH)₂, which is the reaction product of on-going Portland cement clinker hydration. The alkalinity of the bulky cement matrix increases and creates favourable conditions for the consequent geopolymerisation process by means of high alkaline NaOH as shown in equations [1] and [2]:

Ca(OH)₂+Na₂CO₃ → CaCO₃+2NaOH    [1]

Calcite CaCO₃ (Eq. 1) and gypsum CaSO₄×2H₂O (Eq. 2) are generated as by-products. The interaction of gypsum with C₃A present in Portland cement clinker results in the formation of ettringite C₃A×3CaSO₄×32H₂O as demonstrated by equation [3]:

C₃A+3 CaSO₄×2H₂O+26 H₂O → C₃A ×3CaSO₄×32H₂O    [3]

Reaction mechanism (Eq. 1) is quicker but yields calcite only, while the presence of sulphate ions promotes ettringite formation and consequently a denser matrix with the increased early strength (11).

GBFS and siliceous fly ash are predominantly vitreous materials with amorphous structures dissolved in the presence of alkaline NaOH, Na₂SiO₃, Na₂SO₄ and Na₂CO₃ or KOH, K₂SiO₃, K₂SO₄ and K₂CO₃. After dissolution, alkali-activated alumino silicates, geopolymers are formed. Geopolymers possess a three-dimensional structure expressed by the formula Me_n[-(SiO₂)_z-AlO₂]_n×wH₂O, where Me is Na⁺ (N) or K⁺ (K), which in the framework balances the negative charge of Al(OH)_4-; n is the degree of polycondensation and z means 1, 2, 3 or >>3 (15). As a result, N-A-S-H or K-A-S-H gels are formed, which in the presence of Portland clinker may contain some amount of Ca to get a (C)-N-A-S-H gel. The curing conditions of fresh geopolymers are very important. After a short period, commonly between 6 and 12 hours of steam autoclaving or curing in dry air of about 60 °C to 80 °C, the three-dimensional structure of the geopolymers gets stronger. In contrast, hydration of the Portland cement clinker results predominantly in the formation of C-S-H gels, whose microstructure contain some amount of Al to get C-(A)-S-H gels and portlandite Ca(OH)₂ formed to a smaller extent. Portlandite promotes the pozzolanic properties of fly ashes to get secondary C-S-H and C-A-H gels and the latent hydraulic properties of granulated blast furnace slag to produce secondary C-(A)-S-H gels. Solid solutions of hydrotalcite minerals of Mg₆Al₂CO₃(OH)₁₆×4H₂O kind and C₄AH₁₃, the hydrogarnets C₃AS_3-nH_n and C₆AFS₂H₈, strätlingite C₂ASH₈, ettringite and sometimes carboaluminates C₃A.CaCO₃H₁₁ are also created.

Novel hybrid cement binders therefore contain both kinds of gels, C-(A)-S-H as well as (C)-N-A-S-H, whose collective chemical interaction results in the formation of a final (N,C)-A-S-H gel (11). Recently it has been found that hybrid cements based on a high content of fly ash from coal combustion (>70 mass %) and low content of Portland cement clinker in the presence of an alkaline activator give rise to the development of three different gel-bonding environments, tentatively attributed to C-(A)-S-H, C-A-S-H and (N,C)-A-S-H type gels (16). The related findings showed that the C-S-H/N-A-S-H mixture of the formed gels did not precipitate in a pure state but rather that their composition was affected by the presence of dissolved species (17). In the presence of aluminium C-S-H gel development is specified by next developmental sequence: C-S-H → C-(A)-S-H → C-A-S-H. In the presence of calcium, the N-A-S-H gel evolved as: N-A-S-H → (N, C)-A-S-H → C-A-S-H. This last conversion is not complete in these systems because the amount of calcium present is thought to be insufficient (17). Due to the intensive formation of gel-like hydration products hybrid cements are more chemically resistant to aggressive media in comparison with traditional OPC (18–20). The main disadvantage of ordinary geopolymers based on N-A-S-H or K-A-S-H gels lies in the relatively short initial setting time, which is being reduced by increasing alkalinity and steam autoclaving or dry curing. Contrary, their significant benefit is very high heat and fire resistance up to temperature elevation of 1100 °C (21). According to the gained knowledge (19), original strength of hybrid cement is almost doubled after the exposure to 800 °C and even to 1000 °C in contrast with OPC showing steady and continual strength losses as the temperatures rise up.

