Investigation of pitchstone fines as a novel supplementary cementitious material for Portland cement based construction products
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Environmental implications associated with the manufacture and consumption of Portland cement (PC) presents a major challenge to the construction industry. For every tonne of PC manufactured, an equivalent amount of carbon dioxide is generated as greenhouse gas emissions. The use of supplementary cementitious materials (SCMs), also known as pozzolans, for the partial replacement of PC aids the reduction in consumption of PC. SCMs, as environmentally friendly ‘green solutions’, also provide performance-driven engineering properties of long-term strength development and enhanced durability. Siliceous and aluminous industrial by-products, such as fly ash (FA), silica fume (SF) and ground granulated blast furnace slag (GGBFS) are used as SCMs to partially substitute PC in order to reduce the consumption of PC in cement-based construction materials. In this study, pitchstone fines (PF) which are the by-product of the production of expanded perlite from a naturally occurring pitchstone deposit in the state of Queensland in Australia are investigated. PF are produced in the crushing stage of the operation where particulates of less than 0.5 mm, which constitute as much as 30% of the pitchstone rock, are discarded causing a potential waste problem. PF being an amorphous aluminosilicate material was investigated as a potential SCM. Initial investigations on an as received grade of PF, passing 150 µm mesh, produced favourable results for small substitutions of PC (10%); however, a significant water demand was noted in the production of mortars, otherwise the poor workability reduced homogeneous compaction in the moulds and resulted in inconsistent samples with low compressive strength. Two factors, reduced particle size and improved flow, were identified as critical for the improvement of mortar properties. In order to demonstrate this, a fine grade PF with an average particle size 10 pm was prepared. The water demand for the finer grade PF was significant and superplasticiser was added to improve flow. After accelerated ageing at elevated temperature the strength of mortars containing increasing PF additions up to 40% demonstrated increased strength. In order to investigate the further potential of PF, PF mortars were compared to FA mortars (FA being an industry accepted SCM). The PF was graded to a similar particle size distribution to the FA, and was found to produce similar strength. Based on the ASTM standard criteria for classification as a pozzolan, strength activity index (SAI), a relative measure requiring the strength to be within 75% of the control 100% PC mortar, both PF and FA were found to fulfil the criteria at 20% additions at 7 and 28 days ageing. Mortars with 40% additions approached the SAI criteria only at significantly longer periods of ageing (91 days). A further 10 pm grade PF was prepared by bead milling to investigate the standard water curing of PF mortars and compared to a similarly graded FA. Both mortar types produced with 20% and 40% addition levels significantly surpassed the strength of the control PC mortars. The susceptibility of PF and FA mortars to sulphate attack was investigated by immersing mortar cubes in deionised water and 1 M sodium sulphate. The degree of sulphate attack was monitored by mass gain and compressive strength measurement after 182 days of immersion. The 100% PC control mortar showed significant susceptibility with a large increase in mass gain and a reduction in strength. Both the PF and FA substituted mortars were found to be significantly more resistant to the sulphate solution with lower mass gains and significant improvements in strength relative to the control mortar in deionised water. In the PF and FA mortar samples, ettringite was identified by XRD analysis suggesting that its formation mitigated the effects of sulphate attack. The investigation of PF as a pozzolan particularly in comparison to FA demonstrated that PF in mortar and concrete exhibited the positive attributes of strength and durability required. This experimental investigation proved that PF may be used as a SCM for the partial replacement of PC. From an engineering perspective, PF, which are hitherto unknown as a SCM for PC-based construction materials, are a viable option for adoption in the manufacture of PC-based construction materials, eventuating in value added benefits of strength improvement and increased resistance to chemical attack. Since the PF studied are currently a by-product of mining, the use of this naturally occurring SCM also helps mitigate the environmental impact at the mine site.
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