Perlite is an amorphous glass deposit near volcanoes. Perlite normally exists in the form of lightweight, highly porous particles with a frothy-like microstructure. It is the micro, inter-connected air bubbles (or pores) inside perlite particles which create an excellent heat resistance (it can withstand a rising temperature even above 1,000°C). These micro bubbles simultaneously result in the capacity to absorb water and expand to multiple times the original volume (from 5 to 20 times) and obviously, weaken. (The more water perlite absorbs, the less heat resistant it becomes.)

In concrete, perlite is normally used as a partial replacement of fine aggregate to improve fire, heat, and acoustic insulation. Perlite concrete therefore finds itself particularly useful for roof insulation as well as other non-structural applications. However, perlite concrete performs very poorly when it comes to load-bearing capacity. As perlite absorbs a huge amount of water, expands, and weakens, it critically impacts the strength and durability of the final concrete structures [1]. As an illustration, if the expanded perlite is used as a replacement for fine aggregate (i.e., sand), only 4% replacement by weight of sand can cause a drop of compressive strength of as much as 40%. At 20% replacement by weight, concrete can lose up to 80% of its compressive strength (compared to concrete of the same mix design and the same ingredients, without perlite) [2].

To enhance structural performance of perlite concrete, current research suggests the following:

• Modify perlite by impregnating it with a hydrophobic agent (to reduce its absorptivity and strengthen its structure)
• Lower dosage of perlite aggregate (for example, maximum 10% by weight of sand)
• Apply a lower water-to-cementitious material ratio in the mix, and
• Use a high percentage of supplementary cementitious materials such as ground granulated blast furnace slag or silica fume [3].

For the fireproof house you intend to build in South Australia, it is important to discuss with your design consultant and work out the most appropriate strategy to apply perlite concrete.

If it is desirable to use perlite concrete for the entire house, including load-bearing elements, then it is necessary to consult a professional concrete supplier in your area. They may be able to provide you with a customized, especially careful mix design which best suit your need.

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Reference(s):

[1] I. B. Topçu and B. Işikdaǧ, “Effect of expanded perlite aggregate on the properties of lightweight concrete,” J Mater Process Technol, vol. 204, no. 1-3, pp. 34-38, Aug. 2008, doi: 10.1016/j.jmatprotec.2007.10.052.

[2] O. Sengul, S. Azizi, F. Karaosmanoglu, and M. A. Tasdemir, “Effect of expanded perlite on the mechanical properties and thermal conductivity of lightweight concrete,” Energy Build, vol. 43, no. 2-3, pp. 671-676, Feb. 2011, doi: 10.1016/j.enbuild.2010.11.008.

[3] A. el Mir, S. G. Nehme, and J. J. Assaad, “Durability of self-consolidating concrete containing natural waste perlite powders,” Heliyon, vol. 6, no. 1, Jan. 2020, doi: 10.1016/j.heliyon.2020.e03165.

In terms of sustainable living, building a house near big trees can bring benefits at multiple fronts:

• The trees act as a natural barrier. They protect the house from extreme weather events such as heatwaves and hailstorms, slowing down the degradation process of external components, thereby extending the house’s service life.
• The trees are a natural air conditioner. They harmonize temperature variation inside the house. This not only supports healthy residents but also reduces energy consumption (i.e., for air heating and cooling) significantly.
• Big trees are a natural water storage and an effective bushfire shield. They shield the house from radiant heat and filter flying embers or debris. Plus, big trees limit the growth of shaded vegetation and grass. This reduces available fuel, fire intensity and fire spread accordingly.

However, there are certain risks associated with building houses near big trees. For example:

• Trees grow over time. In some cases, tree root can reach a distance over 100 m. The spread of root system creates a disturbance in foundation, mostly in the form of an uplift, which can cause cracking or distortion of footings, beams, walls, columns etc.
• Falling branches due to strong winds can cause physical damages to roof and even residents.

