BUILDING WITH PUMICE: Making blocks out of pumice, lime and concrete

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Pumice P07.GIF

Everyone would like to live in his own home. Many people in industrialized countries occupy apartments in multistorey buildings.

This situation involves lots of problems for which there’s no immediate solution. This book is intended to stimulate interest in a line of approach to such problems by describing how to build simple, inexpensive houses out of a very commonplace raw material, namely pumice, i.e. volcanic glass or hardened volcanic froth. Such homes can be constructed on a self-help basis by individual builders, as cooperative efforts, or on an industrial scale. Which building components can be made of pumice and how those components can be put together to make a house is described in the following chapters.

We will be referring throughout to a basic model with roughly 30 m² floor space, for which the requisite building material costs approximately US . Such houses could serve well and be affordable as a minimum size dwelling for a family of six or less (Fig. 2). The described home-construction systems can be enlarged, built onto and/or modified at will, depending on the prevailing architectural style, the family’s space requirement and their given financial situation. The building materials for the house do not have to be bought or made all at once, but can be accumulated or put together little by little. With a bit of handicraft skill, it’s relatively easy to make the most of the building material assuming, of course, the builder has access to and knows how to handle pumice (or volcanic ash), cement, water and a few elementary tools.

The best way to tackle the job is for several prospective home builders to team up with each other to jointly plan, organize and implement their own building projects.

The main purpose of this book is to give practical information on the use of pumice as a building material and on organizing one’s own home building project. Naturally, no individual solutions can be offered for problems concerning the purchase of property or the financing, obtaining a building permit or actual construction of the house.

Building material for a house can be made from any number of raw materials, such as straw, reed, rocks, soil, wood, metal, depending on what’s available within a reasonable distance, for which climate the house is being built, and which culture-dependent conceptions it will have to incorporate. Pumice, too, is a good raw material for use in making building members. Pumice is not found everywhere, but only in the vicinity of extinct or still-active volcanoes, e.g. in Central America, East Africa, East Asia and Europe (Fig. 3).

Pumice P08.GIF
Figure 3

Europeans have always used pumice in residential buildings and industrial structures and continue to do so. As a building material in general it’s very popular, particularly in the near vicinity of the deposits.

The dissemination of knowledge and the transfer of technology concerning the production of pumice building materials should help developing countries establish their own indigenous production of inexpensive, versatile building materials. This book hopes to stimulate the utilization of existing resources in the form of volcanic ash/pumice deposits while also providing practical guidance for the production of building members for low cost homes.

Turning pumice into building material

All pumice building members can be made using simple craft skills. No complicated (and therefore expensive) machinery is needed.

What are needed most are a wood or metal formwork, a wheelbarrow, a shovel, a trowel and a level area for shaping and drying the pumice building members. Cement or lime, sand and water must also be available.

Producing one’s own pumice building members can always be recommended where the raw material is sufficiently inexpensive or, even better, available free of charge and the building is to be put up on its own, where there is solid foundation soil, and the builder/owner has some skill and prior experience in handling building materials.

It’s important to know that pumice building members are very durable if made properly and that they’re particularly suitable for dry climates.

As mentioned above, the materials needed to build a pumice home with 30 m² floor space cost roughly US . Add to that, of course, the cost of the property and any wages paid to helpers or contractors. For making one’s own wall members from pumice, the raw material should be available within a radius of 30 km (Fig. 4).

Pumice P09.GIF
Figure 4: Maximum distance between deposit and building site

Further information on rules and regulations governing home construction can be obtained from:

  • cooperative building societies
  • building authorities
  • credit institutions
  • architects
  • missions

What is pumice?

Pumice is a very porous form of vitrified volcanic rock, usually of very light colon. Its true density, i.e. the density of the powdered material, amounts to between 2 and 3 kg/ dm³ and its bulk density, i.e. the density of the loosely piled material, amounts to between 0.3 and 0.8 kg/dm3. In other words, pumice is very light. It has roughly the consistency of a mixture of gravel and sand, with light, porous individual granules that normally either float on water or sink only slowly. Pumice particles are either round or angular and measure up to 65 mm in diameter. Only particles in the 1 -16-mm size range should be used to obtain good building material.

Pumice P10.GIF
Figure 5: A volcanic eruption

In addition to light-coloured pumice, there are also various dark-coloured forms referred to as lava, tuff, etc. They, too, can be used as building material, but the light-coloured pumice processes better, as described in Chapter 2.4.

