86,843 research outputs found
THAUMASITE: EVIDENCE FOR INCORRECT INTERVENTION IN MASONRY RESTORATION
No full textThaumasite, as well as ettringite, are compounds which are increasingly found as deterioration products of cementitious materials subjected to sulfate attack. Thaumasite, and especially ettringite, have been abundantly reported in relation to concrete deterioration as well as, more recently, to the deterioration of cementitious mortars for masonry and for plasters. In particular, the problem appears serious in the field of repair of historical buildings, where the cementitious mortar can easily deteriorate just because of the formation of ettringite and thaumasite. However, although thaumasite is responsible for deterioration, in most cases, it may not be detected since it can be partially or almost completely removed by atmospheric agents. Many causes can be responsible for the presence of sulfates in masonry. At the same time, mortar and plaster are, in most cases, sources of calcium carbonate. Moreover, masonry is typically a porous material, which can be easily permeated by water, either rising groundwater or falling rainwater. Therefore, when cement based materials, which are in turn sources of calcium aluminates and calcium silicates, are used as binders, all of the ingredients necessary to cause thaumasite formation are present. Consequently, a compatibility issue emerges, which if not kept into proper account, will lead sooner or later to ineffective intervention
Effect of hydrophobic admixture and recycled aggregate on physical-mechanical properties and durability aspects of no-fines concrete
No full textNo-fines concrete with a compressive strength in the range 7–30 MPa at 28 days of curing were optimized by changing the water/cement ratio from 0.41 to 0.34 and the aggregate/cement ratio from 8 to 4. Some mixtures were also repeated with the addition of a hydrophobic admixture and prepared by fully replacing the ordinary aggregate with recycled aggregate to evaluate durability effects. High susceptibility to carbonation was observed for all the no-fines mixes studied. The use of recycled aggregate increases capillary water absorption (about 50%); however, the related decrease in durability could be easily counteracted with the use of a hydrophobic admixture
Calcium sulfoaluminate and alkali-activated fly ash cements as alternative to Portland cement: study on chemical, physical-mechanical, and durability properties of mortars with the same strength class
No full textThere is an increasing interest towards the development of alternative binders for the manufacture of sustainable mortars and concretes. Ordinary Portland cement (OPC) is the most commonly used material in construction, even if its production process is highly polluting. Both calcium sulfoaluminate (CSA) and alkali-activated cements (AAC) are potential alternative binders to be used in both structural (R3 class, with Rc " 25 MPa) and non-structural applications (R1 and R2 classes, with Rc " 10 MPa and Rc " 15 MPa, respectively) according to UNI EN 1504-3. This paper reports the hydration mechanisms and the evolution of porosity of OPC-, CSA- and AAC-based binders. The properties of fresh and hardened mortars, belonging to the above-mentioned mechanical strength classes, were evaluated and compared with particular emphasis on durability properties in terms of capillary water absorption, drying shrinkage, and resistance to sulfate attack. The results show that CSA-based mortars exhibit the lowest drying shrinkage due to their highest elasticity modulus. AAC mortars are characterized by the highest water vapor permeability and the lowest capillary water absorption for the highest presence of large pores
(>3000 nm)
Use of no-fines concrete as a building material: Strength, durability properties and corrosion protection of embedded steel
No full textAlthough no-fines concrete has been generally used for paving applications, it could be considered in other non-structural application, as reinforced panels, thanks to its acoustic, thermal and permeability properties. In this work, mechanical, durability-related properties and the protection provided by no-fines concretes to embedded steel against carbonation induced corrosion have been investigated on mixtures with compressive strength in the range 7-30 MPa. Additional protections, such as a mixed-in hydrophobic admixture, the coating of cement paste on the reinforcing bar or the use of galvanized or stainless steel bars, are also considered. Results show that, although no-fines concrete is susceptible to fast carbonation and it cannot provide long-term passivation to embedded steel, it may prevent corrosion in elements exposed to the atmosphere and sheltered by rain. In case of frequent contact with water, additional protections are required, preferably based on the use of corrosion resistant bars