Instead of cutting down a healthy big gum tree to clear the land, there are ways to mitigate the risks above. We suggest you discuss with your design engineers to include the following considerations in the house design:

• Use stronger footings. This may include stone, stiffened raft or pile systems (e.g., sand piles, concrete piles, timber piles etc.). The piles are driven to at least a few metres below the ground. Together, they increase stability and load-carrying capacity of the whole footing system. In addition, these piles help densify the soil and reduce potential ground movement.
• Beware of windbreak trees. This is particularly critical when the newly built house and the gum tree are aligned on a major wind direction. Under strong and regular winds, the risks of tree toppling or branch falling directly onto the house are very high. To reduce the risks, locate the house to a different position and/or ensure a safer distance to the tree. A regular inspection to cut down old branches before wet season can be helpful too.
• Be tree friendly. Let the tree grow naturally unless it has an incurable disease. If the tree is felled prior to the house construction, degrading root system over time will lead to unstable ground or uneven settlement. This in turn causes instability or damages for the house in the near future.

There are certain risks associated with what you intend to do.

• When you cast a new concrete slab directly onto the top of an existing wall footing, the slab imposes additional weight on the footing, which may compromise its stability, not to mention a potential damage to the footing itself.
• The new concrete slab tends to deform (e.g., as longitudinal contraction or expansion) under temperature change and moisture loss, which may affect the wall's stability.
• The top corner of the footing where the slab is cast on acts as a disturbing point due to high stress concentration. When the ground (i.e., sub-grade) under the slab settles, the slab is highly likely to crack in the vicinity of this point.

However, it is not impossible to proceed with your construction idea. If the slab is de-bonded to the footing, properly jointed and reinforced, and the footing is checked to ensure it is safe under the additional weight of the slab, then it can work just as well as other structures in your house.

It is unclear how you defined concrete sustainability in your question. From our very basic understanding, concrete of the lowest embodied carbon is the most environmentally friendly and therefore, the most sustainable.

Most of the embodied carbon in concrete comes from cement. In normal-class concretes, cement only takes up about 13-16% by total weight, but its embodied carbon contributes to above 90% the total level. This percentage is even higher in special-class including high strength concretes (above 95%).

To reduce the embodied carbon, a number of industrial by-products, also known as supplementary cementitious materials (SCM's), are being used to partly or fully replace cement. Fly ash, for example, can replace up to 35% cement in concrete while also enhancing concrete durability. Similarly, ground granulated blast furnace slag can be used to replace up to 60% cement in concrete.

Most commercial concrete at supermarkets (i.e., dry bagged concrete) contains a moderate amount of SCM's, which is good (even though Australian suppliers rarely disclose the actual amount). In addition, when it comes to cement type, blended cement or limestone cement (i.e., Type GB or GL complying with Australian standard AS 3972) contains more environmental values than the others.

You can contact your local concrete suppliers to clarify the amount of SCM's and cement type used in their products. You can also discuss a “tailored” concrete mix with them to suit better your sustainability goal (which probably will cost you a bit more).

Firstly, your brick-concrete house has stood strong for over 40 years of normal service. This is a great indication of sustainability when it comes to strength, durability and weatherproof. Congratulations!

Secondly, if you consider sustainability in terms of embodied carbon, then timber has some distinct advantages.

• Timber does not emit carbon, but in fact is one of the most effective natural carbon storages throughout its service life.
• The production and treatment of timber only produce a small fraction of carbon emission, compared to the manufacturing processes of other materials.

Plus, recycling rate of timber at the end of its service life is high.

Plus, timber is a lightweight material, which is highly beneficial for your new room:

• The new room will exert extra but less intense loads on existing footing(s) and existing wall(s) of the main house, meaning a lower risk of structural instability or damage for the main house.
• Light weight also means less foundation settlement under the new room. This helps ease the complication of jointing or connecting the new room to the main house. It also reduces potential issues at locations where the new room and existing wall(s) meet such as cracking or joint opening. Future repair, or maintenance, will also be cheaper and easier.