Pumice has the following chemical composition:

silica SiO2 approx. 55%
alumina Al2O3 approx. 22%
alkalies K2O+Na2O approx. 12%
ferric oxide Fe2O3 approx. 3%
lime CaO approx. 2%
magnesia MgO approx. 1%
titania TiO2 approx. 0.5%

Pumice originates during volcanic eruptions, when molten endogenous rock is mixed with gases before being spewed out (Fig. 5). The light, spongy particles are hurled up and carried off by the wind. As they cool and fall back to Earth, the particles accumulate to form pumice rock or boulders. Sometimes the molten rock is too heavy to be ejected, in which case it flows out and collects at the foot of the volcano as a compact, fairly homogeneous, usually somewhat less porous rock formation. Most such lava deposits can be cut up into natural stone blocks for direct use in construction work.

Where is pumice found?

Most pumice is found on the downwind side of volcanoes (Fig. 6).

Pumice P11.GIF
Figure 6: Pumice deposits on the downwind side of a volcano

The average deposit is loose, with a layer thickness ranging from 50 to 300 cm. Pumice should always be extracted under expert supervision and not haphazardly; otherwise, the results will look like Figure 7. The thickness of the pumice strata decreases with increasing distance from the center of the eruption.

The size of pumice particles ranges from superfine powder (0-2 mm) to sand (2-8 mm) to gravel (8-65 mm). The particle porosity can reach 85%, meaning that 85% of the total volume consists of “air” and only 15% of solid material. Its high porosity gives pumice good thermal insulating properties and makes it very light.

Old pumice deposits in areas with once-active volcanoes are covered with a 0.2-1 m thick layer of humus. When quarrying it, care must be taken to ensure that no humus is mixed into the pumice. If a large area is being mined, e.g. for a housing project, the humus should be replaced afterwards to prevent erosion and consequent ecological damage.

Additional site-specific information on pumice deposits is available from the various national geological institutes and/or soil research offices.

Pumice P12.GIF
Figure 8: Pumice extraction

What properties does pumice have?

Pumice has excellent properties. As a building material it is:

  • very light
  • inexpensive
  • refractory
  • resistant to pests
  • easy to work with
  • sound-absorbent
  • heat-insulating
  • temperature-balancing (Figs. 9, 10 and 11)

But it also has some negative properties like:

  • the lower compressive strength of pumice concrete, as compared to concrete containing other, heavier aggregates
  • the tendency of its edges and corners to break off more easily than those of heavy concrete
  • its lack of frost resistance when wet

Consequently, pumice building material should not be used for:

  • foundations
  • components with constant exposure to water, e.g. in showers
  • components subject to heavy traffic, e.g. stair treads and floor tiles

Pumice P13.GIF
Figures 9, 10, 11

How can pumice be made into building members?

A few expedients that facilitate working with pumice are required for turning it into building members, e.g.:

– some means of hauling the pumice from the deposit to the building site (Fig. 12);

Pumice P14A.GIF
Figure 12: Various means of transportation

– various tools like a wheelbarrow, shovel, buckets, saw, hammer, nails, spirit level, a folding rule, trowel, plumb bob, set square, plastic sheeting, etc. (Fig. 13).

Pumice P14B.GIF
Figure 13: Various tools

– an adequate supply of natural pumice (amounting to, for example, about 5600 kg, or 7 m³ for a house with 30 m floor space). A wheelbarrow holds about 0.15 m³, meaning that about 45 wheelbarrow loads would be needed to build the house;

– wooden moulds for bricks, moulds, etc. and/ or a press for making cavity blocks (Fig. 14: cf. Figure 34, p. 31).

In addition, a roofed-over, level work area is needed. The pumice being processed should have a particle-size distribution of 1 – 16 mm. The requisite cement should be Portland cement with normal compressive strength, to which lime or pozzolana can be added. Pumice building members can also be made exclusively with lime, as described in Chapter 3. The cement and lime must be kept dry, and there should be enough on hand to last for a full week of work. The gauging water should be clean; unpolluted rainwater is well-suited. How to make the moulds is described in Chapter 3.

In general, pumice building members are classified as lightweight concrete, since they are produced and processed in a similar manner, the main difference being that the aggregate—namely the natural pumice—is very light, porous and water-absorbent, so that such material has to be worked somewhat differently than normal-weight concrete.

As a rule, natural pumice is first saturated with water and then mixed with cement or lime, poured into the prepared moulds, compacted (either manually or by mechanical means), removed from the mould and stored to set and cure.

