Petrolab has now been undertaking Mundic assessment for 25 years. This is the first of a two part post looking at how the Mundic Guidance has developed over those 25 years starting with the initial political drivers and aggregate uncertainty.
The “Mundic” problem first began to surface in the mid to late 1980’s being recorded in the House of Common’s Hansard in 1982. As can be seen from this early exchange the problem was initially underestimated, being pegged at just 200 – 300 houses, and only in the Truro area. In 1983 the then Hon. Mem. for Truro Mr David Penhaligon in summarizing an eventually unsuccessful case for repair grants made a cogent summary of the state of knowledge and the impact it was having on peoples lives. However, by 1988 matters seemed barely to have moved on and even by 1990 there was still great uncertainty for policy makers as this interchange between the subsequent Hon. Mem. for Truro Mr Matthew Taylor and the then Secretary of State for the Environment Mr. Chope highlights.
Mr. Chope – “…The mechanism of “Mundic” in aggregates is little understood and, as the article makes clear, there is confusion within the construction industry…”
The uncertainty covered two main areas. Firstly, what were the different aggregates being used across Cornwall and Devon. Some block works and quarries were well known and research could relatively quickly confirm some obvious concrete types. Others were far more uncertain and the ingenuity applied by many builders to the sourcing of suitably graded aggregate posed a significant challenge. For this question local expertise at Camborne School of Mines proved invaluable. The then Head of Geology Dr Alan Bromley’s comprehensive overview is still the best and current resource used by the Royal Institute of Chartered Surveyors for Mundic guidance. Dr Alan Bromley would found Petrolab in 1991.
Secondly, after recognizing the different aggregate types how to then determine which were potentially deleterious. For this question the initial uncertainty led to understandable caution in the 1st edition of the Mundic Guidance published by the RICS (1994), where essentially all aggregates containing sulphides, or metalliferous mining and ore processing wastes were classified as potentially deleterious. However, this initial caution meant that aggregate types were deemed potentially unmortgageable which until then, and subsequently, have shown no evidence of concrete degradation (e.g. slate from the great Delabole quarry). By the time of the second edition (1997) some of this uncertainty had been resolved and several notable aggregate types became classed as stable at stage 1. The recommended reclassification of these aggregates as stable have since been reconfirmed by the 3rd edition of the Mundic guidance (published 2015).
Perhaps the two most interesting of these aggregates from a pyrite-bearing point of view are granite hosted mine wastes and the Penlee dolerite.
Granite-hosted Mine Waste
Granite-hosted mine waste covers two main block types produced from the former South Crofty mine waste in Pool, near Redruth and from Wheal Reeth mine waste near St Ives. Other distinctive examples have been observed (e.g. around the villages of St Neot and Indian Queens). There may have been other local granitic mine-waste sourced on occasion. Although definitely a mine waste the low total sulphide content and predominance of unreactive silicate and oxide phases in the aggregate usually makes this stable. The blockwork is often a pale pink due to hematite eroded from the aggregate material which stains the binder. Importantly this hematite hasn’t formed in-situ from pyrite oxidation but was an initial component of the aggregate and therefore presents no evidence of degradation.
On occasion sulfide content can be moderate but even then the mine waste would normally be considered stable due to the high degree of sulfide encapsulation within stable silicate phases where reaction and dissolution of the binder is not possible. Since 1997 granite hosted mine wastes are considered stable at Stage 1 unless there is evidence for very high sulphide content and in-situ aggregate degradation.
Typically pink granite mine waste concrete. The aggregate was slightly poorly graded and arsenopyrite and pyrite were both observed. However, overall sulphide grades were very low, encapsulation was high and there was no evidence of concrete degradation. Therefore this sample could be classed as stable at Stage 1.
Penlee Dolerite
The Penlee dolerite is one of the most interesting regular aggregates. Penlee dolerite was extracted from the Penlee or Gwavas quarry at Newlyn for nearly 100 years and until the mid-1970s was exported in large quantities by sea to Netherlands and Germany. However, concerns over the sulphide contents eventually led to the closure of the quarry in the mid-1980s. During its operating life Penlee dolerite was widely used in blockwork in Penzance, Newlyn and surrounding villages all the way up to Marazion, St Ives, Carbis Bay and Hayle. It was also regularly used in structural concrete being found in bridges across Cornwall. Pyrite content is normally <1.0 wt% but can reach up to 2.0%. However, buildings containing Penlee dolerite have only very rarely shown visible signs of concrete degradation and in each case there was mitigating circumstances of poor building maintenance that exacerbated the problem.
The stability of this aggregate is mainly related to encapsulation of pyrite within dolerite aggregates preventing the possibility of reaction and oxidation. However, pyrite may also be present as liberated grains free to react within the binder. The most interesting aspect of this is that in nearly every instance the pyrite shows no signs of reaction. This is thought to be due the coarse-grained nature of the pyrite (i.e. high volume to surface ratio) and well crystallized form making the pyrite relatively unreactive – (bearing in mind that these concrete blocks have stayed in place for the best part of 100 years with no pyrite oxidation). Contrast this with pelite mine waste or high sulphide complex mine waste where exposed pyrite regularly shows oxidation and associated concrete degradation. Since 1997, and ratified in 2015, Penlee dolerite are considered stable at Stage 1.
Cross section through a Penlee dolerite core. Sulphide grades were low and there was no sign of in-situ oxidation or degradation. Pyrite was observed as above, highly encapsulated in coarse dolerite pieces. The sample could be passed as stable at stage 1.
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