Influence of complex additives based on sulfoaluminate cement on the properties of composite binder

. One urgent issue today is reducing CO 2 emissions worldwide. In particular, the study of using sulfoaluminate cement to create less CO 2 is a suitable solution. The paper presents compound additives' influence on the composite binder's standard consistency. Binders such as sulfoaluminate cement, natural gypsum, and tripoli are used as additives. The resulting binder's standard consistency was determined per GOST 310.3-76. Box-Wilson central composite design method was used to simulate the test results. The results obtained are presented as the surface of the second-level regression equation, which describes the dependence of the mixed binder's standard consistency on the additives' content. Research results have been obtained from compositions of composite binders with maximum and minimum standard consistency. Based on the analysis of experimental data, conclusions were drawn about the possible mechanism of the effect of the introduced additives on the consistency of the cement paste.


Introduction
Every year, global CO2 emissions are increasing rapidly. According to Rosstat data, in 2019, each year, the industrial sector emitted 233.6 million tons of CO2 [1]. Carbon dioxide is formed during cement production, accounting for approximately 5% of global gas emissions [2]. In this regard, the use of low-cement concretes in constructing buildings and structures and cement that produces less CO2 is gaining popularity [3]. Sulfoaluminate cement (SAC) is a special binder used when it is necessary to achieve high-strength concrete or mortar in the shortest possible time (up to 6 hours of hardening). The main mineral of sulfoaluminate cement is calcium sulfoaluminate 3СаО·Al2O3·CaSO4 [5], during sulfoaluminate cement production of which CO2 emissions into the atmosphere are 2-2.5 times lower than during the production of the main clinker minerals (alite, belite) of ordinary Portland cement [4]. Sulfoaluminate cement has gained wide application in producing fast-setting, fast-hardening compositions and stressing and expanding cement. The initial and final setting of such cement can take up to 10 minutes from the initial mixing of the cement with water. Concrete and reinforced concrete products based on sulfoaluminate cement, in addition to achieving high strength in the shortest time and self-stressing, also have increased density and sulfate resistance, which determines their use in the construction of hydraulic facilities [6].
In practice, using SAC in large volumes can be economically unsuitable and difficult to implement. In Russia, only the Podolsk-JSC cement factory produces sulfoaluminate and sulfoaluminate-belite cement in small quantities. Other plants do not produce sulfoaluminate and sulfoaluminate-belite cement. It is, therefore urgent to develop a composite binder that has all the advantages of SAC and allows you to control the properties of the cement stone based on it by varying the amount of SAC in a binder.
The composition of the mixed binder based on Portland cement (PC) and sulfoaluminate cement includes pozzolanic additives and gypsum. The standard consistency of the composite binder "PC-SAC" increases from 27% (for PCS) to 40.5% (SAC replaces 40% PC) [7]. The mobility of cement pastes, determined using a shaking table, also such as depends on the SAC content: for 100% SAC it is 116 mm; with the introduction of 40% SAC it is 153 mm, and with 20% SAC it is 169 mm [8]. During SAC hydration, a large amount of ettringite is formed, so the cement paste quickly loses its mobility [9][10][11][12]. In addition, in the "alite-belite-yelemite" systems in a medium rich in aluminum ions, the hydration of belite proceeds more actively with the formation of new complex calcium hydro-aluminosilicates [4]. With the introduction of 10% SAC, the onset of setting occurs 3.2 times faster, and the end of setting is 2.5 times faster than for Portland cement paste without additives [8,13].
Materials such as fly ash, slag, microsilica, tripoli, diatomite, etc. are often used as pozzolanic additives, which makes products based on them environmentally friendly [14]. When finely ground blast furnace slag is added to sulfoaluminate cement in 50% by weight of the binder, the standard consistency of the cement paste increases by 4% compared to SAC without additives [15]. At the same time, the additive does not have a significant effect on the setting time. However, it allows the acceleration of concrete curing [16]. With the introduction of fly ash into the SAC composition, the viscosity of the cement paste also increases. Moreover, the lower the water-binding ratio (W/B<0.6), the more reduced the mobility of the cement paste [17].
Analysis of the references showed that tripoli is a highly porous zeolite-containing polymineral rock of sedimentary origin [18]. Tripoli is widely used in construction; for example, in the production of Portland slag cement, part of the slag is replaced with tripoli (no more than 10% by weight of cement) and gypsum-cement-pozzolanic binders [19,20]. In this paper, tripoli from the Khotynets deposit was used, the mineral composition of which is represented by zeolites: clinoptilolite, montmorillonite, and opal-cristoballite. The chemical composition is represented mainly by silicon oxide SiO 2 (71%) and aluminium oxide Al 2 O 3 (8%) [21].
Based on the above analysis, it was found that the study of the influence of inorganic additives on the consistency of cement paste made from Portland cement-based composite binder with the addition of clinker sulfoaluminate, tripoli, and gypsum is necessary. To simulate the test results, an orthogonal second-order Box-Wilson second-order composition plan was used.

