THREE-PARAMETRIC MODEL FOR ASSESSMENT

 OF THE 1-DAY STRENGTH OF A SILICATE COMPOSITE

Violeta J. Petkova

Central Laboratory of Physico-Chemical Mechanics,

Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 1, 1113 Sofia,

E-mail: malahit@bgcict.acad.bg

Received: 20.07.01; Cited: 9.10.01

 Abstract

Polynomial models describing the mechanical properties of composite mortars, including a new combined admixture in their formulation, have been developed on the basis of three-factor planned experiment. The obtained mathematical relationships are presented in graphic form. They represent the influence of the proportions between the admixture ingredients and the water/cement ratio on the changes in the 1-day compressive strength of the composite in the broad range of the factor space with high values of the investigated parameter. The obtained results can be used for the solution of a number of technological and technical problems connected with the application of the new composite elements.

Key words: silicate composite, combined admixture, compressive strength, planned experiment.

The application of different admixtures provides an alternative possibility of extending the production methods of silicate composite elements. The combined admixtures are suitable in the cases where accelerated hardening and high mechanical parameters are necessary. Different industrial sub-products (by-products or wastes) can be used as fillers of the cement matrix. Their rational utilization leads to improved ecology of the environment. The compressive strength is considered to be the main factor in such cases according to the technological and construction requirements in connection with the application in building practice, while the 1-day strength is considered as a criterion for comparative assessments related with the investigation and choice of optimal solutions concerning the development of new high-strength and economical non-metallic composites.

The possibility of obtaining a new composite material (CM) based on cement [1, 2] is proved using the above mentioned parameters. The formulation includes a new combined admixture (CA). Its components – alkaline (A) and pozzolanic (P) one, are cheap and available waste raw materials. The admixture is in conformity with the requirements of the Bulgarian State Standards (BDS EN 196-5 and BDS EN 196-6) [1] with respect to its dispersity, chemical composition and activity. It can be added both to pure clinker cements and to lower-quality plant cements with restricted alite content.

The experimental results from the preliminary investigations of CM show the substantial advantages of the admixture. It accelerates the hydration and structure formation of the slag-cement stone and contributes to considerable increase of the mechanical strength. The assessment of the influence of some important parameters is made by means of suitable cybernetic method in order to predict the compressive strength kinetics for one-year period of hardening [2]. The obtained regression models show that quantitative relationships exist between the formulation components (combined admixture, Portland cement) and the water/cement ratio. At the same time, high values of the investigated characteristics have been reached.

The present report considers the data reflecting the three-factor influence of the main parameters – the ingredients A and P composing of the combined admixture and the water/cement ratio, on the changes of the 1-day strength. The possibility for effective choice of suitable optimal solutions is shown with the variation of the quantitative participation of the accepted factors, as well as their contribution to the development of a silicate composite with high early strength. 

Materials and methodology of research

Multiple regression analysis has been applied in the presented investigations for the assessments of the 1-day compressive strength. The method does not ignore the complex physico-chemical processes taking place during the hydration and hardening of the composite material. Portland cement PC35 D20 has been used for the purpose, its mineral composition being С₃S=50 %, С₂S=28 %, С₃А= 8 % and С₄АF=10 %, the dispersity of grinding being 11.1 %. The cement meets the requirements of BDS EN 196-1.

The experiments were carried out with composite mortars with work composition 1:3. The used CA has a definite grain-size composition and activity [1]. Its two components are mixed and homogenized with the Portland cement in dry state. The test series were performed using a medium-grained artificial sand. The latter represented well-designed mixture of granulated acidic slag and flotation sterile material. In this way, the basic aim of the author was reached to effectively utilize more secondary by-products from metallurgy in combination with ionized in a constant electric field mixing water with рН = 10-11 [1].

The choice of the single factors - Х₁ (W/C), Х₂ (A), Х₃ (P) and their variation levels in natural and coded coordinates are shown in Table 1. The alkaline and pozzolanic ingredients of the combined admixture are expressed in percent from the total mass of the samples. The selected proportions allow the real and exact prediction of the complex relationship between both formulation parameters, the water/cement ratio and the 1-day strength of the new composite.

 Table 1.

The plan of the experiment was developed with 27 test points, Table 2. Three tests of the 1-day compressive strength were made for each point. The calculations were performed using the EXEL electronic sheets and the LINEST multiple linear regression function. The chosen three factors varied within the limits of -1 to +1.

The unknown target function Rc =f (X₁, X₂, X₃) describes the relationship between the compressive strength at the age of 1 day, the quantities of the components A and P and the value of the corresponding water/cement ratio. It is determined as a second order polynomial:

Rc = b_o + S b_iХ_i + S b_iiХ_i² + S b_ijХ_iХ_j                                                                           (1)

Table 2