Potentiometric method for determination of hydraulic activity of quartz-containing powders

Introduction. An important aspect of concrete mixture preparation technology is the exploration of methods to effectively utilize the properties of aggregates (fillers) in hardening multi-component systems. The most common binder composition is a mixture of a binder system with crushed quartz-containing polymineral sand. It is known that micro silica is classified as an active mineral additive (filler), the properties of which are characterized by the amount of Ca2+ ions absorbed from a saturated lime solution. Literature sources describe two types of analytical methods for determining pozzolanic activity. The first type that trace the change in the content of Ca(OH)2 over time during the pozzolanic reaction. The second type that aim to determine the physical and mechanical characteristics of the material under study. These characteristics reflect its ability to bind free calcium oxide into stable hydrate compounds. Previously applicability of the potentiometric method of analysis using an ion-selective electrode with pCa-function was demonstrated for the assessment of pozzolanic activity of highly dispersed materials. It should be noted to note that this electrochemical method of analysis which is functionally related to the concentration of the determined component in the tested solution, is very efficient in terms of expressiveness and labour intensity. The objective of the research is to validate the method for micro silica systems obtained by mechanical grinding of raw materials of polymineral sands from different deposits. The experiments were conducted using sands from the Arkhangelsk region deposits. The quartz sand (QS) was chosen as a reference object to establish the nature of change in the functional relationship of the measured potential of the electrode system from the volume of added Ca(OH)2 solution. Methods and Materials. The polymineral sand of the Krasnoflotsky-West deposit is categorised as alluvial marine modern Quaternary sediments (am IV). The sand of the Kholmogorskoye deposit is quartz-feldspathic sand, which belongs to alluvial modern Quaternary sediments (a IV). The mineralogical composition of polymineral sands was determined by means of the semi-quantitative method, utilising the binocular microscope MBS-1. Fine sand powders were obtained by dry mechanical grinding in a Retsch PM100 planetary. The specific surface area of the obtained fine powders was measured by the Kozeny-Karman gas permeability method on a PSKh-10a apparatus. The potentiometric analysis was conducted using an apparatus comprising an electrode pair, consisting of a measuring electrode with pCa-function and a reference electrode connected to an Expert-001-3.0.1 ionometer. The reaction system was stirred using a magnetic stirrer. In order to ascertain the sorption capacity of silica-containing raw materials, a lime solution with a concentration and a suspension of fine sand powders were prepared in advance. Concurrently, the EMF (potential) of the system was measured, with constant stirring. The concentration of calcium ions, Cact , was then calculated based on the previously established calibration dependence, using the potential values (mV) obtained. In order to determine the activity of fine mineral powders of silica, the dependence between the values of the given concentration of calcium ions in lime solution when adding a certain volume of lime to the suspension of the studied sands and the concentration values obtained by potentiometric measurements was plotted. Results and Discussion. The obtained powders of quartz-containing polymineral sands, which had been finely dispersed, had the following specific surface values: quartz sand – 1020±31 m2/kg, Krasnoflotsky West – 465±14 m2/kg and Kholmogorskoye – 450±14 m2/kg. The given data on the specific surface of powders of natural origin allow for a comparative analysis of experimental potentiometric measurements. Concurrently, the dependence of the measured potential on the concentration of calcium ions in the solution for quartz sand suspension can be utilised as a reference, given the significantly high specific surface of the powder, thereby resulting in a more pronounced dependence E = f(Vadd ). Relationship obtained the dependence of the measured potential of the electrode system on the volume of added Ca(OH)2 solution in a suspension of quartz sand (as a reference sample) and distilled water (as an idle experiment) shows that the nature of the change in the functional relationship of the analysed two samples (QS and H2 O) is different. Concurrently, it can be observed that the incorporation of a lime solution into water (in accordance with the anticipated outcome) results in a nearly constant augmentation in the measured potential values, the magnitude of which is proportional to –lgCCa . Change of potential of electrode pair in suspension of quartz sand powder has character different from that mentioned above for water. This functional dependency can be divided into three periods. The initial period is characterised by an induction phase, during which the electrode potential displays a linear increase in where stabilisation of potential values is observed when calcium hydroxide is added to the reaction system, is directly related to the adsorption (pozzolanic) effect of finely dispersed quartz. The third and final period is characterised by the growth of the electrode potential value due to the appearance of excess Ca2+ ions in the solution. The data presented demonstrate that the powder from the Krasnoflotsky-West deposit exhibits the highest pozzolanic and adsorption activity in relation to calcium ions. The dependence has a pronounced three-stage character, as determined by us through conducting a similar experiment with quartz sand powder. Given that quartz-containing powders with high dispersion exhibit sorbent properties discrepancy in the dependencies may signify a quantitative variation in the active adsorption centres of the studied powders, consequently resulting in a disparity in their degree of filling. This fact is a kinetic factor in the process of solid-phase concentration of the determined cations. The results obtained demonstrated that the sorption capacity of calcium oxide in the powders of the studied sand deposits was practically equivalent. However, the character of the functional dependence of the change in the determined concentration of calcium hydroxide in the solution relative to its added value was different. This observation suggests the potential for distinct mechanisms underlying the sorption processes. Conclusion. The paper demonstrates that the potentiometric method of determining the sorption capacity of calcium oxide can be used as an express method for powders of quartz-containing polymineral sands. It is demonstrated that to accurately calculate the specified parameters under experimental conditions that ensure full reproducibility, it is essential to conduct an experiment using a suspension of quartz sand (as a reference sample) alongside a comparative experiment in which the dissolution water serves as the measurement object. The functional dependence of the change in the determined concentration of calcium hydroxide in the solution relative to its added value can be used for comparative evaluation of the mechanism of sorption processes for powders of different material nature.