Sustainable modified pozzolanic supplementary cementitious materials based on natural zeolite, fly ash and silica fume

The present study examined the effectiveness of pozzolanic supplementary cementitious materials (P-SCMs), such as fly ash (FA), super fine zeolite (SFZ) and silica fume (SF), in reducing the energy intensity and CO2 emissions of concrete based on ordinary Portland cement. Optimization of the composition of the P-SCMs was carried out by the simplex-lattice Scheffe method according to the criterion of pozzolanic activity. The type and level of P-SCM replacement was analyzed to improve cement paste properties and target CO2 reduction in concretes. It is shown that the combination of pozzolanic materials of different origin and granulometric composition helps to improve the quality of bended cements and speed up the concrete production process. Blends of pozzolanic P-SCMs results in environmentally friendly concrete with a significant positive impact on the environment. This can be considered the main initiative of sustainability of concrete production.


Introduction
An important problem according to the priority areas of the European Green Deal is ensuring carbon neutrality, introducing innovations, modernization and greening of industry.According to the low-carbon development strategy presented by the European Cement Association (CEMBUREAU), it is necessary to reduce carbon dioxide emissions at every stage of the production and technological chain -from the production of clinker, cement and concrete to construction.Already by 2030, it is planned to reduce CO 2 emissions during the production of cement by 30%, and at the stage of concrete production and construction -by 40% [1][2][3][4][5].
Solving the problem of energy efficiency and reducing CO 2 emissions in building production is largely determined by the search for technological and ecological ways to replace a part of Portland cement clinker with supplementary cementitious materials (SCMs).Due to the rational selection of SCMs in the production of low-carbon cements, technical, ecological and economic benefits are achieved [6][7][8].Another way is to add SCMs directly to the concrete mixture instead of a part of Portland cement, which helps to reduce the cost of concrete, increase its strength in subsequent periods of hardening, and increase durability.In this case, replacing each kilogram of clinker in concrete with supplementary cementitious materials allows reducing CO 2 emissions by 0.6-1.0kg [9][10][11][12].
One of the main non-clinker constituent, which has been used in cement production for many years, is granulated blast furnace slag (GBFS) [13,14].At the same time, rapidly growing consumption of Portland cement as well as low availability of ground granulated blast furnace 1254 (2023) 012004 IOP Publishing doi:10.1088/1755-1315/1254/1/012004 2 slag (widely used as SCM) serious steps should be taken to find alternative solution for cement industry.Therefore, there is a growing interest in the use of a by-product of thermal power plants -fly ash.This product is an amorphous vitreous pozzolanic material and, when mixed with cement, significantly affects the strength development of the cementitious composites.Fly ash increases the durability and strength of concrete at a later age, but at the same time reduces the early strength of concrete.It also reduces the amount of water-reducing admixtures required to obtain the same consistency [15][16][17].However, the availability of fly ash is also limited in some parts of Europe.Therefore, it is recommended to add other types of pozzolans to the composition of so-called "green" cements together with fly ash.At the same time, with an increased amount of pozzolans, they significantly affect the strength of concrete.Silica fume significantly increases the strength and durability of concrete, but it has a high water demand, so it requires the use of an increased amount of water-reducing admixtures [18,19].
Significant resources in the European region of natural pozzolans -zeolite tuffs -make it possible to solve the problem of regional application of SCMs [20][21][22].High-silica zeolite tuffs (the main mineral is clinoptilolite (Na, K) 6 [Al 6 Si 30 O 72 ]24H 2 O) have unique characteristics, such as a high specific surface area and the ability to exchange cations.The introduction of natural zeolites allows binding excess alkalis (Na + , K + ) into insoluble hydroaluminosilicates as part of inorganic complexes.