What sets pumic material apart from normal-weight concrete is that pumice concrete is usually soil-moist, i.e. used with relatively little gauging water and only small amounts of fine-grain aggregate—enough to cover the pumice particles with cement paste, but not enough to fill the cavities between the particles of aggregate. Consequently, pumice building components normally have a porous not quite smooth surface like that of nor mar-weight concrete. If so desired or necessary, e.g. for facade tiles, fine aggregate like sand can be added to obtain a smooth surface.

What kind of buildings can be made of pumice?

Pumice-based material can be used for building various kinds of structures:

  • single-storey homes
  • apartment buildings (up to four storeys)
  • workshops and storehouses
  • schools

Pumice P15.GIF
Figure 15: What kind of buildings can be made?

This book deals with the construction of single-storey homes, for which pumice building materiel can be made into (cf. Fig. 15)

  • pumice concrete solid blocks (solid pumice bricks),
  • pumice concrete cavity blocks,
  • pumice tiles,
  • pumice panels/planks,
  • in-situ pumice concrete,
  • special-purpose pumice building members (cf. Chapters 4.6 and 5)

Chapter 3 describes how prefabricated pumice wall members can be used for building houses.

Pumice-plank and pumice-panel homes are houses made of prefabricated members. After laying the foundation, the individual members (mainly the wall members) are prepared and used to erect the house on the foundation slab. This mode of construction is expecially well-suited for collective self-help measures in which several families wish to build the same kind of house, because erection of the plank or panel walls requires the work of several people at once (Fig. 16). One of the main advantages is the comparatively short erection time.

Pumice-concrete brick houses are built in a similar manner to heavy-clay brick houses, i.e. the masonry consisting of relatively small pumice bricks is built up on a solid foundation in the traditional manner. This method yields very individual homes and serves well for renovating or expanding existing homes.

Precast pumice-concrete building members

This chapter offers some practical self-help information on how to make and use simple pumice building components and members.

The following activities are explained:

  • making simple pumice-concrete solid bricks
  • making simple pumice-concrete cavity blocks
  • making simple pumice-concrete wall panels
  • making wall-length reinforced pumice concrete hollow-core planks

Such building members can be made using elementary do-it-yourself techniques without complicated tools and implements and may then be used for building a simple home.

The essential raw material is, of course, pumice. Consequently, the first step should be to find out where the raw material can be obtained, either by quarrying it or buying it from an inexpensive source. Then comes the decision as to how well the Chapter 2.3 conditions are met, and whether or not one’s own handicraft skills and available time will suffice for making the pumice concrete needed for the prefabrication work (solid or cavity bricks, planks or panels).

Pumice P17.GIF
Figure 17: Pattern for sketching out a self-help builder’s home.

In preparing one’s own pumice-concrete home building project, the following checklist could be valuable:

My property has an area of … m².

Pumice is available within a radius of … km. I have the means to buy and haul cement and lime. 1 bag costs US ..
There is an adequate supply of water located … km away.

I either own or can borrow the following tools:

  • shovel
  • pick
  • hammer
  • bucket(s)
  • wheelbarrow
  • trowel
  • nails
  • boards
  • saw

I have either made concrete before or know a mason and one or two friends who would be willing to help me make the building members and erect my house.

Enter your own ideas for a house in Figure 17. There are many ways to design a floor plan, depending mainly on the nature of the property upon which the house is to be built. Figure 18 shows several examples of common floor plans as a guideline. Fill in the following list as a basis for calculating the cost of construction:

The house I am planning to build has :

….. m² floor space,
….. m² wall area,
….. windows measuring ….. cm by ….. cm,
….. doors measuring ….. cm by ….. cm, a floor made of …..
….. m² roof made of ….. other important characteristics:

The property for the house will cost an estimated US …..

In order to calculate the quantities of building material needed for the house as planned, the following technical data must be known:

– 1 m³ pumice concrete contains:

  • 3 bags of cement (= 150 kg)
  • 600 kg pumice material
  • 250 litres of water

– The same cubic meter of pumice concrete will yield:

approx. 500 solid bricks (24 — 11.5 — 7 cm), approx. 120 cavity
blocks (40 — 15 — 20 cm with 2 cavities),
approx. 25 pumice panels (100 — 50 — 7 cm), 12 wall planks (200 — 50 — 10 cm, with cavities),

or, in other words:

– One bag of cement (50 kg), 200 kg pumice and 80 litres of water are needed to make 0.33 m³ pumice concrete.

