Investigation of strength and durability performances of pumice substituted geopolymer mortars

Cement is the most important and widely used binding material in the construction industry. However, the Portland cement industry has significant adverse environmental impacts, including the extraction of millions of tons of clinker raw materials and the high levels of carbon dioxide (CO2) emissions released during the high-temperature production of clinker. This industrial process, which accounts for approximately 7% of global CO2 emissions, is considered one of the most environmentally damaging production processes today. Approximately 700-760 kilograms of CO2 equivalent emissions are released during the production of one ton of Portland cement. In the light of these facts, researchers are working to find new solutions that are both environmentally friendly and cost-effective in the long term. Researchers are striving to develop alternative binders in the construction sector. Geopolymer technology has emerged as a sustainable alternative in the construction industry. The lower greenhouse gas emissions of geopolymers compared to Portland cement offer significant potential, especially against climate change. Extensive research has been conducted on the use of waste materials and industrial by-products in the production of Portland cement to reduce environmental impact and promote sustainability. These studies have aimed to develop new binding systems by partially or fully replacing traditional Portland cement with by-products having pozzolanic properties, such as fly ash, ground granulated blast furnace slag, and silica fume. Fundamentally, silica-rich and alumina-rich sources and pozzolanic materials form the basis of geopolymer technology. These sources are the building blocks that enable the formation of Si-O and Al-O bonds. The properties and proportions of these materials are the most critical factors determining the final properties of geopolymers. When reactivated with chemical activators in an alkaline environment, these materials provide a sustainable alternative binding system to traditional cement production. Fly ash and slag-based geopolymers have been the subject of extensive research and practical applications worldwide. These materials have significant potential for recycling industrial waste and developing sustainable construction materials. In contrast, pumice-based geopolymers are a relatively unexplored area. The limited number of scientific studies on this topic worldwide indicates that pumice-based geopolymers have great potential for structural utilization. This situation presents a significant opportunity for researchers to work on and investigate pumice-based geopolymers. Especially in regions where pumice is abundant and locally available, such studies can be of great importance both scientifically and industrially. This study investigated the mechanical performance and durability of alkali-activated mortars based on fly ash and slag containing different proportions of basaltic pumice powder. The mechanical performance and durability were evaluated under curing conditions of 7, 28, and 56 days, with 8M, 12M, and 16M sodium hydroxide concentrations, at temperatures of 200, 400, and 600°C, and in 5% sulfuric acid and 5% magnesium sulfate solutions. Compressive strength, flexural strength, and ultrasonic pulse velocity tests were performed to determine the macroscopic changes caused by high temperatures and exposure to 5% sulfuric acid and 5% magnesium sulfate. In addition, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analyses were conducted to observe microscopic differences in the samples. The study found that the addition of basaltic pumice powder decreased the compressive strength of mortars. Increasing the amount of sodium hydroxide (NaOH) and extending the curing period increased the compressive strength. The lowest strength was observed in mortars containing 50% pumice and cured for 7 days, while the highest strength was achieved in pumice-free mortars cured for 56 days. It was understood that pumice-containing mortars require longer curing times to reach their full potential due to the longer time required for the pumice particles to fully react. Although the increase in the amount of NaOH generally increased the strength, it had a limited effect on the strength loss of mortars exposed to high temperatures. This situation indicates that 8M NaOH concentrations may be more suitable in terms of economy. The addition of pumice up to 30% has shown that it can be used without significantly reducing the strength of mortars against high temperature effects. These results support that pumice is a suitable additive material for high temperature resistance in slag and fly ash-based alkali-activated mortars. Compressive strengths exposed to 5% sulfuric acid and 5% magnesium sulfate showed that the strenght loss decreases as the pumice content increases. This effect becomes more pronounced with 50% pumice substitution. Strength loss is due to the degradation of alumina-silicate bonds due to 5% sulfuric acid and magnesium sulfate exposure. Pumice-containing samples exhibited better durability performance than the pumice-free samples, while 12M geopolymer samples perform better than 8M and 16M samples in 5% sulfuric acid and 5% magnesium sulfate environments.