The pozzolanic activity of SCMs, which is evaluated by their reactivity, mainly depends on three parameters -the nature of the activity (hydraulic, pozzolanic), the chemical composition of SCMs and their dispersion, while SCMs particles smaller than 10 micrometers are reactive [23][24][25].The effectiveness of the increased dispersion of artificial and natural pozzolans is confirmed by the development of ultrafinely dispersed supplementary cementitiouse materials that belong to superpozzolans and provide accelerated binding of calcium hydroxide -a product of the hydrolysis of the alite phase of Portland cement clinker [26,27].Such interaction of superpozzolana with products of hydration of Portland cement clinker leads to a decrease in porosity, which helps to increase the strength, corrosion resistance of concrete and determines its durability.However, a high content of finely dispersed SCMs causes an increase in the water consumption of concrete, which leads to a loss of early strength [28][29][30].
The physical approach opens significant prospects for improving the technical properties of building composites.It consists in not changing the chemical and mineralogical composition of Portland cement clinker, but reducing the water-binder ratio (W/B) of multicomponent cementitious systems and increasing the packing density of binder grains in the cement paste with the help of highly effective superplasticizers, especially of the polycarboxylate type [31,32].
The creation of modified concretes using multi-component SCMs involves the optimization of their compositions due to the intensification of pozzolanic reactions in the concrete cementitious matrix.The effectiveness of such an idea lies in the maximum disclosure of the synergistic role of highly dispersed pozzolans in the composition of low-carbon cementitious systems, which will ensure a directed effect on the processes of regulating the properties of modified concrete and mortars.The assessment of the indicator of the impact of multi-component SCMs on the environment allows to determine their suitability for the production of low-carbon concrete.In advanced EU countries, CO 2 emissions are reduced to 83.4 kg of CO 2 per 1 ton of concrete.One of the main directions of reducing the E CO 2 indicator is the replacement of a part of Portland cement CEM I type in concrete with multicomponent mineral additives, which is a relevant approach to achieve sustainability in construction industry [33][34][35].
In recent years, significant attention of researchers and manufacturers has been attracted by alkali-activated binders, which provide significant potential for the development of promising technologies [36][37][38].Alkali-activated cementitious materials are characterized by accelerated hardening and are at the epicenter of a new and necessary transition from modern Portland cement to eco-cements of the future.In particular, this technology may soon reach such a stage of development that it will serve as a link for the development of clinker-free alternative binders [39][40][41][42].
The purpose of the work is the rational design of the granulometric and material composition of combined pozzolanic SCMs, which, when combined with admixtures of superplasticizing action, create the possibility of obtaining eco-efficient modified blends with specified quality indicators and a reduction in CO 2 intensity per product unit.
The chemical composition of fly ash and zeolite was determined using an ARL 9800 XP Xray spectrometer (Thermo Electron SA, Switzerland).The specific surface of Portland cement and SCMs was determined on Blaine's device.Determination of the strength of blends was carried out in accordance with DSTU B EN 196-1:2015, bleeding in accordance with DSTU B EN 196-3:2015, DSTU B EN 196-6:2015.Pozzolanic activity was determined according to EN 450-1:2009, ASTM C593-06 [43].According to the methodology based on Italian method, the strength indicators should be R f ≥ 0.5 MPa. and R c ≥ 2.5 MPa.
The optimization of the composition of pozzolanic supplementary cementitious materials (P-SCMs) was carried out by the simplex-lattice Scheffe method according to the criterion of pozzolanic activity according to EN 450-1:2009.The optimal ratio between P-SCMs components was determined using a simplex-lattice Scheffe "blend-properties" plan to ensure an even spread of experimental points in the factor space.The Gibbs concentration triangle was used in the analysis of the experimental plan.The criterion of pozzolanic activity of P-SCMs according to EN 450-1:2009 was used as the target function.The criteria for the effectiveness of the modifiers (plasticizing and water-reducing effects) were determined according to DSTU B V.2.7-69.