– Thus, 1 bag of cement is enough for making:

165 solid bricks (24 — 11.5 — 7 cm),
40 cavity blocks (40 — 15 — 20 cm with 2 cavities),
8 pumice panels (100 — 50 — 7 cm),
4 wall planks (200 — 50 — 10 cm with cavities)

For a house with 30 m³ floor space, the following quantities are needed:

2500 solid bricks (24 — 11.5 — 7 cm) or
500 hollow blocks (40 — 15 — 20 cm with 2 cavities) or
64 pumice panels (100 — 50 — 7 cm) or
36 wall planks (200 — 50 — 10 cm with cavities)

Pumice P19.GIF
Figure 18: Selection of four 30 m² plans


How is pumice processed?

The pumice gravel is screened to separate the coarse and fine fractions and remove soil contamination. Then, the pumice is mixed with carefully measured amounts of cement and water to produce a batch of lightweight concrete. Careful mixing is very important for ensuring that the pumice concrete will be of uniform quality.

The mixture is filled into moulds (the dimensions of which vary, of course, depending on what kind of building member is being made) and then compacted by shaking and tamping. Then, the moulds are carefully removed and the block (or plank, panel, brick, etc.) is laid out to dry. After four or five days, the individual pieces can be stacked and left to cure and dry for at least another four days. After another 20 days, they are sufficiently transportable and can be used any time after that. Walls made of pumice members should be rendered/stuccoed to obtain a smooth finish and keep water out of the masonry. (The processing of pumice building members is shown schematically in Figures 19 and 20.)

Pumice P20.GIF
Figure 19: Production process for pumice building members (part 1)

The proper mixing ratio is achieved as follows: first, put together a suitable particle size blend. The heavier the end product should be, the more fine material and cement you will need.

Pumice P21.GIF
Figure 20: Production process for pumice building members (part 2)

The consistency of the mixture should always be such that the large particles touch each other, providing mutual support, while the fine aggregate materials more or less fill in the spaces in between. Good pumice cement usually consists of four parts mixed pumice, one part Portland cement and one part clean water. Mix the parts by hand or in a mixing machine until the material takes on the appearance of soil-moist light weight concrete of uniform colon.

Use the mixture as quickly as possible (within 30 minutes at the most) and do not let it even begin to dry out beforehand. In most cases, the described mixing ratio will be just right. If, however, the pumice is already moist and/or has a less-than-optimal particle-size composition, add more pumice, sand, cement or water as necessary (cf. Fig. 21).

Pumice P22.GIF
Figure 21: Proper moisture content of pumice-concrete

Heed the following points in preparing your pumice concrete:

  • Use only clean pumice
  • Saturate the pumice with water prior to mixing
  • Use only new cement
  • First mix the presaturated (soil-moist) pumice with cement; then add water and mix thoroughly to obtain a moldable mix
  • Compact the mixture well, but not excessively
  • Keep precast building members out of the sun and cover them with, say, wet cement bags to keep them from cracking
  • Keep building members out of the rain
  • Let pumice bricks, blocks, planks and panels dry for at least 28 days, or one month, prior to use
  • Stack building members on a level base
  • Handle them carefully to avoid breaking off their edges
  • Remember that pumice building materials can also be made with lime instead of cement

3.1.1 Making building blocks from pumice and lime

Building blocks can be made of natural pumice and lime. Indeed, such blocks used to be quite common. However, careful consideration must be given to the characteristics of the lime.

In the first place, use only hydraulic—or better—eminently hydraulic lime. Dolomitic or magnesium lime, i.e. lime with a somewhat grey colour, is preferable to fat lime, i.e. chalk-coloured lime, for making good pumice. Lime blocks, thanks mainly to the fact that the grey types, as the name implies, contain more magnesium, which reacts with the silica fraction give the finished product superior strength properties. On the other hand, whatever lime is used should contain as little salt as possible, particularly in the form of sulfuric acid, because salt causes efflorescence and detracts from the blocks’ mechanical strength.

To obtain pumice-lime blocks with strength values exceeding 20 kg/cm²:

– the exact chemical composition of the lime and all pumice materials under consideration should be ascertained by way of careful chemical analysis, and

– sample blocks and compression strength test specimens should be prepared.

In general, the following mixing ratios are recommended:

1 m³ pumice (slightly moist, but not dripping wet)
150 kg hydraulic lime gauging water as necessary or
3 m³ pumice (slightly moist, but not dripping wet)
250 kg hydraulic lime
100 kg Portland cement gauging water as necessary

The latter batch should yield about 1000 pumice-concrete solid bricks measuring 25 — 12 — 10 cm and displaying a compression strength of roughly 25 kg/cm² after approximately 3 months’ curing time.