Calcium sulfoaluminate cement and fly ash-based geopolymer as sustainable binders for mortars
No full textThis work investigates the hydration behaviour and the physico-mechanical properties of mortars based on calcium sulfoaluminate (CSA) cements and fly ash-based geopolymers (GEO) as alternatives to ordinary Portland cement. According to the EN 1504-3, mortars were prepared in order to reach three compressive strength classes, namely R1, R2 and R3 (R1 with Rc ≥ 10 MPa, R2 with Rc ≥ 15 MPa and R3 with Rc ≥ 25 MPa). CSA mortars were prepared by using sulfoaluminate cement alone (R3) or in mixture with a limestone filler (R1 and R2); GEO mortars were manufactured by alkali-activation of coal fly ash and calcium aluminate cement with a sodium silicate and potassium hydroxide water solution. The hydration behaviour was evaluated on pastes submitted to differential thermal-thermogravimetric and X-ray diffraction analyses. Mortars was analysed through mercury intrusion porosimetry; their mechanical properties were evaluated in terms of compressive strength and dynamic modulus of elasticity. Furthermore, capillary water absorption and drying shrinkage tests were carried out in order to evaluate their durability. Due to the rapid ettringite formation, CSA-based mixtures reached their maximum compressive strength values faster than the corresponding GEO mortars. Results showed that the lower modulus of elasticity of GEO mortars causes the higher drying shrinkage. Moreover, the lower porosity exhibited by GEO mortars was responsible for the lower water capillary absorption
Influence of Different Aggregation States on Volatile Organic Compounds Released by Dairy Kluyveromyces marxianus Strains
No full textKluyveromyces marxianus has the ability to contribute to the aroma profile of foods and beverages since it is able to produce several volatile organic compounds (VOCs). In this study, 8 dairy K. marxianus strains, previously selected for their adhesion properties, were tested for VOCs production when grown in different conditions: planktonic, biofilm-detached, and MATS forming-cells. It was shown that biofilm-detached cells were mainly able to produce higher alcohols (64.57 mg/L), while esters were mainly produced by planktonic and MATS forming-cells (117.86 and 94.90 mg/L, respectively). Moreover, K. marxianus biofilm-detached cells were able to produce VOCs with flavor and odor impacts, such as ketons, phenols, and terpenes, which were not produced by planktonic cells. In addition, specific unique compounds were associated to the different conditions tested. Biofilm-detached cells were characterized by the production of 9 unique compounds, while planktonic and MATS forming-cells by 7 and 12, respectively. The obtained results should be exploited to modulate the volatilome of foods and beverages and improve the production of certain compounds at the industrial level. Further studies will be carried out to better understand the genetic mechanisms underlying the metabolic pathways activated under different conditions
Comparison between surface and bulk hydrophobic treatment against corrosion of galvanized reinforcing steel
No full textThe effectiveness of bulk hydrophobic treatment against corrosion of galvanized steel reinforcement in
concrete specimens with w/c=0.45 and w/c=0.75 was compared with that of surface treatment, even in the
presence of cracks 0.5 and 1 mmwide in the concrete cover. In this case surface hydrophobic treatments were
applied both before and after cracking as a preventive and a restorative method against reinforced concrete
deterioration, respectively. The obtained results in terms of water absorption, electrochemical measurements,
chlorides penetration, and visual observations carried out on reinforced concrete specimens during the
exposure to wet–dry cycles in 10% NaCl solution showed that bulk hydrophobization is the most effective
treatment in improving the corrosion resistance of galvanized steel reinforcements in concrete also in the
presence of cracks. Surface hydrophobization is very effective just in the first few exposure cycles to the
aggressive environment and when used as a restorative method which is able to cancel the deleterious effect
of cracks only 0.5 mm wide
Effetti Della Terminazione Conservativa Delle Colture Di Copertura Sulla Produzione Dello Zucchino e sul Controllo Delle Infestanti: Primi Risultati Del Progetto ORWEEDS
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