1. Introduction
The constituent materials of cement-based materials usually include cement, water, aggregates, admixtures and additives. The binder is a continuous phase in the cement composite and its pore structure is a critical factor, affecting the transport of fluids including gases, liquids and chemical substances in concrete [
1]. Materials which are designed to perform in aggressive chemical conditions must be of sufficient durability and should have the capacity to withstand the physical and chemical conditions to which they are exposed, throughout their service life [
2]. Cracks have a negative impact on concrete structures, and affect their performance in different ways, including weakening the structure as a result of reduced mechanical properties, or lowering the durability as a result of the penetration of harmful agents into the structure resulting in degradation of reinforcing steel or concrete [
3].
Crystalline admixtures can increase the durability of cement-based materials, especially in the case when these materials are exposed to aggressive environments. The catalytic reaction of the chemicals in the crystalline admixture, occurs as long as there is moisture in the cement-based materials. The crystalline admixtures react with calcium hydroxide and other products resulting from the hydration of the cement [
4]. Calcium carbonate is the main healing product which, during 28 days, is able to close cracks up to 400 μm in width [
5]. Reiterman et al. [
6] confirmed the ability of the repair mortar containing a crystalline admixture to penetrate the concrete surface and form a thin layer with reduced permeability. Crystalline admixtures can positively influence the ability of cement mortars and concrete in reducing the transport of water and chemical aggressive liquids, with improvements in durability [
7]. The availability of self-healing technologies, and the control and repair of early-stage cracks in concrete, where possible, could prevent the permeation of fluids, a driving factor for deterioration, thus extending the service life of reinforced concrete structures [
8].
Four types of pores occur in cement-based materials. Closed air voids (entrapped air) ranging in size from 1 to several mm and caused by poor compaction have a negative impact on strength. Closed macro-pores with a typical diameter of 10 to hundreds of μm (entrained air) added to accommodate technological needs, decrease the strength of composites as well. Capillary pores form a connected network of mesopores among hydration products, unhydrated cement grains, fines and aggregate, with an average size varying from 10 to 10 μm. These influence mainly durability, because they enable gas and liquid transport by means of diffusion, capillary suction, sorption and ion transport. The finest pores with an average size of 0.5 to 10 nm are called gel pores and are found within the structure of the calcium-silicate-hydrate (C-S-H) phase. These pores are too fine to have any major influence on the strength of the composites, although they directly affect durability and largely shrinkage and creep as well [
1,
9,
10,
11,
12].
Crystalline admixtures (CA) are hydrophilic and they react easily with water, in contrast to water-repellent or hydrophobic products [
13]. The crystalline formations produced by CA become a permanent part of the cement matrix [
11,
14]. The crystallization process, according to the American Concrete Institute (ACI) [
14], follows Equation (1), where a crystalline promoter M
xR
x reacts with tricalcium silicates and water, to produce modified calcium silicate hydrates, together with a pore-blocking precipitate M
xCaR
x − (H
2O)
x.
Most supplementary cementitious materials are considered as pollutant by-products, so their reuse would address and partly solve other environmental problems, such as their landfilling. One the most popular additions is fly ash and several researchers [
15,
16,
17] have noted that cementitious materials, which incorporate the addition of fly ash, have a better performance when compared to materials produced only with Ordinary Portland Cement (OPC). High amounts of fly ash can be used in combination with Portland cement, for the production of concrete covering the range of design strengths which are typically required in practice [
18]. Mortars prepared using sustainable cements with fly ash and exposed to Mediterranean climatic conditions, showed adequate service properties in the short term (90 days), similar to or even better than the properties of mortars made with ordinary Portland cement [
19,
20].
The addition of fly ash has also shown improved performance of cement-based polymer mortars [
21]. Waste limestone in concrete, including Globigerina waste limestone, as a partial replacement of Ordinary Portland Cement has been reported to improve the mechanical properties of concrete but also the long-term durability performance determined through water and gas permeability and chloride ion penetration tests [
20].
According to the basic requirement No.7 for construction and building materials with CE marking [
22], these products have to meet the limits for the appropriate quality of the indoor environment and at the same time minimally burden the external environment. Furthermore, it is necessary to choose suitable energy sources (using renewable energy sources) and also consider the appropriate quality of the product after installation in the building and take into account the economic context necessary for design, operation and disposal (recycling).
The aim of the research presented in this paper was to design mortars using landfilled fly ash, which was produced at the coal-fired power plant in Marsa, Malta, waste Globigerina limestone and crystalline admixture to improve the properties of mortars. The aim was to investigate the effect of the cement substitution by the fly ash and waste limestone filler, together with the addition of crystalline admixture on the durability of mortars for chloride aggressive environments. The properties were determined after curing at a high relative humidity for 540 days. The mechanical properties of the material were analyzed, together with durability performance through various tests including permeable porosity, capillary suction, rapid chloride ion penetration and chloride migration coefficient.
4. Conclusions
In the research presented in this paper, it was concluded that both the mechanical properties and the durability properties of polymer cement mortars are improved by replacing 20% of the cement with the Bengħisa fly ash and by the introduction of crystalline admixture in the mix. Overall, it can be observed that mixes containing secondary raw materials (waste Globigerina limestone, fly ash) showed better performance and have improved properties than the reference mortar, especially in the permeable porosity test, the rapid chloride penetration test, the Chloride migration coefficient test and capillary suction test. This results in a reduction in waste disposal and the potential of recycling of these waste materials, leading also to a reduction in the consumption of new resources. The best resistance to chloride ion penetration was shown in the case of polymer cement mortar FA20-CA, where fly ash and a crystalline admixture were used. Monitoring of the physical and mechanical properties and durability was supported by an analysis of the microstructure and it was shown that, under suitable conditions (high relative humidity), needle-shaped crystals filling micropores form in the porous structure of the mortar, which prevent the penetration of chloride ions into the internal structure of the mortar. The pozzolanic reaction of the fly ash was also observed, with broken cenospheres and evidence of the formation of C-S-H phases on the surface. Therefore, it has been shown that the use of the fly ash in combination with crystalline admixtures improves the performance of the polymer–cement mortars in coastal areas exposed to chloride ion penetration and the resistance to aggressive environments.