Hybrid Cements and Construction Elements Based on Alkaline Activation with Sodium Sulfates from Fly Ash and Construction and Demolition Waste.

This article demonstrates the possibility of producing hybrid cementitious materials (pastes, mortars, concretes, and precast elements) based on fly ash (FA) and construction and demolition wastes (CDW) using alkaline activation technology. Sodium sulfate was used as an activator and fine and coarse aggregates were obtained from CDW residues. An addition of Portland cement (OPC) (10 to 30%) allowed for improvement in the mechanical behavior of the hybrid cements and them to be cured at room temperature (25 °C). The FA and CDW cementitious materials obtained compressive strengths of 37 MPa and 32 MPa, respectively. The compressive strength of FA and CDW alkali-activated concretes at 28 days of curing was 22 MPa and 18 MPa, respectively, which identifies them as structural concretes according to NSR-10 title C in Colombia. The potential use of these concretes was validated by obtaining and classifying precast materials.

Assessment of the Corrosion of Steel Embedded in an Alkali-Activated Hybrid Concrete Exposed to Chlorides.

Hybrid alkali-activated cements (HAACs), also known as cements with high percentages of alkali-activated supplementary materials, are alternative cements that combine the advantages of ordinary Portland cement (OPC) and alkali-activated systems. These cements are composed of a minimum of 70% precursor material and a maximum of 30% OPC mixed with an alkaline activator. This article evaluates the corrosion performance of reinforced HAAC concrete based on fly ash (FA) under exposure to chlorides (FA/OPC, 80/20). Its performance is compared with that of a binary alkali-activated cement (AAC) based on FA and granulated blast furnace slag (GBFS) (FA/GBFS, 80/20). The tests performed on the concrete matrix correspond to the compressive strength and permeability to chloride ions. Using accelerated corrosion techniques (impressed voltage) and electrochemical tests after immersion in 3.5% NaCl, the progress of the corrosive process in the reinforcing steel is evaluated. The FA/OPC exhibit a better corrosion performance than the FA/GBFS concrete. At the end of the exposure to chlorides, the FA/OPC hybrid concrete presents the best performance, with a 49% lower corrosion rate than that of the FA/GBFS. Note that according to the polarization curves, the values of the proportionality constant B in the alkaline-activated concretes differ from the values recommended for concrete based on OPC.

Alkali-Activated Hybrid Cements Based on Fly Ash and Construction and Demolition Wastes Using Sodium Sulfate and Sodium Carbonate.

This article demonstrates the possibility of producing alkali-activated hybrid cements based on fly ash (FA), and construction and demolition wastes (concrete waste, COW; ceramic waste, CEW; and masonry waste, MAW) using sodium sulfate (Na2SO4) (2-6%) and sodium carbonate (Na2CO3) (5-10%) as activators. From a mixture of COW, CEW, and MAW in equal proportions (33.33%), a new precursor called CDW was generated. The precursors were mixed with ordinary Portland cement (OPC) (10-30%). Curing of the materials was performed at room temperature (25 °C). The hybrid cements activated with Na2SO4 reached compressive strengths of up to 31 MPa at 28 days of curing, and the hybrid cements activated with Na2CO3 yielded compressive strengths of up to 22 MPa. Based on their mechanical performance, the optimal mixtures were selected: FA/30OPC-4%Na2SO4, CDW/30OPC-4%Na2SO4, FA/30OPC-10%Na2CO3, and CDW/30OPC-10%Na2CO3. At prolonged ages (180 days), these mixtures reached compressive strength values similar to those reported for pastes based on 100% OPC. A notable advantage is the reduction of the heat of the reaction, which can be reduced by up to 10 times relative to that reported for the hydration of Portland cement. These results show the feasibility of manufacturing alkaline-activated hybrid cements using alternative activators with a lower environmental impact.

Eco-House Prototype Constructed with Alkali-Activated Blocks: Material Production, Characterization, Design, Construction, and Environmental Impact.

The interest of the construction industry in alkali-activated materials has increased to the extent that these materials are recognized as alternatives to ordinary Portland cement-based materials in the quest for sustainable construction. This article presents the design and construction of a prototype of an eco-friendly house built from concrete blocks produced using alkali activation technology or geopolymerization. The prototype meets the requirements of the current Colombian Regulations for Earthquake Resistant Buildings (NSR-10) and includes standards related to the performance of the materials, design, and construction method for earthquake-resistant confined masonry of one- or two-story buildings. The alkali-activated blocks were obtained from different precursors (aluminosilicates), including a natural volcanic pozzolan, ground granulated blast furnace slag, fly ash, construction and demolition waste (concrete, ceramic, brick, and mortar), and red clay brick waste. The physical-mechanical characterization of the alkali-activated blocks allowed their classification according to the structural specifications of the Colombian Technical Standard NTC 4026 (equivalent to ASTM C90). The global warming potential (GWP) or "carbon footprint" attributed to the raw materials of alkali-activated blocks was lower (25.4-54.7%) than that of the reference blocks (ordinary Portland cement concrete blocks). These results demonstrate the potential of alkali-activated materials for application in the construction of eco-friendly houses.

Resistance to Chemical Attack of Hybrid Fly Ash-Based Alkali-Activated Concretes.

The environmental impacts related to Portland cement production in terms of energy consumption, the massive use of natural resources and CO2 emissions have led to the search for alternative cementitious materials. Among these materials, alkali-activated cements based on fly ash (FA) have been considered for concrete production with greater sustainability. In the present article, the chemical durability properties (resistance to sulphates, chloride permeability, and resistance to carbonation) of a hybrid alkali-activated concrete based on fly ash-ordinary Portland cement (FA/OPC) with proportions of 80%/20% were evaluated. It is noted that the FA was a low-quality pozzolan with a high unburned carbon content (20.67%). The results indicated that FA/OPC concrete had good durability with respect to the OPC concrete, with 95% less expansion in the presence of sodium sulphate and a 2% strength loss at 1100 days, compared with the 56% strength loss of the OPC concrete. In addition, FA/OPC showed lower chloride permeability. On the contrary, the FA/OPC was more susceptible to carbonation. However, the residual compressive strength was 23 MPa at 360 days of CO2 exposure. Based on the results, FA/OPC, using this type of FA, can be used as a replacement for OPC in the presence of these aggressive agents in the service environment.