The second disadvantage of ordinary geopolymers based on the N-A-S-H or K-A-S-H gels that one has to take into the consideration lies in the increased susceptibility to shrinkage (22). Water glass-activated slag mortars and concretes are usually characterized by substantial autogenous and drying shrinkage, which represent a restriction of the widespread use of alkali-activated slag systems as alternative binders to traditional OPC in the structural elements (22). The shrinkage-reducing admixture based on polypropylenglycol reduces autogenous shrinkage by 85% and drying shrinkage by 50% in water glass-activated slag mortars (22).

Shrinkage is characteristic feature for geopolymer and OPC-based concrete. Shrinkage-reducing admixture such as ye'elimite 3CA.CaSO₄; or calcium sulphoaluminate cements based on 3CA. CaSO₄, C₂S, CaSO₄ and CaO or a mixture of C₃A and CaSO₄ or a mixture of calcium aluminate cement CA and CaSO₄, which provide the formation of expansive ettringite or a mixture of hard-burnt CaO and H₂O providing expansive Ca(OH)₂ can be applied in the concrete as the expansive addition. Hybrid cement takes advantage of material properties of traditional cement and inorganic geopolymer resulting in such benefits of utility properties that the hybrid cement can partly replace energy- and environmentally-intensive OPC (23).

This paper is focus on the basic properties of ready-mixed concretes containing hybrid cement. Expansion-reducing and mainly shrinkage-reducing properties of H-Cement in concrete cured either in water at temperature of 20±1 °C or dry air conditions of 20 °C/60% R. H. and 40 °C/15% R.H are next important observations of this article.

The novel hybrid cement (H-Cement) is composed of fly ash, granulated blast furnace slag (GBFS) and the highly alkaline waste water concentrate from red mud deposit as well as 20 mass % of Portland cement clinker. Chemical compositions of H-Cement and its raw material components are reported in Table 1 and Table 2, respectively.

The composition of H-Cement was adjusted on the minimum content of SiO₂, CaO, SO₃ and Na₂O of 40.0 mass %, 20.0 mass %, 3.0 mass % and 2.0 mass %, respectively. Highly alkaline waste water concentrate was obtained by the evaporation and electrodialysis of standard waste water occurring in the deposit. The concentrate was neutralised by sulfuric acid H₂SO₄ with precipitation of solid Na₂SO₄. The measured pH values before and after neutralisation were 13 and 5 to 7, respectively. The XFR analysis confirmed 99 mass % chemical purity of Na₂SO₄ when using apparatus SPECTRO X-LAB 2000. Main minerals containing fly ash are quartz SiO₂, mullite 3Al₂O₃×2SiO₂, hematite Fe₂O₃ and magnetite Fe₃O₄. GBFS consists mainly of 90% vitreous phase. The content of GBFS in the hybrid cement was estimated by microscope counting method. The sample, taken from a particular GBFS size fraction is examined in transmitted light (magnification degree 100) and counts at least one thousand slag grains in all. The GBFS grains of glassy fracture are clear, homogeneous and transparent for light. The GBFS grains of crystalline phase are darker not fully transparent. The calculation is as follows: % of glassy phase in GBFS=(Number of glassy, transparent grains of GBFS/Total number of all grains of GBFS) ×100. The portion of glassy phase was also checked by the procedures specified in CEN/TR (24) and EN Standard (25). H-Cement contains 19.3 mass % and OPC 87.59% of clinker minerals Mineralogical composition, determined by XRD technique at BRUKER AXS D8 Advance device supplemented by Rietveld analysis (26, 27) of both cements is reported in Table 3.

Portlandite Ca(OH)₂ (CH) is slowly formed from free lime C_free due to partial hydration by air humidity. Next abbreviations mean quartz SiO₂ (Q), free lime CaO (C_free), cubic C₃A (C₃Ac), orthorhombic C₃A (C₃Ao), periclase MgO (M). The expression ^xCaSO₄ gives the total sum of 4.44 mass % gypsum (CaSO₄×2 H₂O) +2.42 mass % bassanite (CaSO₄×0.5 H₂O) +1.52 mass % anhydrite (CaSO₄) +0.28 mass % calcite (CaCO₃), which are the main constituents of calcium sulphate used as the setting time regulator. The total amount of crystalline phases coming from clinker minerals in H-Cement is 20.0 mass %. Majority portion is constituted by the amorphous inorganic geopolymer, which is not detectable by XRD technique. The total content of crystalline phases of OPC clinker minerals is 91.34 mass % opposite to that of H-Cement showing only 20 mass % occurrence of crystalline phases of clinker origin.

The following conclusions are applicable to the particular test cements and test methods employed:

H-Cement is suitable for the production of ready-mixed concrete compressive strength classes up to C30/37. The plasticizer to achieve extended workability is based on lignosulfonate or polycarboxylate base; the most plasticity efficiency gives the combination of both admixtures.