It is extremely important to realize and act on the fact that pumice-lime bricks need a much longer curing time than do pumice-cement bricks. They should be allowed to cure a good three to six months to develop adequate stability and compressive strength prior to transportation and use.

Accordingly, it’s better to make solid bricks than cavity blocks out of pumice-lime mixes, since the thin walls of the latter are much more susceptible to breaking and therefore require more caution in their manufacture and use.

What can you make with pumice?

Once the pumice-concrete mixture consisting of pumice, cement and water has been properly prepared, it can be poured into various moulds to produce different kinds of wall members, e.g. pumice-concrete tiles/panels and reinforced pumice-concrete hollow-core planks (cf. Fig. 22).

Pumice concrete should not be used for making building members that will be exposed to heavy wear and tear, e.g. stairs, nor is it suitable for building members that are liable to have constant contact with moisture.

3.2.1 Pumice concrete

Lightweight pumice concrete is made in the same manner as normal-weight concrete, except that natural pumice takes the place of sand and gravel. To make pumice concrete from the basic materials, pumice, cement and water, follow these steps:

  •  The first step after the raw pumice is delivered to the intended production site is to remove any humus and other impurities by screening or desilting as necessary.
  •  The second step is to establish the particle-size spectrum of the pumice material. To obtain a good pumice concrete, the particle -size distribution should be about 1-16 mm, i.e. the pumice should have roughly 40 per cent particles measuring 1 – 3 mm in diameter, 25 per cent particles measuring 3 -7 mm in diameter and 35 per cent particles measuring 7-16 mm in diameter.

Pumice P24.GIF
Figure 22: Four pumice-concrete building members

If the particle-size distribution of the raw material does not approximately correspond to the above, it will have to be screened as shown in Figure 23.

Frequently, it will suffice to screen off the particles that are larger than 16 mm, perhaps replacing them with sand.

Pumice P25A.GIF
Figure 23: Screening the raw material

  •  The third step is to add cement and water to the pumice gravel to produce pumice concrete, preferably with the aid of an electric or diesel-powered mixer. If none is available, the concrete can be mixed just as well with a shovel on a clean base or in some kind of big tub (Fig. 24).

Pumice P25B.GIF
Figure 24: Hand-mixing system

How much cement and water are needed depends greatly on the physical condition of the pumice material, especially its inherent moisture and particle-size distribution. As a rule of thumb though, four parts pumice to one part cement and one part water is about right (Fig. 25).

Pumice P25C.GIF
Figure 25: Volume indication of quantities

Pumice concrete should be soil-moist, i.e. it should have no excess water. The moisture level is right if the mould surrounding the concrete can be removed immediately after compacting without having the shaped piece fall apart (Fig. 26).

Pumice P26A.GIF
Figure 26: Immediate removal of forms possible

3.2.2 Pumice concrete solid bricks/blocks

The least complicated kind of wall member for do-it-your-self production by people with little or no handicraft experience is the simple solid pumice brick (Fig. 27). The dimensions can be chosen at will, but adhering to a standard commercial brick format is recommended. If the bricks are to be used for repairing existing walls, they naturally should be of the same size as the bricks or blocks in the old masonry.

Pumice P26B.GIF
Figure 27: Pumice-concrete solid brick.

Elementary-type pumice-concrete bricks are best suited for use in filling out concrete skeleton structures, but are also good for putting up self-supporting walls. Particularly in areas where no loam or clay is found, pumice bricks serve well as alternative wall-building members, assuming, of course, that natural pumice is available (Fig. 28).

The production of pumice concrete solid blocks measuring 49 — 24 — 15 cm is described below. Such blocks are easy to make in a self-help situation.

First, make a simple wooden mould with inside dimensions corresponding to the desired block format (Fig. 29). Normal, smoothly planed boards or square timbers make good box-mould building material. In making the box mould, be sure that it will be easy to remove from the freshly compacted block, i.e. that it is either easy to take apart and put back together or has such smooth inside faces that the block slips out easily.

Pumice P27.GIF
Figure 29: Wooden mould for pumice-concrete solid brick, including board

Place the box mould on a smooth, level base, or better yet on a smooth backing board. Try to have a large number of such boards on hand, depending on how many blocks are to be produced in a certain length of time.

Pour the pumice concrete into the mould(s) and compact it by tamping with a wooden or iron compactor (Figs. 30a and 30b). Smooth off the top with a lath (strike board). If the concrete is soil-moist, the box mould can be removed immediately after the concrete has been compacted (Figs. 30c and 30d). Clean it with water for immediate reuse. If the pumice concrete mixture is right, the freshly compacted block, the so-called “green compact” will not lose its shape, i.e. crumble or sag.

