Portland cement is an essential component for the daily life. It is one of the most consumed materials in the world. The global cement consumption was expected to grow from 448 kg/person in 2009, to 539 kg/person in 2012 and to 645 kg/person by 2017 (1). The cement industry in the Arab world represents ∼6% of the world production which amounts to ∼3.3 Bt cement/year (2). 168 cement plants with a production capacity of 235 Mt cement/year are presently found in this area and shall expand to ∼310 Mt cement/ year within the next five years. Oil and natural gas formulates 98% of the energy sources needed for the power plants; the cement industry alone consuming 2.1% of the primary fuel of the total industrial sector, with an energy cost of ∼40–60% from the total production expenses (3).

On the other hand, the climate change is directly related to the CO₂ emission. On the global level, the cement industry produces ∼1.4 Bt CO₂/ year, i.e. ∼6% of the total CO₂ production and ∼4% of the global warning (4, 5). 54% from the CO₂ are provided from the calcination process, 34% from the fuel combustion and ∼12% from the electricity. An average of 0.83 tons of CO₂ is emitted per ton of cement with 80% clinker (6).

There is therefore an urgent need to reduce the energy consumption and the CO₂ emission in the cement sector. Recent data revealed that a good utilization of waste materials might reduce the CO₂ emission by up to 9% (7).

One way to achieve these targets is to limit the consumption of Portland cement clinker by enhancing the production of blended cements within the scope of the international standard specifications such as EN 197-1, EN 14216 and ES 4756 (8–10). Most of the cement replacement materials cited in the specification are industrial wastes of suitable latent hydraulic properties. Several authors were concerned with the production of the low energy belite cements using wastes materials of environmental benefits, such as fly ash (11–14), rice hull ash (15) or slag (16).

Other solutions deal with the partial substitution of the high-lime cement by the lower temperature belite phase with the stabilization of the more active belite polymorph, the α′-phases (17, 18). Belite-type of cement also referred as “green” is of great economic and environmental interest. It reduces the CO₂ and the energy consumptions due to the lower firing temperature and the lower limestone content compared to alite cement. The energy demand is estimated to be 15–20% less than that needed for ordinary Portland cement at a lime saturation factor of 80–85% (19, 20). A drop in the lime saturation standard of the kiln meal from 100 to 88% results in a reduction in fuel requirement from 3800 kJ/kg to 3400 KJ/kg in the energy – and raw materials dependent CO₂ emission by 10% (21).

To overcome the lack of early strength, the high belite cements are blended with calcium sulfoaluminate or sulfoferrite (22, 23). Belite cements with limited hydraulic reactivity are interesting for massive constructions thanks to their limited heat of hydration.

The activation of a low temperature clinker through the stabilization of the α‘- belite modification adds value to the cement manufacture process and leads to better durability.

Another important aspect related to the reduction of the production costs is the use of alternative raw materials and/or alternative fuels (24–29). The extent of this subject is diverse and needs deeper understanding.

The aim of the present paper deals with the use of alternative raw materials to contribute to the reduction of energy, CO₂ and production cost. In this work, the effect of replacing parts of the clays and limestone with red brick powder and egg shells on the quality of the clinker and the respective cement is investigated.

The raw materials used were limestone, sandy and fatty clays quarried from the quarries of Tourah cement plant, Suez Cement Group, Cairo, Egypt. Iron scales supplied from Alexandria Steel Company were utilized as a corrective material. Red brick powder was available from the red brick factories located in the surrounding of the cement factory and the egg shells were obtained from the alimentation market.

The oxide composition of the raw materials was determined by means of X-ray fluorescence using a Philips apparatus PW 2404 Type X-CEM. The trace elements of the red brick powder and the egg shells were detected by carrying out the scanning mode of the XRF apparatus on the samples. The alkali contents were estimated with the help of a flame photometer Jenway PFP7.

The mineralogical composition of the raw materials was defined by means of X'PERT MPD Philips X-ray diffractometer and their thermal behavior was measured by differential thermal and thermogravimetric analyses on a 60H Shimadzu equipment.

Three mixes were designed to prepare raw meals of the high belite cement with a lime saturation factor of 77%, silica and alumina modulus of ∼2.2 and ∼1.2 respectively (Table 1): The reference mix (M_1ref) was prepared from 77% limestone, 11% sandy clays, 11% fatty clays and 1% iron scales. In the second mix (M₂) the fatty clays were totally replaced by 10% red brick powder and the amount of sandy clays was raised to 12%. Finally the third mix (M₃) was composed of 20% sandy clays, the fatty clays were reduced to 2% and 10% of limestone was replaced by egg shells.

