PRECONDITIONS FOR THE APPLICATION OF PETRAŠIŪNAI QUARRY DOLOMITE SCREENINGS AND DOLOMITE POWDER IN CONVENTIONAL AND SELF-COMPACTING CONCRETE MIXES

The objective of these experimental studies is to evaluate (verify) the possibility of applying by-products (0/2 mm fraction dolomite screenings or dolomite powder) obtained in the process of producing crushed dolomite from Petrašiūnai dolomite quarry rocks in concrete technology. A rational application of this material expands the range of concrete mix aggregates and provides an integrated use of dolomite rock by consuming less attractive by-products of dolomite processing. The article discusses the possibilities of using the above introduced dolomite by-products in concrete applications and gives a preliminary assessment of physical, mechanical and technological characteristics of commercial and technological concrete with dolomite screenings. 0/2 mm fraction dolomite screenings from Petrašiūnai dolomite quarry with an average density of 2600 kg/m3, a bulk density of 1690 kg/m3, a bulk porosity of 39.1%, fine particle content (contamination with dust and clay particles) of < 4,9%, a specific surface of 1085 cm2/g determined by Blaine tester were used for experimental study. The physicalcharacteristics of dolomite powder and dolomite screenings additionally crushed in a lab ball mill were similar: an average density of 2600 kg/m3, a bulk density of 1210 kg/m3, a bulk porosity of 53,5% and a specific surface after additional milling of 3030 cm2/g and 4070…4200 cm2/g respectively. Dolomite particles have a rough, conchoidal and porous surface, however, their form is close to cubic or even oval while their edges are less sharp (mechanically grated) compared to granite or other crushed stone screenings. Therefore, dolomite particles bond very well with cement stone and almost do not increase water demand for producing a paste of normal consistency and do not weaken the rheological properties of the mixes. Dolomite screenings or dolomite powder from Petrašiūnai quarry have stable mineral composition, but the XRD patterns of rock provide little information: although dolomite peaks are prevailing, quartz and feldspar peaks can also be noticeable. Moreover, ferrous minerals (pyrite, limonite) are present in dolomite, nevertheless, so few particles of these impurities are so small (< 0.2 mm in diameter) that they pose no risk of the potential destruction of concrete. Energy consumption of crushing dolomite screenings to reach the fineness of cement particles is much lower compared to crushing granite screenings. The crushed granite screenings are 1.5–2 times finer compared to the fineness of dolomite screenings crushed for the same time. Besides, the fineness of carbonate rock powder can be easily adjusted by changing crushing time. Therefore, dolomite screenings is a very perspective raw material for producing concrete micro-aggregates. No pozzolanic behaviour of dolomite screenings and dolomite powder during the short-term curing of cement stone or concrete under normal (room) temperature conditions were observed, and therefore a rational application of these mineral admixtures in conventional concretes would be only as substitutes for fine aggregate (sand) and only partly for cement. Most probably, dolomite powder can behave as a weak pozzolanic admixture at higher temperatures (above 50 oC); however more detailed studies are required to prove this supposition. The powdered dolomite admixture does not increase water demand for obtaining the paste of normal consistency but improves the structure of cement stone pores and frost resistance. The crushed dolomite screenings reduce the compressive and bending strength of concrete cured under ordinary temperature conditions; however, a small content of these admixtures (up to 15 ÷ 20 per cent of cement mass) can be recommended for self-compacting concrete and other fine-grained concrete mixes because the deterioration of the mechanical characteristics of cement stone is insignificant, i.e. about 10 ÷ 12 per cent. Dolomite screenings substituting for sand (or a part of sand) in conventional Portland cement concrete improve the granular composition of the mix, increase the content of fine (0.063 ÷ 0.25 mm) fractions and grow in the compressive strength of such concrete by 12 per cent. Such concrete has a better structure dominated by closed pores. Therefore, fine aggregate from dolomite screenings (or with them) is recommended for Portland cement mixes or cement grouts.