Pumice P28.GIF
Figure 30: Forming pumice-concrete solid bricks

Give the green blocks four or five days to harden before stacking or otherwise handling them. Subsequently, they will require another four days of hardening before they can be transported. All in all, a curing time of 28 days, i.e. one month, is required before they can be placed.

The dimensions 50 — 25 — 12 cm and 30 — 24 — 11.5 cm make a good choice for commercial-scale production of handstruck blocks/bricks, because one and the same kind of block/brick can be used for putting up a 30 cm thick wall, a 24 cm thick wall or an 11.5 cm thick wall.

3.2.3 Pumice concrete cavity blocks

With a little practice and skill, pumice concrete cavity blocks are also easy to make in small quantities. The size of the wooden mould is more or less a question of personal preference, but a 49 — 24 — 15 cm format with two cavities is recommended (Fig. 31). With a view to facilitating placement of the blocks, it’s advisable to leave the cavities open at one end only. That way, the mortar is easier to distribute around the supporting surface without having it fall into the cavities (Fig. 32). Since the blocks are supposed to be removed from the moulds immediately after they are compacted (so that the wooden moulds are immediately available for reuse), the inside of the moulds should be made as smooth as possible. Some sort of sheet metal lining serves exceptionally well. Considering the handmade nature of the finished blocks, either round plastic tubing or blocks of wood would be the best choice for use as cores for forming the cavities, since both are easy to twist out of the green product without damaging the cavities.

Pumice P29A.GIF
Figure 31: Wooden mold for two-cavity blocks

The production of concrete cavity blocks requires careful work to avoid damaging the corners and edges of the blocks when the moulds are removed. The main things to watch for are that the pumice concrete is neither too dry nor too wet and that it’s carefully compacted.

Pumice P29B.GIF
Figure 32: Spreading mortar on a cavity block

  • Follow this procedure for making two-cavity pumice-concrete blocks:
  • Place the wooden mold on a support (wooden board)
  • Cover the bottom of the mould with about 2 cm of pumice concrete (Fig. 33a)
  • Put the core pattern (for plastic tubes or wooden blocks) on the mould (Fig. 33b)
  • Insert the tubes or blocks for the cavities
  • Remove the pattern
  • Fill the remainder of the mould with pumice concrete and compact it well (Figs. 33c and 33d)
  • Then, slowly and carefully pull the plastic tubes or wooden blocks out of the mould and remove the mould itself (Fig. 33e)

Pumice P30.GIF
Figure 33: Forming pumice-concrete two-cavity blocks

Leave the block on the board to dry for 4 – 5 days. Then stack the blocks to harden for another 4 days. After a total of 28 days, the blocks will have cured sufficiently for transportation and use. Handle the blocks with care, because they break more easily than solid blocks.

If you wish to produce large numbers of cavity blocks, use either steel molds instead of wooden moulds or, better, a simple hand-operated mechanical press that compacts the blocks and ejects them from the moulds.

Pumice P32A.GIF
Figure 36: Filling the corners with concrete and reinforcing bars

Since cavity blocks have relatively thin walls (approx. 2 – 3 cm), the pumice concrete should have a maximum particle size of about 10 mm, i.e. any fraction above 10 mm will have to be screened out of the pumice gravel prior to mixing the concrete. Screening can be accomplished using simple wire screens with mesh sizes of 10 mm (approx. 3/8″) and 7 mm (approx. 1/4″). The recommended mixing ratio reads:

  • 2 parts pumice, 1-6 mm in diameter
  • 2 parts pumice, 6 -10 mm in diameter 1 part (Portland) cement

The advantage of cavity blocks is that they weigh less than solid blocks/bricks, which also means that less pumice concrete (and, hence, less cement) is consumed in making enough blocks for a wall of a given size. An additional advantage is that the cavities situated at the corners of the house can be filled with concrete and reinforcing bars to yield a strong framework’ which can be very important in areas subject to earthquakes (Fig. 36). To do so, ram the reinforcing bar (or some other round tool) through the block bottoms to get wall-length cavities at the corners (Fig. 37).

Pumice P32B.GIF
Figure 37

Pumice concrete cavity blocks are useful above all else for filling out skeleton structures, but they are also suitable for making load-bearing walls. Different house-building systems based on cavity blocks are discussed in Chapter 4.3.

3.2.4 Pumice wall panels

How self-help builders with little or no training can use pumice to make simple wall panels measuring 100 — 50 — 5 cm or 100 — 50 — 7 cm is described below. Such panels can be used in any of several time-tested special-purpose house-building systems.