The raw materials were separately dried, finely ground and sieved up to a residue of 15–17% on a 90 µm sieve. They were intimately mixed and pressed as pellets 40 mm diameter. The pellets were burned 15 minutes in a platinum crucible at 1350 °C. At the end of the burning period the clinkers obtained were quenched in air at room temperature with an approximate cooling rate of 850 °C/ minute then finely ground to a fineness <90 µm, their free lime content indicating their burnability (30) was measured according to the method of Javellana et al. (31).

The oxide composition of the clinkers was determined by means of X-ray fluorescence; (XRF, PANalytical; Model Magix Pro PW 2440, PANalytical). Their phase identification was performed with EVA software and Rietveld refinements were performed using TOPAS 3, Bruker-AXS Panatical X'Pert. Supplementary diffractograms were obtained at low scanning modes in the 2 theta region from 25 to 35° then from 35 to 40° on the X'PERT MPD Philips X-ray diffractometer.

The boron concentration in the clinkers was checked by means of atomic absorption apparatus model GBC Avanta.

The clinkers were treated with KOH-sucrose to dissolve C₃A and C₄AF for 10 minutes (27) leaving the silicate phases (C₃S and C₂S) in the solid residues according to Gutteridge (32). The trace elements of residues were determined using the scanning mode of the X-ray fluorescence apparatus as in the case of the raw materials.

The morphology of fractured surfaces was examined with the secondary electron mode by means of Quanta 250 FEG scanning electron microscope as well as on the back scattered mode with EDAX on Zeiss EVO 50XVP, analyzer Bruker AXS XFlash Detector 410.

The gypsum content added to each clinker was estimated from the Na equivalent and C₃A in the clinker as follows: Naeq = Na% + 0.658 K%, the optimum SO₃ = 1.841 + 0.095 C₃A + 1.636 Naeq (33). 6.9 to 7.1% of gypsum was added to the three clinkers to obtain SO₃ contents of 3.22 to 3.28%. The cements obtained were finely ground to a Blaine specific surface area of 3500 cm²/g as well as 4500 cm²/g. Their oxide and phase composition were determined as in the raw meal and their mechanical and physical properties were determined according to EN 196-1 to 6.

The compressive strength results were monitored with of a compression tester Toni Technik Model 2010.010 with an accuracy of 0.04 ± 0.01%.

The manufacture of low lime cement aims to reduce the energy consumption, the green house effect as well as the production cost. It is realized through the decrease of the LSF of the clinker. Lowering the LSF from 100 to 75 leads to a 12% reduction in the theoretical heat requirement and 6% in CO₂ emission (34). The lower limit of the LSF should be higher than 75% to avoid the formation of alite-free clinker and C₁₂A₇ instead of C₃A (35).

The LSF of the clinkers produced in this work is ∼77%. It was successfully used for the formation of high belite clinker with 60–67% C₂S, and 5–10%.C₃S. The formation of the high belite clinkers is supported by the morphology of the phases in the SEM micrographs.

The differentiation between the various belite polymorphs is difficult but recent work was indicative with the use of synchrotron analysis (36–38).

The present study indicates a probable stabilization of the α′-belite polymorph in the clinkers made of alternative raw materials due to the weakness of lamina and striae in the clinker grains of Clik₂ and Clik₃.

According to the literature, the α′-belite phase is stabilized by P₂O₅ (39, 40) and concentration of up to 1.0% P₂O₅ by mass in the clinker do not have an adverse effect on its properties (40). In this work the concentrations of the phosphate in the three clinkers is below 1% and lay in the range of 0.11–0.17%. Furthermore the atomic absorption analysis proved the absence of boron, omitting thus its possible role in the formation of the α′ polymorph reported in the literature (26, 38). The presence of alkali oxides with an almost equal amounts in the three clinkers; (Na₂O = 0.21–0.28%, K₂O = 0.22–0.28%) reflects its insignificant role in this phenomena. 0.1% Na₂O was reported to affect the stabilization of the active belite polymorph positively (38).

However, the significantly high concentrations of strontium (1272 ppm) in the residue of Clik₂ made of red brick powder could be a strong indication for a role played by this element in the stabilization of the α′-belite phase. Previous work was published on the strontium stabilized belite and on the α′-β transformations in strontium oxide-doped dicalcium silicate (41–43). Zirconium is the next highest concentration in Clik₂ residue and amounts to 252 ppm. With reference to the literature (26, 44) a high ZrO₂ concentration does not affect the melt process.

The strontium concentration detected in Clik₃ made of egg shell (544.5 ppm) is lower than those of Clik₂ and Clik_1ref. The morphology of the clinker grains attributed to the possible stabilization of the active belite phase could be due to the barium content (321 ppm) as previously reported in the literature (21).

The higher 28 days strength values of the second cement prepared from Clik₂ is attributed to a higher alite content but the contribution of the more reactive α′-belite modification to the strength values is not excluded.

The following conclusions can be drawn for the present work:

Red brick powder and egg shells can be successfully used in the cement manufacture process