Materials used
The purpose of the paper is to study the influence of different content of dispersing modifying additives on the rheological properties of composite cement. The materials used are shown in Table 1. The mineral composition of sulfoaluminate cement (SAC) according to TU 5745-008-00281306-18 with a calcium sulfoaluminate content of at least 50% and a belite content of C 2 S of more than 35%, C 4 AF of not more than 10%. Mixing water meets the requirements of GOST 23732-2011.

Methods
The test standards are given in Table 2. The influence of the composite binder's composition on the cement paste's standard consistency was studied using the method of mathematical planning of the orthogonal central 2nd order. The regression equation of the experimental model has the form: In which: b o , b i , b jj and b ju -regression coefficients; x j , x u -represent a factor; x j x udemonstrates the interaction between factors, k -number of input factors.

Results and Discussion
Composite adhesives of different compositions are obtained by the following: first, mixing Portland cement (PC), sulfoaluminate cement (SAC), natural gypsum (G) and tripoli (Tr) to a homogeneous state. After that, Studying the effect of complex additives on the properties of the composite binder, the mathematical planning method of orthogonal centers of order 2 was used with the number of experiments N=15, α = 1.215. Based on the analysis of scientific literature, the following factors were selected to study affecting the properties of composite cement: sulfoaluminate cement (SAC), amount of gypsum (G), amount of tripoli (Tr) calculated as a percentage of the total mass of the composite binder. The results of the experiments are presented in table 4, where: y is the response function representing the standard consistency of cement paste (%). ; ; (1) As a result, the following regression equations were obtained: y = 40,61 + 0,334x 1 -0,013x 2 + 2,213x 3 + 0,094x 1 x 2 -0,031x 1 x 3 -0,094x 2 x 3 -0,031x 1 x 2 x 3 + 0,305(x 1 ) 2 + 0,220(x 2 ) 2 + 0,305(x 3 ) 2 (2) To check the coefficients of the regression equation (2), additional experiments were carried out in the amount of three experiments in the center at x 1 =x 2 =x 3 =0.
The calculated value was compared with the tabular ratio F table (Appendix 7 [23], Table  P4 [24] for the significance level p = 0.05 and the numbers of freedom degrees of the reproducibility variance f 2 = 3 and f1 the number of freedom degrees of the adequacy variance: f 1 = N -v. The resulting ratio F calc =5.503 < F tabl =9.3 allows us to accept the hypothesis of the adequacy of the regression model. The response surface for the regression equations (8) is presented in Figs 1.
After analyzing equation (8) and Figure 1 (a, b), we can conclude that the factor x 2 (G) has practically no effect on the standard consistency of the composite binder, the factor x 1 (SAC) and x 3 (Tr) affect to the standard consistency. However, there are no interaction coefficients, which means that the proportions between the factors do not affect the water requirement of the binder. The dependence of y on the factor x1 (SAC) is linear with no quadratic term. With an increase in the amount of sulfoaluminate cement in the binder, corresponding to the normalized value of the factor x 1 , the response increases for any values of the other factors. For the x 3 (Tr) factor, the linear term has the largest effect on the response, with a significant regression coefficient value of 2.213. Analyzing Figure 1, one can note a trend towards an increase in standard consistency with an increase in x 3 (Tr). This is because the tripoli has a high specific surface (6355 cm 2 /g), high porosity, and absorption capacity and significantly increase the water requirement of the binder. The obtained values of standard consistency vary from 37.52% to 43.71%, which are presented in table 6.   Figure 2 shows that when adding complex additives based on expanding (SAC + gypsum) and pozzolanic (Tripoli) to Portland cement, the water requirement of the binder increases by about 1.42 to 1.65 times.

Conclusions
After analyzing the data obtained during the experiment, we can draw the following conclusions: 1. Using the experimental planning method, a regression equation describes the relationship between the target functions depending on the amount of additive content: y = 40.61 + 0.334x 1 + 2.213x 3 , and images of the surfaces of equation (8) were obtained using computer programs.