The main merit of the relatively small format is that it makes the panels relatively light and accordingly easy to produce, haul and handle—just right for do-it-yourselfers. A panel width of 50 cm and length of 100 cm combine well for a 2.00-m wall height, and openings for doors and windows can be made by simply leaving out a number of panels at the appropriate places.

To make such pumice-concrete panels, proceed as follows:

Make a simple wooden box mould out of 5-8 cm thick boards. If a large number of panels are needed, it would be a good idea to make several identical moulds. That way, the panels can be stacked to save space. The long sides of the panels are supposed to be grooved. To make the grooves, use strips of wooden trim or plastic tubing (Fig. 38a). Later on, when the panels are being placed, the grooves must be filled with mortar to obtain strong joints. For details on wall construction with pumice-concrete panels, refer to Chapter 4.4.

Pumice P33.GIF
Figure 38: Forming pumice-concrete panels

The panel-making area must be absolutely level. Each panel should have its own support made of smooth sheet-metal or wood. If nothing else is available, smooth paper or plastic sheeting can be laid out under each mould/panel, as long as the ground is perfectly level.

Considering the size of the panel, it would be a good idea, but not absolutely necessary, to include some form of iron reinforcement consisting of, say, a lattice arrangement of 10 mm (3/8″) reinforcing bars sized to match the panels’ dimensions. Any panel that will be subject to bending stress (sag), though, should have at least two such bars running lengthwise with several bends/curves (Fig. 38b).

Pumice P34.GIF
Figure 39: Mould for hollow-core planks

For poring the panels, prepare a soil-moist pumice-gravel concrete, consisting of four parts pumice gravel to one part cement, and fill the wooden frame with it as described in Chapter 3.2.1. Place the reinforcing lattice such that it “floats” at the centre of the panel; smooth the surface of the panel with a strike board or trowel (Figs. 38c and 38d). Leave the panels on the ground to set and harden for about five days, after which they can be handle and stacked. Then give them 25 days to cure prior to transportation and placement. In loading the panels for transportation, be sure to protect them against impact and bending, i.e. it’s better to arrange them in an upright position instead of laying them flat.

If the floor in question will not be subject to heavy loads, reinforced pumice-concrete cavity planks can be used in place of the reinforced hollow girders (cf. Fig. 40, Chp. 3.2.5). Planks up to 10 cm thick, however, can only be used to span not more than than 3.0 m. In homes of simple construction, however, such reinforced planks can serve well, as long as careful attention is given to reinforcement, installation and handling, in addition to clarification of the acceptable span width with the aid of a stress analyst (structural engineer).

Pumice P35.GIF
Figure 40: Forming pumice-concrete hollow-core planks

The prime use for such simple building panels is for filling in skeleton structures, although they can just as well be used for repairing existing walls and building new houses. Consider for example the house described in Chapter 4.4. It consists of a skeleton made of channel-section steel into which the panels are inserted. An alternative example consists of a load-bearing wooden framework and inserted panels.

Pumice P36.GIF
Figure 40 (2)


Pumice P37.GIF
Figure 40 (3)

3.2.5 Reinforced pumice-concrete hollow -core planks

Compared to the simple type of panel described in the preceding chapter, it takes somewhat more skill, tools and technical equipment to produce reinforced pumice-concrete hollow-core planks. Consequently, this approach is more suitable for collective self-help building projects than for individual homes. Since easy handling of building members is an important criterion in connection with self-help building projects, care should be taken to avoid making excessively large planks that could not be carried by hand. A maximum length of 250 cm and a maximum width of 50 cm are recommended. The planks used in the model homes discussed in Chapter 4.5 measure 220 — 50 — 10 cm. Planks of that size are just small enough to be carried and placed by four workers.

A relatively large area is needed for producing hollow-core planks. Especially the casting area has to be absolutely level, hard-wearing and easy to clean. The plank moulds should be made of solid wood, because they will have to be used repeatedly (Fig. 39). Longitudinal cavities are necessary to save weight. To make them, place plastic tubes or steel pipes in the moulds and pull them out after the planks have been compacted. To cast the planks, place the fully assembled wooden moulds on a perfectly smooth and level floor panel or on ground covered with plastic sheeting. Alternatively, the floor panel can be coated with used oil before the moulds are filled. Soil-moist pumice concrete prepared as described in Chapter 3.2.1 should be used for making the planks.