However, there is no perfectly sustainable construction material. Keep in mind the following factors during your decision-making process:

• The production of timber requires living forests to be cut down, meaning a portion of the natural air purification mechanisms (carbon absorption) has been removed. Even though replanting will be done afterwards to fill in the gap, young trees will need some time to be able to achieve the previous carbon absorption level. According to a model for “sustainable timber production” in New South Wales, Australia, released in 2012 by New South Wales Department of Primary Industries [1], this “some time” could mean 30 years.
• As trees become more mature, their carbon absorption rapidly accelerates. A study in 2017 from a research group in Germany, conducted on 3 species of untouched trees in South America's tropical rainforests indicated that: If the trees are allowed to grow untouched (i.e., without trimming or cutting), in the last quartile of their lifetimes, each tree can absorb an average amount equivalent to 40% to 50% the total carbon it absorbs throughout its life [2]. Note that trees are normally cut down to produce commercial timber from year 25 to 40. These trees, if leaving untouched, can live to 100 years or more. Their environmental benefits when they live and become more mature, therefore, can be exponentially higher.
• For some iconic forests in Australia, such as Tasmanian's giant Eucalyptus Forest, letting the trees grow old naturally could also mean significantly reducing fire risk. The finding was reported by scientists from The University of Tasmania, Australia, in 2021 [3].
• Timber has a low thermal mass, meaning that if you stay in a timber house, you will feel colder in winter and hotter in summer compared to living in a brick or concrete house. It also means that timber houses require higher energy input for air conditioning, or more costly insulation.

It may be more beneficial for you to consider a good combination of different construction materials, rather than prioritising any of them. Regarding the second part of your question, advice on “best practice” is only appropriate after key design parameters for the room have been chosen, such as material, size, shape, location etc. We suggest you get back to us when more details are available.

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Reference(s):

[1] F. de A. Ximenes, B. H. George, A. Cowie, J. Williams, and G. Kelly, “Greenhouse gas balance of native forests in New South Wales, Australia,” Forests, vol. 3, no. 3, pp. 653-683, 2012, doi: 10.3390/f3030653.

[2] M. Köhl, P. R. Neupane, and N. Lotfiomran, “The impact of tree age on biomass growth and carbon accumulation capacity: A retrospective analysis using tree ring data of three tropical tree species grown in natural forests of Suriname,” PLoS One, vol. 12, no. 8, Aug. 2017, doi: 10.1371/journal.pone.0181187.

[3] J. M. Furlaud, L. D. Prior, G. J. Williamson, and D. M. J. S. Bowman, “Fire risk and severity decline with stand development in Tasmanian giant Eucalyptus forest,” For Ecol Manage, vol. 502, p. 119724, Dec. 2021, doi: 10.1016/J.FORECO.2021.119724.

Cover should be sufficient to protect the steel mesh or bars inside against wearing and, more importantly, environmental attacks such as chloride corrosion. Corrosion not only causes a gradual reduction of load-bearing capacity but also severely impacts concrete durability. Established knowledge indicates that reducing cover by half can quadruple corrosion rate [1]. On the other hand, increasing cover from 20 mm to 30 mm can double maintenance-free concrete life in service [2].

For your driveway slab, 20 mm cover above SL 92 mesh does not seem enough. (Note that the cover is measured from the finished concrete surface of the element to the nearest surface of the main steel mesh/bars, refer to the illustration above). According to Australian standard AS 3727.1:2016 “Pavement Residential”, the top cover should be at least 30 mm.

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Reference(s):

[1] “Durable concrete structures CEB design guide second edition,” 1989.

[2] “CIA Z7/04-2014 Good Practice Through Design, Concrete Supply and Construction,” 2014.

Cracking in concrete slabs may look normal but there are ways to minimize the risk. In your case, cracking started from the vicinity of the tree and propagated further into the slab. It is likely that:

• The soil around the tree is damper and softer, leading to either a large or an uneven deformation, or settlement, of the foundation under the slab surrounding that area.
• As the tree grows, its root system spreads farther and becomes thicker, causing a disturbance, or uplift force, under the slab.

If the slab you built was reinforced with steel mesh, then the steel can hold all the pieces together to keep the cracks as narrow as they can be, and you will be able to use it for a long time to come.

In order to minimize the cracking risk, we suggest you consider the following steps next time you build your slabs:

• Divide your slab into smaller segments using control joints, for example those in the diagram above. Especially, the area around the tree (i.e., the shaded segment) should be completely separated from adjacent segments by an isolation joint. This segment should have more steel reinforcement arranged over its whole area.
• Embed a tree root barrier surrounding the tree when it is still small, to at least 1 m into the ground, to guide the root growth - deeper rather than wider.