Mechanical and durability properties of mortars prepared with untreated sugarcane bagasse ash and untreated fly ash
Introduction
The use of pozzolanic materials in concrete or mortar mixtures as partial replacement of cement provides a satisfactory solution to environmental concerns and to the loss of durability that some reinforced concrete structures have. Studies have shown that pozzolanic materials have a significant amount of amorphous silica in their chemical composition. When pozzolanic materials are added to the cement, this silica (SiO2) reacts with the free lime from the hydration of the cement and new silicate hydrate products are formed, thus improving the mechanical and durability properties of the concrete [1].
The use of industrial waste as pozzolanic additions in concrete or mortar mixtures, and in particular the use of fly ash (FA), has been studied extensively. FA is a by-product of the combustion of pulverized coal in thermal power plants and is gathered by electrostatic precipitators from the combustion gases before they are discharged into the atmosphere. Only flying particles produced from the burning of the coal are attracted to the precipitators. Consequently, most of the FA is formed by silica and alumina rich particles with only a small amount of unburnt coal particles. As a result, FA reacts effectively with hydration products to form more dense and resistant cementitious products, improving in this manner the long-term mechanical and durability properties of concrete.
Research on FA shows that its presence improves the workability of mortars and concretes in their fresh state. In a their hardened state, the presence of FA also improves structural properties; however, this improvement occurs at later ages than in the mortars and concretes which have no FA [2].
FA improves the impermeability and durability of concrete. Some researchers have found that the replacement of 20% of cement with FA in concrete mixtures improved compressive strength and significantly delayed the corrosion initiation period due to the decreased permeability to chloride ions [3], [4].
As a result of the variability of both the coal composition and the burning conditions, the chemical properties of FA vary greatly and this variability has a significant impact on the properties of the cementitious materials where FA is used.
Grinding the FA and high-temperature curing have shown to reduce the variability and to improve the mechanical properties of mortars [5]. Recalcination can also be used to improve its pozzolanic activity. Finally, the addition of Calcium Hydroxide and Sodium Silicate to accelerate the cement early hydration and promote setting and hardening of mortars has been investigated [6]. There is an extensive developing body of knowledge on the effects of post-treated FA on alkali-silica reaction resistance, permeability modification, and corrosion resistance of Portland cement based concretes.
Recently, attention has been placed on the study of agricultural waste materials whose use has been shown to improve the mechanical and durability properties of reinforced concrete and mortar.
Some authors have reported that Sugar Cane Bagasse Ash (SCBA) has substantial amounts of Silica (SiO2), Alumina (Al2O3) and Ferric Oxide (Fe2O3) [1], [7], [8] and that these account for over 70% of the constituents of SCBA, indicating that SCBA can also be used as a mineral admixture.
According to Frias et al. [9], the method of capturing the SCBA and the conditions under which it is calcinated influence the chemical and mineralogical composition and pozzolanic material properties.
Cordeiro et al. [10] found that at 800 °C Cristobalite and Calcium were formed. The production of SCBA under controlled conditions helps to prevent the formation of crystalline phases and in this way increases its pozzolanic activity.
Some researchers have shown that a high percentage of the loss on ignition (LOI) of the SCBA has a negative effect on its pozzolanic activity. For example, the development of the compressive strength of mortars with an addition of SCBA with a high LOI value was less than for SCBA mortars with low LOI values [11].
Cordeiro et al. [12] analysed the influence of the particle size of the SCBA on the density and compressive strength and found that the reduction in the particle size of the SCBA resulted in an improvement in the pozzolanic activity.
It has been reported an increase in the compressive strength of concrete and mortar mixtures with an addition of SCBA and it has been also demonstrated that the optimal percentage of cement replacement is 20% [1]. On the other hand, Hernandez et al. [8] observed that a poor curing time of mortar specimens with an addition of SCBA had a negative effect on the compressive strength.
The effect of the addition of SCBA on the properties that determine the durability of mortar and concrete mixtures has received attention in recent research. Ganesan et al. [1] found that the replacement of 20% of cement with SCBA decreased permeability and penetration of chloride ions. Meanwhile, Hernandez et al. [8] found that the higher the content of the SCBA, the higher the content of chlorides in surface layers of mortar specimens, which led them to conclude that the addition of 10% and 20% of SCBA reduced the diffusion coefficient by about 50%.
The results obtained by different researchers show that when post-treated, SCBA and FA have significant pozzolanic activity and can be used as additives in concrete and mortar mixes, because they help to improve mechanical and durability properties.
Unfortunately, all the methods used to activate FA and SCBA demand a lot of energy, making it necessary to investigate whether or not the impairment of the properties of mortars or concretes caused by the use of untreated, “practically as received” ashes, is tolerable.
Therefore, this research proposes to evaluate the mechanical and durability properties of minimally treated SCBA and FA as a partial replacement for cement in mortar mixtures. Compressive Strength (CS), Ultrasonic Pulse Velocity (UPV), Electrical Resistivity (ER) and Rapid Chloride Permeability (RCPT) tests were carried out. The correlations between mechanical and durability testing results were analysed as well.
Section snippets
Experimental design
To evaluate the effects of untreated sugarcane bagasse ash (UtSCBA) and untreated fly ash (UtFA) mortars were prepared containing these materials at three levels. Tests of these mortars in fresh and hardened states were carried out. Details of the experimental design and experimental procedures for results in the hardened state are summarized in Table 1.
Materials
The materials used in this research consisted of Ordinary Portland Cement R-40 (OPC) according with the NMX-C-414-ONNCCE Mexican Standard. The
Chemical analysis
Table 3 presents the major oxides for OPC, UtSCBA and UtFA. The ASTM 618 establishes that for Class N and F pozzolans, the minimum value of the sum of SiO2 + Al2O3 + Fe2O3 should be higher than 70% and for pozzolans Class C higher than 50%.
For the UtSCBA and UtFA under study the sums are 88.27% and 93.95%, respectively, which are higher than the value recommended by the Standard to consider the materials as pozzolans. The results are consistent with those from the literature which have been
Conclusions and recommendations
Based on the results of the tests the following conclusions can be drawn:
- 1.
The addition of 10% untreated SCBA had the following effects on the properties of the mortars: a decrease in the Compressive Strength at early ages but an increase in strength at later ages to a level similar or higher than the control; a decrease in the Ultrasonic Pulse Velocity at each of the ages studied; an increase in the Electrical Resistivity at all ages, but especially at 90 days, when the increase was 2.4 times
Acknowledgments
The authors are grateful for the financial support of the Consejo Nacional de Ciencia y Tecnología of Mexico (CONACyT), the Instituto Politécnico Nacional of Mexico (IPN), COFAA-IPN, PIFI-COFAA-IPN Program, CIIDIR-IPN-Oaxaca. The authors also thank the Universidad Autónoma de Nuevo León (UANL), the Faculty of Civil Engineering of the UANL (FIC-UANL) and the Institute of Civil Engineering of the UANL (IIC-FIC-UANL), as most of the testing was carried out in these institutions.
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