Results and discussions
Highly dispersed SCMs such as super fine zeolite and microsilica are characterized by increased water demand -42.5 and 55.0 %, while having low bleeding (3.0 and 1.0 %, respectively).Fly ash has a lower dispersion compared to other pozzolanic materials.Fly ash is characterized by particles of the correct spherical shape, which provide a plasticizing effect due to the "roller bearing effect".Therefore, its water demand is reduced (27.0 %), but it is characterized by significant bleeding (34 %).
According to EN 450-1:2009, the strength activity index (SAI) after 28 days should be more than 75%, and after 90 days -more than 85%.As can be seen from the figure 1, super fine zeolite and silica fume reach the corresponding indicator of Psai ≥ 75% already after 2 days of hardening.The SAI of fly ash meets the requirements of EN 450-1:2009, but is significantly lower compared to super fine zeolite and silica fume.According to ASTM C593-06, the mortar strength for super fine zeolite is 5.2 MPa (requirement R c ≥ 4.15 MPa), while for fly ash it is 4.5 MPa.According to the methodology based on Italian method [26], the strength indicators of lime-pozzolan paste in the ratio lime:pozzolan=1:3, for microsilica and super fine zeolite after 28 days of hardening reach 4.6/9.5 MPa and 3.2/7.9MPa, respectively, and for fly ash -0.6/1.8MPa.
The optimal compositions of complex pozzolanic supplementary cementitious materials blends FA -SFZ -SF are determined by the indicator of Strength Activity Index (SAI).The combined effect of components on the pozzolanic activity of P-SCMs after 28 and 90 days of curing was investigated using the Scheffe simplex-lattice plan "blends-property" method.For the three-component mixture "SFZ (X1) : FA (X2) : SF (X3)" each point of the triangular diagram corresponds to one P-SCM ratio.The regression equation, taking into account the significance of the pozzolanic activity coefficients of the P-SCMs blends after 28 and 90 days, is as follows: The analysis of the response function surfaces (figure 2) allows to establish that for the twocomponent P-SCMs-2, the optimal values of the pozzolanic activity indicators are achieved at the ratio FA : SFZ = 0.46 : 0.54, which after 28 and 90 days are respectively Psai 28 = 94 % and Psai 90 = 126 %.At the same time, for the ternary mixture P-SCMs-3, the maximum SAI values are achieved at the ratio FA : SZF : SF = 0.35 : 0.40 : 0.25, with Psai 28 = 99 % and Psai 90 = 134 %.When the content of fly ash increases over 60 mass.% in the FA -SFZ -SF pozzolan blends, a decrease in the pozzolanic activity of the combined SCMs is observed.Therefore, the combination of highly active pozzolans (SFZ and SF) with fly ash, which has a plasticizing effect, allows to obtain a three-component pozzolan additive P-SCMs-3, which provides optimal workability and strength of binder.
Studies of the effects of the three-component complex pozzolanic additive P-SCM-3 on the physical and mechanical properties of binders have established that with an increase in the content of P-SCM-3 from 25 to 75 mass.% in the composition of blends, there is an increase in the specific surface from 5900 to 7700 cm 2 /g, an increase in water demand by 5.0 %, an extension of the hardening period and a decrease in the bleeding coefficient from 14.4 to 6.8 %.Early strength of blends with a content of 25 mass.% of P-SCM-3 is reduced by only 13 % compared to CEM I.With the content of the combined pozzolanic additive P-SCM-3 increases, the early strength decreases proportionally.With a content of 50 mass.%P-SCM-3 in the composition of the blend after 28 days of hardening, the difference in strength is 13 %, that is, due to pozzolanic reactions, a certain increase in strength is achieved.To obtain pozzolanic blends of strength classes 32.5 and 42.5, the effective range of replacing Portland cement CEM I with a combined pozzolanic SCMs is 25...50 mass.%.
Significant reserves of improving the physical and mechanical properties of pozzolanic blends are achieved due to the use of LS and PCE plasticizing admixtures.As can be seen from the table 1, when introducing 1.0 mass.%PCE to blend of 50 mass.%CEM I 42.5R + 50 mass.%P-SCM-3, the flow ability of a standard cone (W/C=0.50)increases from 144 to 290 mm, and when 0.8 mass.%LS is introduced, it increases to 225 mm.For the LS-modified blend, the waterreducing effect is ∆ W/C = 14 %, and when modified with 1.0 mass.%PCE -∆ W/C = 22 %.At the same time, the blend modified by PCE is characterized by higher compressive strength (R c2 = 20.6 MPa and R c28 = 54.4MPa).By mixing CEM I 42.5R and 50 mass.%P-SCM-3 low-carbon pozzolanic cement type CEM IV (SSA=6850 cm 2 /g) was obtained.When tested according to DSTU B EN 196-1:2015, the compressive strength after 2 and 28 days is 15.0 and 44.8 MPa, which corresponds to CEM IV/B 42.5.For this pozzolanic cement, the water demand is 31.5%, the bleeding is 9.1%.Thus, optimization of the granulometric and substance composition of blends allows to obtain lowcarbon blended cements of strength class 42.5.Using the method of differential calorimetry, it was established that for this pozzolanic cement the heat of hydration after 24 h decreases by 1.55 times compared to CEM I 42.5 R and is 149 J/g, that is, according to DSTU B EN 197-1:2015, this pozzolanic cement refers to CEM IV/B 42.5-LH.
Carbon dioxide emissions of cement and concrete are significantly reduced with the combined use of FA, SFZ and SF.For blended pozzolanic cements (clinker factor -0.50) CO 2 emissions are reduced to 456 kg/t, which is 45% less compared to Portland cement CEM I 42.5 R. At the same time, the total energy costs for grinding pozzolana cement (32 kW•h/t) are 40% smaller compared to CEM I.When using complex pozzolanic supplementary cementitious materials, optimization of the properties of binder is ensured: workability, standard and early strength, durability, cost, impact on the environment.

Conclusion
Modified multicomponent pozzolanic supplementary cementitious materials (P-SCMs) containing fly ash, super fine zeolite, silica fume, in combination with superplasticizer admixtures, can be effectively used to improve both fresh and hardened properties of concrete.Fly ash is commonly used with silica fume in concrete production to compensate for the slow strength development of fly ash at early ages.The use of super fine zeolite instead of silica fume or its part opens up new opportunities for improving the technological and technical properties of concrete.Technologically optimized blends based on combined pozzolanic SCMs become a rational solution to the problem of improving the energy efficiency of building production.Sustainability assumes that the combination of SCMs with pozzolanic action of different dispersions, based on fly ash, super fine zeolite and silica fume, ensures reduction of CO 2 emissions and cost of concrete production.

Figure 2 .
Figure 2. Isolines of effects of the composition of P-SCMs "SFZ -FA -SF" on pozzolanic activity after 28 days (a) and 90 days (b).

Table 1 .
Influence of modifiers on the physical and mechanical properties of the blend of 50 mass.%CEMI + 50 mass.%P-SCM-3.The amount of modifier, % W/C Flow, mm R c2 , MPa R c7 , MPa R c28 , MPa