Sand has been considered to be something of an immeasurable quantity. There are many indications that this view is no longer valid and that the limiting of natural aggregates usage is doubly justified. Firstly, the extraction of natural aggregates is expensive and has a huge impact on the environment. The main issues in sand and gravel mining are the large areas that are affected, ground water level changes, illegal mining, unsuitability of desert and marine sand, and costs of transport. Secondly, metallurgical waste can be used as a substitute for natural aggregates. This is doubly beneficial—the waste is recycled and the use of natural aggregates is reduced. Waste is stored in landfills that take up large areas and there is also the possibility of ground and groundwater pollution by hazardous compounds. The research presented in this article focuses on the technological conditions of using metallurgical waste in its original form and as a component of recycled concrete aggregate (RCA). The use of metallurgical sludge waste or crushed or round RCA to produce concrete deteriorates the consistency and does not significantly affect the air content and density of the concrete mix. RCA lowers the density of hardened concrete. Metallurgical sludge waste or RCA usage adversely affect the absorbability and permeability of concrete. Concrete containing metallurgical sludge waste is of higher compressive strength after 7 and 28 days, with up to 60% of waste as a sand replacement. RCA concrete achieved higher compressive strength also.

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The presented paper aims to describe the influence of accelerating admixtures on the properties and microstructure of cement pastes and mortars. Blended slag cement CEM II/B-S containing two different clinkers (differing amounts of siliceous and aluminous phases) and four types of accelerators (calcium nitrate, sodium hydroxide, cement kiln dust, and crystal seeds) were used in research. Compressive strength tests (after 12, 24, 48 h of curing), Scanning Electron Microscope (SEM) observations together with an Energy Dispersive Spectroscopy (EDS) analysis, Mercury Intrusion Porosimetry (MIP) tests, and X-ray diffraction (XRD) analysis were conducted. Results have shown that SEM and EDS examination of the microstructure of cement pastes modified with accelerating admixtures at the observed points did not reveal differences that would be sufficient to explain the changes in compressive strength. Still, the increase in amorphous phase content indicates a faster hydration reaction rate for all pastes modified with accelerating admixture. It is backed up also by lower non-hydrated compounds content. All admixtures accelerate the hydration reaction of calcium silicate phases of cement, but only NaOH and cement kiln dust (CKD) influence the aluminate phase reaction rate. The pore volume is independent of the clinker type, while the pore size distribution is not.

Modern concrete technology includes mineral additives and chemical admixtures usage. It is caused by their beneficial influence on properties of concrete mix and hardened concrete. Accelerating admixtures for concrete are commonly used for shortening of time demanded for demoulding and repeat use of forms in precast facilities. They allow to conduct works during low-temperature season. Main advantage of accelerating admixtures is enhancement of early strength of concrete. Alas they may cause decrease of long-term strength and durability of concrete or increase its shrinkage One of the most popular mineral additives is ground granulated blast furnace slag (GGBFS). It is non-clinker main constituent of CEM II, CEM III and CEM V. GGBFS may be also used as additive with latent hydraulic properties for concrete. GGBFS as constituent of concrete increases consistency, long-term strength and durability, and decreases hydration heat evolution. Early compressive strength of concrete with GGBFS is lower than for Portland cement concrete. Accelerating admixtures and ground granulated blast furnace slag show advantages and disadvantages that can be equalized. In early terms calcium nitrate and crystal seeds enhanced compressive strength. Their efficiency is similar. Cement kiln dust also caused increase of compressive strength but not as much as former ones. Sodium hydroxide caused great increase of compressive strength after 12 hours but not in longer terms. In case of cements rich with C3A the compressive strength in early stage of hardening is shaped by C-S-H phase and well-developed ettringite crystal skeleton. In spite of minor differences in non-modified and calcium nitrate modified cement pastes microstructure, the compressive strength of calcium nitrate modified mortars is significantly greater in comparison to non-modified ones. The greatest compressive strength was achieved by mortar modified with crystal seeds. Responsible for this increase is more well-developed C-S-H phase. Mortars modified with sodium hydroxide are weaker after 2 days of curing in comparison to non-modified mortar. It is caused by sparse ettringite crystal skeleton. Microstructures of non-modified and modified with cement kiln dust (CKD) cement pastes are similar. It is connected with similarity of chemical composition of CKD and Portland clinker. The compressive strength of CKD modified mortars is slightly greater than non-modified one.