Based on experimental results, mathematical models were elaborated to predict the development of compressive strength of concrete with fly ash replacement percentages up to 30 %. Strength of concrete with different types of cement (CEM I 42.5, CEM I 32.5, CEM III 32.5), after 2, 28, 90, 180 days of curing, have been analysed to evaluate the effect of addition content, the time of curing and the type of cement on the compressive strength changes. The adequacy of equations obtained was verified using statistical methods. The test results of selected properties of binders and hardened concrete with fly ash are also included. The analysis showed that concrete with fly ash is characterised by advantageous applicable qualities.

Biomass combustion is a significant new source of green energy in the European Union. The adequate utilization of byproducts created during that process is a growing challenge for the energy industry. Biomass fly ash could be used in cement composite production after appropriate activation of that material. This study had been conducted to assess the usefulness of mechanical and physical activation methods (grinding and sieving), as well as activation through the addition of active silica in the form of silica fume, as potential methods with which to activate biomass fly ash. Setting time, compressive strength, water absorption and bulk density tests were performed on fresh and hardened mortar. While all activation methods influenced the compressive strength development of cement mortar with fly ash, sieving of the biomass fly ash enhanced the early compressive strength of cement mortar. The use of active silica in the form of silica fume ensured higher compressive strength results than those of control specimens throughout the entire measurement period.

Abstract. The influence of cyclic freezing and thawing of concrete upon its microstructure is investigated in this research. The following methods were applied: microscope analysis of specially prepared fragments of concrete specimens, capillary pulling up water under atmospheric pressure, thermal analysis of hardened cement paste separated from concrete specimens. Tested concrete did not contain any chemical or mineral admixtures; W/C ratio was constant for all specimens (0,65). Indexes of mean radius of capillary pores λ and indexes of homogeneity of capillary pores α were measured after various numbers of freezing cycles. Experimental results enable to describe the phenomena occurring in concrete subjected to frost.

Biomass fly ash is a growing challenge for combustion by-product (CBP) management. This research was conducted to investigate the influence of activation by sieving through a 63 µm sieve and a 125 µm sieve on fresh and hardened cement mortar properties. Sieving increased the CaO content by 9.3 percentage points (p.p.) in the oxide composition of the fly ash. The 28-day Strength Activity Indices increased by 24.9 p.p. A 25% replacement rate of cement with fly ash sieved with a 63 µm sieve increased the 2-day compressive strength of mortars by 24% when compared with untreated fly ash. The 90-day compressive strength results of cement mortars with a 15% replacement rate of cement with fly ash sieved with a 63 µm sieve were similar to the control specimen results. The utilization rate of biomass fly ash can be increased to 15% of binder mass without the detrimental effect of the mechanical properties of cement mortar. SEM and TG analyses showed that activated biomass fly ash promoted the growth of the C-S-H phase and ettringite.

No abstract