First, pour a 2 or 3 cm thick layer of pumice concrete into the properly prepared mould and carefully tamp it with a broad hand-held compactor. Even better results can be achieved with a roller, e.g. a steel pipe filled with concrete (Figs., 40a and 40b). Try to get the surface as level as possible. Next, insert the pipes or tubing through the holes in the short ends of the moulds (Fig. 40 c). Place thin reinforcing rods (do not forget to have them ready) between the core tubes/ pipes (Fig. 40d). Now, fill out the interspaces with a second layer of pumice concrete that just barely covers the pipes/tubes. Again, carefully tamp the concrete with a broad compactor (or use a roller). Then, pour the third and last layer of pumice concrete, compact it, and strike off the surface with a straightedge lath, subsequently smoothing it over with a trowel (Figs. 40e and 40f). Now, carefully twist the pipes/tubes out of the mould and remove the mould from the green plank. Leave the planks on their bases to set and harden for about seven days. After that, they will be durable enough for lifting and carrying. They must be transported in an upright position (as opposed to lying flat) and will require a total of 28 days curing prior to use (Fig. 41). With a view to achieving uniform quality, the planks should, if possible, be prepared in series in a small production installation. That, in turn’ will require the availability of several identical moulds and pumice concrete of uniform quality.

Reinforced pumice-concrete hollow-core planks serve well as filler members in various types of frame construction. A simple model house made of load-bearing hollow-core planks is described in Chapter 4.5.

3.2.6 Special-purpose pumice-concrete building members and their applications

Channel blocks can be very useful (Fig. 42) as form blocks for peripheral tie beams, as lintels for doors and windows and as filler blocks for anchoring steel door hinges, wall ties, etc. (Fig. 43).

Pumice P38.GIF
Figure 43: Channel form block in a tie-beam configuration

Channel blocks are made in much the same manner as cavity blocks, except that the core (block of wood) is not placed at the centre, but flush with one side of the mould. The walls of channel blocks should be at least 3 or 4 cm thick to make them strong enough to cope with the pressures that arise in connection with pouring and compacting the pumice concrete.

Closed, square hollow blocks serve primarily as form blocks for columns and as chimney blocks (Fig. 44). The blocks must be carefully aligned during placement, or there will be danger that the concrete could push them out of line, resulting in a crooked column. Such blocks serve well as chimney blocks if the clear cross section measures at least 10 — 10 cm and the walls are at least 5 cm thick.

Pumice P39A.GIF
Figure 44: Chimmey block

Naturally, attention should be paid to dimensional accuracy in fabricating the blocks in order to obtain straight, well -functioning chimneys.

Pumice P39B.GIF
Figure 45: Masonry corner with channel block serving as support fromwork

Fine-grained pumice concrete can also be used to make diverse kinds of vent blocks that provide through-wall ventilation without letting in sizable vermin or other uninvited guests (Fig. 46). Such blocks also serve as ornaments and in the construction of ventilated storerooms. They are made in a manner similar to that used for producing cavity blocks, as described in Chapter 3.2.3. However, we recommend not trying to make blocks of very complicated shape, because pumice blocks are never as smooth as those made of normal-weight concrete.

Pumice P39C.GIF
Figure 46: Vent block

Yet another application for pumice-concrete blocks are intermediate floors. So-called pumice-concrete “hollow floor fillers” can be used in constructing ribbed floors (Fig. 47), e.g. when there is a shortage of form – work material, since such floors consist exclusively of prefabricated members.

The load-bearing beams, i.e. “lattice girders with concrete flanges” are suspended between the walls in a carefully aligned arrangement, with spacing to accommodate the hollow floor fillers. Then the fillers are placed side by side on the concrete flanges of the lattice girders. Check the visible under face, the seating, the end blocks, etc. and install any supplementary reinforcement that may be considered necessary. After that, place a 5 cm-thick layer of pumice concrete over the fillers. The main function of the pumice in such floors is to minimize concrete consumption and reduce the weight burden in the tensioned zones of the floor.

With the requisite accuracy of static analysis, orderly installation and a small-scale industrial production mode, self-help groups can manufacture so-called “beam floors with pumice-concrete hollow-core plank fillers.” The precast hollow planks should measure about 30 — 30 cm, with a length of 3 – 4 m, and have structural-iron reinforcement in their tension zone. They’re placed side by side then filled with concrete. This yields a very sturdy floor that will carry relatively heavy loads, depending on the span width, reinforcement, and the thickness of the pumice-concrete hollow girders.

This article was excerpted from Building with Pumice (GTZ, 1990, 86 p.). Reposted from Appropedia. © 1990. Open Access.

November 23, 2013 |

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