Modern aquascaping has a strong focus on using hardscape elements and having ways to attach hardscape securely and efficiently allows for complex hardscape to be build quickly.
A couple of points on preparing wood & rocks; for wood pieces which have bark which can easily peeled off, the bark should be removed so that the glue can contact directly with the hard wood core. For rocks that are very dusty, clearing off excess rock dust may allow better contact for certain joining methods.
Gel super glue (gel cyanoacrylate)
Gel super glue is good for small joints. It does cure faster than silicon, but does not fill gaps effectively, so good contact area is necessary. A filler such as powdered aquasoil or wood chips can be also be used to fill gaps. It requires a few minutes to cure. It does leave white stains on hardscape when used underwater (it will also cure underwater, and thus can be used to glue plants to hardscape in filled tanks).
Here gel super glue is applied to two wood pieces; however, it needs to be held in place for a couple of minutes for the glue to set. Glue is nearly invisible after bonding.
It takes another 5 to 15 minutes for the glue to harden sufficiently to bear weight. When hard scape is taken apart, the residue is visible but small.
Gel super glue is good for connecting small pieces of hardscape which has good contact with other hardscape piece. It is easy to hide and if the contact is good, can bare quite a bit of weight when left to fully cure. However, it is best used on pieces that rest on the hardscape, as the glue takes a couple of minutes to set (if the piece cannot support its own weight you would need to hold it in place for a few minutes).
Smaller joints here are connected with super glue, while larger joints were connected using liquid super glue and cigarette filter. Total construction time is less than an hour for this aquascape.
Gel super glue is also used to attach plants to hardscape. All plants on hardscape in this picture were attached using super glue.
Kneadable expoxy / Epoxy putty
This comes in paste that consists of two different chemical components. It is then kneaded by hand into a soft putty that can then be used to connect hardscape. It holds very strongly after hardening - it hardens to the consistency of rock. However, it is best used on non-dusty hard scape surfaces which can affect its adhesion. It is good for heavy duty work due to the strong bond. As the paste itself is pliable and deforms to wrap around and fill in uneven pieces of hardscape, it can be used reliably even when contact between two hardscape pieces is poor; such as on porous rock or rough wood.
It does take some time to cure (though it can even cure when wet). The bond starts to harden in a couple of minutes after the two components are mixed. However, it takes about 10 mins to set and longer to harden completely. If the joint does not carry a huge amount of weight, it can be work on after the initial hardening after 10 to 15 minutes.
The other downside is that the epoxy leaves very obvious marks on hardscape upon removal. These can be chipped off using dremel tool or pry off using a screw driver. Epoxy can also be costly to use in large amounts, and takes more handling time (components need to be kneaded just before usage for every joint).
Generally this method gives strong joints, but may be troublesome to use throughout a hardscape project. I would reserve it for large joints that I want a strong bond, especially for joints that need to carry some weight. Multiple smaller joints would be worked faster with super glue.
There are some quality differences between brands, some brands are sticker and adhere better to porous/uneven surfaces.
Kneadable epoxy come in tubes consisting of two compounds. Kneading them together to form a uniform mixture starts the bonding/hardening process.
Due to the ability to fill gaps, uneven pieces that will be difficult to join using gel super glue can be joined using kneadable epoxy. The increased contact area ensures a strong bond. It sets in a couple of minutes, so you'd need to hold the pieces together for some time if they can't hold their position on their own. It takes time (an hour) for the bond to harden fully to weight bearing capacity.
As it is a more obvious filler, it should be used at an angle that is concealed from view if possible - if not it can be covered over with moss later on. As the epoxy sticks to the rock ferociously, it can be a pain to remove when taking the hardscape apart.
The larger rock pieces in this tank were joined using epoxy putty.
Liquid super glue and cigarette filters/Nylon material
This interesting approach was pioneered in Indonesia. When liquid super glue is added to cigarette filter material (cellulose acetate) it causes the sponge like material to harden instantly. The cigarette filter acts as a contact filler between two hardscape parts before liquid super glue is added, so even uneven surfaces can be connected easily with this method. Due to the instant curing of the mixture (faster than any method here), it allows one to quickly build complicated hardscape structures even at odd angles or uneven joints.
Liquid super glue also reacts with other plastic materials such as Nylon. However, it does not react with tissue paper or cotton in the same fashion.
This is my favourite method for connecting hardscape pieces as it allows one to connect hardscape pieces at odd positions easily due to the quick curing time. If a larger piece of cigarette filter is used, the joint can also take a decent amount of weight. This allows one to work quickly from piece to piece, as each piece sets immediately.
The downside of the this method is that the hardscape will be stained with the remains of the cigarette filter/glue mixture upon deconstruction. The combination also produces fumes which are unpleasant, so a well-ventilated area is advised. The white cigarette filter joint is also obvious unless used at hidden angles. Some aquarists rub aquasoil/coffee powder onto such joints to mask the white coloration.
The strength of the joint also depends on the type of liquid super glue used. Some produce a significantly stronger bond with the cigarette filters. Additionally, liquid super glue can be used on its own in a similar fashion to gel super glue above; as long as there is some contact area between the 2 surfaces, it will work. Liquid super glue has the advantage of curing significantly faster than gel super glue.
Liquid super glue is applied to joints sandwiching a cigarette filter. A chemical reaction occurs immediately and the joint sets within seconds. Due to the white filter color, it is best used at less visible angles or covered with moss. Some aquascapers also rub aquasoil or coffee grains into the joint to disguise it.
The joint is weight bearing after mere seconds. Due to this quick joining method, elaborate hardscapes can be built within hours, with hard scape pieces connected in quick succession. If the joints are hidden the cigarette filters will not be obvious; the ones that cannot be hidden are easily covered with moss later on.
Aquascaping foam/insulation foam
This is a more recent product in the market and is used popularly in terrarium and paludarium setups as a filler and bond between rock pieces. It comes in grey/black and other rock colors.
It comes in canisters that emit a stream of expanding foam that sticks to surfaces and hardens after some time. As it hardens into a foam, it is not as strong a bond as epoxy or superglue unless there is a lot of contact surface area. However, it is great to fill gaps between rocks - this is its main usage in aquascaping.
You can (should) pat down the foam to reduce its buoyancy. It has the ability to float hardscape if used in large amounts and not patted down.
Aquascape foam sprays out as a foam that contains a lot of air. It is difficult to control a precise amount, so it is best reserved for larger joints rather than fine work. After being pat down, it sets quickly in a couple of minutes. The foam disguises well as it comes in rock colors.
In this tank, the left and right pillars has been connected using aquascaping foam.
This is similar to the glue that forms a tank's seals. The downside of silicon is that it requires a long curing time, which means that if we are building a structure that requires joints to cure fast and bear weight to continue the next step of building immediately, this method will not be suitable. It sets in a few minutes, so it can be suitable to just prevent stuff from moving about too much. However, Silicon takes 24 hours to cure fully to full strength.
This is best used for simpler hardscape designs where rocks/wood are supported by gravity in their respective positions, and the glue is just used to prevent the hardscape from moving about too much. The entire hardscape can be then glued down at once, then left to dry. The joints will also not hold a great amount of weight unless the contact area is significant and clean. The advantage is that silicon is rather easy to remove upon deconstruction of the hardscape.
I would use this is scapes where the entire structure holds its form due to gravity, and all parts can be glued down at once together and then left to dry.