There are five common adhesives or cements which work to turn sand into sandstone at low temperatures: plaster of paris, portland cement, slaked lime, clay, and sodium silicate.
Plaster of paris (dehydrated gypsum, i.e. calcium sulfate hemihydrate) is soft and weak. It is inert until exposed to water; then it sets in about 45', producing a fair bit of heat. It will set underwater, is nontoxic, and is a bright white. If used outdoors, rain will gradually erode it. It introduces under 100μm of surface roughness. On MercadoLibre 30 kg goes for AR$162 (at AR$18.20/US$, that’s US$8.90 or US$0.30/kg).
Portland cement (calcium silicates with some aluminum and iron silicates) is strong and hard. It takes about 6 hours to set, which can happen underwater, but part of the setting process involves absorbing CO₂ from the atmosphere. It is highly alkaline and can cause chemical burns. It has a substantial thermal coefficient of expansion, which can cause cracks. The results are not suitable for high-temperature use (e.g. metal casting) because the trapped water will cause steam explosions. It is typically gray. On MercadoLibre 50 kg goes for AR$142 (US$7.80 or US$0.26/kg); the white grade costs about three times as much.
Slaked lime (calcium hydroxide) can be mixed with portland cement or applied alone. It sets entirely, over the course of hours to days, by absorbing carbon dioxide from air, and consequently will not set underwater. It’s nearly as strong and hard as portland cement, but has a much smaller thermal coefficient of expansion. It’s even more alkaline. The finished material is porous, although it can be waterproofed with soap, forming materials known as qadad and tadelakt. It forms a brilliant white. On MercadoLibre 25 kg goes for AR$67 (US$3.70 or US$0.15/kg).
Clay is a class of phyllosilicate hydrate minerals that plasticize significantly with the addition of more water, absorbing the water and expanding, then contracting again and hardening as the water evaporates. This process can happen very rapidly, but the strength of the resulting material is very low. However, the product can then be fired in a kiln, first dehydrating the clay and then sintering the clay particles into a ceramic. Bentonite clays are the traditional adhesive in the greensand used in metal casting; they are among the most plastic and expansive. Ball clays and kaolin fire to a bright white color. 25kg of bentonite costs AR$180 on MercadoLibre (US$9.90 or US$0.40/kg).
Sodium silicate is a class of mixtures of silica and sodium oxide; it sets over the course of days by absorbing carbon dioxide, and can be hardened instantly by the application of concentrated carbon dioxide gas or liquid acids. Some grades are very alkaline, but those are more difficult to find. It dissolves in water, forming a dense solution; generally this requires high temperatures and pressures, so it is usually sold predissolved. I haven’t compared its strength to the other cements mentioned above. It’s sold on MercadoLibre as “bloqueador silicato” for AR$1670/20ℓ (US$91 or US$4.60/kg), although I think I’ve found somewhat lower prices.
There are other cements, including magnesium oxychloride, magnesium phosphate, magnesium oxysulfate, calcium aluminate, and bacterially-decomposed urea with calcium salts, and wollastonite with phosphoric acid. Unfortunately, I don’t know where to get the ingredients here, or they have other disadvantages such as the smell.
Construction sand itself goes for AR$575/m³, and a m³ is about 2.4 tonnes. This is US$32 (US$0.013/kg). So to the extent that you can replace binder with sand filler, you can drop the materials cost of your printed object almost proportionally, while increasing its strength; with binders mostly in the US$0.15–0.40/kg range, except for sodium silicate’s US$4.60, the sand doesn’t comprise half the cost of the object until the binder is down to 3%–10% of the mix, and generally none of these binders are so awesome as to allow you to use so little binder.
For 3-D printing, whether powder-bed or nozzle-deposition, the optimal material would be free; remain liquid or powder forever while in storage, but harden instantly as soon as it was activated; would be very strong; and would be colorless, so that you could pigment it however you liked.